Electrodes to Monitor Titrations

An electrode sensitive to both cationic and anionic surfactants can be made from a glass tube sealed at the bottom with a membrane of poly(vinylchloride) containing a high percentage of plasticizer, 40% or more. Dissolved in the plasticized PVC is a surfactant ion pair, such as hexadecyltrimethylammonium dodecylsulfate. The tube is filled with a dilute solution of anionic surfactant with a small amount of a chloride salt, into which a silver/ silver chloride electrode is inserted. For potential measurements in solutions, the circuit is completed with a reference electrode such as saturated calomel. Commercial surfactant-selective electrodes based on PVC membranes have been available since approximately 1990. 

The PVC-membrane electrode can be made in different forms. A particularly mgged version is the coated-wire electrode, in which the PVC covers a length of metal or graphite rod (30,119). A similar form of solid state electrode is made by applying the PVC membrane to a substrate of conductive epoxy resin (120,121). A flow-through electrode suit- 

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An ion-selective field effect transistor device has been invented to serve as an anionic or cationic surfactant-selective electrode. In this case, a plasticized PVC membrane is made incorporating a surfactant salt. This membrane is cast over the gates area of a field effect transistor, the whole (except the sensing surface) is encapsulated in epoxy resin, and a suitable measuring circuit is connected. Application is similar to the polymer membrane electrodes described above (124-126). 

A specially made electrode is now commercially available which is much more tolerant of solvents than the conventional PVC-membrane electrodes. It is in the form of a coated-wire electrode, made with the ion pair incorporated into a silicone polymer by chemical reaction (1,127). This permits use in titration media where high organic solvent concentrations are necessary, as for example when the sample matrix is oily or when an ionic surfactant must be determined in the presence of a very high concentration of nonionic surfactant. The electrode can even be used if two phases are present, analogous to the two-phase titration discussed at the beginning of this chapter. As of this writing, the electrode is only available from Metrohm, but presumably competitive products will be available. 

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