2PHA206 Chemistry for Pharmacy (II) - James Smith

The combination pH electrode
Based on thin glass bulb-membrane, which has an
attraction for some cations
Cations, e.g., H + , are adsorbed on to the silicate matrix,
resulting in a charge separation altering the junction
potential:
E = E o + RT ln(a H
+
+ k H
+
,Na
+
. a Na+ )
nF
K H
+
,Na
+
= potentiometric selectivity coefficient of H +
and Na + (determined by type of glass used). Value low
for commercial pH electrodes – therefore electrode
responds only to H + .