Arsenic acid: chemical properties, formula. Highly hazardous substances

The impact of various chemicals on the human body is ambiguous. Most of us 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 health. They are divided into several classes. The arsenic acid considered in this article is one of such toxic chemical compounds. According to the currently accepted classification, it enters the second class of increased danger, along with chloroform, lead and lithium compounds. We will study the properties of arsenic acid in more detail.

The structure of the molecule and the aggregate state of matter

This compound under normal conditions has a crystalline structure. As a tribasic, arsenic acid, the formula of which is H ₃ AsO ₄ , has both middle and acid salts. For example, potassium hydrosarenate - K ₂ HAsO ₄ , sodium dihydroarsenate - NaH ₂ AsO ₄ , lithium arsenate - Li ₃ AsO ₄ . By calcining arsenic acid, arsenic hemic pentoxide is obtained, called arsenic anhydride. Its white transparent crystals form a vitreous mass that is poorly soluble in water.

Dissociation

H3AsO ₄ , along with formic acid and lead hydroxide, is a moderately weak electrolyte. Thus, in the ionization table of the most important acids, ortho-arsenic acid has three dissociation constants: 5.6 x 10 ^-3 , 1.5 x 10 ^-7 and 3, 89 x 10 ^-12 . These indicators quantify the strength of the acid. In accordance with the dissociation constants, in the series of inorganic acids, H ₃ AsO ₄ occupies a position between chromic and antimony acids. Russian chemists-experimenters AL and IL Agafonov formulated a mathematical expression in which the dependence of the first and second dissociation constants of arsenic acid on the temperature in the range from 0 ° C to 50 ° C was derived.

Features of chemical properties

The degree of oxidation of the arsenic atom, which is part of the acid molecule, is +5. This indicates the fact that the compound itself, in chemical reactions with other substances, exhibits oxidative properties. Thus, when it interacts with potassium iodide acting as a reducing agent, we find arsenious acid H ₃ AsO ₃ in the acid medium among the reaction products. Recall that arsenic acid, whose formula H ₃ AsO _4, is tribasic, and therefore, in reactions with alkalis or insoluble bases, it can give three types of salts: averages, hydro- and dihydroarsenates. A qualitative reaction to the ion AsO ₄ ^3- in analytical chemistry is the interaction of arsenic acid itself or its salts with soluble silver salts, for example, with nitrate. As a result, precipitation of Ag ₃ AsO ₄ Coffee color.

Iodometric method for the determination of arsenic acid

In analytical chemistry, an important task is the discovery of chemical compounds in the solutions under study. Arsenic acid, whose chemical properties we considered earlier, can be detected by the micromethod of iodometry. To 1 ml of its solution is poured the same volume of 4N. Solution of hydrochloric acid and 1 ml of a 4% solution of potassium iodide. Sesquioxide arsenic As ₂ O _{3 is formed, the} mass of which, with strict observance of quantitative volumes of reagents, is always the same and equal to 0.5746 mg.

Oxidative ability of arsenic acid

As is known, H ₃ AsO _4, like orthophosphoric acid, is an electrolyte of medium strength. Its white transparent crystals spread out in air and have the composition 2H ₃ AsO ₄ x H ₂ O. Its salts formed by alkali metals (both average and acidic) have a pH in water solutions greater than 7. Lithium, potassium, sodium and ammonium arsenates Are readily soluble in water, and the remaining average salts do not dissolve in it. Arsenic acid is a good oxidizer. In oxidation-reduction reactions, it is reduced to arsenic acid or arsine.

H ₃ AsO ₄ + 2e + 2H ⁺ = H ₃ AsO ₃ + H ₂ O

H ₃ AsO ₄ + 8e + 8H ⁺ = AsH ₃ + 4H ₂ O

In addition, arsenic acid readily oxidizes various metals, sulphite and iodide acids, as well as hydrogen sulphide.

Production of arsenic acid

Under laboratory conditions, H ₃ AsO ₄ can be obtained by the reaction of arsenic sesquioxide with nitric acid on heating. In products, oxide of trivalent nitrogen and H ₃ AsO _{4 is} found _. Another way of obtaining this is the dissolution of arsenic oxide in water. Often, for its preparation, simultaneous oxidation and hydrolysis of trialkylarsenites with a hydrogen peroxide solution heated to 50 ° C is used. At the same time, water and alcohol are removed from the reaction mixture. The solution is then evaporated and arsenic acid of special purity is obtained. In nature, raw materials for the production of arsenic acid are minerals: arsenolite and arsenopyrite, the deposits of which are rich in the Chelyabinsk and Chita regions of the Russian Federation.

Application of H3AsO4

Considering the fact that ortho-arsenic acid is one of the strongest poisons. Its use in industry and everyday life is limited. Salts are more common - arsenates, whose toxicity is much less than that of H ₃ AsO ₄ itself _. Thus, in the wood industry together with zinc sulphate and sodium salt of pentachlorophenol, arsenic acid is used for wood processing. This method minimizes the loss from the destruction of cellulose by fungal infections and larvae of woodworm beetles. In medicine, H ₃ AsO _{4 is} used as part of the drug "Atoxil" for the treatment of protozoal infections, such as giardiasis, balantidiasis, isospore.

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 foods containing spores of protozoa, through insect bites or through sexual intercourse. Arsenic acid is used as a starting material in the production of optical glasses, as well as in electrical engineering. The H ₃ AsO ₄ derivative, its sodium salt, 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 the inflamed nerve when it is removed from the dental canal.

The effect of acid on the human body

As already mentioned, H ₃ AsO ₄ enters the second class of increased danger - highly hazardous substances. Deadly is the dosage of both the acid itself and its salts in the range of 15 to 150 mg per kilogram of body weight. Along with the general poisonous effect, arsenic acid causes necrosis of the skin and mucous membranes of internal organs: lungs, stomach, intestines.

In the laboratory, when carrying out experiments with arsenates and H ₃ AsO ₄ , the use of protective gloves is mandatory, and the experiments themselves are carried out under the hood. In case of intoxication at the cell level, its enzymatic system is violated, as enzymes are inactivated. In the human body, poisoning with arsenates leads to paresis and even paralysis. In oncology, chemotherapy with non-compliance with the dosing regimen registers cases of poisoning with myarsenol and novarsenol. The first help in poisoning with salts of arsenic acid is the immediate washing of the stomach (for example, with a solution of unithiol or preparations of silicon dioxide).

To prevent acute renal failure, a hemodialysis procedure is prescribed. As an antidote, in addition to the 5% solution of unithiol, Strizhevsky's antidote can be used. Before the emergency ambulance arrives at home, a solution of citric acid can be used to reduce the level of intoxication, then induce vomiting and rinse the stomach. All medical measures must be carried out with strict strict bed rest under the supervision of a doctor.