Lead Arsenate 45 



tion and consequently were all repellent of 

 each other, gradually floe together, the 

 Brownian motion slows down, and as the 

 particles continue to coalesce the Brownian 

 motion ceases and the solid separates out as 

 a precipitate. 



The arsenate of lead formd is a mixture of 

 two compounds; one acidic having the sym- 

 bol PbHAs04, and one basic represented by 

 the formula Pb3As204 • Pb20HAs04. These 

 two compounds are in equilibrium, 



5PbHAs04 + HOH ^ Pb50H(As04)3 

 + 2H3ASO4, 



the acidic one being gradually changed to the 

 basic one as the arsenic acid formd in the 

 reaction is decanted off during the washings. 

 The change to the basic compound, however, 

 is very slow as with very slight concentra- 

 tions of arsenic acid such as accumulate in 

 the wash water the action stops and no more 

 basic compound is formd until the superna- 

 tant wash water is replaced by fresh. As an 

 example of the slowness of the change it 

 required, in an experiment by McDonnell 

 and Graham of Washington, D. C, con- 

 tinuous washing for a year to change two 

 grams of the lead acid arsenate to the basic 

 substance. 



A piece of blue litmus paper added to the 



