This is the oldest method for determining oxidability. Based on the oxidation of water samples with potassium permanganate in an acidic solution (Kubel method). Using the example of phenol oxidation, the process can be represented by the following diagram:
4 MnO 4 - + C 6 H 6 O + 4 H + = 6 CO 2 + 4 Mn 2+ + 5 H 2 O
So, they take a precisely measured amount of KMnO 4 and carry out oxidation. The excess permanganate is then bound with oxalic acid:
2 MnO 4 - + 5 H 2 C 2 O 4 + 6 H+ = Mn 2+ + 10 CO 2 + 8 H 2 O
Then the excess oxalic acid is titrated with potassium permanganate to a faint pink color.
This method is mainly used in the analysis of drinking and lightly polluted surface waters with oxidizability< 10мг О/л. С большей ошибкой можно определять перманганатную окисляемость при окисляемости < 100 мг О/л (при этом пробу предварительно разбавляют).
KMnO 4 is a stronger oxidizing agent than K 2 Cr 2 O 7, but under milder conditions of oxidation with permanganate (lower concentration, lower boiling time), many organic substances (alcohols, ketones, fatty acids, amino acids) are not affected by KMnO 4 at all or are oxidized to a small extent. Other substances: phenols, maleic acid are almost completely oxidized to CO 2 and H 2 O. If a mixture of such contaminants is present in the sample, it is obviously impossible to draw a conclusion about the actual content of organic impurities based on the permanganate consumption.
Permanganate oxidability is 40–60% of the true oxidability of organic substances in the sample. Recently, permanganate oxidation is increasingly giving way to the determination of a more accurate COD indicator.
Biochemical oxygen demand (BOD)
The methods considered make it possible to determine the total content of organic contaminants, regardless of whether they can be oxidized by microorganisms under natural conditions. To assess the self-purifying ability of a water body, you need to know the content of biochemically soft substances in the water, i.e. substances that are easily decomposed by microorganisms.
BOD is the amount of elemental oxygen in mg required for the oxidation of organic substances in 1 liter of water under aerobic conditions as a result of biochemical processes occurring in water. Thus, BOD reflects the total content of biochemically oxidizable organic impurities. Since organic impurities are partially oxidized by microorganisms to CO 2 (with the consumption of oxygen), and partially consumed to create biomass, BOD is always less than COD, even if only easily oxidizable organic substances are present in the water.
Let's calculate the specific theoretical COD (TPC sp.) of casein:
C 8 H 12 O 3 N 2 + 16 O = 8 CO 2 + 2 NH 3 + 3 H 2 O
M=184 g - 16×16 g
1 mg - TPK ud.
TPK ud. = 16×16/184 = 1.39 mg O/mg casein
Let's calculate the specific theoretical BOD (taking into account the proliferation of microorganism cells):
C 8 H 12 O 3 N 2 + 6 O = C 5 H 7 O 2 N + NH 3 + 3 CO 2 + H 2 O
M=184 g - 6×16 g
1 mg - BOD spec.
BOD ud. = 6×16/184 = 0.522
As can be seen from the above example, TPC(COD) > BOD.
There are two methods for experimental determination of BOD:
Dilution method lies in the fact that the process of biochemical oxidation of organic substances is monitored by the decrease in the amount of oxygen introduced into the sample bottle during the incubation of this sample. To do this, measure the oxygen content in the sample at 3,5,10, etc. day.
The name of the method comes from the fact that the water being tested is diluted with clean water, free of organic impurities, so that the oxygen it contains is sufficient to completely oxidize all organic substances. To do this, use the results of preliminary determination of COD, conditionally assuming that BOD » ½ COD. This is how the approximate BOD (BOD orient.) is found.
Water contains about 9 mg/l O 2. In order to be able to determine the remaining oxygen with sufficient accuracy after incubation, there must be at least 4 ÷ 5 mg/l left. Therefore, BOD orient. divided by, i.e. by 5 or 4 and find the required degree of dilution.
After dilution, the water is poured into flasks and the O 2 content in one of them is determined. The remaining flasks are incubated in the dark without oxygen. Having determined the O2 content on a certain day, the BOD is determined by the loss of oxygen. Depending on the duration of sample incubation, when determining BOD, a distinction is made between BOD 5 (biochemical oxygen consumption for 5 days) and total BOD. (total biochemical oxygen consumption).
The determination of BOD 5 in surface waters is used to assess the content of biochemically oxidizable organic substances, the living conditions of aquatic organisms, and as an integral indicator of water pollution (see table). BOD 5 values are also used to monitor the efficiency of wastewater treatment plants.
Table. Values of BOD 5 in reservoirs with varying degrees of pollution
It has been established that when water bodies are polluted with domestic wastewater with a relatively constant composition and properties, at the end of the fifth day of incubation, 70% oxidation of organic substances occurs, which can be oxidized biochemically. Therefore, previously it was justified to determine BOD 5 = 70% of total BOD. . Now, when substances that are difficult to biochemically oxidize, or substances that inhibit the biochemical oxidation of organic impurities, enter water bodies with industrial wastewater, the definition of BOD 5 loses its meaning, because sometimes by day 5 the process of biochemical oxidation is just beginning (the lag phase may be due to the gradual adaptation of microorganisms to toxicants). Therefore, monitoring services are moving from the definition of BOD 5 to the definition of BOD total. .
Total biochemical oxygen demand (BOD total) is the amount of oxygen required to oxidize organic impurities before the onset of nitrification processes. The amount of oxygen consumed to oxidize ammonia nitrogen to nitrites and nitrates is not taken into account when determining BOD. For domestic wastewater (without significant industrial admixtures), BOD 20 is determined, assuming that this value is close to the total BOD.
For a more correct determination of BOD, the total oxygen content in sample bottles is determined by 5, 7, 10, etc. day. When the change in oxygen content stops, determine the total oxygen consumption and the total BOD value. To prevent the consumption of oxygen for the oxidation of ammonia nitrogen, in this case an inhibitor is added to the samples - a nitrification suppressor.
Second method lies in the fact that the process of biochemical oxidation is monitored by the decrease in the content of organic substances in the sample. COD is a measure of organic matter content, so BOD is determined by the difference between the results of COD determination before and after incubation.
During the biochemical decomposition of organic substances, they are partially oxidized to CO 2 and H 2 O, and partially converted into biomass. If the amount of organic substances at the beginning of biochemical oxidation is expressed by the amount of oxygen that is required for its complete oxidation, i.e. the COD value of the liquid and solid phases at the beginning of incubation (COD n.f. + COD n.t.), and the content of organic substances at the end of the process (unoxidized and converted into biomass) is also presented in the form of oxygen required for their oxidation (COD k. g. + COD k.t.), then the difference will be equal to BOD:
BOD = (COD n.t. + COD n.t.) - (COD k.t. + COD k.t.),
An inhibitor (for example, ethylene thiourea) is also introduced to suppress nitrification.
If at the beginning and at the end of incubation the COD is determined separately for the liquid and solid phases, then the following indicators can be calculated that characterize the self-purification ability of the test water:
A = COD of liquid /COD n.g. - expresses what part of the organic substances present in the sample is not subject to biochemical oxidation at all.
B = – characterizes the amount of biomass that is formed in the process of biochemical oxidation (biomass growth).
B = BOD t /COD n.g. – characterizes the relative amount of biochemically soft substances.
Time t is selected according to the BOD - time curve (see Fig. 2), highlighting the steepest rising section.
Г = – characterizes the relative amount of biochemically hard organic substances.
The sum of indicators A+B+C+D = 1.
Fig.2. Kinetics of BOD.
The concepts “biochemically soft” and “biochemically hard” are closely related to rate of biochemical oxidation. The process of biochemical oxidation proceeds in accordance with the laws of first order reactions, i.e. the rate of oxidation is proportional to the amount of unoxidized substance remaining.
Goal of the work. Learn to determine the oxidability of water using the permanganate method and carry out primary processing of the results.
Reagents and solutions.
1. 0,01 N solution of KMnO 4 (it is better to prepare from fix channels; in the absence of the latter, take a sample equal to the molecular weight of KMnO 4 divided by 5 and 100, i.e. 158/(5100) = 0.316 g, and dissolve in a volumetric flask by 1 l; the titer of the solution is unstable; with each determination of oxidation it is set anew).
