Development of Karl Fischer Reagents

Chapter I: Basic Knowledge
at a time to a flask or beaker that is exposed to the air. Since Karl Fischer
instantly reacts with atmospheric moisture, the end-point would never be
reached.
2. At present there is no method that allows the color change at the endpoint
of Karl Fischer titration to be detected visually. The only method is
to detect minute amounts of surplus iodine electrically.
!
Helpful hint -1
How is the end-point detected electrically?
The volumetric and coulometric titration systems currently on the market
employ controlled current voltage detection (constant-current polarization
voltage detection). A constant current of 1-30uA is applied to two platinum
electrodes, and the end-point is detected by measuring the voltage
(polarization voltage) between the electrodes. If the titration solution has a
relatively high water content, a polarization voltage of 300-500mV will be
produced. The voltage varies according to the type of solvent used and the
sample. In general, high voltages are produced in the case of substances
with low conductivity, such as hydrocarbons.
As the Karl Fischer titration continues and the end-point approaches, the
voltage will suddenly drop to 10-50mV. With volumetric analysis, the endpoint
is deemed to have been reached after the voltage has remains at this
level for a specific period of time. With commercially available titration
systems, the period is 30-60 seconds. With coulometric analysis, an endpoint
voltage is specified, and the end-point is reached when the voltage
falls below that level.
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