DDT has been incorporated in the pyrethrum-Freon aerosol, which has been widely used by the armed services and is now on the civilian market. Because of the low solubility of DDT in Freon-12 it is necessary to add an auxiliary solvent to the formula. Cyclohexanone and certain aromatic hydrocarbons derived from petroleum have been used for the purpose.

Insecticidal preparations containing DDT have a longer lasting residual effectiveness than other commonly employed insecticides, but the length of the effective residual periods varies considerably. In field tests on agricultural crops, the residual effect is relatively short. Under certain conditions (indoors or protected from sunlight and wind) where the physical environment is more uniform than under agricultural conditions, much longer periods of effectiveness have been reported. The type of surface is important, as would be expected in a contact poison. When DDT is applied to screens, its action lasts longer on copper screening than on iron. Iron has been shown to decompose DDT.

Whenever a new chemical is produced for large-scale use as an insecticide it becomes necessary to develop analytical procedures for it. DDT has five chlorine atoms in its molecular structure. They constitute 50 percent of the compound by weight. One of the chlorine atoms is readily removed as hydrogen chloride and is termed labile or hydrolyzable chlorine.

Methods for the determination of total chlorine or of hydrolyzable chlorine content are generally used for the analysis of DDT residues or preparations containing DDT. Neither of these methods is specific for DDT, since other compounds containing chlorine interfere. Considerable discretion must be used in interpreting results obtained with either of these methods especially when minute amounts of chlorine, such as are found in spray or dust residues, are determined.

Several methods that are specific for the determination of DDT have also been developed. One of these, which has found considerable use, is based on the fact that when DDT is treated with a mixture of concentrated nitric acid and sulfuric acid, a tetranitro derivative is formed. This product when treated with sodium methylate yields a blue color with pure p,p'-DDT and a wine-red color with o,p'-DDT. Intermediate colors are obtained when technical DDT is determined.

The methods so far developed have proved useful, but further work is needed in this field.

THE AUTHORS

H. L. Haller, a chemist in the Bureau of Entomology and Plant Quarantine, is a native of Cincinnati, Ohio. He has been with the Department since his discharge from the Army during the First World War, with the exception of a few years when he was with the Rockefeller Institute for Medical Research as an associate in chemistry. In his present position he is project leader of the researches on plant insecticides and synthetic organic insecticides. In collaboration with his colleagues he has published about 125 papers on naturally occurring and synthetic organic insecticides. For their studies on the determination of the structure of rotenone, Dr. Haller and Dr. F. B. LaForge in 1933 received the Hillebrand prize, given annually by the Washington Chemical Society.

Ruth L. Busbey, a chemist in the Division of Insecticide Investigations, Bureau of Entomology and Plant Quarantine, has been engaged in research on insecticides since 1930. She has conducted synthetic and analytical studies on organic insecticides and chemical investigations on fumigants, especially hydrocyanic acid fumigation to control California red scale on citrus trees.