Karl Fischer Titration: An Overview
Karl Fischer titration is a titration method that uses volumetric or coulometric titration to determine the quantity of water present in a given analyte. This method for quantitative chemical analysis was developed by the German chemist Karl Fischer in 1935. Karl Fischer reagent consists of iodine, sulphur dioxide, a base and a solvent, such as alcohol. In the aqueous environment, the Bunsen reaction between iodine and sulphur dioxide is the basis for the reactions of Karl Fischer reagents.
Karl Fischer Titration Principle
The principle of Karl Fischer titration is predicated on the oxidation reaction between iodine and sulphur dioxide. Water reacts with iodine and sulphur dioxide to form sulphur trioxide and hydrogen iodide. This reaction is named the Bunsen reaction. When all the water is consumed that’s the endpoint.
Karl Fischer Titration Formula
Karl Fischer titration is a technique for the determination of moisture content. This method is based on a reagent which reacts with water and converts the water into a non-conductive chemical. Water determination:
\[{\rm{water}}\,{\rm{content}} = \dfrac{{SF \times 100}}{{{\rm{mg}}\,{\rm{of}}\,{\rm{test}}\,{\rm{preparation}}}}\]
where S is the volume in mL of the reagent consumed and F is the water per mL of the reagent.
Karl Fischer Titration Instrumentation
Karl Fischer titration is based on iodide reaction, which means water reacts with iodine. The apparatus consists of an automatic burette, a back titration flask, a stirrer, and equipment for amperometric titration at a constant voltage or potentiometric titration at a constant current.
Coulometric KF analysis requires only one, iodide-containing solution. Iodine needed for KF reaction is produced by anodic oxidation of iodide from solution and the end point is detected electrochemically. Coulometric determination is best suited for samples with less than 1% of water.
Electrolysis Cell of Coulometric Moisture Metre
Volumetric determination is suitable for the determination of water content of less than 1% of water. The sample is dissolved in KF solvent (usually methanol based) and therefore, the iodine is added as a part of the KF reagent containing sulphur dioxide and iodine dissolved in pyridine and methanol. The endpoint is determined potentiometrically.
Karl Fischer Titration Reaction
The alcohol reacts with sulphur dioxide and base to form an intermediate alkyl sulfite salt, which is then oxidised by iodine to an alkyl sulphate salt. This oxidation reaction consumes water.
Step 1:
\[C{H_3}OH + S{O_2} + RN \to \left[ {RNH} \right]S{O_3}C{H_3}\]
Step 2:
\[{H_2}O + {I_2} + {\left[ {RNH} \right]^ + }S{O_3}CH_3^ - + 2RN \to {\left[ {RNH} \right]^ + }S{O_4}CH_3^ - + 2{\left[ {RNH} \right]^ + }{I^ - }\]
Important Questions and Answers
Karl Fischer is a widely used method for the determination of water in the pharmaceutical industry. Karl Fischer is employed to determine the only water content present in the sample but no other volatile impurities in the samples. In interviews on quality control of pharmaceutical industries, it's generally asked. Karl fischer titration interview questions and answers are listed below:
1. What are the two methods of water determination?
Ans:
Volumetric Water Determination
Coulometric Water Determination
2. What is the difference between volumetric Karl Fischer determination and coulometric Karl Fischer determination?
Ans: The titrant is added directly through the burette in the volumetric determination while titrant is generated electrochemically within the titration vessel in coulometric titration.
3. What is the basis of Karl Fischer titration?
Ans: The basis of Karl Fischer titration is predicated on the oxidation reaction between iodine and sulphur dioxide. Water reacts with iodine and sulphur dioxide to produce sulphur trioxide and hydrogen iodide. An endpoint is reached when all the water is consumed.
Key Features of Karl Fischer Titration
The volumetric Karl Fischer titration method is employed to determine water content by adding iodine-containing titrant to the sample which is then dissolved or dispersed in a suitable solvent.
Karl Fischer titration is a redox reaction which uses the consumption of water during the reaction to measure the amount of water in a sample.
It is the reference method for water determination because of its specificity, accuracy, and speed of instrument.
The optimum pH range of the sample solution is 5.5 to 8.
FAQs on Karl Fischer Titration
1. Why is methanol used in KF titration?
Consistent with the KF principle, the medium should need lower alcohol or organic solvent and lower alcohol is methanol. The iodine reacts with water, the consumed iodine value is water content.
\[C{H_3}OH + S{O_2} + RN \to \left[ {RNH} \right]S{O_3}C{H_3}\]
\[{H_2}O + {I_2} + {\left[ {RNH} \right]^ + }S{O_3}CH_3^ - + 2RN \to {\left[ {RNH} \right]^ + }S{O_4}CH_3^ - + 2{\left[ {RNH} \right]^ + }{I^ - }\]
2. Why is DST (disodium tartrate) used in KF solution?
DST is used as a primary standard in the Karl Fischer reagent because it is easily reacted with the KF reagent and it has a highly stable crystalline structure.
3. What electrode is used in KF titration?
The foremost advanced KF titration technology uses a double platinum electrode for electrochemical indication of the endpoint, but visual and photometric indicators are also used.
