December, '19] 



ROBINSON: LIME IN LIME SULFUR 



431 



cent of the arsenic was found in a soluble form thus increasing the 

 tendency to cause burning of foUage and other injury. Furthermore 

 the decrease in the sulfur content indicates excessive loss of that active 

 element. The total polysulfid content was reduced from 8.8004 grams 

 to 5.8367 grams or over 35 per cent. In other words, by mixing lead 

 hydrogen arsenate and summer strength Hme sulfur the efficiency of 

 the latter is decreased more than 35 per cent while severe burning of 

 fohage may occur owing to the presence of arsenic as a soluble salt. 



The changes that occur in the composition of the lead hydrogen 

 arsenate mixed with the lime sulfur as shown by analyses of the lead 

 arsenate residue likewise emphasize the need for adding some inert 

 material that will diminish the chemical reaction. Table II gives the 

 analyses of both types of arsenates before and after mixing with lime 

 sulfur and treated as described above: 



Table II. Analyses op Lead Hydrogen and Basic Arsenate Before and After Mixing with 



Lime Sulfur 



These results substantiate those given in Table I. Both sulfur and 

 lime have been removed from the lime sulfur in the reaction and the 

 lead from the lead arsenate to form sulfide of lead. From the analysis 

 the decrease in the quality of the insecticide is self-evident. 



Although the foregoing results show that the basic lead arsenate may 

 be advantageously mixed with lime sulfur, since it is not available, the 

 hydrogen arsenate must be used. Hence the importance of adding 

 some inert material in order to prevent, as much as possible, the devi- 

 talizing reaction. During investigations on the calcium arsenate by 

 the writer^ it was observed that slaked lime, Ca(0H)2, prevented al- 

 most entirely any decomposition of both calcium arsenate and lime 

 sulfur when mixed. Consequently it was thought that lime might 

 have a similar effect when used with lime sulfur and lead arsenate 

 combination spray and the reaction tend toward the formation of the 

 basic lead arsenate according to the equation: 



3 PbH As04+Ca(OH)2 = Pb3(As04).>+CaHAs04+2 HoO 



To this end a study was made using both pure material, prepared in 

 the laboratory, and commercial samples. The lime was obtained by 



• Robinson, R. H. The Calcium Arsenates, Jour, of Agri. Res., Vol. 13, Apr. 29, 

 1913. 



