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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.

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