Exactly 0.01 N solution of oxalic acid H 2 C 2 O 4 2H 2 O (prepared from fixanals).
25% sulfuric acid solution (by volume).
Utensils and equipment.
1. 300 ml conical flasks - according to the number of samples.
Glass capillaries or beads are placed at the bottom of the sample flask before oxidation to ensure uniform boiling of the liquid in the flask.
25 ml titration burette.
Pipettes: 5 ml for a 25% sulfuric acid solution, 10 ml for an oxalic acid solution.
A 100 ml graduated cylinder or pipette for measuring the required sample volume.
Electric stove or gas burner.
General information. Water always contains organic substances in the form of suspensions, colloidal or molecular compounds. Currently, there is no sufficiently reliable method to completely isolate organic matter from water and evaluate it quantitatively; therefore, indirect methods are more often used to judge the content of this substance in natural waters. The most common assessment of the amount of organic matter is based on the oxidability of water. Under oxidability understand the amount of oxygen consumed for the oxidation of organic matter (under certain conditions) contained in 1 liter of water.
Depending on the oxidizing agent used, permanganate (oxidizer KMnO 4) and chromate or dichromate (oxidizer K 2 Cr 2 O 7 in sulfuric acid) oxidation are distinguished.
The permanganate method for determining oxidability is the most widely used, especially in fish farming, due to its simplicity; the bichromate method requires more time and labor. By comparing the values of dichromate and permanganate oxidation, one can judge the quantitative composition of organic matter in water.
Determination principle. Oxidation is carried out by boiling the test water for exactly 10 minutes with a solution of potassium permanganate added to the sample. When boiling, atomic oxygen is released from KMnO 4 and easily oxidized organic substances are oxidized by this oxygen. This process can occur in both acidic and alkaline environments. Usually in fresh waters oxidation is carried out in an acidic environment, in sea waters - in an alkaline environment.
2KMnO 4 + ZN 2 SO 4 = K 2 SO 4 + 2MnSO 4 + ZN 2 O + 5O (19)
For better oxidation, it is necessary to add a solution of potassium permanganate to the sample in excess, i.e., so that after boiling the sample remains colored pink. If, when boiling with KMnO 4, the color of the sample becomes brownish, the oxidation should be repeated all over again, diluting the test water by 2.5 or 10 times. The permanganate that did not oxidize the organic matter is reduced with a solution of oxalic acid added to the sample in an amount equivalent to the amount of potassium permanganate solution poured into the water sample before boiling it.
2КМnO 4 + 5С 2 Н 2 О 4 + 3Н 2 SO 4 - К 2 SO 4 + 2МnSO 4 + 10СО 2 + 8Н 2 О (20)
The amount of oxalic acid remaining in the sample, after reducing the excess KMnO4, is titrated with a solution of potassium permanganate. The amount of potassium permanganate solution used for titration is used to judge the oxidability of water, with 1 ml 0.01 N solution of KMnO 4 is equivalent to 0.08 mg of atomic oxygen used for the oxidation of organic matter in the sample.
Progress of determination. Take 100 ml of the test water into a conical flask, add 5 ml of a 25% sulfuric acid solution and heat to boiling. At the very beginning of boiling, add 10 ml of 0.01 to the sample N solution of KMnO 4 and then boil the sample for exactly 10 minutes (it is advisable to measure the time using a watch with a second hand or a stopwatch). It is necessary to ensure that the boiling is uniform and not very violent. For this purpose, before heating the flask with the sample, glass beads or capillaries, well washed with distilled water and dried, are placed on the bottom of this flask.
If a brown tint appears during boiling, the sample should proceed as indicated in the principle of determining oxidability by this method.
At the end of boiling, 10 ml of 0.01 is introduced into the flask with the sample N solution of oxalic acid, mix its contents and titrate the discolored liquid to 0.01 N solution of KMnO 4 until a stable light pink color appears.
Determination of the correction factor to the titer of the CM solutionnO 4 . To determine the correction factor for the normality of the KMnO 4 solution, add 10 ml of exactly 0.01 to the just titrated sample that has not yet had time to cool. N oxalic acid solution and titrate to 0.01 N potassium permanganate solution.
The correction factor is calculated using the formula: K=10/n (*), where P - the amount of ml of potassium permanganate solution used for titration of 10 ml is exactly 0.01 N oxalic acid solution.
Calculation of results. During the determination process, records should be kept in a table (see Table 3) (Appendix).
The amount of oxidability is calculated using the formula:
O = mg/l, (21)
where A 1 is the amount of KMnO 4 solution added to the sample at the beginning of boiling ml; A 2 - amount of KMnO 4 solution used for sample titration, ml; K is the correction factor to the titer of the potassium permanganate solution; B - quantity exactly 0.01 N oxalic acid solution added to the sample after its oxidation, ml; 0.08 - amount of oxygen equivalent to 1 ml 0.01 N KMnO4, ml; V - sample volume taken for analysis, ml.
Calculation results should be rounded to 0.01 mg/l.
The order of work. They study the principle of determining the permanganate oxidation of water in laboratory conditions. Permanganate oxidation of water is determined. Prepare a report.
Report.
CONTROL QUESTIONS
What is water oxidability?
Name the methods for determining oxidability. What are their advantages and disadvantages?
In what units is oxidability expressed?
What is the principle of determining oxidability using the permanganate method?
What is the relationship between the amount of KMnO 4 solution used for titration of the sample and the value of oxidation?
What should you do if, during the process of boiling a sample with a solution of potassium permanganate (during oxidation), the solution in the flask turns brown? What does this indicate?
LABORATORY WORK No. 7
DETERMINATION OF WATER HARDNESS
Goal of the work. Learn to determine water hardness and carry out initial processing of the results.
Reagentsand solutions.
1.0,02 N a solution of Trilon B (disubstituted sodium salt of tetrabasic ethylenediaminetetraacetic acid) is prepared from fixanals. In the absence of fixatives, take 3.72 g of this salt (Na 2 H 2 C 10 H 12 O 8 N 2 2H 2 O) and dissolve it in a 1-liter volumetric flask, gradually bringing the total volume of the solution to the mark.
Buffer solution (dissolve 20 g of NH 4 Cl in distilled water, add 100 ml of concentrated ammonia solution and bring the total volume to 1 liter with distilled water).
Indicator solution (chromgen black) (0.5 g of El-00 indicator is dissolved in 10 ml of buffer solution and the volume is adjusted to 100 ml with ethyl alcohol).
4. Exactly 0.02 N It is advisable to prepare magnesium sulfate (MgSO 4 7H 2 O) from fixanal. Dishes.
1. 300 ml conical flasks - according to the number of samples plus one.
Pipettes: 5 ml for buffer solution; by 25 ml or 10 ml to measure the required sample volume (if it is less than 50 ml); per 10 ml for MgSO 4 solution.
Dropper for black chrogen.
100 ml graduated cylinder.
General information. Overall hardness natural water shows the content of cations of divalent alkaline earth metals and, above all, calcium and magnesium. These elements enter the water during the dissolution of carbonates or as a result of biochemical processes occurring in the upper layers of the soil cover. The hardness of fresh water varies widely. Swamp water and water from ponds filled primarily with precipitation (1-2°N) are especially soft. A hardness of 20°Nem and higher is typical for groundwater and for rivers, ponds and lakes located on soils with easily leached salts Ca and Mg, especially if the waters of these reservoirs are rich in CO 2, which promotes the dissolution of carbonates.
In some southern regions there are lakes whose water hardness reaches 100°N and above.
The amount of calcium and magnesium equivalent to the amount of carbonates and bicarbonates in water is called carbonate hardness. Non-carbonate hardness- the difference between total and carbonate hardness - shows the amount of alkaline earth metal cations corresponding to the anions of mineral acids: chloride, sulfate, nitrate ions, etc.
For fish ponds, water that is too soft and has low alkalinity is undesirable. To meet the need of aquatic organisms for Ca and Mg, water is needed with an alkalinity of at least 2 mg/l and a hardness of about 5° Hem.
