Karl Fischer (moisture analysis) is one of the most frequently used analyses in laboratories.
A Karl Fischer chemical reaction takes place in the presence of a base and a solvent.
A typical solvent could be methanol, and a base imidazole.
A chemical reaction takes place between Iodine (I) and water (W) with the reactants being in a 1:1 ratio between (I) and (W).
Once the iodine in the Karl Fischer reagent is determined, the unknown concentration of water in the sample can be determined.
As 1x(I) reacts with 1x(W), the amount of (I) used during the titration must be equal to the unknown amount of water present in the sample.
The volumetric method is used for ppm levels up to 100 per cent water - 'high-level moisture'.
The coulometric method is used for moisture levels in the range of 10 micrograms to 10mg of water in a sample - 'low-level moisture'.
With a coulometric Karl Fischer titration, the amount of water present is determined by measuring the amount of current generated during the titration (Coulombs).
Coulombs are a measurement of current (amps) multiplied by the titration time in seconds.
There is a relationship between the iodine (I) used in the titration, the sample's water content (W) and the current.
According to Faraday's Law, 2 x 96,485 Coulombs are needed to generate 1 mole of iodine, and this iodine subsequently reacts in a 1:1 ratio with the water in the KF reaction.
Coulometric KF titration is preferably carried out within the pH range 4-7.
Instead of dispensing KF reagent as in volumetric KF titration, the Metrohm KF instrumentation actually generates the reagent inside the reaction cell.
A current flows through the reagent, generating iodine at the anode electrode.
The Metrohm instrument detects the end of the titration (end-point).
It then calculates the moisture content.
The electrode system consists of anode and cathode platinum electrodes that conduct current through the cell, generating iodine at the anode, which reacts with the water in the sample.
The platinum indicating electrode determines when and if iodine generation is necessary to react with any moisture in the sample.
It does this by voltametrically sensing the presence of water, and continues to generate iodine until all the water in the sample has been reacted with - the KF end-point.
From this titration, the onboard microprocessor in the Metrohm instrument calculates the total amount of current consumed in completing the titration and the time to completion in seconds.
Based on the relationship between coulombs and iodine, the exact amount of iodine generated is determined.
Since water reacts in a 1:1 ratio with iodine, the amount of water can be calculated.
Volumetric KF determinations are preferably carried out between pH 4 and 7.
The Metrohm KF Titrator dispenses the KF reagent into the reaction cell, detects the end- point of the reaction between the KF reagent and water, and calculates the moisture content in the sample.
The exchange unit (burette) dispenses the KF reagent, which contains iodine, for reaction with the water present in the reaction cell.
The platinum measuring indicator electrode, in this setup, determines whether or not iodine needs to be added to react with any water present.
As before, the water is detected voltametrically, and as long as the electrode detects water, it will electronically instruct the burette to keep adding reagent.
When all the water has reacted, the burette stops dispensing and the end-point is reached.
The onboard microprocessor records the volume of KF reagent dispensed, then calculates how much water was present in the sample, based on the concentration of iodine in the KF reagent.
For water content determinations on solids that are either insoluble, react with the Karl Fischer reagent or for other reasons cannot be transferred to the titration vessel directly for either coulometric or volumetric analysis, Metrohm offers KF ovens.
These ovens thermally eliminate the moisture in the samples and transfer it to the KF reaction vessel by means of an inert carrier gas.
Metrohm also offers a system for the automation of up to 35 samples, for either volumetric or coulometric determination, with an oven for each sample.
In the 774 oven sample processor, samples are placed in vials with rubber seals, which are then inserted into a rack.
Stainless steel needles pierce the rubber seal to introduce and remove a carrier gas; at the same time the vial is pressed into the hot oven.
Dry air or nitrogen is passed through the sample, and the gas transfers the released water quantitatively to the titration cell contained in the sample changer.
The moisture can now be determined easily by volumetric or coulometric KF titration, without any sample preparation, in less than five minutes per sample, and there is a considerable reduction in KF reagent use.
