Understanding Karl Fischer Moisture Analysis

Karl Fischer (Moisture analysis) is one of the most frequently used analysis in the QA laboratories. This article seeks to explains them.

Karl Fischer (Moisture analysis) is one of the most frequently used analysis in the QA laboratories however many people have not been taught the fundemental principals of this technique - Here we try and explain them :- What is a Karl Fischer chemical reaction? This reaction taking place in the presence of a base and a solvent.

(a typical solvent could be methanol, and a base imidazole).

How does the Karl Fischer determination work? A chemical reaction takes place between Iodine (I) and water (W) with the reactants being in a 1 to 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.

i.e 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.

Volumetric or Coulometric determination methods? The Volumetric method is used for ppm levels up to 100% water i.e "high level moisture".

The Coulometric method is used for moisture levels in the range of 10 micrograms to 10mgs of water in a sample i.e"low level moisture".

Coulometric Karl Fischer.

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 samples water content (W) and the current.

According to Faradays Law :- 2 x 96,485 Coulombs are needed to generate 1 mole of iodine and this iodine subsequently reacts 1 to 1 with the water in the KF reaction.

How does a Coulometric titration work? Coulometric KF titration's are preferable carried out within the pH range 4-7.

STAGE 1.

Iodine generation 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.

STAGE 2.

The Metrohm instrument detects the end of the titration (end-point).

STAGE 3.

The Metrohm instrument subsequently calculates the moisture content.

Components of a Coulometric Titrator.

The electrode system - this consists of an anode and cathode platinum electrodes which conducts current through the cell, generates iodine at the anode, which then subsequently 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 then continues to generate iodine until all the water in the sample has been reacted with the KF end-point.

From this titration the on board 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 the 1:1 ratio with iodine, the amount of water can be calculated.

Volumetric Karl Fischer.

How does volumetric Karl Fischer Titration work? Volumetric KF determinations are preferentially carried out between pH 4-7 In volumetric KF titration the Metrohm KF Titrator controls 3 steps :-.

1.

It dispenses the KF reagent into the reaction cell.

2.

It detects the end point of the reaction between the KF reagent and water.

3.

It calculates the moisture content in the sample.

Components of a Metrohm Volumetric Titration :- Exchange unit (burette), this part of the system dispenses the KF reagent, which contains iodine, for reaction with the water present in the reaction cell.

The platinum measuring indicator electrode, in this set up, determines whether or not iodine needs to be added to react with any water present i.e 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 based on the concentration of iodine in the KF reagent, it calculates how much water was present in the sample.

Karl Fischer Moisture Determination in Difficult Samples.

For water content determinations using the Karl Fischer method 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 KF Ovens drive off thermally the moisture in the samples and transfer it to the KF reaction vessel by means of and inert carrier gas.

Automatic Oven Sample Processing.

Metrohm offer a unique system for the automation of up to 35 samples, for either volumetric or coulometric determination, with a new oven for each sample.

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 led through the sample and the gas transfers the released water quantitatively to the titration cell contained in the sample changer.

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