Determination principle. One of the most common methods is the trilonometric determination of total hardness with a black chromogen indicator. A certain amount of indicator solution is added to the sample, having previously brought the reaction of the medium to pH 10 with an ammonia buffer. In this case, complexes of the indicator with the cations Ca 2+ and Mg 2+ are formed, and first calcium enters the compound as the more active one, then magnesium:
Na 2 Hind + Ca 2+ 
CaHInd+2Na+ 
(22)
Na 2 Hind + Mg 2+ 
MgHInd + 2Na + (23)
The magnesium complex with the indicator has a wine-red color, as a result of which, after adding the indicator, the entire solution turns wine-red, and then the test liquid is titrated with a solution of Trilon B. There is a transition of calcium and magnesium cations from the complexes with the indicator to the complex with Trilon B, since the complexes of Ca 2+ and Mg 2+ - with the indicator are more dissociated than the complexes of these cations with trilon:
CaHInd + Na 2 H 2 (tril.) = CaH 2 (tril.) + Na 2 HJnd; (24)
MgНInd + Na 2 H 2 (tril.) = MgH 2 (tril.) + Na 2 HJnd. (25)
As a result of this transition, the indicator is restored, and the titrated solution at the equivalent point changes color to blue, since the black chromogen has a different color at different pH values of the medium.
pH 6.3 pH 11.5
NaH 2 Jnd 
Na 2 HJnd 
Na3Jnd (26)
Wine red Blue Yellowish gray
Progress of determination. The amount of sample is taken depending on the hardness value: if the hardness does not exceed 5 mmol/l, take 100 ml of water; if the hardness is 5-10 mmol/l, take 50 ml; with a hardness value of 10-20 mmol/l, the volume of water for analysis is 25 ml and, finally, with water hardness exceeding 20 mmol/l, the sample volume will be 10 ml. In all cases except the first, the total sample volume should be brought to 100 ml with distilled water. When using this method to determine hardness, it should be borne in mind that some metals, such as iron, aluminum, if their content in water exceeds 20 mg/l, and copper if its content is more than 0.3 mg/l, interfere with the determination, creating a blurred color transition at the end of the titration. To eliminate the influence of copper, add 1 ml of a 2% solution of Na 2 S to the water before determination. Place the sample in a conical flask, add 5 ml of a buffer solution and 10 drops of a black chromogen indicator solution and titrate with a solution of Trilon B until the wine-red color changes to blue. Titrate slowly, constantly stirring the contents of the flask in a circular motion. At the end of the titration, the Trilon B solution is added dropwise with pauses of 10-15 s.
Determination of the correction factor for the normality of the Trilon B solution. Take 10 ml of exactly 0.02 into a conical flask N solution of magnesium sulfate, bring the volume to 100 ml with distilled water, add 5 ml of buffer solution, 10 drops of indicator and titrate with Trilon B solution until the red-violet color changes to blue. The correction factor for the normality of the solution is determined by the formula (*), in which P- amount of Trilon B solution used for titration 10 ml 0.02 N magnesium sulfate solution. The correction factor is calculated with an accuracy of 0.01.
Calculation of results. During the determination process, records are kept in tabular form (following the example of Table 1 (Appendix)).
The value of total stiffness is calculated using the formula:
Sa 2+ +Mg 2+ = A N TO 1000/ V mmol/l, (27)
where A is the amount of Trilon B solution used for titration of the sample, ml; N - normality of Trilon B solution; TO - correction factor to the normality of the Trilon B solution; V – sample volume taken for analysis, ml.
The results obtained must be expressed in mmol/l and in German degrees. When expressing total hardness in German degrees, it should be borne in mind that previously it was not metal cations, but their oxides that were determined in water. In particular, when determining water hardness, the calcium content was calculated in terms of CaO, with 10 mg of CaO corresponding to one German degree. Hence, 1 mmol/l hardness corresponds to 1 mmol CaO/10, i.e. (28 mg.1)/10 mg = 2.8°Gem.
Results are rounded to the nearest 0.001 mmol/l and to the nearest 0.1° German.
Report. The report on this laboratory work consists of an oral interview with the teacher. The measurement results are presented in the prescribed form.
CONTROL QUESTIONS
What is total water hardness?
What units express the value of total stiffness?
What is the principle of determining the total hardness of water using the trilonometric method?
How to determine the correction factor to the titer of Trilon B solution?
LABORATORY WORK No. 8
DETERMINATION OF CALCIUM CONTENT (Ca 2 *) AND MAGNESIUM(Mg 2+ ) INWATER
Goal of the work. Learn to determine the content of calcium (Ca 2+) and magnesium (Mg 2+) in water and carry out initial processing of the results.
Reagents and solutions.
1. 0,02 N Trilon B solution (for preparation, see laboratory work No. 6).
2N NaOH solution (80 g of chemically pure NaOH is dissolved in 1 liter of distilled water).
Dry indicator mixture (0.5 g of murexide and 9.5 g of NaCl are thoroughly ground in a mortar and stored in a bottle with a ground stopper).
Dishes.
1. 300 ml conical flasks - but the number of samples is plus one.
Pipettes: 2 ml for 2 N NaOH solution, 25 ml to measure the required sample volume if it is less than 50 ml.
25 ml burette.
100 ml graduated cylinder.
General information. The principle for determining calcium content is the same as for the total hardness of water. The calcium content is determined by the volumetric trilonometric method. Murexide (ammonium purpurate) is used as an indicator, which in combination with calcium is colored bright red. The free form of the indicator in an alkaline medium has a purple color. The sample volume taken for analysis depends on the calcium content: 100 ml with a Ca 2+ content of 0.5 to 2.5 mmol/l, 50 ml with a Ca 2+ content of 2.5 to 5 mmol/l and 25 ml when the Ca 2+ content is more than 5 mmol/l.
The concentration of Mg 2+ in water is calculated by the amount of calcium and the value of total hardness.
Progress of determination. Measure the required amount of test water into a conical flask and adjust the volume to 100 ml with distilled water, then add 2 ml 2 N NaOH solution and 30 mg of dry indicator mixture. Titrate the sample with Trilon B solution until the bright red color turns purple. During the titration process, the liquid in the conical flask should be stirred vigorously.
Calculationresults. Records are kept in tables according to the example of Table 1 (attached).
N(Sa 2+ ) = A N TO 1000/ Vmmol/l,(28)
Where A- the amount of Trilon B solution used for titration of the sample;
N - normality of Trilon B solution; K – correction to normality of Trilon B;
V –
In addition to mmol/l, the Ca 2+ content is expressed in mg/l; for this, the result obtained from formula (25) should be multiplied by 20.04 mg, which corresponds to 1 mmol (equiv.) Ca 2+. The accuracy of determining calcium in water by this method is ≈ 1%.
After determining the total hardness and Ca 2+ content in water, a simple calculation can be used to determine the amount of magnesium (Mg 2+) present in it. To do this, determine the Mg +2 content in mmol/l as the difference between the total hardness, expressed in mmol/l, and the calcium content in mmol/l. Then express the Mg 2+ content in mg/l by multiplying the previous result by 12.16.
Report. The report on this laboratory work consists of an oral interview with the teacher. The measurement results are presented in the prescribed form.
CONTROL QUESTIONS
1. Why is the presence of Ca 2+ necessary in the water of fish ponds?
What is the principle for determining Ca 2+ in water?
How to determine the amount of Mg 2+ contained in water?
What units are used to express the amount of Ca 2+ and Mg 2+ in water?
LABORATORY WORK No. 9.
DETERMINATION OF CHLORIDE CONTENT IN WATER
Goal of the work. Learn to determine the chloride content in water and carry out initial processing of the results.
Reagents and solutions.
1. Silver nitrate solution (AgNO3), 1 ml of which 1 mg precipitates Cl ¯ . Take a sample of 4.791 g of crystalline silver nitrate and dissolve it in a volumetric flask with 1 liter of distilled water.
10% chloride-free potassium chromate solution. Take 100 g of K 2 CrO 4 and dissolve it in a small amount of distilled water, then add a few drops of AgNO 3 until a reddish-brown precipitate appears. Allow the solution to settle, then filter and bring its volume to 1000 ml with distilled water.
