Glass electrodes and pH sensors

There are three basic types of selective electrode those based on glass membranes, on inorganic salt solid membranes, and on ion exchange. Other more complex electrodes are sensitive to dissolved gases and enzymes. These various types are now described. 

It is important to see how the potential difference across the membrane, usually of thickness 50 jum, arises. The glass is hydrated to about 50nm depth on each side, regions m and m , the interior of the membrane, region m, remaining dry. There is adsorption and desorption of ions on the hydrated layers. It appears that transport through the membrane is entirely by cations, e.g. Na+ or Li+. 

Consideration of Fig. 13.3 shows that the potential across the membrane is given by 

The first and last terms are differences of interfacial potential originating 

The alkaline error , often found in pH electrodes, arises because in very alkaline solution [Na+] or [K+] is normally very high, making a significant extra contribution to the potential as expressed through the Henderson equation (13.10). Minimization of this error is done by using glass of special composition and with very low selectivity coefficients for Na+ and K+. 

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