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1.3.5 Other electrodes

Electrodes exist for the measurement of many other ions such as Liþ,Kþ,Naþ,Ca^2 þ,

Cland NO 3 in addition to Hþ. The principle of operation of these ion-selective

electrodes (ISEs) is very similar to that of the pH electrode in that permeable membranes

specific for the ion to be measured are used. They lack absolute specificity and their

selectivity is expressed bya selectivity coefficientthat expresses the ratio of the response

to the competing ions relative to that for the desired ion. Most ISEs have a good linear

response to the desired ion and a fast response time.Biosensorsare derived from ISEs

by incorporating an immobilised enzyme onto the surface of the electrode. An important

example is the glucose electrode that utilises glucose oxidase to oxidise glucose

(Section 15.3.5) in the test sample to generate hydrogen peroxide that is reduced at the

anode causing a current to flow that is then measured amperometrically.Micro sensor

versions of these electrodes are of great importance in clinical biochemistry laboratories

(Section 16.2.2). Theoxygen electrodemeasures molecular oxygen in solution rather

than an ion. It works by reducing the oxygen at the platinum cathode that is separated

from the test solution by an oxygen-permeable membrane. The electrons consumed

in the process are compensated by the generation of electrons at the silver anode hence

the oxygen tension in the test sample is directly proportional to the current flow

between the two electrodes.Optical sensorsuse the enzyme luciferase (Section 15.3.2)

to measure ATP by generating light and detecting it with a photomultiplier.

Shielded

insulated

cable

(a) (b)

Glass

stem

Inner electrode

(Ag/AgCl wire)

‘External’

reference

electrode

HCI (0.1 M)

Thin-walled

glass bulb

HCI

solution

(0.1 M)

Ag/AgCI

internal

electrode

Salt bridge

solution

(usually KCI)

Porous plug

Glass

membrane

Fig. 1.1Common pH electrodes: (a) glass electrode; (b) combination electrode.

15 1.3 Weak electrolytes