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. |