Sulfate ions are medium salts of sulfuric acid. Many of these compounds are highly soluble in water. Under normal conditions, the substances are in a solid state of aggregation, have a light color. Many sulfate ions are of sedimentary origin, they are marine and lake chemical sediments.
Structural features
The crystal structure allows the content of complex anions SO42-. Divalent metal sulfates can be identified as common compounds. For example, sulfate ions, combining with cations of calcium, barium, strontium, form insoluble salts. These sediments are minerals that exist freely in nature.
Being in the 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 fall into water bodies during the death of living organisms, the oxidation of terrestrial and aquatic plant creatures. In addition, they are located in underground drains.
A significant amount of sulfate ion is formed in industrial and agricultural effluents.
Low-saline water is characterized by the presence of SO42- ions. There are also stable forms of such compounds that positively affect the mineralization of drinking water. For example, magnesium sulfate is an insoluble compound that accumulates in water.
Value in the sulfur cycle
If we analyze the sulfate ion in water, it is necessary to note its importance for the full circulation in the nature of sulfur and its compounds. Due to the effects of sulfate-reducing bacteria, without access to atmospheric oxygen, it is reduced to hydrogen sulfide and sulfides. Due to the presence of oxygen in soil waters, these substances are converted again to sulfates.
Under the influence of sulfate-reducing bacteria and in the absence of oxygen, they are reduced to sulfides and hydrogen sulfide. But as soon as oxygen appears in natural water, they are oxidized again to sulfates.
In rainwater, the concentration of SO42- ions reaches 10 mg per cubic decimeter. For fresh water, this figure is about 50 mg per dm 3 . In underground sources, the quantitative content of sulfates is significantly higher.
Surface water is characterized by a relationship between the season and the percentage of sulfuric acid ions. In addition, the quantitative indicator is influenced by human activities, reducing and oxidative processes occurring in living nature.
Impact on water quality
Sulfates have a significant impact on the quality of drinking water. Their increased concentration negatively affects the organoleptic characteristics. Water acquires a salty taste, its turbidity increases. The increased content of such anions negatively affects the physiological processes taking place in the human body. They are poorly absorbed into the blood from the intestines. At elevated concentrations, they give a laxative effect, disrupt the digestive processes.
It was possible to establish the negative effect of sulfates on the hair, the 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 sulfate ions and take timely measures to reduce their amount in drinking water. According to the standards, they should not exceed 500 mg per cubic decimeter.
Features of determination of anions in water
The basis of laboratory research is a qualitative reaction to sulfate ion with Trilon B. Titration is carried out according to GOST 31940-12, established for SO42-. To conduct laboratory experiments related to the determination of sulfate anions in drinking and waste water, barium chloride solutions are prepared with a given concentration (0.025 mol per dm 3 ). In addition, the analysis requires solutions: magnesium salts, ammonia buffer, Trilon B, silver nitrate, and black Eriochrom T indicator.
The algorithm of actions during the analysis
The laboratory assistant uses a conical flask, the capacity of which is about 250 ml. 10 ml of a solution of magnesium salt are pipetted into it. Next, 90 ml of distilled water, 5 ml of ammonia buffer solution, a few drops of the indicator are added to the flask to be analyzed, titration is carried out using a solution of EDTA disodium salt. The process is carried out until the color changes to blue from red-violet.
Next, the amount of disodium EDTA solution, which was required for titration, is determined. To obtain a reliable result, it is advisable to repeat the procedure 3-4 times. Using the correction factor, a quantitative calculation of the content of sulfate anions is carried out.
Features of the preparation of the analyzed samples for titration
A simultaneous analysis of two samples having a volume of 100 ml is carried out. It is necessary to take conical flasks designed for 250 ml. In each of them, the laboratory assistant makes 100 ml of the analyzed sample. Then they add 2-3 drops of concentrated hydrochloric acid, 25 ml of barium chloride, put the flask in a water bath. Heating is carried out for 10 minutes, then it is necessary to leave the analyzed samples for 60 minutes.
Then the samples are filtered so that there is no precipitate of barium sulfate on the filter. The filter is washed with distilled water, check for the absence of chloride ions in the solution. To do this, periodically conduct a qualitative reaction with a solution of silver nitrate. If turbidity appears, this indicates the presence of chloride in the solution.
Then put the filter in the flask, where the deposition was carried out. After adding 5 ml of ammonia, stir the contents of the flask with a glass rod, unfold the filter, straighten the bottom. Based on 5 mg of the analyzed ions, 6 ml of disodium EDTA are added to the water. The contents are heated in a water bath, then boiled on an electric stove until the sediment that has got into the water along with the filter is completely dissolved.
The duration of heating should not exceed five minutes. To improve the quality of the analysis, it is necessary to periodically stir the contents of the flask with a glass rod.
After cooling the sample, 50 ml of distilled water, 5 ml of ammonia buffer solution, a few drops of an indicator alcohol solution are added to it. Next, titration with an excess of disodium salt of EDTA of a solution of sulfate or magnesium chloride is carried out until a steady purple hue appears.
Conclusion
Sodium, potassium, sulfate ions are formed in wastewater not only due to various natural processes, but also as a result of human activities. In order for the water used for nutrition not to adversely affect living organisms, it is necessary to monitor the quantitative content of various anions and cations in it.
For example, when titrating samples with Trilon B, it is possible to carry out quantitative calculations of the content of sulfate anions in samples, take specific measures to reduce this indicator (if necessary). In modern analytical laboratories, heavy metal cations, chlorine anions, phosphates, pathogenic microorganisms are also detected in drinking water samples, which, when exceeding permissible concentrations, have a negative effect on the physical and emotional health of a person.
Based on the results of such laboratory experiments and numerous studies, chemists and analysts conclude that the water is suitable for use or that it needs additional purification, the use of a special filtration system based on chemical water purification.