Ammonium Osmyl Oxychloride, (NH₄)₂OsO₃.Cl₂

Ammonium Osmyl Oxychloride, (NH₄)₂OsO₃.Cl₂, is obtained by careful addition of hydrochloric acid to the corresponding nitrite:

(NH₄)₂OsO₃(NO₂)₂ + 2HCl = (NH₄)₂OsO₃Cl₂ + 2HNO₂.

It is an advantage to use a slight excess of acid, as it tends to prevent oxidation from the nitrous fumes. The temperature, however, must not be greatly raised, otherwise the osmyl salt, (NH₄)₂OsO₂.Cl₄, or even the chlorosmate, (NH₄)₂OsCl₆, may result. On very gently warming the nitrous fumes are disengaged, and a yellow crystalline deposit of ammonium osmyl oxychloride is obtained, which is but slightly soluble in water. With caustic potash, ammonia and potassium osmate are formed, the latter imparting to the solution its characteristic red-violet, colour.