Metrohm 794 Basic Titrino - Manual (Page 39)

2.7 Parameters, key
794 Basic Titrino
35
Fix EP's
Fix EP's allow determination of the associated volume value for every inputted measured
value on the titration curve. This function is useful for performing standard methods such
as TAN/TBN determinations. For the evaluation of fix EP's, the pH calibration is advisable.
The volume values of the fix EP's are available for the calculation as C5X:
Fix EP1 ⇒ C51
:
Fix EP9 ⇒ C59 Maximum 9 fix EP's are possible.
pK and HNP evaluation
The following relation (Henderson-Hasselbach), derived from the law of mass action,
exists between the activities of a conjugate acid-base pair in aqueous solutions:
pH = pK + log (a
B
/a
A
)
When the activities a
B
= a
A
, then pH = pK is valid. This value corresponds to the pH at
the half neutralization point and can be taken from the titration curve. For pK determi-
nations, a careful pH calibration is necessary.
The pK value determined is an approximate value owing to the fact that the ionic strength
of the solution has not been taken into account. For more exact values, titrations must be
performed with decreasing ionic strength and the results extrapolated to zero ionic
strength.
The evaluation of pK's in aqueous solutions is limited to
pK>3.5 due to the leveling effect of strong acids in aqueous solutions
pK<10.5 because for weaker acids no inflection points can be found aqueous
solutions.
pK evaluation for polybasic acids and for acid mixtures is also possible.
In non-aqueous solutions, the half neutralization potential (HNP) is often used instead of
pK. The HNP is evaluated accordingly.
A start volume must be smaller than half of the equivalence point volume of the first EP.
The pK/HNP values are available for calculation as C6X.