1912.] PUBLIC DOCUMENT — No. 31. 203 



Experimental Besults. 



In the production of lead arsenates pure chemicals are a 

 prime requisite for a high-grade product. The lead salts should 

 be free from other bases forming insoluble arsenates, and the 

 sodium arsenate (xsa2HAs047H20) from arsenites, carbonates, 

 chlorides and sulfates. Acetate of lead is objectionable as a 

 source of lead in that it readily carbonates on exposure to air. 

 As to concentration of solutions, our exiierience has shown that 

 for salts of such high molecular weight dilute solutions not ex- 

 ceeding /^ molecular (M/5) are preferable. At that dilution, 

 laboratory temperature gives a very finely divided precipitate 

 which is highly desirable from the standpoint of suspension. 

 The arsenate should be run into the lead salt very sloioly with 

 thorough agitation in order to prevent precipitation of lead 

 hydroxide due to the alkalinity of the sodium salt. The re- 

 verse precipitation, lead into the arsenic, proved less satisfac- 

 tory both as to formation and behavior of the precipitate. 

 While arsenic acid is stronger than arsenous, it neutralizes 

 only about one-half the alkalinity of the soda in disodium 

 hydrogen arsenate. 



Neutral Lead Arsenate. — After many attempts, employing 

 di and tri sodium and ammonium arsenates, salts containing 

 arsenic and lead in proper molecular ratio were finally pro- 

 duced according to the following equation : — 



3 Pb(C2H302)23 H2O + 2 Na2HAs047 H2O 



= Pb3(As04)2 + 4 NaC2H3023 H2O + 2 C2H4O2 + 11 H2O. 



To obtain these results it was necessary to prepare the 

 disodium arsenate in order to exclude carbonic acid which 

 was present in the commercial salts purchased. The principal 

 difficulties, however, arose from failure to add the strongly 

 alkaline sodium arsenate slowly and with sufficient agitation to 

 prevent the precipitation of lead hydi'oxide and to maintain an 

 excess of at least 5 per cent, of lead to prevent the formation of 

 the acid salt. The usual precautions as to concentration, tem- 

 perature and thoroughness of washing were carefully observed. 

 The following analyses of two samples show the material to be 

 practically of theoretical composition : — • 



