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Determination of the chemical composition of water

Hydrogen indicator of water (pH) - characterizes the active reaction, determines the natural properties of water and is an indicator of pollution. Natural water usually has a slightly alkaline reaction. An increase in alkalinity indicates pollution or flowering of the reservoir, an acid reaction indicates the presence of humic substances (marsh water) or industrial wastewater. The hydrogen indicator of drinking water (pH) should be between 6.0 and 9.0.

The course of determination. Qualitative reaction (pH) is determined by the indicator. A strip of test paper is immersed in a flask with the test water and its color is compared with the standards of the scale of the universal pH indicator.

Determination of nitrogen-containing substances. An indicator of water pollution by organic substances of animal origin are salts of ammonia, nitric and nitrous acids.

The content of ammonium salts above 0.1 mg / dm3 indicates fresh water pollution, because ammonia is the initial decomposition product of organic nitrogen-containing substances.

Salts of nitrous acid (nitrites) are the products of the oxidation of ammonia under the influence of microorganisms in the process of nitrification. The presence of nitrites in amounts exceeding 0.002 mg / dm3 indicates the limitation of pollution.

Salts of nitric acid (nitrates) are the final products of the mineralization of organic substances. The presence of nitrates in the water without ammonia and nitrites indicates the completion of the mineralization process.

The simultaneous content of ammonia, nitrites and nitrates in water indicates a clear disadvantage of the water source, its constant pollution. Increased amounts of nitrites and nitrates without the presence of ammonia indicate a cessation of pollution at the present time. The presence of ammonia and nitrites in water indicates the recent emergence of a permanent source of pollution. The presence of nitrates in water alone indicates the end of mineralization processes.

Permissible nitrate content in drinking water is not more than 45 mg / dm. 3

Chemical research begins with qualitative reactions so as not to waste time quantifying those salts that are absent in water.

Determination of ammonium nitrogen. The method is based on the ability of ammonia and ammonium ions to form a compound (mercuric ammonium iodide) with the Nessler reagent, which stains the solution in a yellow-brown color. The color intensity is proportional to the ammonia content in the water.

Qualitative determination of ammonium salts

with approximate quantification

The course of determination. 10 cm3 of the test water is poured into a test tube of colorless glass with a flat bottom with a diameter of 13-14 mm, 0.2 cm3 of Rochelle salt (potassium sodium tartrate) and 0.2 cm3 of Nessler's reagent are added. After 10 min, an approximate determination of ammonia nitrogen is performed according to the table. nine.

Table 9

An approximate definition of ammonia nitrogen

As a rule, in pure natural waters it contains 0.01-0.1 mg / dm3 of nitrogen of ammonium salts.

Quantification of ammonium nitrogen

The course of determination.
The volume of water for the study is taken taking into account the results of a qualitative sample with an approximate quantitative assessment. If the content of NH + 4 in water is more than 0.5 mg / dm3, the sample should be diluted.

Table 10

Ammonia content in water depending on the optical density of solutions

To determine the amount of ammonium salts, the previously obtained solution is poured into a 10-millimeter cell of FEK. The determination is made with a blue filter with a wavelength of 400-425 nm. Distilled water is used as a control solution. The result is compared with the data in table. ten.

Determination of nitrite nitrogen. The method is based on the interaction of nitrites and the Griss reagent, which is a mixture of a-naphthylamine and sulfanilic acid in acetic acid, resulting in the formation of a diazo compound colored from pink to intensely red, depending on the concentration of nitrite nitrogen.

Qualitative determination with an approximate quantitative assessment.

The course of determination. 10 cm3 of test water and 0.5 cm3 of Griss reagent are introduced into a flat-bottomed test tube of colorless glass with a diameter of 13-14 mm. They are placed in a water bath at a temperature of 50-60 ° C and heated for 10 minutes. The approximate nitrite content is determined by the table. eleven.

The nitrogen content of nitrites in pure natural waters, as a rule, does not exceed 0.005 mg / dm3.

Table 11

Approximate determination of nitrite nitrogen in water

Quantitative determination of nitrite nitrogen in water

The course of determination. If the nitrite content in water is more than 0.05-0.07 mg / dm3, the sample should be diluted.

2 cm3 of Griss reagent are added to 50 cm3 of the test sample, mixed, placed in a water bath at a temperature of 50-60 ° C, after 10 min, photometry at a wavelength of 520 nm with respect to distilled water to which Griss reagent is added.

The mass concentration of nitrites is found according to the calibration graph.

To build a calibration graph, 0, 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0 cm3 of working standard solution are added to volumetric flasks with a capacity of 50 cm3 and adjusted volume to the mark with distilled water. Get solutions with a content of: 0; 0.002; 0.004; 0.01; 0.02; 0.04; 0.10; 0.20; 0.30 mg / dm3 nitrite.

Next, an analysis and photometry are carried out, as in the study of the sample. Based on the results obtained, a calibration graph is constructed, plotting the mass concentration of nitrites in milligrams of 1 dm3 on the abscissa axis, and the optical density values ​​corresponding to them on the ordinate axis.

The nitrite content is determined by the formula:

X = 50 x C / V (mg / dm3)

where: C is the nitrite content found according to the calibration graph, mg / dm3;

50 - volume of a standard solution (dilution), cm3;

V is the volume of the sample taken for analysis, cm3;

If the sample volume was 50 cm3, then the formula will look like: X = C.
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Determination of the chemical composition of water

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