A solution of sodium chloride, 1 ml of which contains exactly 1 mg Cl -. Take a sample of 1.6486 g of chemically pure NaCl and dissolve it with distilled water in a 1 liter volumetric flask.
Dishes.
1. Conical flasks with a volume of 300 ml - according to the number of samples plus one.
Pipettes: 10 ml for NaCl solution; per 1 ml graduated for K 2 CrO 4 solution.
100 ml graduated cylinder.
General information. Chlorides are the main component of the salt composition of sea water. In land reservoirs their content is insignificant, with the exception of some lakes located on saline soils or predominantly fed by highly mineralized groundwater. Such reservoirs are found in some places in the southern territories of Ukraine and Russia and in Central Asia. The content of chlorides in these reservoirs reaches hundreds and even thousands of milligrams per 1 liter, although usually their amount in land waters does not exceed 100 mg/l. If the increased chloride content cannot be explained by hydrometeorological conditions, it serves as an indicator of foreign contamination. In terrestrial water bodies, chlorides are determined to obtain the value of mineralization and pollution; in the seas and oceans, the salinity of water is calculated from the amount of chlorides.
Determination principle. Determination of chlorides in fresh water by the argentometric method is based on the precipitation of chlorine anions with a solution of silver nitrate during titration. A solution of potassium chromate is used as an indicator. The chemical essence of what is happening can be represented by the reaction equation:
NaCl + AgNO 3 = AgCl↓ + NaNO 3. (29)
As soon as all the chlorine anions are bound, the reaction begins:
K 2 CrO 4 + 2AgNO 3 = Ag 2 CrO 4 + 2KNO 3. (30)
The formation of the salt Ag 2 CrO 4 will be indicated by a transition in the color of the solution from lemon yellow to reddish, which does not disappear with shaking.
The principle of determining chlorine and salinity of sea water is similar to the principle of determining chlorides in fresh water. However, the technique for determining the chlorine content of sea water has many significant differences from the technique for determining chlorides in fresh water bodies.
To determine chlorine, prepare a solution of silver nitrate of such a concentration that the burette reading when titrating sea water approximately corresponds to the chlorine value. For example, for water with normal (35%) salinity, the chlorine content of which is 19.38% (specific gravity 1.02674), the concentration of silver nitrate will be:
(4.791151.02674) / 2 =36.92 g/l,
where 4.791 is the amount of AgNO 3 precipitating 1 g of chlorine, provided that 15 ml of sea water is taken for titration.
To prepare 1 liter of solution, take a sample of AgNO 3 in the amount of 3.71 g, taking into account the presence of impurities in the AgNO 3 salt, which is produced industrially.
When determining the chlorine content of sea water, special burettes and pipettes are used. These burettes differ from conventional ones by the presence of devices for automatically filling and setting the solution to zero division. Each division of the burette has a volume of 2 ml; the whole division is in turn subdivided into 20 fractional divisions. This allows you to count down during titration with an accuracy of 0.01 ml. Burettes are available in several types for determining different chlorinities.
Pipettes are automatic and have the same capacity - 15 ml. A 15 ml sample is placed in a special titration glass with a thick oval bottom with a capacity of about 300 ml. During titration, the sample is thoroughly mixed with a glass rod.
To check the titer of a silver nitrate solution and determine the chlorine content in the test water, it is necessary to titrate a sample of sea water with a precisely known chlorine value (the so-called “normal” water) under the same conditions as the sample. Normal water is ocean water whose chlorine content is precisely determined and corresponds to the average salinity of the ocean (35%). Normal sea water is produced in sealed glass containers with a capacity of 250 ml, the label of which indicates the exact chlorine value.
Progress of determination. Take 100 ml of the test water into a conical flask (if the water is more turbid, filter it), add 1 ml of a 10% solution of K2CrO4 and titrate with the AgNO3 solution with constant stirring until a stable reddish tint appears. It is advisable to titrate with two “witnesses” (in one flask there is a sample with only K 2 CrO 4, in the other there is a slightly overtitrated sample).
Determination of the correction to the titer of solution AgNO 3 . The correction to the AgNO3 titer is determined using an accurate NaCl solution, 1 ml of which contains 1 mg Cl¯.
Place 10 ml of NaCl solution in a conical flask, adjust the volume to 50 ml with distilled water, add 0.5 ml of a 10% solution of K2CrO4 and titrate with a solution of silver nitrate until the color of the solution changes. The correction is determined by the formula (*), where n is the number of ml of AgNO 3 solution used to titrate 10 ml of NaCl solution.
Calculation of results. All entries when performing work are made in a table similar to Table 1 (Appendix).
C(Cl¯) = (A 1 TO 1000)/ V mg/l (31)
where A is the amount of AgNO 3 solution used for titration of the sample, ml;
1 - the amount of Cl¯ that precipitates 1 ml of AgNO 3 solution during titration;
TO- correction to the titer of the AgNO 3 solution; V- volume of sample taken for analysis.
The results of titration when determining the chlorine content of sea water are calculated using the formula:
WITHl % = a + TO (32)
Where A- corrected burette reading (counting taking into account the burette correction after sample titration), TO- titration correction, which is found from the “Oceanographic tables”. Knowing the chlorine content, the salinity of water is found in the “Oceanographic Tables”; the value of which (g/kg) is expressed in ppm (%).
Report. The report on this laboratory work consists of an oral interview with the teacher. The measurement results are presented in the prescribed form.
CONTROL QUESTIONS
What determines the content of chlorides in water?
What is the principle for determining chlorides in fresh water?
What is the relationship between the amount of AgNO 3 used for sample titration and the Cl¯ content in water?
What is the salinity of sea water and how is it determined?
5. What units are used to express the amount of chlorides contained in water, as well as the value of salinity?
LABORATORY WORK No. 10.
DETERMINATION OF SULPHATE CONTENT IN WATER
Goal of the work. Learn to determine the content of sulfates in water and carry out initial processing of the results.
Reagents and solutions.
Barium chromate (BaCrO 4), chemically pure.
2,5 N hydrochloric acid solution. Take 2.08 ml of concentrated hydrochloric acid and adjust the volume to 10 ml with distilled water.
10% potassium iodide solution.
5% ammonia solution. Take 20 ml of a 25% ammonia solution and adjust the volume to 100 ml with distilled water.
0,05 N sodium thiosulfate solution. Take 12.40 g of NaS 2 O 3 5H 2 O and adjust the volume to the mark with distilled water in a 1 liter volumetric flask.
1% starch solution.
Reagents for determining the correction to the titer of Na 2 S 2 O 3 solution.
Dishes.
1 Measuring cylinders with a capacity of 200 and 100 ml.
2 Conical flasks with a capacity of 300 ml - according to the number of samples plus one.
Filter paper with blue stripe.
Pipettes: 1 ml 3 pcs. (for solution of HCl, ammonia and starch); for 10 ml 3 pcs. (for HCl solution, KJ, KJO 3).
Burette with a capacity of 25 ml.
Volumetric flasks with a capacity of 250 and 100 ml.
General information. Sulfuric acid salts (sulfates) are present in most fresh water bodies in quantities up to 20-30 mg/l. Most sulfates are found in reservoirs located on soils containing CaSO 4 and saline ones, in particular in some southern regions of Ukraine and the European part of Russia.
Sulfates themselves do not have a significant effect on the existence of living organisms, but their high content with an abundance of organic substances in the water can lead to the formation of hydrogen sulfide.
An increased amount of sulfates (more than 20-30 mg/l) indicates foreign contamination of the reservoir. For this purpose, sulfates are studied in water bodies subject to fishery use. In addition, determining the amount of sulfates is necessary for a complete picture of the composition of water and to obtain the value of its mineralization.
The principle of the volumetric method is the precipitation of sulfate ions with barium chromate BaCrO4:
K 2 SO 4 + BaCgO 4 = BaSO 4 ↓ + K 2 СгО 4 (33)
The amount of released chromate ions, equivalent to the precipitated amount of sulfates, is determined iodometrically, after adding KJ and HCl to the sample. The amount of iodine released will be equivalent to the amount of CrO 4 2 ¯ anions in the solution:
2K 2 CrO 4 + 16HCl + 6KJ = 10KCl + 2CrCl 3 + 8H 2 O + 3J 2 (34)
The released iodine is titrated with sodium thiosulfate solution:
J 2 + 6Na 2 S 2 O 3 = bNaJ + Na 2 S 4 O 6. (35)
The amount of sodium thiosulfate used for titration is used to determine the content of sulfate ions in the water.
