VOLUMETRIC TITRATION

General remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent. It is usually quoted in mg of water per ml of Karl Fischer reagent.
Modern reagents allow direct titration of water in the sample. The sample may be
introduced directly into the KF cell or after an extraction or dissolution with a suitable
solvent. The water concentration of the solvent must be determined previously in
order to be subtracted from the sample analysis.
In Radiometer Analytical titrators all these operations are simplified and the different
results are accounted for automatically.
The volumetric titration of water allows the analysis of water concentrations between
0.1% and 100%. If an aliquot contains less than 1 mg of water, coulometric
determination will result in a more accurate result. For reasons of precision, the
titre of the titrant should be chosen so that the titration is completed with a titrant
demand between 1 and 10 ml.
End point determination
The end point of the reaction is generally based on the detection of a slight excess
of iodine which occurs when water is no longer present in the KF cell. The iodine
excess can be indicated visually, photometrically or potentiometrically. The
potentiometric method is the most common for the majority of titrators currently on
the market.
Radiometer Analytical titrators allow the use of direct or alternating current. The
indicating electrode geometry and frequency of the alternating signal have been
optimised. The instrument does not operate with instantaneous potential values
but uses half the difference between two consecutive measurements.
E =
E(t) - E(t-1)
2 (IV)
In a conventional system with dc current, the electrodes are polarised,
and become the site for reactions other than the reaction iodine to iodide. This
leads to a drift in the potential difference between the electrodes and an end point
that may be erroneous.
- 6 -
Two curves for the same electrode are shown. Both have the same
amplitude but one is with direct current whereas the other is with alternating current.
It can be seen that with ac the potential is stabilised whereas with dc the potential
increases, showing the appearance of reactions other than the reaction iodine to
iodide.
The amplitude of the direct or alternating current and the value of the set point are
modified according to the resistivity of the reactive medium. Radiometer Analytical
offer more than a simple end point titration. The instrument controls the speed of
reagent addition in order to maintain the indicating electrode potential at a constant
value, thus an excess of iodine is never observed. This reaction control is achieved
with a self-adapting PID algorithm(1). The only input parameter is the maximum
allowed speed that only depends on the reaction kinetics of the reagent used.
Radiometer Analytical has tested most available reagents and the default value is
valid for the most commonly used ones. The table below gives the maximum advised
speed for the tested reagents.
Thanks to this principle, the titrator compensates the water introduced into the KF
cell by determining a drift value which is subtracted during the titration. In this way,
more accurate results are obtained, especially for low water contents.
(1) PID: Proportional Integral Derivative