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Calibration of pH meters and glass electrodes at the National Physical - page 2 / 7

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reference electrode or as a single pH electrode. Glass electrode contains an internal reference electrode (usually Ag/AgCl), an internal reference buffer (usually with pH 7) and a glass membrane.

    pH meter is basically a high impedance voltmeter with facility for converting the output voltage into pH, at measuring temperature, or at other chosen temperature, by temperature compensation.

    Direct pH readings are possible after adjustment of a pH meter. According to the International Vocabulary of Basic and General Terms in Metrology [3], adjustment is an operation of bringing a measuring instrument into a state of performance suitable for its use. For a pH meter adjustment, at least two so-called technical buffers are necessary [2].

    If the electromotive force (emf) values corresponding to the two technical buffer solutions S1 and S2 with pHS1 and pHS2 are E1 and E2, respectively, the pH value of the unknown (test) solution can be calculated from its measured E by the following formula:

.                           (3)

where k' is the practical slope of the glass electrode [4]:

                                               (4)

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