Manganese Chromates

The normal salts have not been isolated. When an alkali chromate is added to a solution of a manganese salt, some of the manganese undergoes oxidation, and the brownish-black precipitate formed, in addition to manganous and manganic chromates, is liable to contain manganate and permanganate, and trivalent chromium. If the reaction takes place in the cold, the alkali metal is found in the precipitate, but this can generally be removed at boiling temperature - most readily in the case of sodium.

The existence of most of the double salts which have been described in the literature must be considered as extremely doubtful, but by adding manganous chloride to a concentrated solution of potassium chromate, potassium manganese chromate, K₂Mn(CrO₄)₂.2H₂O, has been obtained by Groger as a red crystalline precipitate.

Mercury Chromates. - Mercurous chromate, Hg₂CrO₄, is obtained in the crystalline form as glittering red leaflets, when the amorphous precipitate produced by adding potassium dichromate to a solution of a mercurous salt is dissolved in boiling 5N nitric acid and the solution allowed to cool. It is also formed by adding potassium chromate to a solution of mercurous nitrate;4 the yellowish-brown amorphous precipitate first formed is unstable, and changes - without change of composition - into the crystalline form, the colour of which varies from yellow to bright red, depending on the fineness of the particles. The salt is not decomposed by water in the cold; it is converted by ammonia into the chromate of Millon's Base, (OHg₂NH₂)₂CrO₄. The formation of mercurous chromate affords a useful means of estimating chromium gravimetrically.

Basic salts have been described. By boiling the normal salt with water, or by treating the freshly formed amorphous precipitate with excess of mercurous nitrate solution, the salt, Hg₆Cr₂O₉, or 2Hg₂CrO₄.Hg₂O, results. If, during the gradual transition of the normal salt from the amorphous to the crystalline form, water is added so that hydrolysis occurs, the salt, Hg₈Cr₃O₁₃, or 3Hg₂CrO₄.Hg₂O, is produced. The basic salt, Hg₆CrO₆, or Hg₂CrO₄.2Hg₂O, has been stated to result from the action of sodium hydroxide on the normal chromate, but this is denied by Fichter and Oesterheld, who maintain that the mercurous oxide formed is uncombined with chromate.

The double salt, K₂Hg₂(CrO₄)₂, has been described by Groger, who obtained it as golden yellow non-crystalline spangles containing absorbed potassium chromate, by adding a solution of mercurous nitrate to a concentrated solution of potassium chromate. Water decomposed it, giving a basic salt.