100 



Fig. 1. — Average per cent 

 of initial residue remaining on 

 apple leaves at intervals after 

 spraying with several insecti- 

 cides. 



6 10 13 



DAYS AFTER SPRAYING 



20 27 34 45 



100 



Fig. 2. — Average per cent 

 of initial residue remaining at 

 intervals after red clover was 

 sprayed with several insecti- 

 cides. 



7 14 



DAYS AFTER SPRAYING 



Table 1. — Vapor pressure at 25°C. and per cent loss of 

 residue from apple leaves in 10 days for five chlorinated hydro- 

 carbon insecticides. 



Inseaicide 



Lindane 



Aldrin 



Chlordane 



Dieldrin 



DDT 



residue studie.s in\t)lving apples, peaches, soybeans, al- 

 falfa, and clover, showed that under varied conditions 

 of exposure the residues of lindane, aldrin, chlordane, 

 dieldrin, toxaphenc, and DDT tend to disappear in the 

 order named, and usually in about the same relationship. 

 It was shown also that in some cases, at least, the per 

 cent of the initial deposit remaining at intervals was a 

 straight-line logarithmic function of time (Fig. 1 and 

 2). Evaporation was not clearly established as an impor- 

 tant factor in residue loss, but data were presented to 



sh(i\s th.it tiicrc was "a considerable loss of even slowly 

 volatile materials during the actual spraying process." 

 Later the apparent correlation between the reported 

 vapor pressures of the compounds in cjuestion and the 

 order in which their residues tended to disappear (Table 

 I ) led to the conclusion that volatility might be an im- 

 portant factor in determining residue persistence. 



EVAPORATION STUDIES 



In the light of the above observations the writer and 

 his associates set about to explore the ramifications of 

 and, if possible, determine the probable importance of 

 this relatively unevaluated factor. 



Throughout the investigations reported, the chemical 

 analysis procedure utilized was that devised by Stepanow 

 (1906:4056-4057) with the modification suggested by 

 I'leck (1947:319-324). Umhoefer (1943:383). and Cald- 

 well &: Moyer (193'':3H-39). The method involves the 

 conversion of (jrganic chlorine to the chloride ion. which 

 is subsequently determined by the Volhard titration 



