Potentiometry of alkoxylates

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Abstract
The response characteristics of barium ion-selective electrodes to various alkoxylates have been investigated in aqueous and aqueous-ethanol test solutions in the presence and absence of barium ions and in the presence of sodium and potassium salts. The observed potentiometric response is an increase in the e.m.f. of the barium ion-selective electrode of up to about 100 mV, according to the amount of added alkoxylate in the 2 × 10–5–10–3 M range. The response is linear with log[alkoxylate], but is characterised by a break in the linearity, which is attributed to the critical micelle concentration (CMC) of this class of non-ionic surfactants. The alkoxylates studied include nonylphenoxypoly (ethyleneoxy) ethanols (NPs) of the Antarox CO series, namely, CO-430, -630, -730, -850, -880 and -890; octylphenoxypoly (ethyleneoxy)ethanols (OPs), namely, Antarox CA-620 and Triton X-100; a sorbitan-9-octadecanoate poly (ethyleneoxy)ethanol, namely, Tween 80; alkylpoly (ethyleneoxy)ethanols, namely, Dobanol 25–7, Lutensol AO7 and Synperonic 7; a fluoroalkyl poly(ethyleneoxy)ethanol, namely, Monflor 51; poly(ethylene glycol)(PEG) 1540; and poly(propylene glycol)(PPG) 1025. CMCs were characterised in all instances except for PEG 1540, where the increases in the e.m.f. values were smaller than for the other materials studied. Anionic surfactants gave small decreases in e.m.f., but the cationic surfactant, benzyldimethylhexadecylammonium chloride, gave a large increase in e.m.f. characteristic of a cationic response. Exposure to alkoxylates reduced the lifetime of the barium ion-selective electrodes.

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