Karl - Fisher Titration

There are two different approaches to KF titration:-
1. Volumetric method originally developed in 1935
2.Coulometric method (the modern way) such as that used in all Cou-Lo titrators.

In the volumetric technique the active ingredient, Iodine, is introduced by volume addition of a reagent.

In the coulometric technique the Iodine is produced by electrolysis and the amount of current is measured.

In volumetric method is usually used for high water contents and also for samples such as foods which contain high water and also require large sample amounts for representative analysis.
Typically used for water contents %.

Coulometric technique is 1000 times more sensitive and will detect down to microgram level. Much better for low water contents samples, ease of operation etc.

Volumetric titration reaction

The determination is based on the iodometric titration of sulphur dioxide in water.
I2 + SO2 + 2H2O ↔ 2HI + H2SO2
To shift the equilibrium to the right the acid is neutralized with an Imidazole-buffer.
The complete reaction involves also the solvent, methanol. So the description of the Karl Fisher titration is done by the following two reactions. Where RN is the base used in the mixture.
CH3OH + SO2 + RN → [RNH]SO3CH3
H2O + I2 + [RNH]SO3CH3 + RN → [RNH]SO4CH3 + 2[RNH]I

Coulometric titration reaction:

The main compartment of the titration cell contains the anode solution plus the analyte. The anode solution consists of an alcohol (ROH), a base (B), SO2 and I2. A typical alcohol that may be used is methanol or diethylene glycol monomethyl ether, and a common base is imidazole.
The titration cell also consists of a smaller compartment with an (anode) immersed in the anode solution of the main compartment. The two compartments are separated by an ion-permeable membrane.
The Pt anode generates I2 when current is provided through the electric circuit. The net reaction as shown below is oxidation of SO2 by I2. One mole of I2 is consumed for each mole of H2O. In other words, 2 moles of electrons are consumed per mole of water.
B·I2 + B·SO2 + B + H2O → 2BH+I− + BSO3
BSO3 + ROH → BH+ROSO3−
The end point is detected most commonly by a bipotentiometric method.

A second pair of Pt electrodes are immersed in the anode solution. The detector circuit maintains a constant current between the two detector electrodes during titration. Prior to the equivalence point, the solution contains I- but little I2. At the equivalence point, excess I2 appears and an abrupt voltage drop marks the end point.
The amount of current needed to generate I2 in order to reach the end point can then be used to calculate the amount of water in the original sample.

The advantages of coulometric KF are:-
•Fast analysis - Most results available in less than 1 minute, many in less than 30 seconds.
•Low reagent usage - Multiple samples can be analysed on just one charge of reagent.
•Simple operation -Press START, inject the sample, read the result.
•No reagent calibration - No chemical calibration required.
•Direct readout - Results displayed and printed automatically.
ALL coulometers operate the same way.

10.71 coulombs of electricity will titrate 1 milligram of water.