M. Marchand 07i the Reduction ofChromate of Lead. 533 



Many persons imagine that in organic analysis with chroraate of 

 lead it is necessary to use a very strong heat in order to perfect the 

 operation. This is a mistake, for carbon as well as hydrogen very 

 easily reduces the chromate. If however it is required to libe- 

 rate oxygen, then the temperature must be very great, and the 

 salt must be fused. This circumstance, as may be easily conceived, 

 is inconvenient and liable to introduce error. 



When chromate of lead is heated in a current of hydrogen gas, it 

 commences to glow at a heat far below redness, and a quantity of 

 water is formed. The yellow colour of the salt disappears ; it be- 

 comes black, and very small metallic globules are disseminated 

 through the mass. 



3'049 grammes lost 0*307 gram, or 10'07 per cent, of oxygen ; 

 this loss may be increased by a continued and strong heat. 



1"91 grammes lost in another experiment 0"224 gram., or 11'8 

 per cent, oxygen. At the commencement of the reduction of this 

 portion the temperature was kept moderate, by which it lost 0*2045 

 grm., or 10" 7 per cent. Oxygen .gas was then passed over it 

 while in a heated state. At a low degree of heat, the mass burnt 

 with great brilliancy, turned brown, at least partly so, which was 

 very evident on cooUng. It absorbed 0'133 grm. oxygen, which 

 calculated for the original quantity (1'91) amounted to 7 per cent.; 

 a small quantity of water was formed during the operation, amount- 

 ing, however, to but a few milligrammes. It would therefore 

 appear that hydrogen was condensed in the pores of the reduced 

 mass, but in a small quantity ; heating in a stream of carbonic acid 

 gas would have entirely driven it out. The oxidated quantity 1*839 

 grm. which had lost from the first 3*7 per cent, of oxygen, was 

 again reduced in hydrogen gas, by which it lost 0*152 grm., this 

 upon the whole quantity is equal to 8 per cent. Upon heating in 

 oxygen gas the same appearances again took place, and the mass 

 absorbed 0*128 grm., therefore, a simdar quantity as before. A 

 subsequent reduction at a very high temperature occasioned a loss 

 of 0*128 grm. while a repeated oxidation only gave an increase 

 of 0*119. This was again driven out by hydrogen, but without any 

 further decrease of oxygen. 



If we examine these experiments, keeping in view the results, 

 we shall find that 1*91 grm. lost 0*224 grm. equal to 11*8 per 

 cent, while the last oxidation was only 1*19 grm. or 6*2 per cent. 

 Chromate of lead contains 19*54 per cent, of oxygen, equal to four 



atoms, Cr Pb. 11*8 per cent, is equal to 2*4 atoms or nearly |^ths of 

 the whole amount of oxygen. This would be 12*2 percent. The 

 reduction, if complete, would convert all the oxide of lead into the 

 metallic state and the chromic acid into the state of oxide of chrome. 

 2 Cr + 6 O 2Pb + 2 O 

 3 2 



2 Cr + 3 O 2 Pb. 

 By oxidation about half of the oxygen is recombined ; this takes 

 place the more readily when the metallic lead is in a finely divided 



