46 Preparation of Substances 



mixture after the material has been well 

 washt will turn red slowly showing the pres- 

 ence of a sUght concentration of acid. This 

 is the arsenic acid hydrolyzed oE from the 

 acid arsenate. 



The addition of any acid changes the 

 equilibrium from right to left forming the 

 acidic arsenate with the elimination of the 

 basic compound. Most of the lead arsenate 

 pastes on the market are made with lead 

 nitrate and the acidity of the system is 

 sufficient to produce a mixture consisting 

 mostly of the acid arsenate with smaller 

 proportions of the basic compound. 



Inasmuch as the acidic arsenate consists 

 of approximately 33 per cent AS2O5 and the 

 basic approximately 23 per cent the analysis 

 of the compound will show the proportions 

 of the two arsenates present. For example, 

 a mixture of the two analyzing 30 per cent 

 AS2O5 is nearly all acidic and contains very 

 Httle of the basic compound, while one 

 analyzing 28 per cent is composed of equal 

 parts of the two arsenates, basic and acidic. 

 These facts can be applied to the analysis of 

 two commercial samples of arsenate of lead 

 which are here given, one made from, lead 

 nitrate which contains a strong acid and the 

 other made from lead acetate containing a 

 weak acid. 



