During the last two years, a new selective herbicide has been in extensive commercial use, in cereal crops, for the control of certain weeds which are not susceptible to the long-established "hormone" weedkillers such as MCPA and 2,4D. This herbicide* is composed essentially of an aqueous solution of the alkali metal salts of trichlorobenzoic (TBA) and methylchlorophenoxyacetic (MCP) acids together with formulants added to assure satisfactory field performance. The principal physiologically active constituents of the mixture are 2,3,6-trichlorobenzoic acid and 4-chloro-2-methyl phenoxyacetic acid (4C2M) and methods were therefore required for the specific determination of these acids.
The acidic impurities in the mixture result from the manufacturing processes used for its constituent technical MCPA and technical TBA. Technical MCPA normally contains 85-1oo% 4C2M, the principal acidic impurities being 6-chloro-2-methyl phenoxyacetic acid (6C2M) 4,6-dichloro-2-methylphenoxyacetic acid (46C2M), 2-methylphenoxyacetic acid (2M) and the corresponding chlorinated cresols. Glycollic acid may also be present. Technical TBA usually contains a minimum of 4o% 2,3,6-TBA, possible impurities (depending on the manufacturing process employed) including the 3 monochloro-acids, 6 dichloro-acids, 5 isomeric trichloro-acids, 3 tetrachloro-acids and pentachlorobenzoic acid.
The determination of 236 TBA in MCPA-TBA mixtures will be described in this part of the series. Part II describes the determination of 4C2M. * Commercial name "Fisons 18-15". Anal. Chim. Acla, 23 (196o) 271-281 * Throughout this paper these acids will be referred to by the position of their chlorine atoms (e.g. "236" = 2,3,6-trichlorobenzoic acid). ** All paxtition coefficients are expressed organic solvent: aqueous solution.