INSECTICIDES AND REPELLENTS 5 



Methods of Analysis 



The setting point of not less than 89° C. is a simple and effective 

 check on the quality of technical DDT and is used in specifications as 

 a criterion of its purity. It is determined from the cooling curve of 

 a molten sample under specified conditions. 



DDT is determined quantitatively by the total organically bound 

 chlorine method (A. O. A. C. I), 2 in which the sample is refluxed with 

 metallic sodium in isopropanol. The resulting inorganic chlorides 

 may be determined by any of the recognized procedures — Volhard and 

 its modifications, electrometric, or amperometric. Total chlorine may 

 also be determined by a Carius combustion, with subsequent analysis 

 for the inorganic chloride. The factor for total chlorine to DDT is 2. 

 The total-chlorine determination is not specific for DDT; so it is of 

 value only when it is known that DDT is the only chlorine compound 

 present. 



Another method, but also not specific for DDT, is known as the 

 labile, or hydrolyzable, chlorine method (Wichmann et al. 15). The 

 sample is refluxed with alcoholic alkali with the result that hydrogen 

 chloride is eliminated and the chloride may be determined as inorganic 

 chloride. The DDT may be calculated from the labile chlorine by 

 use of the factor 9.9441. 



The p,p' -isomer content of technical DDT may be ascertained by a 

 crystallization method described by Cristol et al. (3). Ethanol (75 

 percent in water) is saturated with p,p'-DDT at 25° C. The tech- 

 nical DDT is added and dissolved by refluxing. When cooled to 25° 

 C, the p,p' isomer of the added DDT will crystallize and may be 

 weighed. 



A colorimetric method (A. O. A. C. 1) for small quantities is based 

 on the formation of a tetranitro derivative that reacts with sodium 

 methoxide to give a blue color for the p,p' isomer and a reddish purple 

 for the o,p' isomer. This method is primarily for spray-residue and 

 other analyses where small quantities of DDT are present, but it has 

 application also in technical and formulation analyses. It is rather 

 specific, but certain other insecticides, such as methoxychlor, Dilan, 

 and TDE, interfere. 



METHOXYCHLOR AND TDE 



A large number of DDT analogs have been synthesized, and 

 some have shown a high degree of toxicity to insects. Two that are 

 commercially available and are employed in the control of insects 

 affecting man are methoxychlor (l,l,l-trichloro-2,2-bis(^-meth- 

 oxyphenyl) ethane) and TDE (l,l-dichloro-2,2-bis(^-chlorophenyl)eth- 

 ane). 



CH 3 0'< \ H 



\_ // \| 



C— CC1 3 C— CCI2 



CH3 °\ / 



Methoxychlor TDE 



2 Italic numbers in parentheses refer to Literature Ciced, p. 91. 



