13 Feb

Karl Fischer

Karl Fischer Titration is a specific and accurate method for water determination.  The reaction is:

B·I2 + B·SO2 + B + H2O → 2BH+I + BSO3
BSO3 + ROH → BH+ROSO3

Where B is a base such as Imidazole and ROH is an alcohol such as anhydrous Methanol.  When all the H2O has reacted, there is no more free I as it reacts to form I2 .  Without I to carry current there is a steep voltage drop which indicates the endpoint.  The voltage drop is measured by a second pair of anode and cathode – the method is bipotentiometric.

In a coulometric titration the anode solution consists of the alcohol,  the base (B), SO2  I2 and the analyte (sample containing water).  The cathode is in a smaller compartment containing the anode solution but no analyte.  The anode compartment and cathode compartment are separated by an ion-permeable membrane.  The Pt anode generates I2 when current is applied.  This I2 reacts with the H2O to produce the I ion.  When no more water is present the voltage drops abruptly – this is the endpoint.  The amount of current required to reach the endpoint determines the amount of water in the sample.

Volumetric titration involves adding I2 to the solution using a buret.  The volume of I2 required to complex all of the H2O is directly related to the amount of water in the sample.

Some of he advantages 0f Karl Fischer titration are:   The method is accurate, it is specific for water, it works over a wide range of moisture, it can measure moisture in solids liquids or gases, and it can be automated.  Mettler Toledo and Lab Synergy are two  companies who manufacture automated Karl Fischer titrators.

Robin Prymula