Chemical elements
    Physical Properties
    Chemical Properties
      Hydrogen Selenide
      Selenium Fluorides
      Selenium Monochloride
      Selenium Tetrachloride
      Selenium Monobromide
      Selenium Tetrabromide
      Selenium Chlorobromides
      Selenium Oxyfluoride
      Selenium Oxychloride
      Sulphur Selenium Oxytetrachloride
      Selenium Oxybromide
      Chloroselenic Acid
      Selenium Dioxide
      Selenious Acid
      Selenium Trioxide
      Selenic Acid
      Perselenic Acid
      Selenium Sulphoxide
      Selenotrithionic Acid
      Diselenotrithionic Acid
      Selenopentathionic Acid
      Selenium Nitride
      Nitrosylselenic Acid
      Phosphorus Subselenide
      Phosphorus Monoselenide
      Tetraphosphorus Triselenide
      Phosphorus Triselenide
      Phosphorus Pentaselenide
      Phosphorus Chloroselenide
      Selenophosphates and Oxyselenophosphates
      Carbon Diselenide
      Carbon Subselenides
      Carbon Oxyselenide
      Carbon Sulphidoselenide
      Cyanogen Monoselenide
      Cyanogen Diselenide or Selenocyanogen
      Cyanogen Triselenide
      Selenocyanic Acid
      Ammonium Selenocyanate
      Caesium Triselenocyanate
      Copper Selenocyanate
      Lead Selenocyanate
      Magnesium Selenocyanate
      Mercurous Selenocyanate
      Mercuric Selenocyanate
      Potassium Selenocyanate
      Silver Selenocyanate
      Sodium Selenocyanate
      Zinc Selenocyanate
      Silicon Selenide
    Detection and Estimation

Selenotrithionic Acid, H2SeS2O6

In the preparation of potassium selenosulphate as just described, a certain small amount of a potassium selenotrithionate, K2SeS2O6 (potassium salt of Selenotrithionic Acid, H2SeS2O6), is obtained, which generally crystallises first on evaporation of the solution at the ordinary temperature. The yield is much better if potassium hydrogen sulphite is also present with the selenium and normal potassium sulphite.

The corresponding free acid is formed by the interaction of aqueous solutions of sulphurous and selenious acids, the former in excess:

3H2SO3 + H2SeO3 = H2SeS2O6 + H2SO4 + 2H2O.

In the description of the properties of selenious acid it was mentioned that the acid is reduced by sulphurous acid in the presence of hydrochloric acid. In the absence of hydrochloric acid the separation of selenium may be far from complete or even entirely lacking, on account of the formation of selenotrithionic acid.

Selenotrithionic acid is known only in aqueous solution and in the form of its salts. The solution is not very stable and on exposure to light gradually deposits selenium. Evaporation also causes decomposition. Hydrochloric acid in considerable quantity causes decomposition (compare above) with liberation of selenium:

H2SeS2O6 = H2SO4 + SO2 + Se.

The selenotrithionates are rather more stable than the acid, but they yield a precipitate of selenium when warmed with hydrochloric acid. As the barium salt is soluble in water, no precipitate is produced on the addition of barium chloride.

From the general behaviour of the acid and its salts the constitution is presumably

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