The response characteristics of calcium ion-selective electrodes to various anionic surfactants have been investigated in test solutions in the presence and absence of calcium ions. Particular attention has been given to the effect of sodium dodecylsulphate (SDS) on the response of PVC membrane calcium ion-selective electrodes based on calcium bis[di(4-octylphenyl)-phosphate] sensor and dioctyl phenylphosphonate solvent mediator.
The effect of added anionic surfactant, even at about 2 × 10–5M, is to lower the e.m.f. response of electrodes when in contact with the various test solutions. However, replacement of the dioctyl phenylphosphonate solvent mediator by decan-l-ol diminishes the surfactant ion interference to a considerable extent, but the amount of decan-l-ol introduced into the electrode membrane needs to be restricted in order to preserve calcium ion selectivity.
The mechanism of anionic surfactant interference seems to involve a calcium-surfactant interaction at the electrode surface and possibly involving dioctyl phenylphosphonate rather than complexation of calcium ions in the bulk solution.
Alkylsulphates exhibit the trends expected of a homologous series, and those of alkyl chains shorter than 6 carbon atoms give little interference. A “C16—C18 olefin sulphonate,” although having good surfactant properties, also shows little interference below about 5 × 10–3M.
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