Karl Fischer Titration

Karl Fischer, a German physicist, published a method for determining the water content of materials in 1935. This was a titrimetric method for determining sulphur dioxide in aqueous solutions based on the Bunsen reaction: 

H2SO4+ 2 HI = SO2 + I2 + 2 H2O

Karl Fischer discovered that if too much sulphur dioxide is supplied, the same reaction may be used to determine the amount of water in a sample by titration of the generated acids. Pyridine, which was apparently “simply there in the rack,” was his preferred base. The titration is known as the Karl Fischer titration in his honour (KF titration for short).

Both the original stoichiometry and the reagents were altered in the years that followed.

Fischer described the wrong molar ratio reaction, presuming an aqueous Bunsen reaction with methanol acting alone as a solvent.

Smith, Bryant, and Mitchell remedied the error by discovering that pyridine serves simply as a buffer, resulting in the following reaction:

H2O+ I2  + [RNH]++SO3CH3-+ 2 RN=  [RNH]+ SO4CH3–+ 2RNH+I–

Initially, the titration was done manually. The presence of brown colour from the additional excess of iodine signified the endpoint. Not only was it slow, but it was also unsuitable for colourful samples.

KF titration is now automated and widely utilised in numerous industries for water determination.

Titration Techniques of Karl Fischer

Karl Fischer uses two alternative methods to determine water: volumetric KF titration and coulometric KF titration.

Volumetric KF Titration

Water content can be determined by volumetric determination down to 1 percent of water. The sample is dissolved in a KF Solvent (typically methanol), and the iodine is added as part of a KF Reagent that includes sulphur dioxide and iodine dissolved in pyridine and methanol. Potentiometric analysis is used to determine the endpoint.

Only one iodide-containing solution is required for coulometric KF analysis. Anodic oxidation of iodide from solution produces the iodine required for the KF reaction, and the endpoint is monitored electrochemically. Coulometric analysis is best suited for samples containing less than 1% water.

Coulometric titrator Karl Fischer

In the titration process  the sample is transferred to a titration vessel, dissolved, and then titrated in both procedures. Some samples are insoluble in any appropriate solvent, and others can generate Karl Fischer reagent side reactions. This is where a volumetric or coulometric titrator and an oven can be used together. The sample is cooked in the oven, and the water released is transferred to a titration cell by a dry gas flow, where it reacts with KF reagent. This is an ideal solution for samples that are poorly soluble, substances that can produce solvent side reactions, and extremely hygroscopic samples where handling the sample in the laboratory can lead to falsely higher results.

Karl Fischer Titration Reagents and Applications 

Reagents for Karl Fischer titration come in a variety of forms, depending on the user’s needs and environmental awareness. The stronger base (and less stinky) imidazole can be used instead of pyridine, and ethanol can be used instead of methanol as a more “green” solvent. Additives for fats and oils, reagents for determining low water content, solubilizers for poorly soluble compounds, and buffer solutions for extremely alkaline or strongly acidic samples are among the auxiliaries available.

Karl Fischer titration is a dependable and stable method for direct analysis of water content in a variety of industries. 

1.It is used in the food industry to determine the water content of fruit juices, honey, flour, noodles, chips, and cocoa powder.

2. In the petroleum industry to determine the water content of various oils, gasoline, kerosene, and petroleum;
3. In  the cosmetic industry to determine the water content of shampoos, creams, lipstick, and toothpaste; and in the pharmaceutical industry to determine the water content of raw materials, active substances, lyophilized substrates, tablets, ointments, and oils. 

4.It’s used to figure out how much water is in silk, wool, wood, paper, and even building materials like zeolite and cement.

The list goes on and on.

This is simply another example of how, with the correct technique and reagents, Karl Fischer titration can be used to determine the water content of nearly any sample. It’s no surprise that titration is a preferred method in any laboratory or enterprise.

Conclusion 

We conclude that the oxidation process between iodine and sulphur dioxide is the basis for Karl Fischer titration. Sulphur trioxide and hydrogen iodide are formed when water combines with iodine and sulphur dioxide. When all of the water has been consumed, an endpoint has been reached.