Instant Notes: Analytical Chemistry

By using one of the standards described above, for example, 0.05 M potassium

hydrogen phthalate, which has a pH at 25oC of 4.008, we may eliminate E*and

measure the activity of hydrogen ions, and hence the pH, in the unknown

solution X.

pH(X) =pH(S) +(ES-EX)/(RTln(10)/F)

pH control It is often necessary to control the pH of a solution, especially if hydrogen ions
are being generated or consumed, or if the nature of the species being analyzed
changes with the pH. A few examples will illustrate this problem.

(i) In recording the UV spectrum of a solution of a weak acid, such as a

phenol, the peak maxima occur at different wavelengths in an acid medium

compared with those in a basic medium. Comparisons may best be made

using a constant pH buffer.

(ii) In complexometric titrations (see Topic C7), such as the determination of

magnesium by EDTA, the complex is formed readily and completely at

high pH, so the titration is carried out using an ammonia-ammonium

chloride buffer to keep the solution at pH 10.

(iii) As noted in Topic C3, the fluoride electrode detects free F-ions very well,

but OH-ions interfere, and H+ions form undissociated H 2 F 2. It is therefore

essential to make measurements in a buffer of about pH 5-6.

pH is most often controlled by performing the analysis in a suitable buffer

solution. Occasionally, where a reaction produces an acid (or alkali) the tech-

nique of pH-stat titrationmay be used. Here, the acid produced is detected, and

sufficient alkali added to return the pH to the optimum value.

C4 – pH and its control 79

Hg–Hg 2 Cl 2 paste

Saturated KCl

Porous ceramic plug

Ag–AgCl electrode

Buffer solution

Glass membrane

pH meter

Unknown solution

Fig. 4. pH measurement system.