﻿6 BULLETIN 1147, U. S. DEPARTMENT OF AGRICULTURE. 



The two most important determinations to be made on lead ar- 

 senates are the total arsenic oxid and the water-soluble arsenic oxid. 

 The total arsenic oxid of an acid lead arsenate usually varies from 

 31 to 33 per cent, and the water-soluble arsenic oxid is less than 0.3 

 per cent in a good grade of commercial acid lead arsenate. 



Robinson and Tartar (87) reported analytical results on commercial 

 lead arsenates and described various tests used to determine the 

 forms in which the lead and the arsenic are combined, as well as the 

 extent to which these forms exist in such substances. 



In acid lead arsenate the ratio by weight of arsenic oxid to lead 

 oxid is theoretically 1 to 1.94. According to the results of the 

 analysis (Table 3), however, this ratio is somewhat higher in com- 

 mercial lead arsenates, showing that a slight excess of lead oxid 

 (litharge) had been used in their manufacture in order to make sure 

 that no uncombined arsenic acid would be left in the product. A 

 small amount of carbon dioxid, which had been introduced in the 

 litharge, was found in the acid lead arsenates tested. This is of no 

 practical significance. In all but three of the powdered samples the 

 moisture content was less than 0.35 per cent. The water of consti- 

 tution of acid lead arsenates is theoretically 2.58 per cent. The 

 results by difference show differences slightly greater than the theo- 

 retical figures, but in no case are they of any magnitude. The per- 

 centages of arsenic oxid and lead oxid, together with the low per- 

 centage of water-soluble arsenic oxid, indicate that the commercial 

 acid lead arsenates examined were good and stable products. 



BASIC LEAD ARSENATE. 



The early investigators recognized ''basic," or "sub," arsenate of 

 lead and applied the term "neutral lead arsenate" to PbHAs0 4 , 

 which is the present commercial acid lead arsenate. They also ap- 

 plied the term "neutral lead arsenates" to lead pyro arsenates, which 

 are not commercial products, and therefore will not be discussed here. 

 McDonnell and Smith have printed a report on p3Toarsenates (27). 

 As a result of another investigation on basic lead arsenates, these 

 authors (28) report the existence of a basic arsenate having optical 

 and crystallographic properties similar to those of mimetite, from the 

 analytical data apparently hydroxy mimetite, containing one mole- 

 cule of water of crystallization. One or two manufacturers of in- 

 secticides sell, generally on special order, what is commercially called 

 "T. P." arsenate. 



Basic lead arsenate may be prepared as follows: Produce basic lead 

 acetate by the action of acetic acid on lead or lead oxid, usually 

 litharge. Then mix it with arsenic acid, thus forming basic lead ar- 

 senate. Basic lead arsenate may also be made by the reaction of 

 sodium arsenate, litharge, and nitric acid, or by the action of ammonia 

 on acid lead arsenate. It has the following theoretical composition: 

 As,0 5 (23.2 per cent) , PbO (75 per cent) , and water of constitution and 

 crystallization (1.8 per cent). The specific gravity of this substance 

 was found by McDonnell and Smith (28) to be 6.86. 



Only two samples (Table 3, Nos. 28 and 21) of commercial basic 

 lead arsenate (a powder and a paste) were secured on the market. 

 While these showed somewhat greater variations from the theoretical 

 than did the acid lead arsenates, both are relatively pure compounds. 



