Development of Karl Fischer Reagents

I-1. Questions & Answers about Karl Fischer Reagents
Q4
[Principle]
16
Please outline the principles and procedures involved in
coulometric titration.
With coulometric titration, electrolytic oxidation is achieved by adding the
sample to an iodide ion solution, instead of the iodine contained in the Karl
Fischer reagent and used as the titration agent for volumetric titration. As
shown in the following formula, electrolytic oxidation produces iodine at the
anode. The electrolytic solution immediately begins to act as a Karl Fischer
reagent and reacts with the water in the sample. At the end of the process,
the excess iodine can be detected electrically.
2I - - 2e � I2
According to Faraday's laws, the iodine is produced in proportion to the
quantity of electricity. This means that the water content can immediately be
determined from the Coulombs required for electrolysis. To produce 1 mole
of iodine requires 96485 x 2 Coulomb {current (amperes) x time (seconds)}.
Since 1 mole of iodine reacts with 1 mole of water, the amount of electricity
required to cause a reaction with 1mg of water will be as follows.
96485 � 2/18020 = 10.71 Coulombs
This means that if electrolysis takes 1 second with a current of 107mA, the
amount of water can be quantified as 10ug. Normally, titration systems sold
over the counter carry out electrolysis at 300-400mA. This design allows
titration at the rate of about 30ug per second.
Memo " Faraday's Laws"
The laws relate to electrolysis.
(1) The mass of an electrolyzed substance deposited on an electrode is
proportionate to the amount of electricity.
(2) The quantity of a substance deposited by a given amount of
electricity is proportionate to its chemical equivalent weight.
Faraday constant = 96485 C/mol