Selenic acid is an inorganic substance consisting of a selenate anion and a hydrogen cation. Its chemical formula is H 2 SeO 4 . Selenic acid, like any other compound, has unique properties, due to which it has found wide application in certain areas. And this should be described in more detail.
General characteristics
Selenic acid belongs to the class of strengths. Under standard conditions, it looks like colorless crystals that dissolve well in water. This substance must be wary, since it is poisonous and hygroscopic (it absorbs water vapor from the air). This compound is also a powerful oxidizing agent. Other characteristics can be identified in the following list:
- The molar mass is 144.97354 g / mol.
- The density is 2.95 g / cm³.
- The melting point reaches 58 ° C, and the boiling point reaches 260 ° C.
- The rate of dissociation constant is -3.
- Solubility in water is achieved at 30 ° C.
Interestingly, selenium acid is one of the few substances that can dissolve gold. This list also includes cyanide, Lugol's solution and aqua regia. But of the acids, it is so unique.
Receiving a substance
Most often, selenic acid is synthesized according to the formula SeO 3 + H 2 O → H 2 SeO 4 . It displays the interaction of water and selenium oxide. It is an inorganic substance that is readily soluble in acetic anhydride, sulfuric acid, and sulfur dioxide. By the way, the selenium compound can decompose into oxide and water under the influence of phosphoric anhydride (P 2 O 5 ).
In addition, acid is also obtained as a result of reactions when the main substance interacts with chlorine or bromine water. Here are the formulas for selenic acid in these two cases:
- Se + 3Cl 2 + 4H 2 O → H 2 SeO 4 + 6HCl.
- Se + Br 2 + 4H 2 O → H 2 SeO 4 + 6HBr.
But these are not the last ways. There are two more. Obtaining selenic acid from selenium is possible due to its reaction with hydrogen peroxide. It looks like this: SeO 3 + H 2 O 2 → H 2 SeO 4 .
Application
Now you can talk about him. Why is the production of selenic acid so important? Because without it, the synthesis of its salts is impossible. They are better known as selenates. About them will be discussed later.
The use of selenic acid as an oxidizing agent is very common, since it exhibits much more properties in this process than sulfuric. Even if diluted. If in sulfuric acid the electrode potentials are approximately ~ 0.169 V, then in selenium this indicator reaches ~ 1.147 V. And every person who does not even understand chemistry will notice the difference.
Needless to say, if selenic acid easily oxidizes hydrochloric acid, but also dissolves gold, as a result of which a selenate of this metal is formed, which is a red-yellow liquid.
Ammonium selenate
The formula for this salt is (NH 4 ) 2SeO 4 . This substance is represented by colorless crystals. They dissolve well in water, but not in acetone or ethanol. Show the general properties of salts.
They are used as insecticides. These are the substances used to kill insects. Ammonium selenate is actively used in pest control. But you need to use it with extreme caution, since this substance is especially toxic. But that’s why it’s effective.
Barium Selenate
Its formula is BaSeO 4 . This salt, unlike the previous one, does not dissolve in water. But it reacts with sulfuric acid, as a result of which selenium and barium sulfate are formed. So it is of particular interest. After all, barium sulfate is an X-ray positive substance that is actively used in radiology.
This compound is not toxic. It increases the contrast obtained during the x-ray image. Sulfate from the digestive tract is not absorbed, and does not enter the bloodstream. It is excreted with a chair, so that it is harmless to humans. This substance is used as a suspension inside, together with sodium citrate and sorbitol.
Beryllium Selenate
This salt with the formula BeSeO 4 forms crystalline hydrates. The substance itself is formed very interestingly. It is the result of amphoteric beryllium hydroxide in selenic acid. Dissociation results in the formation of colorless crystals, which decompose upon heating.
Where are the notorious hydroxides used? Usually they are used as raw materials to obtain beryllium. Or used as a catalyst for polymerization reactions and Friedel-Crafts.
Gold Selenate
This substance has the following formula: Au 2 (SeO 4 ) 3 . It looks like small yellow crystals. Naturally, this “salt” does not dissolve in water. It can only be affected by hot concentrated selenic acid. Selenium oxide does not form as a result of this reaction, but a reddish-yellow solution appears.
The "golden" salt is also soluble in nitric and sulfuric acids. But hydrogen chloride can destroy it.
Getting gold selenate is quite quick and easy. For the reaction, a temperature of 230 ° C is sufficient.
Copper selenate
The formula of this salt looks like this - CuSeO 4 . This substance is white, water-soluble (but not ethanol) crystals, which also form crystalline hydrates.
This salt is prepared according to the following formula: CuO + H2SeO 4 → 40-50 ° C CuSeO 4 + 2 . Dissolution of the oxide in selenic acid is reflected here, as a result of which water is also released. By the way, the resulting crystalline hydrates subsequently lose part of H 2 O. For this, it is sufficient to increase the temperature to 110 ° C. And if it is above 350 ° C, then the crystalline hydrate will even begin to decompose.
Sodium selenate
This is the last salt in the formation of which the discussed acid is involved. Its formula is Na 2 SeO 4 . This compound is of particular interest since it is the result of the interaction of an alkali metal and a strong acid. Salt, by the way, is soluble in water, and also forms a crystalline hydrate.
Get it in many ways. The most common involves dissolving selenium in hydrogen peroxide. According to the formula, it looks like this: Se + 2NaOH + 3 2 2 → Na 2 SeO 4 + 4 2 .
They also resort to the oxidation of sodium selenite, carried out using hydrogen peroxide, electrolysis or oxygen. But the simplest method involves the interaction of the acid under discussion with sodium carbonate. It is sometimes replaced with hydroxide.
This is perhaps the most actively used selenate. It is used as a medicine. According to ATX, sodium selenate is a mineral supplement. However, not a single medicine with its content has been registered either in the USA or in Russia. But in Latvia and Denmark there are such drugs. The same "Bio-Selenium + Zinc", for example. A great tool to strengthen the immune system.
But selenate is part of the biological additives sold in Russia. The same "Supradin Kids Junior" contains it in an amount of 12.5 mcg per tablet.
In general, it is through the use of this substance that doctors recommend treating selenium deficiency in the body. But, of course, before use it is necessary to undergo a medical examination and consultation.
As you can see, even if selenic acid is not used in its pure form, the significance of its derivatives in chemistry, medicine and industry is obvious.