Instant Notes: Analytical Chemistry

E(cell) =E*(F-) -(RT/F) ln (a(F-)) -E(SCE)

or, at 25oC:

E(cell) =k -0.0257 ln (a(F-))

If a cell is constructed with a fluoride ISE combined with a SCE, calibration with

solutions of fluoride ions of known concentration enable the value of k to be

found. When the same cell is filled with an unknown solution, the fluoride ion

concentration may be determined. This is an example of direct potentiometry.

Other crystalline ion-selective electrodes use AgCl for Cl-, Ag 2 S for Ag+and

S^2 - , Ag 2 S +CuS for Cu^2 +and many more.

Liquid membrane electrodes are shown in Figure 3(a). Typically an

organophilic porous plastic membrane separates the internal reference from the

sample solution. Another reservoir contains a liquid ion-exchanger, which satu-

rates the pores in the membrane. The calcium electrode uses calcium dodecyl

phosphate [(RO) 2 PO 2 ] 2 Ca where R is the dodecyl radical, dissolved in dioctyl

phenylphosphonate.

The electrode response is given by:

E(Ca^2 +) =k +(RT/2F) ln (a(Ca^2 +))

Ion selective field effect transistors (ISFETs)

One disadvantage of the glass electrode for measuring pH is its fragility. A

modern development uses a field effect transistor where the gate surface is

coated with a material, often aluminum oxide, whose surface is sensitive to pH.

As the potential at the surface changes with pH, the current induced through

the transistor varies. A temperature diode, incorporated in the electrode

C3 – Potentiometry 71

Ag electrode

Leads to meter

Reference

electrode

Internal

solution

Glass electrode

O-ring

Gas-permeable

membrane

Liquid organic

ion exchanger

Porous plastic

membrane with

liquid ion exchanger

Aqueous solution

saturated with

AgCl and MCl 2

(a) (b)

Fig. 3. (a) Liquid ion exchange membrane electrode for M^2 +ions. (b) Gas sensing electrode

using glass ISE.