Liquid-liquid distribution of ion associates of anions with 4-[4-alkyl(aryl)aminophenylazo]pyridines and their use as spectrophotometric reagents for anionic surfactants

Abstract

A series of 4-[4-alkyl(aryl)aminophenylazo]pyridines and 4-(4-dialkylaminophenylazo)pyridines in which the alkyl groups are CH₃, C₂H₅, C₃H₇ and C₄H₉ and the aryl group is C₆H₅ were synthesised and examined as colour-changeable extractants for acids. The fundamental physico-chemical constants for the azo dyes and their ion associates were determined by spectrophotometry. The contribution of a methylene group to the distribution coefficient of the azo dye (R), log K_D(R), and to the extraction constant of the ion associate, log K_ex, was found to have a value of 0.5–0.6, i.e., the greater was the number of methylene groups, the larger were the values of log K_D(R) and log K_ex. A parameter expressing the extractability of the protonated cations of the azo dyes and of several organic and inorganic anions as ion associates was obtained, based on the assumption that the extraction constant of an ion associate, log K_ex, can be divided into a cationic and an anionic contribution, C and A, respectively. The order of extractability of the anions studied was: ClO₄^– < SCN^– < picrate < (Ethyl Orange)^– < C₁₂H₂₅SO₃^– < C₁₂H₂₅OSO₃^– < C₁₂H₂₅–C₆H₄–SO₃^– < SSS^–(di-2-ethylhexylsulphosuccinate). Anionic surfactants could be extracted quantitatively into chloroform, 1,2-dichloroethane and o-dichlorobenzene with the dibutyl derivative of the azo dye: the molar absorptivity was ca. 5.5 × 10⁴ l mol^–1 cm^–1 at 555 nm. The azo dye, once used, can be recycled and used repeatedly for further determinations.