18 LEAD ARSENATE. 



In numerous trials with pure salts it was found that the latter 

 reaction occurs almost theoretically, though a small amount of the 

 tri-plumbic arsenate is usually formed. 



With lead acetate, however, there are other conditions which 

 affect the reaction, probably temperature, concentration, method 

 of mixing, etc. In several cases when pure chemicals were used 

 the resulting product was found to be principally the plumbic hydro- 

 gen arsenate. Most of the samples examined in which the acetate 

 was used in the preparation consisted mainly of the tri-plumbic 

 arsenate, Pb 3 (AsO 4 ) 2 . This contains theoretically 74.40 per cent 

 of lead oxid (PbO), and 25.60 per cent of arsenic oxid (As 2 O 5 ). 



As may be calculated from the reaction previously given, it will 

 be found that by using pure crystallized lead acetate (58.81 per cent 

 PbO) and crystallized sodium arsenate (36.84 per cent As 2 O 5 ) there 

 will be required to make 1 pound of tri-plumbic arsenate 1.296 

 pounds of lead acetate and 0.695 pound of sodium arsenate, or 

 64.55 per cent of lead acetate and 35.45 per cent of sodium arsenate. 



Plumbic hydrogen arsenate, PbIIAsO 4 , contains theoretically 

 64.26 per cent of lead oxid (PbO); 33.15 per cent of arsenic oxid 

 (As 2 O 5 ), and 2.59 per cent of water of constitution. 



Calculating the amount of lead nitrate (67.35 per cent PbO), and 

 sodium arsenate (36.84 per cent As 2 O 5 ) required to make 1 pound 

 of this compound from the second reaction given, the following result 

 is obtained: 0.954 pound of lead nitrate, and 0.900 pound of 

 sodium arsenate, or 51.43 per cent of lead nitrate, and, 48.57 per cent 

 of sodium arsenate. 



However, formulas can not be given based on technical or even 

 on pure salts, for a number of reasons: 



(1) Pure salts are too expensive to use. 



(2) The technical grades show considerable variation in composition, as has been 

 shown. 



(3) Allowance must be made for other salt-forming compounds in the sodium 

 arsenate, notably chlorids, which use up some of the lead salt. 



(4) The lead salt should be in slight excess to insure rendering all of the arsenic 

 insoluble. 



(5) Under the varying conditions which exist at the time of making, the reactions 

 do not proceed as indicated by theory. 



In regard to the last reason, it may be said that even if the exact 

 chemical composition of the salts were known and the correct propor- 

 tions calculated to satisfy the reaction were mixed together, it would 

 seldom, if ever, result in a complete combination of the lead and 

 arsenic radicals. The only way to proceed, therefore, is either to 

 add lead salt considerably in excess of the theoretical amount, or to 

 add the lead salt gradually and test from time to time to see when 

 it is in excess. The latter method is much the better one. In a few 

 of the published formulas attention is called to the necessity of having 