Progress of determination. The required amount of test water is usually
200 ml, but if the sulfate content is very high, take a smaller volume and dilute the sample with distilled water to 200 ml, place it in a conical flask, add 1 ml 2.5 N HCl solution and heat to boiling.
Carefully pour 500 mg of BaCrO4 into the boiling sample (otherwise steam and liquid may escape) and continue boiling for a few more minutes until the lemon-yellow solution turns light orange.
Then neutralize with a 5% ammonia solution, adding it drop by drop until the orange color changes to lemon yellow, and cool to room temperature. The cooled sample with the sediment is transferred to a 250 ml volumetric flask and the total is adjusted to the mark with distilled water, repeatedly rinsing the flask with the sample with distilled water and pouring this water into the volumetric flask.
The contents of the volumetric flask are mixed and filtered through a dense ashless filter (with a blue stripe) into a 100 ml volumetric flask, discarding the first portions of the filtrate. 100 ml of the filter liquid is transferred to a conical flask, 10 ml of a 10% KJ solution and 10 ml of 2.5 are added N HCl solution and after 5 minutes titrate the released iodine 0.05 N thiosulfate solution, determining the end of the titration using starch.
Determination of the correction factor to the titer of sodium thiosulfate solution.
The correction factor is determined by 0.01 N KJO3 solution as follows. Pipette 10 ml of 0.01 into a conical titration flask N KJO 3 solution, add 0.5 g of crystalline KJ (weighed on a technochemical balance) and 1 ml of H 2 SO 4 solution or 2 ml of 25% H 2 SO 4 or concentrated HCl. The contents of the flask are stirred, then the released iodine is titrated with a solution of sodium thiosulfate, stirring continuously, until it has a faint straw color. To more accurately detect the end of the titration, add 1 ml of a freshly prepared starch solution into the flask: starch in the presence of free iodine turns the solution blue. Titrate the sample until it becomes discolored with one drop of thiosulfate solution.
Titration when determining the correction factor to the concentration of the Na 2 S 2 O 3 solution is carried out twice. The discrepancy in burette readings during parallel titration should not exceed 0.03-0.05 ml. Correction factor K determined by the formula (*), where n is the number of ml of Na 2 S 2 O 3 solution used for titration of 10 ml of KJO 3 solution.
The arithmetic mean value n from two parallel titrations is substituted into formula (*).
Calculation of results. Records during the determination process are kept in a form similar to Table 1 (Appendix). The content of sulfates in water is expressed in mg/l and mmol(equiv.)/l. The calculation formula looks like:
WITH (SABOUT 4 2 ¯) = (2.5 A TO 1,6 1000)/ V mg/l (36)
where A is the quantity 0.05 N sodium thiosulfate solution used for titration of 100 ml of filtrate, K - correction factor to the normality of the sodium thiosulfate solution; 1.6 - amount of mg SO 4 2 ¯, equivalent to 1 ml 0.05 N sodium thiosulfate solution, see reaction equations (33), (34), (35); 2.5 is the conversion factor necessary, since only 100 ml was taken from the total sample volume (250 ml) after precipitation of sulfates for titration; V- volume of sample taken for analysis.
To convert the amount of mg/l of sulfates to mmol(equiv)/l, the value obtained from formula (36) is divided by 48.03, which corresponds to 1 mmol(equiv) of the SO 4 2– anion.
Results for sulfate determinations should be rounded to 0.001 mmol(eq)/L and 0.1 mg/L.
Report. The report on this laboratory work consists of an oral interview with the teacher. The measurement results are presented in the prescribed form.
CONTROL QUESTIONS
1.What does the large amount of sulfates contained in natural water indicate?
2. What methods exist for determining the amount of SO 4 2 ¯ in water?
3. Explain the principle of determining sulfates by the volumetric method.
4. What is the relationship between the amount of Na 2 S 2 O 3 solution used for titration of the sample and the content of SO 4 2 ¯ in it?
5. In what units is the content of sulfates in water expressed?
LABORATORY WORK№11.
DETERMINATION OF HYDROGEN INDICATOR (pH)
NATURAL WATER
Goal of the work. Learn to determine the pH value of natural water.
Instruments and reagents.
Phosphate mixture (KH 2 PO 4 and Na 2 HPO 4 2H 2 O) for preparing solutions with pH values from 5 to 8.
Boroborate and borate-alkaline mixture (Na 2 B 4 O 7 10H 3 BO 3 ; Na 2 B 4 O 10H 2 O and NaOH) for the preparation of buffer solutions with pH values more than 8.
The following organic dyes are used as indicators for pH ranges: methyl red at pH 4.4-6.0; bromothymol blue at pH 6.0-7.6; cresol red at pH 7.6-8.2; thymol blue at pH 8.2-9.0.
Buffer solutions are usually prepared in test tubes, which are sealed after preparing the scale. The exact pH value in the scale tubes is determined using the electrometric method. Ready-made scales of buffer solutions for determining pH are commercially available.
pH meter, electrolytic cell with glass and silver chloride electrodes.
General information. The hydrogen index of water (pH) is a value characterizing the activity or concentration of hydrogen ions and is numerically equal to the negative decimal logarithm of this activity or concentration, expressed in mol/dm 3: pH = -lga H + = - lgC H + , where a H + is the activity of hydrogen ions, CH + is the concentration of hydrogen ions.
In water, the concentration of hydrogen ions is determined by electrolytic dissociation according to the equation: H 2 O 
H + + OH ¯ . In this case, the concentration of hydrogen ions can be calculated from the equilibrium constant of the dissociation process:
TO d = C N + WITH HE ¯ /WITH BUT
where СН + and СН¯ are the concentrations of hydrogen ions and hydroxyl group, respectively, mol/dm 3 . Since the degree of dissociation of water is very small, without introducing a significant error, we can consider the concentration of undissociated water molecules to be a constant value and combine it with Kd into one constant:
K w = K d Sno.
In this case, the equation will take the form; Kw = Sn + Son¯. The quantity K w is called ionic product of water and is constant for a given temperature, so for 25°C K w = 10 -14.
The pH value plays an important role in determining water quality; in river waters its value usually ranges from 6.5 to 8.5, in precipitation - from 4.6 to 6.1, in swamp waters - from 5.5 to 6, 0, in ocean water from 7.9 to 8.3, in mine and ore waters it sometimes reaches 1.0, and in the water of soda lakes and thermal springs - 10.0. The concentration of hydrogen ions is subject to seasonal fluctuations - in winter the hydrogen index for most river waters is 6.8-7.4, in summer 7.4-8.2.
The concentration of hydrogen ions is of great importance for the chemical and biological processes occurring in natural waters: the development and vital activity of aquatic plants, the stability of various forms of migration of elements, the degree of aggressiveness of water towards metals and concrete, etc. depend on the value of the hydrogen index.
To determine the pH value, the electrometric or colorimetric method is used. The electrometric method gives more accurate results.
Determination of water pH. The potentiometric (electrometric) method for determining the pH of water with a glass electrode is the most universal and accurate. Devices that measure pH using this method are called pH meters. Most commercial pH meters allow measurements with an accuracy of 0.05-0.02 pH units in waters with a wide range of mineralization and containing colored and suspended substances.
Operating principle The pH meter is based on measuring the potential difference that occurs at the boundaries between the outer surface of the glass electrode membrane and the test solution on one side and the inner surface of the membrane and a standard acid solution on the other, since the internal standard solution of the glass electrode has a constant activity of hydrogen ions, potential on the other the surface of the membrane does not change and the measured potential difference is determined by the potential that arises at the boundary of the outer surface of the electrode and the test solution.
The measurements are carried out relative to the potential of another electrode, called the reference electrode. As the latter, an electrode is selected whose potential is practically independent of the activity of hydrogen ions, for example calomel or silver chloride.
The most common types of pH meters for measuring the pH of surface waters are “pH-121” and “pH-47M”.
