Metrohm 781 pH/Ion Meter - Manual (Page 151)

9 Annex
780/781 pH/Ion Meter, Manual
141
Notes
For a pH calibration at least two or more different buffers should be
used; these should cover the intended measuring range (DIN 19268).
For example, you can obtain Metrohm buffer solutions for pH 4, 7 and 9
as ready-to-use solutions or concentrates (see Optional Accessories,
Section 9.6).
A pH electrode is an ion-selective electrode which responds to hydro-
gen ions (H + ) with great selectivity. This is the reason why the above
linear relationship between the measured potential U and the pH value
based on it can be assumed. A variation may be caused by the so-
called alkali error. This describes the interference by high concentra-
tions of alkali ions (e.g. 0.1 M Na + ) which can occur at high pH values
(> pH 12).
The temperature compensation of the pH/Ion Meters during a pH
measurement for the calculation of the pH value is based on a tem-
perature-corrected slope (conversion via the definition of the Nernst
constant UN, see p. 138). This method is based on the assumption that
the intercept of all theoretical Nernst slopes lies at pH 7.0 and 0 mV
(isothermal intercept point). However, this is not always the case. The
measuring uncertainty increases with the temperature difference be-
tween calibration and measurement. Therefore, calibration and meas-
urement should be performed at the same temperature wherever a high
degree of accuracy is required. In this case DIN 19268 also stipulates
that calibration and measurement are to be carried out at the same
constant temperature.
The temperature compensation considers only the temperature de-
pendence of the Nernst slope. In addition, the pH values of buffers and
samples are also temperature dependent. The temperature depend-
ence of the buffer is corrected by means of the stored buffer series (see
Section 9.4). But no such information is usually given for the sample.
Therefore, the pH should be measured at the 'original' sample tempera-
ture instead of adjusting the sample temperature to the buffer tempera-
ture. For high accuracies it is recommended that you calibrate at the
samples temperature. During the calibration take care of a correct cor-
relation of the temperature specific buffer pH values.