Page 391 - Instrumentation Reference Book 3E

374 Chemical analysis: electrochemical techniques

Figure 17.31 Schematic diagram for ion-selective monitor. CourtesyABB Instrument Group.

would happen if there were insufficient solution
in the reservoir.
This shows that the selectivity of potassium
Ion-selective electrodes available and application over sodium for the potassium electrode is
areas There is a very wide range of electrodes about 3851, i.e., U(2.6 x It is important
available. Not only are there many specific ion to note that selectivity coefficients are not con-
monitors but several manufacturers now market stant, but vary with the concentration of both
standardized modular assemblies which only primary and interferent ions, and the coefficients
need different electrodes, different buffer solu- are, therefore, often quoted for a particular ion
tions, and minor electrical adjustments for the concentration. They should be regarded as a
monitors to cope with many ion determinations. guide to the effectiveness of an electrode in a
Table 17.9 shows the ion-selective electrodes particular measurement and not for use in pre-
available for the more common direct poten- cise calculations, particularly as quoted selectiv-
tiometric determination of ions. ity coefficients vary by a factor of 10 or more.
Ion-selective electrodes, as their name implies, For accurate work the analyst should determine
are selective rather than specific for a particular the coefficient for himself for his own type of
ion. A potassium electrode responds to some solution.
sodium ion activity as well as to potassium, and Direct potentiometric determination of ions
this can be expressed as: by means of ion-selective electrodes has many
+
Emeasured = constant * Slog(apotasslum+ K~N,-) applications. Examples are determination of
pH, sodium, and chloride in feedwater, conden-
where K is the selectivity coefficient of this elec- sate, and boiler water in power stations; cyanide,
trode to sodium and 0 < K < 1. fluoride, sulfide, and chloride in effluents, rivers,
Thus the fraction K of the total sodium activity and lakes; fluoride, calcium, and chloride in
will behave as though it were potassium. The drinking water and sea water; bromide, calcium,
smaller the value of K, the more selective that chloride, fluoride, iodide, potassium, and sodium
electrode is to potassium, Le., the better it is. To in biological samples; calcium, chloride, fluoride,
identify a particular selectivity coefficient the and nitrate in soils; sulfur dioxide in wines and
data are best written in the form: beer; chloride and calcium in milk; sulfide and