Metrohm 916 Ti-Touch - Manual (Page 362)

28.8 Evaluations (EVAL)
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346 ￭￭￭￭￭￭￭￭
916 Ti-Touch
Measured value (measuring mode Upol):
Input range
–200.00 - 200.00 µA
Selection off
Default value off
Measured value (measuring mode T):
Input range
–20.0 - 150.0 °C
Selection off
Default value off
Time:
Input range
0.0 - 999,999.9 s
Selection off
Default value off
Volume:
Input range
0.00000 - 9,999.99 mL
Selection off
Default value off
Fixed EP2 at…Fixed EP9 at
See Fixed EP1 at.
28.8.2 pK value and half neutralization potential evaluation (EVAL pK/
HNP)
The pK value (pH measuring mode) or the half neutralization potential (U
measuring mode) can be determined for DET and MET titrations.
The activities of conjugated acid-base pairs are linked by the so-called
Henderson-Hasselbalch equation:
pH = pK
a
+ log (a
B
/a
A
)
If the activities of the acid and the conjugated base are equal (a
A
= a
B
),
then pH=pK
a
. This is the value at the half neutralization point and can be
extrapolated from the titration curve. A careful pH calibration is necessary
for pK evaluations. Nonetheless, the determined pK value is only an
approximation, as the ionic strength is not taken into account. In order to
obtain more accurate values, titrations must be carried out with decreas-
ing ionic strength and the results extrapolated to the ionic strength zero.
pK evaluation in aqueous solution is limited to the range 3.5 < pK < 10.5
because of the leveling effect of strong acids and the lack of jumps with
very weak acids. pK values of mixtures of acids and polyvalent acids can
also be determined.