40 N. S. ENTOMOLOGICAL SOCIETY. 



ARSENATE OF LEAD vs. ARSENATE OF LIME. 



By G. E. Sanders, Field Officer in Charge. 

 Dominion Entomological Laboratory, Annapolis Royal, N. S. 



FOR SOME years past orchardists.when using the almost universally recommended 

 lime sulphur lead arsenate combination have been troubled by black "sludge"in 

 the bottom on the spray tank, preventing the proper agitat on of the poison and 

 •ccasionally clogging the nozzles. 



Consulting Prof. Sexton of the Nova Scotia Technical College in regard to the ac- 

 tion of lead and other arsenates on sulphide solutions, he stated that the "sludge" was 

 undoubtedly lead sulphide, and further that only three arsenates, namely those of cal- 

 cium, barium and strontium, could be added to sulphide solutions without causing a 

 reaction and the formation of sulphides of the metallic bases used. 



In May 1915 R. H. Robinson and H. V. Tarter issued station Bulletin No 128 

 from the Oregon Agricultural College on the arsenates of lead. In this publication the 

 authors state that when dry acid or hydrogen lead arsenate is added at the rate of 1 

 pound to 40 gallons to a solution of lime sulphur diluted at the rate of 1 gal. of commer- 

 cial concentrate to 30 gallons of water, over 25 per cent of the calcium and over 35 per 

 eent of the sulphur are removed from solution and at the same time 5 per cent of the 

 arsenic oxide (As 2 5 ) of the lead arsenate becomes soluble. To quote from their sum- 

 mary, "The study of the reactions involved indicates a practical interchange of cal- 

 cium and lead forming calcium arsenate and lead fulphide respectively, free sulphur 

 being deposited at the same time. The soluble arsenic is then derived from partial so- 

 lution of the calcium arsenate thus formed." 



Lime and arsenates combine in three common forms as arsenates — mono-calcic, 

 bi-calcic and tri-calcic; the first of these, mono-calcic arsenate, is entire 1 y soluble and it 

 is due to the formation of a portion of the calcium arsenate in the form of mono-calcic 

 arsenate that a portion of the arsenic is rendered soluble by the reaction. 



Following this it would seem that one would have as safe a spraying solution as the 

 sulphur lime lead arsenate combination, if arsenate of lime (44 per cent As 2 5 ) con- 

 taining 2.2 per cent of the soluble arsenic were added to lime and sulphur. It would be 

 more economical as 35 per cent of the sulphur would be prevented from going out of 

 solution and, moreover, the sprayer would not be bothered with the lead sulphide 

 "sludge" in holding the poison down and clogging the nozzles. 



The commercial arsenate of lime containing 43 and 44 per cent AsjOs in the form 

 of tri-calcic arsenate is guaranteed to contain less than one per cent of soluble arsenic, 

 so that one would expect a combination of lime and sulphur and commercial arsenate of 

 lime to give less injury to the foliage and fruit than a combination of lime sulphur and 

 lead arsenate, containing an equal quantity of arsenic oxide. In actual field experi- 

 ments this has been shown to be the case. 



On another page of thi^ report we have published an article on "The effect of cer- 

 tain combinations of spraying materials on the set of apples." The following table from 

 that article is here repeated. 



