Page 53 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202
P. 53

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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