The general pH measurement scheme consists of the following operations. The so-called “mechanical zero” of the device is checked and set before turning it on. Turn on the pH meter, and after warming up and setting the “electric zero”, check and adjust the scale using two or three buffer solutions. To do this, glass and calomel (silver chloride) electrodes are placed in a glass with a buffer solution.
A thermometer with a division value of 0.1-0.5°C is placed in the glass. The measurement begins by making sure that there are no air bubbles on the surface of the glass electrode ball. Having measured the pH of the buffer solution, record its value and after 2-3 minutes. Repeat the measurement. If the pH values coincide, the electrode potential is considered to be steady and the scale is corrected according to the instructions for the device. Then similar operations are carried out with the second and third buffer solutions.
After checking and correcting the instrument scale, the glass, electrodes and thermometer are thoroughly rinsed with distilled and then the test water. The latter is poured into a glass and the pH is measured in the same way as in the case of buffer solutions. Measurements are repeated 2-3 times or more at intervals of 2-3 minutes. The last two readings of the device should be the same.
Report. The report on this laboratory work consists of an oral interview with the teacher. The measurement results are presented in the prescribed form.
CONTROL QUESTIONS
1 What is pH value?
What reaction will water have at pH 10 and 5? Why?
What is the pH value of natural waters?
What methods are there to determine pH? Their accuracy?
What is a pH meter? Describe the principle of its operation.
How to determine the pH of water using the electrometric method?
LABORATORY WORK No. 12.
DETERMINATION OF THE CONTENT OF PHOSPHORUS COMPOUNDS IN WATER
Goal of the work. Learn to determine the content of phosphorus compounds in water and carry out initial processing of the results.
Reagents and solutions.
1. Basic standard solution of chemically pure sodium hydrogen phosphate (Na 2 HPO 4 12H 2 O), containing 0.1 mg P 2 O 5 in 1 ml of solution, prepared by dissolving 0.5047 g of this salt in a 1 liter volumetric flask with distilled water.
2. A working standard solution of phosphates is prepared from the main standard solution by diluting it 10 or 20 times (depending on the expected amount of phosphates in the sample).
3. A solution of ammonium molybdate in sulfuric acid [(NH 4) 2 MoO 4 +H 2 SO 4 , for the preparation of which 100 ml of 10% ammonium molybdate and 30 ml of 50% (by volume) sulfuric acid are mixed. To dissolve quickly, the mixture should be heated. If the solution turns out to be cloudy, it must be filtered by first treating the filter paper with a solution of H 2 SO 4 (1:20), and then with distilled water.
The solution of tin (II) chloride (SnCl 2) must be freshly prepared. To prepare, take 0.30 g of metal tin (foil or shavings) and dissolve it in 4 ml of concentrated HCl (when heated in a water bath), add 1-2 drops of a 4% solution of copper sulfate as a catalyst and adjust the volume to 15 ml distilled water. You can also use an undiluted solution of tin chloride; in this case, only 1 drop of SnCl 2 is added to the samples.
37% sulfuric acid solution. 337 ml of concentrated 98% H 2 SO 4 is carefully poured in small portions into 600 ml of distilled water. After cooling, the volume of the solution is adjusted to 1 liter.
Utensils and equipment.
1. Photoelectric colorimeter KFK-2.
300 ml conical flasks - according to the number of samples plus one.
2 ml pipette for a solution of ammonium molybdate in sulfuric acid.
Dropper for tin chloride (P).
50 ml volumetric flasks.
Heating device.
Pipettes for 1 and 2 ml.
General information. Phosphorus compounds are classified as biogenic substances, which also include nitrogen and silicon compounds, and in land reservoirs also iron compounds, which occupy a somewhat special position. These are vital elements closely related to the existence of living organisms (in Greek “bios” - life, “genos” - birth, origin).
Plants mainly have the ability to absorb nutrients directly from water. During the period of rapid development of algae, the consumption of biogenic elements is so intense that their content in water quickly decreases and drops to analytical zero (i.e. to such quantities that cannot be determined by conventional analytical methods). Further development of algae slows down.
If the water contains insufficient nutrients, life in such reservoirs is poor. In fish farms, they resort to artificial fertilization of ponds by adding nitrogen and phosphorus fertilizers to the water (similar to how fields are fertilized). In the oceans, vibrant biological life is observed in those areas whose surface layers are rich in nutrients. These areas are called productive zones.
Unlike gases, the entry of nutrients into the photosynthesis layer occurs from below or is carried out by river runoff.
As part of the waste products of aquatic organisms, their remains previously consumed and transformed nutrients in the form of complex organic compounds settle to the bottom. There they undergo complex chemical and biochemical transformations, mineralize and gradually transform again into forms that can be absorbed by aquatic plants. In other words, the regeneration of biogenic elements occurs at the bottom (with the exception of phosphates, a significant part of which is regenerated directly in the photosynthesis layer).
Most of the substances that sink to the bottom remain forever buried in the thickness of bottom sediments. However, some of them, in the process of vertical mixing of waters, return to the photosynthesis layer, where they are again consumed by algae. The cycle of nutrients is closed.
The concentration of nutrients in water depends on the ratio of the intensity of the processes of their consumption and regeneration, as a result of which it is subject to seasonal and sometimes daily fluctuations. As photosynthesis develops, the content of nutrients in water decreases. During the process of autumn-winter vertical convection, the upper layers of reservoirs are again gradually enriched with biogenic elements. Their content increases with depth.
Land reservoirs, especially rivers, contain, as a rule, more nutrients than seas and oceans, especially in their open parts.
Phosphorus is part of various compounds, both mineral and organic (organic phosphorus). Inorganic phosphorus is represented mainly by phosphoric acid ions H 3 PO 4, phosphates:
N 3 PO 4 
H + + H 2 PO 4 ¯ 
H + +HPO 4 2¯ 
H + + PO 4 3 ¯ (38)
Biogenic elements are contained in natural waters in relatively small quantities, therefore, for their determination, weight and volumetric methods are unsuitable due to their low sensitivity. The main method for determining nutrients photocolorimetrically.
Determination principle phosphates, is based on their ability to form complex salts, colored blue, with Mo(VI) compounds in the presence of tin(II) chloride.
2(MoO 2 . 4MO 3) + H 3 PO 4 + 4H 2 O = (MoO 2 . 4MO 3) . H 3 PO 4 . 4H 2 O (39)
Progress of the analysis. To 100 ml of the test water, filtered through a thick paper filter, or to a smaller volume brought to 100 ml with distilled water, add 2 ml of a 37% sulfuric acid solution and boil for 30 minutes. The volume of test water is maintained by adding distilled water within the range of 50-90 ml. After cooling the solution, transfer it to a 100 ml volumetric flask and adjust the volume to the mark with distilled water. Add 1 ml of ammonium molybdate solution, mix and 5 minutes after stirring, add 0.1 ml of tin (II) chloride solution, then mix again and after 10-15 minutes photometer.
Construction of a calibration graph. Pipette 0.0 into 50 ml volumetric flasks; 0.5; 1.0; 2.0; 5.0; 10.0; 20.0 ml of a working standard solution of sodium hydrogen phosphate Na2HPO 4 (1 ml - 0.001 mg PO 4 3¯) and adjust the volume of the solution to the mark with distilled water. The phosphate content in the resulting solutions will be respectively equal to 0.0; 0.01; 0.02; 0.04; 0.10; 0.20; 0.40 mg PO 4 3¯ in 1 l. Add exactly 1 ml of ammonium molybdate solution to each volumetric flask, mix, and after 5 minutes add 0.1 ml of tin (II) chloride solution and mix. The color intensity is measured after 10-15 minutes, using a red filter (λ = 690-720 nm) and cuvettes with a layer thickness of 2-3 cm. The optical density of the control sample is subtracted from the obtained optical density values and the results are plotted on a graph.
Phosphate content is determined by the formula:
X= (C . 50)/ V,
Report. The report on this laboratory work consists of an oral interview with the teacher. The measurement results are presented in the prescribed form.
CONTROL QUESTIONS
In what form is phosphorus found in water?
Explain the principle of determining phosphates.
