The impact of various chemicals on the human body is ambiguous. Most of the known compounds are either neutral or play a positive role in human life. But there is a group of substances that pose a serious threat to he alth. They are divided into several classes. The arsenic acid discussed in this article is one such toxic chemical compound. According to the currently accepted classification, it is included in the second class of increased danger, along with chloroform, lead and lithium compounds. Let's study the properties of arsenic acid in more detail.
The structure of the molecule and the state of aggregation of matter
This compound has a crystalline structure under normal conditions. Being tribasic, arsenic acid, the formula of which is H3AsO4, has both medium and acidic s alts. For example, potassium hydrogen arsenate - K2HAsO4, sodium dihydroarsenate - NaH2AsO 4, lithium arsenate - Li3AsO4. By calcining arsenic acid, arsenic hemipentoxide is obtained, called arsenicanhydride. Its white transparent crystals form a glassy mass, poorly soluble in water.
Dissociation
H3AsO4, along with formic acid and lead hydroxide, is a moderately weak electrolyte. So, in the ionization table of the most important acids, orthoarsenic acid has three dissociation constants: 5.6 x 10-3, 1.5 x 10-7 and 3, 89 x 10-12. These indicators quantitatively characterize the strength of the acid. In accordance with the dissociation constants, in the series of inorganic acids, H3AsO4 occupies a position between chromic and antimony acids. Russian experimental chemists A. L. and I. L. Agafonovs formulated a mathematical expression in which they derived the dependence of the first and second dissociation constants of arsenic acid on temperature in the range from 0°С to 50°С.
Features of chemical properties
The degree of oxidation of the arsenic atom, which is part of the acid molecule, is +5. This speaks to the fact that the compound itself, in chemical reactions with other substances, exhibits oxidizing properties. So, when it interacts with potassium iodide, which acts as a reducing agent, in an acidic medium, among the reaction products, we will find arsenic acid H3AsO3. Recall that arsenic acid, whose formula H3AsO4, is tribasic, which means that in reactions with alkalis or insoluble bases it can give three types of s alts: medium, hydro- and dihydroarsenates. Qualitative reaction to an ionAsO43- in analytical chemistry is the interaction of arsenic acid itself or its s alts with soluble silver s alts, for example, with nitrate. As a result, we observe the precipitation of Ag3AsO4 coffee color.
Iodometric method for the determination of arsenic acid
In analytical chemistry, an important task is the detection of chemical compounds in the studied solutions. Arsenic acid, the chemical properties of which we considered earlier, can be detected by the micro method of iodometry. To 1 ml of its solution is poured the same volume of 4N. hydrochloric acid solution and 1 ml of 4% potassium iodide solution. Arsenic sesquioxide As2O3 is formed.
Oxidizing power of arsenic acid
As you know, H3AsO4, , like phosphoric acid, is an electrolyte of medium strength. Its white transparent crystals blur in air and have the composition 2H3AsO4 х H2O. Its s alts formed by alkali metals (both medium and acidic) in aqueous solutions have a pH greater than 7. Lithium, potassium, sodium and ammonium arsenates are highly soluble in water, while the remaining medium s alts do not dissolve in it. Arsenic acid is a good oxidizing agent. In redox reactions, it is reduced to arsenous acid or arsine.
H3AsO4 + 2e + 2H+=H3AsO3 + H2O
H3AsO4 + 8e + 8H+=AsH 3 + 4H2O
In addition, arsenic acid easily oxidizes various metals, sulfite and iodide acids, as well as hydrogen sulfide.
Production of arsenic acid
In laboratory conditions, H3AsO4 can be obtained by the reaction of arsenic sesquioxide with nitrate acid by heating. The products contain trivalent nitric oxide and H3AsO4. Another way to obtain is the dissolution of arsenic oxide in water. Often, to obtain it, simultaneous oxidation and hydrolysis of trialkyl arsenites with a solution of hydrogen peroxide heated to 50 ° C is used. Simultaneously, water and alcohol are removed from the reaction mixture. Then the solution is evaporated and arsenic acid of high purity is obtained. In nature, the raw materials for obtaining arsenic acid are minerals: arsenolite and arsenopyrite, the deposits of which are rich in the Chelyabinsk and Chita regions of the Russian Federation.
Using H3AsO4
Given the fact that orthoarsenic acid is one of the strongest poisons. Its use in industry and everyday life is limited. More common s alts are arsenates, whose toxicity is much less than that of H3AsO4 itself. So, in the woodworking industry, together with zinc sulfate and pentachlorophenol sodium s alt, arsenic acid is used for wood processing. This method minimizes losses from the destruction of cellulose by fungalinfections and larvae of carpenter beetles. In medicine, H3AsO4is used as part of the drug "Atoxil" for the treatment of protozoal infections such as giardiasis, balantidiasis, isosporiasis.
It should be noted that the infection of the population with these infections has recently increased dramatically. There are several reasons - for example, infection through food containing protozoan spores, through insect bites or through sexual contact. Arsenic acid is used as a starting material in the production of optical glasses, as well as in electrical engineering. Derivative H3AsO4- its sodium s alt is successfully used in dermatology and phthisiology. Arsenic compounds are used in dentistry (arsenic paste) as a drug used to reduce the pain sensitivity of an inflamed nerve when it is removed from the dental canal.
The effect of acid on the human body
As mentioned earlier, H3AsO4 is included in the second class of increased danger - highly dangerous substances. The lethal dosage is considered to be both the acid itself and its s alts ranging from 15 to 150 mg per kilogram of human body weight. Along with the general poisoning effect, arsenic acid causes necrosis of the skin and mucous membranes of internal organs: lungs, stomach, intestines.
In the laboratory, when conducting experiments with arsenates and H3AsO4 be sure to use protective gloves, andexperiments are carried out under a hood. In the case of intoxication at the cell level, its enzymatic system is disturbed, since enzymes are inactivated. In the human body, poisoning with arsenates leads to paresis and even paralysis. In oncology, during chemotherapy, cases of miarsenol and novarsenol poisoning are recorded if the dosing regimen is not followed. First aid for poisoning with arsenic acid s alts consists in immediate gastric lavage (for example, with a solution of unitiol or silicon dioxide preparations).
To prevent acute renal failure, hemodialysis is prescribed. As an antidote, in addition to a 5% unithiol solution, Strizhevsky's antidote can be used. Before the arrival of an emergency ambulance at home, a solution of citric acid can be used to reduce the level of intoxication, then induce vomiting and gastric lavage. All therapeutic measures must be carried out in compliance with strict bed rest under the supervision of a physician.
