Generally speaking the actual concentration of a broad spectrum of solutes may be measured conveniently by forming an appropriate electrochemical cell. Thus, most electrochemical cells invariably comprise of two electrodes, namely : (a) an Indicator Electrode—the voltage of which solely depends on the thermodynamic activity (i.e., concentration) of one specific component in the solution ; and (b) a Reference Electrode—the voltage of which must be absolutely independent of the nature and composition of the solutions wherein it is immersed. Placing together of these two electrodes in a solution obviously gives rise to an electrochemical cell ; and consequently the voltage thus generated across the electrodes may be determined by connecting it either to a potentiometer or a millivoltmeter that has a sensitivity to measure ± 0.2 mV, besides possessing a high impedence-input of minimum 1012 ohms (Ω).
Under these experimental parameters when an extremely feeble current, of the order of less than 5 pA, is drawn from the electrodes, the e.m.f. of the cell may be expressed as below :
Ecell = E+ – E– + Ej ........(a)
where, Ej = e.m.f. at the liquid junction.
In Eq. (a), Ej may be eliminated completely by emp1oying a saltbridge integral with the reference electrode. In usual practice, the loss of electrons or reduction occurs from the prevailing chemical system at the cathode ; whereas the gain of electrons or oxidation takes place at the anode.