Metrohm 726 Titroprocessor - Manual (Page 122)

7.9 CAL Calibration of pH sensors
726 Titroprocessor, Instructions for use 116 116
7.9 CAL Calibration of pH sensors
Application:
The calibration of a pH sensor is indispensable not only for pH measure-
ments; it is also essential for SET titrations and for the correct determination
of pK values and fixed endpoints.
In contrast to manual sensor calibration, the CAL mode allows the calibra-
tion parameters and individual buffer series to be set down in a method. The
calibration data, like all determination data, can be stored and are available
for subsequent data output; however, they cannot be reactivated.
Calibration principle
During the pH calibration an isotherm (constant temperature) of the pH
value is recorded as a function of the electrode system voltage (electrode
system = sensor + electrode cable + connection socket + measurement
interface) and used for the conversion of the measured voltages into pH
values. This means that all the buffer solutions used for the calibration must
be at the same temperature, even with automatic temperature correction
using a temperature sensor. If possible the calibration should be carried out
at the same temperature as that at which the later measurements will be
carried out.
pH 14
7
0
100
200
300
400
500
-500
-400
-300
-200
-100
U[mV]
d U
d pH
*
pH as
Slope =
dU/dpH
U N (T)
Temperature 25°C
• Without calibration the relative slope of the electrode system will be 1
and pH
as
will be set at 7.
Depending on the requirements for accuracy the calibration can be carried
out with 1, 2 or up to 9 buffer solutions.