208 Agricultural Gazette of N.S.W. [Mar. 2, 1920. 



The Compositions of Various Lead 



Arsenates* 



A. A. liAMSAY, Principal Assistant Chemist. 



An examination has been made of such brands of arsenate of lead us were 

 procurabk^ on the market in September. 1919, and the results arc set forth 

 in the accompanying table. Nine samples in all were examined, four of 

 "these being in " paste " form, and five as " dry powder." Of the paste 

 forms examined, it will be noted that the moisture content ranged from 

 34-7 to 47-1, the arsenic acid content from 13-9 to 19-6, and the lead oxide 

 •content from 33*9 to 39-5 per cent. Of the dry powder forms examined, 

 "the moisture content ranged from 0-2 to 0-8, the arsenic acid from 26 to 29-5, 

 and the lead oxide from fiO-S to 65-3 per cent. 



This range in the amounts of lead oxide and arsenic acid present i-i due 

 "to the different forms and combination of the lead oxide and the arsenic acid ; 

 "that is to say, to the different amounts of diplumbic arsenate and triplumbic 

 arsenate present. 



Arsenate of lead, as sold, consists essentially of diplumbic arsenate, tri- 

 plumbic arsenate, or a mixture of both. 



The compound diplumbic arsenate consists of two molecules or jiavts of 

 lead oxide (the weight of which is represented by 445) united with one 

 molecule or part of arsenic acid (the weight of which is represented by 230), 

 and united with one molecule or part of water (the weight of which is 

 Tepresented by 18). 



This compound, therefore, contains in every 100 parts 04-21 parts lead 

 •oxide and 33-19 parts arsenic acid. If this compound were made into 

 a paste formed from 50 parts powder and 50 parts water, the paste would 

 contain 32-11 parts lead oxide and 16-5i) parts arsenic acid in every 100 

 parts of the paste. 



This compound is sometimes called " acid arsenate of lead," and the term 

 used indicates in chemical nomenclature the type of compound. The term 

 *' neutral arsenate of lead " is applied to that compound which contains 

 three parts of lead oxide united to oi e part arsenic acid, since this is the 

 maximum amount of lead oxide possible in the combination. 



The compound triplumbic arsenate consists, as stated above, of three parts 

 lead oxide (the weight of which is represented by 667-5) united to o e part 

 arsenic acid (the weight of which is represented by 230). This compound, 

 therefore, contains in every 100 parts 74-37 parts lead oxide and 25-63 parts 

 arsenic oxide. If pure trij)lumbic arsenate were formed into a paste, using 

 equal ]>arts of the powder and water, tiie paste would contain 37-19 parts 

 lead oxide and 12-81 parts arsenic acid per cent. 



