Lead Dichromate, PbCr₂O₇

Lead Dichromate, PbCr₂O₇, containing 2 molecules of water of crystallisation, was described by Preis and Ray man, but its existence was subsequently denied.7 When a very concentrated solution of chromic acid (130 grams CrO₃ per 100 c.c.) is electrolysed with a lead anode, a solution is obtained which evolves oxygen when kept, and deposits reddish-brown needles of lead dichromate, PbCr₂O₇. From the volume of oxygen evolved it appears that plumbic dichromate is formed and decomposes thus:

Pb(Cr₂O₇)₂ = PbCr₂O₇ + 2CrO₃ + O.

Mayer obtained lead dichromate by heating lead acetate with chromic anhydride and concentrated nitric acid under a reflux condenser; and Cox prepared the same salt by taking account of the fact that it is prone to hydrolysis and can only be formed in contact with a chromic acid solution of a certain concentration, which is 6.865 gram-molecules CrO₃ per litre at 25° C. Thus by grinding together lead monoxide and chromium trioxide in saturated solution in the proportion of 2 gram-molecules of the former to 5 of the latter, lead dichromate is produced and may be obtained pure after washing with 7N solution of CrO₃. It is a lustrous, red, crystalline powder.