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Glass Ion-Selective Electrodes
The first commercial glass electrodes were manu- factured using Corning 015, a glass with a composition of approximately 22% Na2O, 6% CaO, and 72% SiO2. When immersed in an aqueous solution, the outer approximately 10 nm of the membrane becomes hydrated over the course of sev- eral hours. Hydration of the glass membrane results in the formation of negatively charged sites, G–, that are part of the glass membrane’s silica framework. Sodium ions, which are able to move through the hydrated layer, serve as the counterions. Hydrogen ions from solution diffuse into the membrane and, since they bind more strongly to the glass than does Na+, displace the sodium ions
H+(aq)+ G––Na+(s) < = = = = > G––H+(s)+ Na+(aq)
giving rise to the membrane’s selectivity for H+. The transport of charge across the membrane is carried by the Na+ ions. The potential of glass electrodes using Corn- ing 015 obeys the equation
over a pH range of approximately 0.5–9. Above a pH of 9–10, the glass membrane may become more responsive to other cations, such as Na+ and K+.
Replacing Na2O and CaO with Li2O and BaO extends the useful pH range of glass membrane electrodes to pH levels greater than 12.
Glass membrane pH electrodes are often available in a combination form that includes both the indicator and the reference electrode. The use of a single electrode greatly simplifies the measurement of pH. An example of a typical combination electrode is shown in Figure 11.12.
The response of the Corning 015 glass membrane to monovalent cations other than H+ at high pH led to the development of glass membranes possessing a greater selectivity for other cations. For example, a glass membrane with a composition of 11% Na2O, 18% Al2O3, and 71% SiO2 is used as a Na+ ion-selective electrode. Other glass electrodes have been developed for the analysis of Li+, K+, Rb+, Cs+, NH4+, Ag+, and Tl+. Several representative examples of glass membrane electrodes are listed in Table 11.1.
Since the typical thickness of the glass membrane in an ion-selective electrode is about 50 μm, they must be handled carefully to prevent the formation of cracks or breakage. Before a glass electrode can be used it must be conditioned by soaking for several hours in a solution containing the analyte. Glass electrodes should not be allowed to dry out, as this destroys the membrane’s hydrated layer. If a glass elec- trode has been allowed to dry out, it must be reconditioned before it can be used. The composition of a glass membrane changes over time, affecting the electrode’s performance. The average lifetime for a glass electrode is several years.
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