Learn about the principle, types, and applications of Karl Fischer titration, which is a commonly used titrimetric method for the determination of water, or moisture in different types of samples including solid, liquid, and gas.
What is Karl Fischer titration?
Karl Fischer titration is a titration technique that determines the amount of water contained in a given sample of analyte by using volumetric or coulometric titration. Karl Fischer, a German scientist, discovers this method for quantitative chemical analysis in 1935. Nowadays, The Karl Fischer titrator is a modern automated apparatus that can execute such titrations.
It is a commonly used titrimetric method for the determination of water in different substances. Water determination is significant to ensure product quality and to assure the chemical and physical properties of the product.
For example, in the pharmaceutical industry, the stable content of water is very significant for intermediate powders and granules, by which tablets are prepared. The content of water that is too high or too low reduces the effectiveness of pharmaceutical / medicinal products.
Principle of Karl Fischer titration:
The oxidation process between sulphur dioxide and iodine is the basis for Karl Fischer titration. The chemical equation for the reaction of iodine, sulphur dioxide, and water which is used throughout Karl Fischer titration is explained below.
I2 + SO2 + H2O → 2HI + SO3
Sulphur trioxide (SO3) and hydrogen iodide (HI) are formed when water (H2O) reacts with iodine (I2) and sulphur dioxide (SO2). This basic reaction uses exactly one molar equivalent of water vs. iodine.
Iodine is gradually added to the solution until it reaches an excessive level, indicating the titration's endpoint, which is detected using potentiometry. The reaction is performed in an alcohol solution containing a base, which consumes the generated hydroiodic acid and sulphur trioxide.
Bunsen reaction Karl Fischer titration:
The basic principle behind Karl Fischer titration is based on the Bunsen reaction between iodine (I2) and sulphur dioxide (SO2) in an aqueous medium. Karl Fischer found that this reaction could be modified to determine the amount of water in a non-aqueous method with an excess sulphur dioxide concentration. He employed pyridine as the buffering agent and a primary alcohol (methanol) as the solvent.
The alcohol reacts with SO2 and a base to form an intermediate alkylsulfite salt, which is subsequently oxidized to an alkylsulfate salt by iodine. Water is consumed in this oxidation process.
Different physical and chemical methods are used for determining the content of the water. Of these, the KF titration method has been recognized as a reference for general use. It is described by its high accuracy and specificity workings on a broad range of concentrations from 100% to PPM.
Types of Karl Fischer titration:
Volumetric Karl Fischer titration and Coulometric Karl Fischer titration are the two types of techniques that are used for the determination of water content by Karl Fischer.
Volumetric Karl Fischer titration:
In a volumetric titration, iodine is mechanically introduced into a solvent containing the sample throughout the titration by the titrator’s burette. The volume of KF reagent consumed is used to calculate the amount of water consumed.
This method is used to determine the amount of water content in the range of 100 ppm to 100%. One-component volumetric KF and two-component volumetric KF are the two major types of volumetric KFT reagent systems.
Coulometric Karl Fischer titration:
In a coulometric titration, the titrant is generated electrochemically in the titration cell and the endpoint is detected electrochemically. The anodic oxidation of iodide from the solution provides the iodine necessary for the KF reaction.
The colorimetric method measures the water level much less than the volumetric method. Conventional, or fritted-cell, coulometric KF, and fritless-cell coulometric KF are the two major types of coulometric KFT reagent systems:
Procedure of Karl Fischer Titration:
There are two ways to perform the Karl Fischer titration experiment: Volumetric determination and Coulometric determination.
Requirements:
Karl fisher reagent, dry methanol, and Karl fisher titrator instrument, etc.
Volumetric titrator works on the following three major steps.
- Using a burette, it dispenses the iodinated KF titration reagent into the cell.
- The titration's endpoint detects using a double-platinum pin indicator electrode.
- It calculates the final result based on the amount of Karl Fischer reagent dispenses using the onboard microprocessor.
Coulometric Titrator works on the following three major steps.
- This titration generates iodine at the anode of the titration cell, rather than delivering the reagent as in a volumetric titration.
- The titration's endpoint detects using a double-platinum pin electrode.
- It calculates the final result based on the total charge passed, in Coulombs, using the onboard microprocessor.
Applications of Karl Fischer titration:
- It is most commonly used in the pharmaceutical industries to detect water content and maintain the quality and shelf life of the product
- It is used in the oil industry, to detect the presence of water in products, such as brake oil, transformer oil, kerosene, etc.
- It is used in the food industry since water observation is necessary at different stages of the products.
- The Karl Fischer titrator is used in cosmetic manufacturing to determine the amount of water in different products such as lipstick, shampoos, creams, and toothpaste, etc.
- It is the most important and commonly used technique in different industries such as are chemical, power stations, plastics, gases, paper, wood, silk, wool, paints, and adhesives, etc.
Advantages of Karl Fischer method:
- It provides very accurate and rapid results.
- It is appropriate for solid, liquid, and gaseous samples.
- It works with volatile and non-volatile substances.
- It requires a small amount of sample and has a wide range of determination from 0.001 to 100% or ppm to ng level.
- The availability of the two different methods expands the versatility of the application range.
Preparation of the KF Reagent:
- Take 670.00 ml of methanol and 170.00 ml of pyridine using a measuring cylinder.
- Then accurately weigh 125.00 gm of iodine and pour into the solution and cool it.
- Then pour 100.00 ml of pyridine into a 250.00 m cylinder, keep the solution in an ice bath and pass in dry sulphur dioxide till its volume reaches 200.00 ml.
Instrumentation of Karl fisher titrator:
A Karl Fisher apparatus normally consists of automated burette assembly (a gas-tight syringe with Teflon tipped plunger with Luer fitting), a stirrer, titration flask, reagent dropper, detection electrode, silica tube, keyboard, display, and printer output, etc.
The normal environmental operating conditions are as follows.
Operation: Indoor
Temperature: Ambient to 450C
Relative humidity: 05 to 70 %
FAQ (Frequently Asked Questions):
What is KF titration?
Karl Fischer titration is a redox reaction that measures the amount of water in a sample by measuring the amount of water consumed during the reaction. It is the reference method for determining water, as of its precision, specificity, and rate of measurement.
Why methanol is used in Karl Fischer titration?
Karl Fischer titrations employ methanol as a solvent. Because methanol has a higher hydrophilic than other alcohols and is a more polar solvent hence the analytes dissolves quickly in it.
How do you calculate the Karl Fischer factor?
The water equivalence factor F is calculated using the formula 0.1566 x w/v in mgs of H2O per ml of reagent, where W is the weight in mgs of sodium tartrate and V is the volume in ml of reagent.
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