The experimental determination of pH

22 THE EXPERIMENTAL DETERMINATION OF pH In some instances it may be important to determine the pH of the solution experimentally. According to the accuracy we need and the instrumentation available we can have a choice of several techniques. A few of these will be discussed here. 

The use of indicators and indicator test papers An indicator is a substance which varies in colour according to the hydrogen-ion concentration. It is generally a weak organic acid or weak base employed in a very dilute solution. The undissociated indicator acid or base has a different colour to the dissociated product. In the case of an indicator acid, Hind, dissociation takes place according to the equilibrium 

Some of the indicators listed in Table 1.10 may be mixed together to obtain a so-called universal indicator, and with such the approximate pH of the solution can be determined with one single test. Such a universal indicator may 

Brilliant cresyl Amino-diethylamino-methyl Red- Blue 0-0-10 

Thymol blue (acid) Thymol-sulphone-phthalein Red Yellow 1-2-2-8 

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Figure 6. Plots of density versus temperature for supercritical water as a function of pressure. The conditions employed in the experimental determination of pH for 0.01 M HCl solution are indicated hy the large filled circles. Reprinted fi-om Ref. 5, Copyright (1997) with permission from Elsevier.

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