Karl Fischer (KF) Titration for Water Determination: How to perform

Table of Contents

• Introduction and Outcome
• Principle of Karl Fischer titration
• Reaction of Karl Fischer titration
• Effects of pH on Karl Fischer titration
• Karl-Fischer titration equipment
• The sample size for Karl Fishers titration
• Ingredients of Karl Fischer reagents
• Karl fischer titration procedure
• Applications and case study
• Advantages
• Limitations
• Differences between KF and LOD
• Conclusion
• FAQs

Introduction and Outcome

Karl Fischer Titration is widely used for water determination in various industries such as the pharmaceutical industry, food industry and cosmetic industries. It also plays a vital role during drug development. In this article, I will discuss principle, applications, procedure, KF reagent, KF reaction, sample size, effect of pH, case studies and FAQs. After reading this article, all your doubts will be cleared and your knowledge will increase to the next level.

Principle of Karl Fischer titration

The Karl Fischer titration is widely used for a variety of products for water determination in the various industries. The principle of KF titration is based on the Bunsen reaction between iodine and sulfur dioxide in aqueous medium. Karl Fischer modified this reaction in a non-aqueous system containing excess of sulfur dioxide. He used methanol as a solvent and pyridine as a buffering agent.

Karl Fischer titration

ROH + SO₂ +R’N → [R’NSH]SO₃R + H₂O +I₂ + 2R’N → 2[R’NH]I + [R’NH]SO₄R

1. The alcohol reacts with Sulfur Dioxide and base to form intermediate alkyl sulfite salt which is then oxidised by iodine to an alkyl sulfate salt. This oxidation reaction consumes water.
2. The reactive alcohol is typically methanol or 2-(2-Ethoxy) ethanol
3. Classical Karl Fischer reagent contained pyridine as a base but now pyridine free base are available for use.
4. Frequently used Karl Fischer reagent contains imidazole or primary amine
5. Water and iodine is consumed in 1:1 ratio in this reaction. Once all of the water presents is consumed, the presence of excess iodine is detected volta-metrically by the titrators indicator electrode. That signals the end point of the titration.
6. The amount of water present in the sample is calculated based on the concentration of iodine in the Karl Fischer titration reagent and amount of iodine consumed in the titration

Effects of pH on Karl Fischer titration

The rate of reaction depend upon the pH value of the solvent or working medium. when pH is between 5 to 10, then titration proceeds normally. However, when the pH is lower than 5 the titration speed is very slow. On the other hand, when pH is higher than 8 then the titration rate is very fast, but only due to interfering esterification, which produces’ water resulting in a vanishing end point. Thus, the optimal pH range for Karl Fischer titration is between 5 to 8. The highly acidic or highly basic samples need to be buffered to bring the overall pH in that range.

Karl-Fischer titration equipment

The equipment used in the Karl-Fischer titration consists of a titration vessel, a burette, a Karl-Fischer titration reagent, and the sample being tested. The titration vessel is filled with sample whereas the burette is filled with Karl-Fischer reagent. The burette is then attached to the titration vessel and the Karl-Fischer reagent is allowed to flow into the sample. The amount of water in the sample is determined by measuring the amount of Karl-Fischer titration reagent required to fully titrate the sample.

The sample size for Karl Fischer titration

• For a sample with a large amount of water small amount of sample is taken and for a sample with low amount of water large amount of sample is taken.
• In principle, the sample size should be selected in such a way that the titrant consumption lies between 10 to 90% of the burette reading.

Ingredients of Karl Fischer reagents

The KF reagent contains iodine, sulfur dioxide, a base (imidazole) and a solvent

Karl fischer titration procedure

In the KF titration vessel about 50 ml of methanol is taken and is neutralised with Karl Fischer reagents. Then sample is added as per the respective method and titration is started. The KF instrument automatically detects the end point and gives the result.

Applications and case study

KF titration is used in the following industries for determination of water:

• Pharmaceutical industry
• Food industry
• Pesticide industry
• Research and development centre
• Testing laboratory and
• Environmental analysis

Advantages

The following are the advantages of KF titration for water determination:

1. Fast result
2. No need of special skill
3. Accurate result
4. Acceptable by all regulatory agency
5. Low analysis cost
6. Low maintenance cost

Limitations

It works well between pH 5 to 8

Differences between KF and LOD

The following are the difference between KF and LOD:

Conclusion

The KF titration is a powerful technique for the determination of water in the various industries and it gives fast, accurate and reliable result. I hope this post has cleared all your doubts and increased your knowledge to the next level. Write your opinion and suggestions in the comment section.

Abbreviations

• LOD: Loss on drying
• KF: Karl Fisher

References

• USP
• IP

FAQs

What is the Karl Fischer titration for water content?

In the KF titration, in the titration vessel about 50 ml of methanol is taken and it is neutralised with Karl Fischer reagents. Then sample is added as per the respective method and titrated with KF reagent. The KF instrument automatically detects the end point and gives the result.

Why is KF used in water determination?

It gives fast, accurate and reliable result. Secondly it does not need any special skill. That is why is KF used in water determination.

What is the titration method for water determination?

In the KF titration, in the titration vessel about 50 ml of methanol is taken and it is neutralised with Karl Fischer reagents. Then sample is added as per the respective method and titrated with KF reagent. The KF instrument automatically detects the end point and gives the result.

Which electrode is used in KF titration?

In KF instrument double platinum electrode is used for electrochemical indication of the end-point.

What is Karl Fischer Titration and How Does It Work?

In the KF titration, in the titration vessel about 50 ml of methanol is taken and it is neutralised with Karl Fischer reagents. Then sample is added as per the respective method and titrated with KF reagent. KF reagent contains iodine, sulfur dioxide, a base (imidazole) and solvent.

The alcohol reacts with Sulfur Dioxide and base to form intermediate alkyl sulfite salt which is then oxidised by iodine to an alkyl sulfate salt. This oxidation reaction consumes water. Once all of the water present in the sample is consumed, the presence of excess iodine is detected volta-metrically by the titrators indicator electrode. That signals the end point of the titration.

ROH + SO₂ +R’N → [R’NSH]SO₃R + H₂O +I₂ + 2R’N → 2[R’NH]I + [R’NH]SO₄R

What is the difference between KF and LOD?

The KF method is selective for water and it only gives water value of the same whereas LOD method is not selective method and it gives combine value of water and organic solvent.