16 



LEAD ARSENATE. 



tions for making lead arsenate based on this formula. This is mislead- 

 ing, as such a salt forms below 18 C. (above this temperature it loses 

 water rapidly) and it is not the ordinary sodium arsenate of commerce. 

 The pure crystallized salt, however, is too expensive for the purpose 

 in question and it is necessary to employ the technical grades. These 

 are very cheap and if we can be assured of the absence of objectionable 

 impurities they are just as good as the pure salt for making lead 

 arsenate. In fact they usually contain more arsenic oxid than the 

 crystallized salt, owing to the fact that they have been fused and do 

 not contain water of crystallization, which theoretically amounts to 

 40.4 per cent in the pure salt. Frequently, however, they contain 

 large amounts of impurities, usually sodium chlorid, which lowers the 

 per cent of arsenic oxid. Sodium arsenate sold for technical purposes 

 comes in varying degrees of purity, concerning which there has been 

 much confusion. Two grades commonly on the market are the 50 

 per cent and the 65 per cent grades, which figures refer to the arsenic 

 oxid (As 2 O 5 ) content. In only one sample examined was the arsenic 

 oxid over 45 per cent. 



In Table IV is given the total arsenic oxid and chlorin in the sam- 

 ples of sodium arsenate examined. 



TABLE IV. Composition of sodium ar senates. 



DISCUSSION. 



All of the samples were tested for arsenic present as arsenite, 

 but none was found except traces in two or three of the commercial 

 samples. Nos. 4542, 4548, 4549, 4623, and 4831 are samples of the 

 pure crystallized salt, and all of them have effloresced to a slight 

 extent, which accounts for the arsenic content being a little above 

 the theoretical amount. No. 4547 is comparatively pure and con- 

 tains nearly 8 per cent more arsenic oxid than the crystallized salt, 

 owing to partial dehydration. Nos. 4625, 4643, 4653, and 4717 

 are technical samples and are very impure, containing large amounts 



aFresenius, J. prakt. Chem., 1852, ,56:30. 