What colorimetric methods are applicable for the determination of phosphates? What is their essence?
LABORATORY WORK No. 13
DETERMINATION OF THE CONTENT OF IRON COMPOUNDS IN WATER
Goal of the work. Learn to determine the content of iron compounds in water and carry out initial processing of the results.
Reagents and solutions.
Basic standard solution of ferroammonium alum. 0.8636 g FeHH 4 (SO 4) 2 12H 2 O on an analytical balance, dissolved in a 1 dm 3 volumetric flask in a small amount of distilled water, add 2.00 cm 3 HCl (ρ = 1.19 g/cm 3) and dilute to the mark with distilled water. 1 cm 3 of solution contains 0.1 mg Fe 3+.
2. A working standard solution of FeNH 4 (SO 4) 2 is prepared on the day of analysis by diluting the stock solution 20 times. 1 cm 3 of solution contains 0.005 mg Fe 3+.
Sulfosalicylic acid solution. 20 g of sulfosalicylic acid are dissolved in a 10 cm 3 volumetric flask in a small amount of distilled water and adjusted to the mark with this water.
2M NH 4 Cl solution. 107 g of NH 4 Cl are dissolved in a 1 dm 3 volumetric flask in a small amount of distilled water and adjusted to the mark with this water.
5 Ammonia solution (1:1) 100 cm 3 of 25% ammonia solution is added to 100 cm 3 of distilled water and stirred.
Utensils and equipment.
1 Photoelectric colorimeter KFK-2.
50 ml volumetric flasks.
Pipettes for 1 ml - 3 pcs.
4 50 ml graduated cylinder.
Heating device.
300 ml conical flasks for the number of samples plus one.
General information. Iron is an essential element for life and is part of hemoglobin in the blood. It is contained in water in the form of iron ions Fe 3+ and Fe 2+, the latter are easily oxidized to Fe 3+ ions. A large amount of oxygen is spent on oxidation, so it is necessary to especially strictly monitor the iron content, primarily Fe 2+, in wintering ponds, where an unfavorable gas regime may develop under the ice. When Fe 3+ ions combine with humic acids dissolved in water, a loose brown precipitate forms.
Despite the fact that iron is a biogenic element, its content in quantities exceeding optimal values can be harmful and lead to the death of fish. The harmful effects of excess iron are increased in an acidic environment. Significant amounts of iron can end up in water bodies with industrial wastewater.
Determination principle iron compounds is based on the interaction of Fe 3+ ions in an alkaline medium with sulfanilic acid to form a yellow-colored complex compound. The color intensity, proportional to the mass concentration of iron, is measured at a wavelength of 400-430 nm. The measurement range for the mass concentration of total iron without sample dilution is 0.10-2.00 mg/dm 3 .
Progress of determination. When the mass concentration of total iron is no more than 2 mol/l, 50 ml of the test water is measured using a measuring cylinder (at higher concentrations, the sample is diluted with distilled water), placed in a conical flask, heated to boiling and evaporated to a volume of 35-40 ml.
The solution is cooled to room temperature and transferred to a 50 ml volumetric flask. To the resulting solution add 1 ml of ammonium chloride solution, 1 ml of sulfosalicylic acid, 1 ml of ammonia solution, stirring thoroughly after adding each reagent. Using indicator paper, determine the pH of the solution, which should be greater than 9. If the pH< 9, го прибавляют 2-3 капли раствора аммиака (1:1) до рН>9. The volume of solution in the volumetric flask is adjusted to the mark with distilled water and left to stand for 5 minutes for color development.
The optical density of colored solutions is measured using a violet filter (λ = 400-430 nm) and cuvettes with a working layer thickness of 2.3 or 5 cm in relation to 50 ml of distilled water, to which the same reagents are added as in the test sample. The mass concentration of total iron is determined using a calibration curve.
Construction of a calibration graph. 0.0 is poured into a series of volumetric flasks with a capacity of 50 cm 3; 1.0; 2.0; 5.0; 10.0; 15.0; 20.0 cm 3 of the working standard solution and dilute to the mark with distilled water, mix, and analyze as test water. A scale of solutions corresponding to a mass concentration of iron of 0.0 is obtained; 0.1; 0.2; 0.5; 1.0; 1.5; 2.0 mg/dm3. A calibration graph is constructed, plotting the mass concentration of iron on the abscissa axis, and the corresponding optical density values on the ordinate axis.
Processing of analysis results. If the sample volume taken for analysis is less than 50 ml, then the iron concentration is calculated using the formula:
X= (C . 50)/ V,
where C is the mass concentration found from the calibration curve, mg/dm 3 Fe 3+ ; V - sample volume taken for analysis, ml; 50 - volume of standard solution, ml.
The final result of the analysis is taken as the arithmetic mean of the results of two parallel determinations, the permissible differences between which should not exceed 25% at the mass concentration of iron at the MPC level.
The convergence of analysis results (A) in percentage is calculated using the formula:
Report. The report on this laboratory work consists of an oral interview with the teacher. The measurement results are presented in the prescribed form.
CONTROL QUESTIONS
1 . What are the forms of iron dissolved in water?
2. What is the principle for determining iron dissolved in natural waters?
LABORATORY WORK No. 14
DETERMINATION OF THE CONTENT OF MINERAL NITROGEN-CONTAINING SUBSTANCES
Goal of the work. Learn to determine the content of mineral nitrogen-containing substances in water and carry out initial processing of the results.
This laboratory work consists of three stages:
determination of nitrite content;
determination of nitrate content;
determination of the total content of ammonium and ammonia ions.
General information. Nitrogen is represented in water by the following chain: organic residues 
albuminoid nitrogen (organic) 
ammonia nitrogen (lowest form of mineralization) 
nitrite nitrogen 
Nitrate nitrogen (the highest form of mineralization).
Free nitrogen is inert and is not absorbed by organisms. The presence of nitrogen compounds in water is an important factor in the development of life in a reservoir. Nitrogen is one of the most important nutrients, as it is used by plants to build cells. After the death of plants and animals, nitrogen turns into water as a result of mineralization of organic residues. Nitrous acid is an intermediate product of mineralization.
Aquatic animals release ammonia into the water as a result of protein metabolism. Albuminoid nitrogen enters the water from dead phyto- and zooplankton, which, under the action of microorganisms, turns into ammonia, and then into nitrous and nitric acid.
Nitrites are not stable, so their concentration in natural conditions is extremely low. For most of the year, they are either not found at all in the surface layers, or are contained there in quantities measured in thousandths of mg N/l. The appearance of NO 2 ¯ in large quantities indicates foreign contamination of the reservoir.
We recently received a question in the comments: “ Permanganate oxidation exceeded in a multi-storey building- causes and consequences? A sniff test of the water revealed a rotten smell. And water analysis in the laboratory showed an excess of permanganate oxidation. The house was built in 1970, the pipes have never been changed. We will try to answer the question, at the same time expanding the section “Water” and the subsection ““.
Permanganate oxidation is a measure of the total amount of organic matter in water. It does not show exactly what substances are present, but it does show how much there is in total. The indicator is named according to the method of obtaining the value - potassium permanganate (potassium permanganate) is added to the water sample. Oxidability is because forms of organic substances that are oxidized “all the way” do not interact with potassium permanganate. That is, all substances are oxidized up to this “stop”, and the amount of potassium permanganate consumed is calculated. The result is the value of permanganate oxidation.
Now let's move on to answering the question.
The first conclusion about the excess of permanganate oxidation:
In itself, the excess of the “Permanganate oxidability” indicator simply indicates that there is an excess of organic substances in the water. The indicator does not say whether these substances are good, bad, necessary, unnecessary. There are just a lot of them.
Another thing is where these substances come from and what are the consequences of their excess.
The source of organic matter in water pipes is algae.
Over decades of work, colonies of algae develop on the inner walls of pipes. This is not the usual algae from the river. These are special algae that can live without light and are more or less resistant to chlorinated water. Almost every water supply user can find this algae in their water supply. The inner walls of the toilet are rich in them - the easiest source of verification. A more difficult way is to unscrew the faucet aerator with a key (it’s still worth unscrewing and rinsing it sometimes so that the water flow is greater), and stick your finger inside the faucet. Mucus to the touch is the same microorganisms.
