Sulfate ions: determination of water and soil content

Sulfate ions are the average salts of sulfuric acid. Many of these compounds are readily soluble in water. Under normal conditions, the substances are in a solid aggregate state and have a light color. Many sulfate ions have a sedimentary origin, they are marine and lacustrine chemical sediments.

Features of the structure

The crystal structure allows the content of complex anions SO42-. Divalent metal sulfates can be isolated as common compounds. For example, sulfate ions, combined with cations of calcium, barium, strontium, form insoluble salts. These precipitates are minerals, they exist in a free form in nature.

Being in water

In addition, the sulfate ion is formed during the dissociation of salts, therefore such ions are contained in surface waters. The main source of such compounds are the processes of chemical oxidation of sulfides and sulfur.

In significant quantities, sulfate ions enter water bodies with the death of living organisms, the oxidation of terrestrial and aquatic plant creatures. In addition, they are in underground drains.

In a significant amount, the sulfate ion is formed in industrial and agricultural effluents.

Low-mineralized water is characterized by the presence of SO42- ions. There are also stable forms of such compounds that positively influence the mineralization of drinking water. For example, magnesium sulfate is an insoluble compound that accumulates in water.

Value in the sulfur cycle

If you analyze the sulfate ion in water, it is necessary to note its importance for a full cycle in the nature of sulfur and its compounds. Due to the influence of sulfate-reducing bacteria, without the access of oxygen to the air, it is restored to hydrogen sulfide and sulfides. In connection with the presence of oxygen in the soil waters, these substances are re-converted into sulphates.

Under the action of sulfate-reducing bacteria and in the absence of oxygen, they are reduced to sulfides and hydrogen sulphide. But as soon as oxygen appears in the natural water, they are again oxidized to sulfates.

In rainwater, the concentration of SO42- ions reaches 10 mg per cubic decimeter. For fresh water, this is about 50 mg per dm ³ . In underground sources, the quantitative content of sulphates is much higher.

Surface waters are characterized by a relationship between the season and the percentage content of sulfuric acid ions. In addition, the quantitative indicator affects the economic activities of man, the recovery and oxidation processes occurring in living nature.

Impact on water quality

Sulfates have a significant effect on the quality of drinking water. Their increased concentration adversely affects the organoleptic parameters. The water acquires a salty taste, its turbidity increases. The increased content of such anions adversely affects the physiological processes taking place in the human body. They are poorly absorbed into the blood from the intestines. At higher concentrations, they produce a laxative effect, disrupt digestive processes.

It was possible to establish a negative effect of sulfates on the hair, an irritating effect on the mucous membrane of the eyes and skin. In connection with the danger they pose to the human body, it is important to determine the sulfate ions, to take timely measures to reduce their quantity in drinking water. According to the regulations, they should not exceed 500 mg per cubic decimeter.

Features of the determination of anions in water

The basis of laboratory studies is a qualitative reaction to the sulfate ion with trilon B. Titration is performed in accordance with GOST 31940-12, established for SO42-. To conduct laboratory experiments related to the detection of the content of sulfate anions in drinking and waste water, solutions of barium chloride with a given concentration (0.025 mol per dm ³ ) are prepared. In addition, solutions need solutions: magnesium salts, ammonia buffer, trilon B, silver nitrate, and black eryochrome T indicator.

Algorithm of actions during the analysis

The lab technician uses a conical flask with a capacity of about 250 ml. 10 ml of a solution of a magnesium salt are introduced into it by means of a pipette. Further, 90 ml of distilled water, 5 ml of buffer ammonia solution, several drops of indicator are added to the flask under analysis, titration is carried out with a solution of EDTA disodium salt. The process is carried out until the color changes to blue from red-violet.

Further, the amount of EDTA disodium salt solution required for titration is determined. To obtain a reliable result, it is desirable to repeat the procedure 3-4 times. Using the correction factor, a quantitative calculation of the sulfate anion content is carried out.

Features of preparation of analyzed samples for titration

A simultaneous analysis of two samples having a volume of 100 ml is carried out. It is necessary to take a conical flask, designed for 250 ml. In each of them, the laboratory assistant makes 100 ml of the analyzed sample. Further in them add 2-3 drops of concentrated hydrochloric acid, 25 ml of barium chloride, put the flasks on a water bath. Heating is carried out for 10 minutes, then leave the analyzed samples for 60 minutes.

Then, the samples are filtered so that there is no precipitate of barium sulphate on the filter. The filter is washed with distilled water, the absence of chloride ions in the solution is checked. For this purpose, a qualitative reaction is periodically carried out with a solution of silver nitrate. If there is turbidity, this indicates the presence of chlorides in the solution.

Then, place the filter in the flasks where the precipitation was carried out. After the addition of 5 ml of ammonia, stir the contents of the flask with a glass rod, unfold the filter, spread it on the bottom. Based on 5 mg of analyzed ions, 6 ml of disodium EDTA are added to water. The contents are heated in a water bath, then on an electric stove they boil until the sediment is completely dissolved, which has got into the water together with the filter.

The duration of heating should not exceed five minutes. To improve the quality of the analysis, it is necessary to stir the contents of the flask periodically with a glass rod.

After cooling the sample, 50 ml of distilled water, 5 ml of buffered ammonia solution, several drops of an alcohol solution of the indicator are added to it. Further, an excess of disodium EDTA of a solution of sulfate or magnesium chloride is titrated until a stable lilac shade appears.

Conclusion

Sodium, potassium, sulfate ions are formed in sewage not only because of a variety of natural processes, but also as a result of human economic activity. To ensure that water used for food does not adversely affect living organisms, it is necessary to monitor the quantitative content of various anions and cations in it.

For example, when carrying out titration of Trilon B samples, quantitative calculations of the content in samples of sulfate anions can be made, and specific measures can be taken to reduce this indicator (if necessary). In modern analytical laboratories, cations of heavy metals, chlorine anions, phosphates, and pathogenic microorganisms are also detected in drinking water samples, which, when exceeding permissible concentrations, exert a negative effect on the physical and emotional health of a person.

Based on the results of such laboratory experiments and numerous studies, chemical analysts conclude that the water is suitable for use or that it needs to be cleaned up further, using a special filtration system based on chemical water purification.