Thermo Fisher Scientific CyberScan pH 10/pH 100 - Manual (Page 57)

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14 INFORMATION ON pH MEASUREMENT & ELECTRODE
The previous sections of this manual describe the various features of the instrument and
practical aspects of its operation. This section provides a general description of pH
measurement principles. It also provides tabulations of pH buffer changes with temperature.
Lastly, it includes a list of available accessories.
14.1 pH Measurements
14.1.1 Liquid Junction Potential
The liquid - junction potential is the difference in potential created at the liquid - liquid phase
boundary at the electrode tip, due to dissimilar composition of the test solution and the
saturated KCl electrolyte. The sign and size of the liquid - junction potential depends upon the
composition of the two solutions, the temperature and the geometry of the type of junction
used.
When two solutions of different compositions come into contact, ion diffusion occurs at the
point of contact.
When there is a high liquid junction potential, measured values differ greatly from the true
value.
14.1.2 Asymmetry Potential
Within the glass electrode is an internal electrode that contains a reference solution of pH 7.
When the electrode is placed in a pH 7 solution, the pH values within and outside the
electrode are the same, and no potential ought to be produced. In practice, however, an
electric potential called asymmetry potential, is generated.
It has been shown that asymmetry potential varies with the pH of the solution in contact with
the glass, with age, with temperature, with the shape of the membrane, with the thickness of
the membrane, with impurities in the reference solution, and any imperfections in the glass.
In addition, when the glass membrane dries out, the asymmetry potential increases and leads
to measurement errors. That is why it is important to keep the glass hydrated prior to use, by
soaking the electrode in a buffered solution or tap water. (Do not use de-ionized water). The
purpose of soaking the glass electrode is to obtain a swollen low resistance glass surface and
to stabilize the asymmetry potential of the electrode. This swollen surface is generally spoiled
by drying, prolonged immersion in dehydrating solutions or chemical attack, such as etching
by alkalis or hydrofluoric acid. Hence, it is necessary to work with a well-soaked glass
electrode to ensure a constant asymmetry potential.