Page 55 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

(Test solution) ( reference solution)

It develops at the two surfaces of the glass membrane. It consists
of two potentials, E1 and E2, each of which is associated with one
of the two gel/solution interfaces.
Where 1 refers to the interface between the exterior of the glass
and the analyte solution, 2 refers to the interface between the
interior of the glass and the internal reference solution.

The boundary potential is the difference between these potentials,
E1 and E2

Eb = E1 − E2 = 0.059 log aH1 / aH2

aH1 = H+ activity of the external test solution
aH2 = H+ activity of the internal reference electrode solution

aH2 is constant --- then Eb is a measure of the H+ activity of

external test solution

Eb = K1 + 0.059 log aH1 ( K1 constant due to aH2 )
Eb = K1 − 0.059 pH

Glass pH electrode

It is composed of:

1- pH-sensitive glass membrane
2- Internal reference electrode, Ag/AgCl electrode, ---- for

making electrical contact with the glass membrane

-The glass membrane is made of thin glass bulb sealed onto one
end of heavy-walled glass tube containing the internal reference
electrode

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