Page 54 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102
P. 54
The surface of a glass electrode contains fixed silicate groups
associated with sodium ions, SiO– Na+
For the electrode to operate, it must be soaked in water.
Accordingly, the outer surface of the membrane becomes
hydrated--- as water enters a short distance into the crystal lattice
and causes it to swell. The “interior” of the glass remains dry
+
If glass membrane is placed in an aqueous solution containing H ,
it functions as a result of ion exchange on the surface of a
hydrated layer --- Na+ ions are exchanged for H+ in the solution
ion exchange reaction at surface
SiO– Na+ + H+ ↔ SiO– H+ + Na+
(solid) (solution) (solid) (solution)
To function as a pH sensor, a layer of pH sensitive glass is placed
++
between two solutions containing H . A difference in the H
activities on either side of the glass membrane leads to a
difference in the number of ion pairs that exist, and an imbalance
in the surface charge between the hydrated layers. This results in
a membrane potential (boundary potential)
BOUNDARY POTENTIAL
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