Chromic Borate

Ammonium hydrogen metaborate, (NH₄)HB₂O₄, when added to a dilute solution of chromic chloride, causes the precipitation of a pale green powder.

If borax is used the precipitate is blue, and is soluble in excess of the reagent; with potassium hexaborate, K₂O.6B₂O₃.10H₂O, the precipitate is green. Hebberling gives the composition of the product as approximating to 7Cr₂O₃.4B₂O₃, but the proportion of B₂O₃ present appears to vary with the concentration of the reagent.

Guignet's Green is obtained by heating together potassium dichromate and boric acid and treating the fused product with water. Salvetat assumed that a chromiborate was formed according to the equation:

K₂Cr₂O₇ + 16H₃BO₃ = Cr₂O₃.6B₂O₃ + K₂B₄O₇ + 24H₂O + 3O,

the chromiborate being decomposed by water with evolution of heat. A similar compound, usable as a dye, was obtained by Poussier by heating chromium chloride with calcium borate.