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Lead Dichromate, PbCr2O7

Lead Dichromate, PbCr2O7, containing 2 molecules of water of crystallisation, was obtained by Preis and Rayman, but its existence was subsequently denied by Autenrieth. Elbs and Nubling found that 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 7-N solution of CrO3. It is a lustrous, red, crystalline powder.

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