A determination of the apparent molar absorption coefficients of the cobalt thiocyanate complexes of nonylphenol ethylene oxide adducts

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Abstract
Cobalt thiocyanate is used as a colorimetric reagent to determine traces of polyoxyethylene nonionic surfactants. It has been established that when there is an insufficient number of oxide units per hydrophobe the color intensity is markedly diminished, and with smaller numbers of oxide units the color fails to form altogether. the exact number of oxide units at which the colorimetric procedure fails has not been established previously, due to the unavailability of the pure individual ethylene oxide adducts.The individual ethylene oxide adducts of high purity were obtained by liquid chromatography on silica gel. The mixed solvent used was originally developed for thin‐layer chromatography and was applied without change to column chromatography. The composition of the separated isomers has been determined by infrared, ultraviolet, nuclear magnetic resonance, and mass spectroscopy.The apparent molar absorption coefficients have been obtained for the individual cobalt thiocyanate complexes both in benzene and chlorform. For the low molecular weight adducts studied, the efficiency of color development and extraction into the organic phase has been found to be dependent on the concentration of the cobalt thiocyanate reagent. A saturated aqueous solution of cobalt thiocyanate was found to be preferable and benzene was found to be a more reliable extractant than chloroform.The apparent molar absorption coefficients do not vary linearly with chain length at low molecular weights and the minimum number of ethylene oxide units that will form an extractable color with the saturated reagent was found to be 3.