To fight algae and other microorganisms, water is chlorinated at Vodokanal. In some cities, fluorine is used instead of chlorine, but this does not change the essence of the matter.
Algae is an excellent food source for bacteria—if it can survive in chlorinated water and take root in the algae layer. The older the house, the greater the layer of mucus on the pipes. And the more places where bacteria can hide from chlorination. Bacteria that live in the dark and without air oxygen are usually putrefactive bacteria.
Putrefactive bacteria decompose the algae layer, releasing an unpleasant odor.
That is, a very likely reason for the smell of water in this case is bacteria feasting on the algae layer. These odor molecules may also contribute to excess permanganate oxidation.
Now about the dangers of exceeding permanganate oxidation.
In itself, excess does not threaten anything. To assess the threat, you need to know exactly what organic substances are present - which means additional chemical tests and additional costs. Therefore, it may be cheaper to use complex methods of influence:
- at the house level - chlorination
- at the apartment level there is a water filter.
In multi-storey buildings, water is regularly chlorinated - a much higher concentration of chlorine passes through the pipes than usual. The result is that the algae layer dies along with the bacteria. Naturally, if the algae layer is thick, then the usual increase in concentration is not enough, and you need to increase the dose. Perhaps contacting Vodokanal with the results of a water analysis can help correct the situation.
But the practice of contacting Vodokanals shows insignificant positive results. Therefore, the second way to solve the problem is most often chosen - water filtration.
Recommendations for water filters against unpleasant odors:
Filters at the entrance to the house- cartridges with . They are specifically designed to remove unpleasant odors and tastes from water. I personally like Aquaphor Viking filters. Before installation, you can additionally consult with the manufacturer to see if the filters will solve the problem.
Drinking water filter- Optimally, a mineralizer is also optional, since all kinds of harmful substances are guaranteed to be removed, including microorganisms with algae.
We hope we have answered the question completely. If not, please clarify in the comments!
Permagane oxidability characterizes the content of organic and mineral substances in water that prevent the transformation of iron from divalent to trivalent, which can be oxidized by oxygen. Those. permagane oxidation determines exactly the amount of oxygen that will save the situation, and per one liter of source water. The lower the oxidability, the less cost and effort it takes to convert water into usable water. 1-2 units is a quite good indicator of permagantane oxidation, 4-6 is within the normal range, and higher is an unacceptable indicator.
From permagane oxidation The composition of the water treatment and water purification system for the entire house depends. Even if the chemical composition of the two is the same in terms of iron and organic content, the indicators of permagane oxidation can vary greatly, which will make it possible or impossible to install reagent-free filters in one of the houses.
As a rule, a high indicator of permaganate oxidation indicates the content in water of certain biological substances called iron bacteria (humic acids, plant organic matter, anthropogenic organic matter, etc.). They actively hold ferrous iron in a stable form.
The source of increased water contamination with iron bacteria is in most cases human activity, or, more simply put, waste disposal. Surface waters have a higher oxidability compared to underground waters; they are saturated with organic matter from the soil and organic matter falling into the water. Oxidability is affected by water exchange between reservoirs and groundwater. It has a pronounced seasonality. The water of lowland rivers, as a rule, has an oxidability of 5-12 mg O 2 / dm 3, rivers fed by swamps - tens of milligrams per 1 dm 3. Groundwater has an average oxidation capacity of from hundredths to tenths of a milligram of O 2 /dm 3 . Maximum permissible concentration of drinking water for permanganate oxidation according to SanPiN 2.1.4.1175-02 “Hygienic requirements for the quality of water from non-centralized water supply. Sanitary protection of sources" is 5.0-7.0 mg/dm 3.
There are several types of water oxidation: permanganate, dichromate, iodate. The highest degree of oxidation is achieved by the dichromate method. In water treatment practice, for natural low-polluted waters it is determined permanganate oxidability, and in more polluted waters - as a rule, bichromate oxidation (COD - "chemical oxygen demand").
In such cases, reagent filters are used that allow powerful oxidizing agents (ozone, potassium permanganate, sodium hydrochlorite, etc.) to be introduced in portions. Installing such filters and regularly replacing reagents is, of course, many times more expensive. Conventional aeration is practically ineffective in such cases.
The only rational solution to avoid this problem is to change the location and depth of drilling. Transition to deeper groundwater layers.
From the point of view of the impact on the human condition, with high permaganate oxidation, the most dangerous for humans are large organic compounds, which are 90% carcinogens or mutagens. Organochlorine compounds formed when boiling chlorinated water are dangerous, because they are strong carcinogens, mutagens and toxins. The remaining 10% of large organic matter is at best neutral in relation to the body. There are only 2-3 large organic compounds dissolved in water that are useful for humans (these are enzymes needed in very small doses). The impact of organic matter begins immediately after drinking. Depending on the dose, this may be 18-20 days or, if the dose is large, 8-12 months. And based on logic, the presence of iron bacteria prevents the removal of iron from the water. You can read about the influence of iron on the human body
Chemical designation: permanganate oxidation (PO).
Synonyms: oxidability.
Description: an integral indicator that characterizes the content of reducing agents in water (for example, iron (II)) and organic substances that are completely or partially oxidized by permanganate ion under acidic or alkaline conditions and upon heating. Permanganate oxidability is expressed in mg of oxygen per 1 liter of water, which can be roughly interpreted as the amount of oxygen that is required to oxidize substances in water.
Determination methods: back titration.
Methods used at the MSU Test Center to determine permanganate oxidation in natural environments
Prevalence: permanganate oxidation is due to the presence of a large group of substances and elements in water. It must be remembered that permanganate is not the strongest oxidizing agent, so part of the organic matter may not be taken into account. Contribution to this parameter is made not only by compounds hazardous to health, but also beneficial or neutral, for example:
- glucose or sucrose;
- ascorbic acid (vitamin C);
- polysaccharides.
Rationing
The detection of oxidation values exceeding the maximum permissible values does not in itself provide information about the composition of the water, but gives reason to conduct extensive research to identify the cause of the excess. Hazardous substances that cause excess oxidation include:
- Surfactants (detergents);
- waste products of organisms;
- carcinogens;
- organic acids.
Permanganate oxidation is standardized only in drinking water; a similar parameter is for natural waters of reservoirs and wastewater - When determining it, I use a stronger oxidizing agent, dichromate, and aggressive conditions.
Maximum permissible concentration (MPC) of permanganate oxidation in various water bodies
Benefits and harms
Since permanganate oxidation is an integral parameter, it in itself does not cause harm or benefit to human health. Its main task is to provide the opportunity to quickly notice deviations from the norm and conduct a detailed analysis of a group of organic substances and reducing agents or make a decision on installing filters. This indicator also helps to quickly monitor the quality of the water supply and compliance with the rules of technological processes.
Water purification methods
Ion exchange. It is usually used in combination with ion exchange for other components in water, such as iron: organic substances are able to form highly soluble complex compounds with iron. This complicates the iron removal procedure. In such cases, mixtures of ion exchange resins are used, which sorb both organic substances and iron.
Dosing of oxidizing agents. The permanganate oxidability indicator is effectively reduced by the addition of oxidizing agents to water: these include hypochlorite (often used for disinfection and protection against microbiological contamination of water), hydrogen peroxide, etc. Ozonation of water also helps. This approach can be used to solve a complex of problems - disinfection and reduction of organic matter content.
Not all oxidizing agents are safe for health, even in residual quantities. Before use, make sure that the substance will not harm your body.
Carbon filters. Carbon filters have average filtration efficiency for organic matter. They are most effective in combination with preliminary dosing of oxidizing agents.
Reverse osmosis. Together with other substances, reverse osmosis removes organic matter from water, so it can be used to reduce both permanganate oxidation itself, either on its own or in combination with other purification methods.
Permanganate oxidation characterizes the total amount of organic substances, which should be contained as little as possible in drinking water. Increased values of this parameter indicate the need to conduct more extensive research and search for the source of pollution. The reason for the excess in well water may be its contamination (cleaning is recommended), in water from a well - the addition of groundwater and failure of waterproofing, in the water supply - poor-quality communications or a failure in the water supply filtration system.
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