Direct Potentiometric Measurements
Methods in which the cell potential for the sample solution is compared with that for one or more
standards are rapid, simple and readily automated. The measurement of pH is the most common
application of this type, one or more buffer solutions serving to calibrate the pH-meter (potentiometer).
In all such measurements, calibration involves the evaluation of the constant k' in the equation
where pX is the negative decadic logarithm of the activity of species X (see equation (3.1)). Thus, k' is
determined for a solution of known pX and the value used for the measurement of solutions of
unknown pX, i.e.
In practice, the value of k' is never obtained as such, because the meter is adjusted so that the standard
reads the correct value for its pX, the scale being Nernstian. As k' contains in addition to the reference
electrode potentials, a liquid-junction potential and an asymmetry potential, frequent standardization of
the system is necessary. The uncertainty in the value of the junction potential, even when a salt bridge is
used, is of the order of 0.5 mV. Consequently the absolute uncertainty in the measurement of pX is
always at least 0.001/(0.059/n) or 0.02 if n = 1, i.e. a relative precision of about 2% at best. For the most
precise work a standard addition technique (p. 30) and close temperature control are desirable. All
measurements should be made at constant ionic strength because of its effect on activities. Likewise, the
presence of complexing agents or precipitants must be controlled as this can significantly decrease the
activity of the analyte resulting in gross changes in electrode response at a given total concentration
level.
Potentiometric Titrations
By monitoring the change of Ecell during the course of a titration, where the indicator electrode responds
to one of the reactants or products, the stoichiometric or equivalence point can be located. A plot of Ecell
against volume of titrant added gives a characteristic 'S-shaped' curve (Figure 6.6(a)) owing to the
logarithmic relation between Ecell and activity. If the reactants are in a 1:1 mole ratio, the curve is
symmetrical as shown and the equivalence point is the mid-point of the inflection. This is true for all
acid-base, silver-halide and many other titrations. Where the mole ratio of reactants is not 1:1, an
asymmetrical curve is produced and the equivalence point does not coincide with the inflection point.
The error incurred in assuming the two points coincide is generally less than 1% and can be