No. 4.] GYPSY MOTH — APPENDIX. 



363 



Table No. 4. 



Arsenic 

 Peiitoxide. 



No. 3, chemically pure crystals, from drug store, 

 No. 4, chemically pure crystals, from drug store. 

 No. 5, commercial, 58 to 60 per cent.. 

 No. 7, commercial, 68 to 70 per cent., 



Per Cent. 

 36.86 



36.77 

 37.70 

 47.80 



.75 

 .57 



The Presence of Chlorine in Arsenate of Soda.* 

 There are probably no commercial grades of arsenate 

 of soda wholly free from chlorine but its presence to the 

 amount of 2. or 3 per cent, does no serious harm. If chlorine 

 be present to a greater extent than 4 or 5 per cent., upon 

 the addition of the soluble lead salt to the impure arsenate 

 of soda a considerable quantity of chloride of lead will be 

 precipitated. This but wastes the soluble lead salt, since 

 chloride of lead has practically no value as an insecticide. 

 The reaction in this case may be expressed by the following 

 equation : — 



Pb(C2H302)23H20 + 2NaCl = PbCl2-|-2NaC2H302+3H20. 



The solubility of lead chloride in water at 20° C. is .9712 

 part to 100 (Formanck), and in hot water 1 part to 22 

 (Wittstein).f The arsenate of lead should be precipitated 

 in a dilute solution, and where possible, brook or hydrant 

 water should be used in preference to colder spring or well 

 water. 



Arsenate of Lead., Lead Arsenate. 



Arsenate of lead may be prepared by mixing a solution of 

 arsenate of soda with a solution containing either acetate 

 or nitrate of lead. As previously stated, it consists of a 



* Several other impurities, such as arsenious acid, sulphates, nitrates, etc., also 

 occur in commercial arsenate of soda. The arsenious acid, upon the addition of a 

 soluble lead salt, is precipitated as arsenite of lead, which possesses considerable 

 value as an insecticide. 



t A. M. Corney, "Dictionary of Chemical Solubilities," 1896, p. 206. 



