Metrohm tiamo 2.3 Manual - Manual (Page 939)

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tiamo 2.3 ￭￭￭￭￭￭￭￭ 923
5.6.3.9.10 TET - Fixed endpoint evaluation
Dialog window: Method ▶ TET ▶ Properties... ▶ Additional evaluations ▶ Fixed
endpoint evaluation #
Measured quantity
Selection of the fixed measured quantity to which the associated values
for the other quantities are to be interpolated from the list of measured
points.
Selection Measured value | Time | Volume
Default value Measured value
Fixed value
Value of the fixed endpoint.
Measured value
Range
0.0 ... 60.0 °C
Time
Range
0.0 ... 999999.9 s
Volume
Range
0.00000 ... 9999.99 mL
5.6.3.10 Evaluation
5.6.3.10.1 pK value and Half neutralization potential
Parameters: Method ▶ DET/MET ▶ Properties... ▶ Additional evaluations ▶ pK/
HNP evaluation
The pK value can be determined for pH titrations (DET and MET) and the
half neutralization potential can be determined for U titrations.
The activities of conjugated acid-base pairs are linked by the following
equation (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 strengths are not taken into account. In order
to obtain more accurate values, titrations must be carried out with
decreasing ionic strengths 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 polyva-
lent acids can also be determined.