Polyvinylchloride membrane electrodes

The construction of ISEs used in clinical measurements is of the membrane electrode type, i.e., the ion-sensitive membrane separates the sample from an internal reference electrolyte, which is the site of the internal reference element, usually a silver wire covered by silver chloride. The membrane can be shaped to different forms such as flat, convex, tubular, etc. Sodium sensitive membranes are made from special composition glass, the other ion-sensitive membranes from a polymer matrix such as plasticized polyvinylchloride (PVC) or silicon rubber. The particular selectivity of polymer membranes is first of all due to a small percentage of active material, e.g., valinomycin, dissolved in the polymer. Important secondary effects have been attributed to the type and permittivity of the polymer. The useful lifetime of the sensors also depends on the polymer. The time response [13] may again depend on membrane composition. 

The so-called coated wire electrodes (1) are typical examples. A recent review (2) and a book by Freiser (3) give very detailed accounts of the fabrication and range of coated wire sensors studied. Coated wire electrodes are most commonly formed by dipping a copper wire into a polymer solution which, once dry, leaves the metal covered with plastic. The composition of the plastic is often tailored to reproduce compositions which have been used in membrane electrode configurations. A well-documented example is polyvinylchloride (PVC) impregnated with valinomycin (1), which is selective to ions in solution. An extensive collection of ISE literature titles by Moody and Thomas (4) can be used to locate, inter alia, most new devices of this type. 

Many membranes are made from a film of polyvinylchloride (PVC) in which one or many ion transporters (ionic or neutral) are inserted. Some 40 chelates are used in about a dozen selective membranes (Fig. 18.5). Electrodes used for CIO4 and BF4 are of this type. 

Most polymer membrane ISEs are prepared by dissolving an ionophore in a polyvinylchloride (PVC) membrane. A large variety of plasticisers are used to increase the dielectric constant of the PVC and improve its hydrophilicity. Some membranes have complexes of the ions to be sensed to increase membrane conductivity, such as potassium tetraphenylborate in K -selective membranes. There is an extensive literature on the arcane arts of polymer membranes for electrodes with dissolved ionophores and a good review of this is given by Professor Ronald Armstrong in Section 3.7 of Gabor Harsanyi s book. Polymer films in sensor applications [14]. 

Many commercial selective electrodes include a membrane made of a plasticized polyvinylchloride (PVC) or polysiloxane film in which are disseminated various ion-carriers, ionic or neutral (Figure 19.5). Around 40 chelate molecules are used for around 10 common ions. Electrodes for CLO, BF, NH, Ca + are of this type. 

The apparatus described by Vesterberg (26) had 20 chambers separated by sieve membranes of polyvinylchloride paper. Cooling water was circulated through narrow glass tubes in each cell. The original mixture from the synthesis of carrier ampholytes was poured into the cells. Current was applied via the platinum electrodes in the two end chambers. After about 24 hr the ampholytes had separated, the most acidic ones... 

CA films by using the phase inversion process. These CA films were cast from solvent/nonsolvent solutions to yield size exclusion membranes consisting of a thin permselective outer layer and a more porous sublayer. These membranes permitted the rapid permeation of a 1500-dalton poly (ethylene glycol) ester of ferrocene however the reproducibility of results presents a problem with these CA mem-branes. Christie et demonstrated that thin films of plasticized polyvinylchloride (PVC), normally used for potentiometric ion-selective electrode applications, applied to electrodes over a polycarbonate dialysis membrane offered improved selectivity ratios for the amperometric detection of phenolic compounds and H2O2 in the presence of the common biological interferents, ascorbic acid and uric acid, over those observed at the dialysis membrane alone or at a composite dialysis/membrane. 

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