Metrohm tiamo 2.2 (ProcessLab) - Manual (Page 891)

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tiamo 2.2 (ProcessLab) ￭￭￭￭￭￭￭￭ 875
Time
Range
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Volume
Range
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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 equa-
tion (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 approx-
imation, 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 evalu-
ation 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.
In non-aqueous solutions the half neutralization potential (HNP) is fre-
quently used instead of the pK value. The HNP is evaluated in the same way
as the pK value.