Metrohm tiamo 1.0 Manual - Manual (Page 431)

Commands 407
Fix end point evaluation
on, [ off ]
For a fixed measured value for the fix end point the associated values are
interpolated from the list of measuring points.
Fixed quantity
Measured value
Fixed quantity to which the associated values for the other quantities are to
be interpolated from the list of measured points.
Fix EP# at
-2000.0 ... 2000.0 mV
Fix end point.
With this button additionally to the always shown fix EP1...3 the fix EP4...9
are displayed.
With this button instead of fix EP1...9 only the fix EP1...3 are shown.
Additional measured values
Screenshot and parameters: see DET pH - Additional measured values
Evaluation
pK value and half neutralization potential
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
and even then the determined pK value is only an approximation, as the ionic
strengths are not taken into account. In order to obtain more accurate value, titra-
tions must be carried out with decreasing ionic strengths and the results extrapo-
lated 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 ac-
ids can also be determined.
In non-aqueous solutions the half neutralization potential (HNP) is frequently
used instead of the pK value. The HNP is evaluated in the same way as the pK
value.