We know that to make a successful measurement with an ion selective electrode, t

Question

We know that to make a successful measurement with an ion selective electrode, the electrode potential should respond (ideally) only to the target analyte. With solid-state and liquid-based ion selective electrodes, this is accomplished by selectively transporting the analyte ion over the membrane/solution interface (for example, the LaF3 membrane in a F? ISE allows the transport of F?). However, one electrode we have used, the glass pH electrode, does not work this way. Explain how a glass pH electrode works. Why can we use it to measure pH when H3O+ does not move inside the glass?

Explanation / Answer

A glass pH electrode is an ion-selective electrode comprised of a doped glass membrane that is sensitive to hydrogen ion. The glass membrane facilitates ion-exchange. The internal filling solution in the glass pH electrode is KCl solution (pH-7). Say, the electrode is placed in an analyte which is relatively more acidic. The additional hydrogen ions, replace some of the metal ions from the outer surface of the glass membrane leaving the metal ions in to the analyte solution. Similar exchange of ions takes place between the inner surface of the galss and teh internal filling solution. This generates electrical charge difference between the two sides of the membrane.A potential difference is observed across the memebrane which is indicative of the pH of the analyte solution.

So, a glass pH electrode can be used to measure the pH of the solution as a function of the potential difference deveopled across the glass membrane due to the differential ion exchange.

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