Karl Fischer (K.F.)

Karl Fischer (K.F.)

Karl Fischer (KF) titration is water determination techniques which is industrial scientists. It is performed by volumetric or coulometric measurement techniques.

Principles of Karl Fischer titration:

The KF reaction is based upon an early reaction called the Bunsen reaction, in which sulfur dioxide is oxidized by iodine with the consumption of water during this oxidation.

German scientist Karl Fischer published a method in a year 1935 for determination of water content in samples. This was a titrimetric method based on Bunsen reaction used for determination of sulfur dioxide in aqueous solutions. The original reaction is as below:

SO2 + I2 + 2H2O → H2SO4 + 2HI

The KF titration reaction currently accepted is as follows, and was reached by several advances in understanding the mechanism of the reaction and modification of the original reagent:

The titration reaction is said to reach its endpoint once the iodine stops reacting with the alkyl sulphite intermediate produced after the first half-reaction, resulting in excess iodine in the solution and a colour change. This signals that no more water is present to be used up and therefore the reaction has stopped.

Water and iodine are consumed in a 1:1 molar ratio as shown by the equation. The end point of the titration is shown by a fall in the voltage required to pass a current through a double platinum electrode indicator. The voltage required for this current is high initially, but when iodine is in excess there is a sharp fall in this voltage.

The volume of reagent used up at this point is used to calculate the water content. With KF titration, a sample can be titrated from 100 to 1x106 ppm (water concentration from 0.01 – 100%). It is high sensitive so measure minute amounts of water. This holds good whether the water in the substance is free, emulsified or dissolved water.

Basics of volumetric and coulometric KF titration

Generally two way used for water determination, either volumetric or coulometric. In short, the two methods may be compared as follows:

Volumetric titration:

Reagent type:

Volumetric titration uses the KF reagent containing sulfur dioxide and iodine, with an alkyl hydroxide and a base. This is initially pretitration so that any water contaminating, which is present in the cell from the air is dissolved by the reagent and removed before the sample is added. The sample is then dissolved in the solvent. Through a two-step reaction the iodine reacts with sample water.

 Reagent addition:

The reagent is added drop by drop or even droplet by droplet using a burette, until the solution changes colour, which is when there is excess iodine in the sample because all the water is used up already.

Samples with 0.1 mg to 500 mg of water can be measured this way using the KF reagent. These reagents have specified titters which change over time and must thus be calibrated periodically using certified water standards. This method suitable up more reagent but can handle large amount of the sample volumes.

End point detection

This is detected by a bivoltammetric indicator electrode. The voltage required to keep polarizing current flowing at a defined value between the electrodes of the titration cell drops sharply once the end point is reached because of the presence of excess iodine. The titter of reagent added until this point is used to get the amount of water in the sample.

Coulometric titration

Reagent type

Coulometric titration is highly sensitive for water determination. It is known as an absolute method. The coulometric cell having two compartments, first compartment is anode and second component is cathode. It is extremely sensitive to the presence of water and therefore the cell must be impervious to any moisture from outside.

Pre-titration is a long process in coulometric KF titration. The sample is injected through a septum into the cell which is closed to the outside. The coulometric titration has coulometric KF reagent which have iodide rather than iodine.

  ­_

2I    ---------->  I2 + 2e-

Reagent addition

In the coulometric KF titration method, the titrant is produced in titration cell itself by electrochemical reaction. The cell has a generator electrode polarized by alternating current to generate iodide instead of the titrating burette.

The iodide in the reagent undergoes oxidation to iodine at the anode, and the iodine then reacts with the sample water until the water is all used up. Once the end point is reached, the content of water is calculated from the amount of current required until this point. The benefits of coulometric KF titration is that samples having very little water include:

·         Better sensitivity

·          Higher speed of titration

·          No need to calibrate the reagent each time as the iodine is produced in situ

·          No need to replace the solvent each time

·          Multiple samples can be tested without reloading the reagent

·          Increased economy

 End point detection

Very sensitive indicator used in coulometric KF. A double platinum electrode indicator used for alternating current. The amount of current passed to achieve sufficient electrolytic generation of iodine up to this point is measured and the corresponding water content is calculated.

 Sample water range

Coulometric titration is detect the small amount of water present in gases. Usually a sample likely to have water content up to about 2% of sample, or 200 micro grams of water, is recommended, which would mean a total liquid sample size of 10 millilitres.

The ideal sample is one which contains less than 1% water, with 2 or less millilitres of sample in total, yielding less than 20 micro grams of water. Larger samples fill the cell too quickly and this makes cell cleaning and reagent reloading necessary, increasing the downtime. Volumetric titration is preferred for samples above 10 millilitres and 2% of water.

Solvent range

Coulometric determination is depend upon the solvent range, and if proper solvent is not available KF oven must to be used to remove the water from the sample. space where it is directly measured.

The need for the Karl Fischer oven because more limited volumetric method range of solvents can be used. In general, the volumetric method is preferred when the sample can be dissolved in the available solvents and is likely to contain less than 1% of water.

Standardization of the reagent of KF

• Sodium tartrate may be used as convenient water reference substance 150 mg to 350 mg Sodium tartrate used to standardization for reagent also water 25 to 250 mg water for reference substance to be used.
• Difference between maximum and minimum NMT 1.0 %
• Verification of reagent also verify with the help of Hydranal water standard 1000 µg & 100 µg 
• Acceptance criteria for recovery of Hydranal water standard 1000 µg is 97.5% to 102.5% and Hydranal water standard 100 µg is 90.0% to 110.0%.  
• Factor between 4.0 to 6.0 mg/ml
• 1 ml KF Reagent is equivalent to approximately 5 mg of water.
• KF Reagent protect from light. Store bulk stock of the reagent in a suitably sealed glass stopped container and under refrigeration at suitably temperature.
• Used Platinum electrode in KF Apparatus.

Calculation:

                0.1566 x Weight 

Factor =------------------------

                Volume (mg/ml)

                                   Volume x Factor x 100

Water contain in % =-----------------------------

                                          Weight in mg

Factor of Di-sodium tartrate 2H2O is 0.1566 

2H2O/DST (MW) = 36/230.06

                              = 0.1566

KF Reagent made by two reaction:

Calculation:
KF Reagent factor calculation:
           
               Weight of water x 1000
Factor = -------------------------------
                          B.R                           (Here, B.R=Volume of Karl Fischer reagent consumed in ml)

Water determination calculation:
                            
                              B.R. x Factor x 1000
% Water (w/w) = ------------------------------
                             Weight of sample x 1000
                                                           (Here, B.R=Volume of Karl Fischer reagent consumed in ml)
 
Note: 

• For Ketonic group used Pyridine, Acetonitrile, Di-methyl formamide.
• For Acidic sample  used Imidazole because pH to be set 5.5 to 7.0
• For basic sample used Salicylic acid because pH to be set 5.5 to 7.0