Metrohm 796 Titroprocessor - Manual (Page 124)

7.9 CAL Calibration of pH sensors
796 Titroprocessor, Instructions for use 118 118
• 1-point calibration: the relative slope is set at 1 and the position of the
calibration curve (isotherm) determined by the buffer solution used,
which is equivalent to compensating the asymmetry pH value pH
as
of the
measuring system.
• 2-point calibration: by using two calibration values not only the coordi-
nate pH
as
, but also the relative slope of the calibration curve (isotherm) is
fixed. This is the commonest type of calibration.
• n-point calibration: for more than 2 calibration points (max. 9) the
compensation curve (linear regression according to the method of the
smallest mean-square error) is determined, i.e. the pH
as
and slope of the
best fitting calibration curve (isotherm) through all calibration points are
determined. If n>2 the same buffer value can be used several times and
thus more heavily weighted. However, two measurements cannot be
carried out on the same buffer in sequence.
Drift / Time-controlled measurement acceptance for the calibration
Measurement acceptance during a calibration is carried out according to
the same principles as in a monotonic (
MET) titration or a MEAS mode. In
drift-controlled measurement acceptance the measurement drift of the sen-
sor must be less than a defined amount in order to be accepted as a valid
measurement. If after a defined waiting period has elapsed this measure-
ment drift has not been undercut then the momentary measurement will be
accepted. For time-controlled measurement acceptance the measurement
drift must be switched off. In this case only the set waiting period is decisive
for the measurement.
Verification of the calibration
For the recognition of a measured calibration value its agreement with the
theoretical value is checked. This is done by using the internally stored
buffer tables of the various manufacturers and buffer types. The measured
potential is compared with the theoretical value of the specific buffer taking
the calibration temperature into account.
For a valid calibration value the measured potential may differ from the
theoretical value by
not more than 30 mV.
The temperatures of the buffer solutions must all lie within a range of 2°C.
The various parameters of the CAL mode are described in detail on
pages 178 to 182.