Temperature Effects

The emf, E, of a pH cell may be written

where k is the Nernst factor (2.303RT)IF, and£°' includes the liquid-junction potential and the half-cell emf on the reference side of the glass membrane. Changes of temperature alter the scale slope because k is proportional to T. The scale position also is changed because the "standard" potential is temperature dependent: Eg is usually a quadratic function of the temperature.

The objective of temperature compensation in a pH meter is to nullify changes in emf from any source except changes in the true pH of the test solution. Nearly all pH meters provide automatic or manual adjustment for the change of k with T. If correction is not made for the change of standard potential, however, the instrument must always be standardized at the temperature at which the pH

is to be determined. In industrial pH control, standardization of the assembly at the temperature of the measurements is not always possible, and compensation for shift of the scale position, though imperfect, is useful. If the value of Eg were a linear function of T, it would be easy to show that the straight lines representing the variation of E and pH at different temperatures would intersect at a point, the isopotential point or pHj. Even though E°' does not usually vary linearly with T, these plots intersect at about pH, when the range of temperatures is narrow. By providing a temperature-dependent bias potential of &pHi( an approximate correction for the change of the standard potential with temperature can be applied automatically (1,5).

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