HOME-MADE LEAD ARSENATE. 17 



,{' sodium chlorid, as shown by the high chlorin content. On account 

 of this, none of them is desirable for making lead arsenate. Sample 

 No. 4649 is a technical sample and is unusually high in arsenic oxid. 

 It Is probably composed largely of sodium dihydrogen arsenate 

 (XaH 2 AsO 4 ). 



The analyses here reported show that there is little or no risk in 

 buying the technical grades of the lead salts, but sodium arsenate 

 is much more variable, and when chlorin is present sufficient lead 

 must be added to combine with this as well as with the arsenic. 

 This is a waste of the lead salt, as lead chlorid is not considered of 

 value as an insecticide and therefore the presence of .chlorids is 

 objectionable, particularly in amounts greater than 3 or 4 per cent. 

 The presence of arsenious oxid (As 2 O 3 ) or sodium arsenite is also 

 objectionable, as by uniting with lead it forms lead arsenite, which 

 is more soluble than the arsenate, does not remain in suspension 

 as well, and, as shown by Kirkland and Burgess, a is less poisonous 

 to insects. 



THEORETICAL COMPOSITION OF LEAD ARSENATE. 



As has been pointed out by others, arsenate of lead may mean 

 any of the various lead arsenates, but the most common ones are 

 the tri-plumbic arsenate and the plumbic hydrogen arsenate, rep- 

 resented by the formulas Pb 3 (AsO 4 ) 2 and PbHAsO 4 , respectively. 

 Most of the commercial samples consist of a mixture of these two, 

 the one predominating depending upon the method used in its 

 manufacture. As has been shown by Smith b and Haywood, c 

 when lead acetate and di-sodium arsenate are used for its preparation 

 the following reaction takes place: 



3Pb(C 2 H 3 O 2 ) 2 3H 2 O + 2Na 2 IIAsO 4 7lI 2 O = Pb 3 (AsO 4 ) 2 + 

 4NaC 2 H 3 O 2 3H 2 O + 2HC 2 H 3 O 2 + 1 1 H 2 O. 



Using nitrate of lead and di-sodium arsenate, Smith d gives the 

 reaction thus: 



5Pb(NO 3 ) 2 + 4Na 2 HAsO 4 (H 2 O) n = Pb 3 (AsO 4 ) 2 + 2PbHAsO 4 + 

 8NaN0 3 + 2HN0 3 + n (H 2 O) . 



Haywood* found the reaction to be mainly as follows: 



Pb(NO 3 ) 2 + Na 2 HAsO 4 7H 2 O = PbHAsO 4 + 2NaNO 3 + 7H 2 O. 



o Agriculture of Massachusetts, 1897, p. 379. 

 & Agriculture of Massachusetts, 1897, p. 364. 

 cU. S. Dept. Agr., Bureau of Chemistry Bui. 105, p. 165. 

 <*Loc. cit., p. 365. 

 Loc. cit., p. 166. 

 23904 Bull. 13110 3 



