Principle of Karl Fischer titration
28/09/2022 - Actualizado: 04/04/2023
What is Karl Fischer titration?
- What is Karl Fischer titration?
- Karl Fischer titration principle:
- Type of Karl Fischer titration:
- Karl Fischer titration procedure:
- Karl Fischer titration applications:
- Advantages of the Karl Fischer method:
- Preparation of the KF reagent:
- Instrumentation of the Karl Fisher titrator:
Karl Fischer titration is a titration technique that determines the amount of water contained in a given sample of analyte by volumetric or coulometric titration. Karl Fischer, a German scientist, discovered this method of quantitative chemical analysis in 1935. Today, the Karl Fischer titrator is a modern automated device that can perform these titrations.
It is a commonly used titrimetric method for the determination of water in different substances. Water determination is important to ensure product quality and ensure the chemical and physical properties of the product.
For example, in the pharmaceutical industry, stable water content is very important for intermediate powders and granules, from which tablets are prepared. Too high or too low water content reduces the effectiveness of pharmaceuticals/drugs.
Karl Fischer titration principle:
The oxidation process between sulfur dioxide and iodine is the basis of Karl Fischer titration. The chemical equation for the reaction of iodine, sulfur dioxide and water used during Karl Fischer titration is explained below.
I2 + SO2 + H2O → 2HI + SO3
Sulfur trioxide (SO3) and hydrogen iodide (HI) are formed when water (H2O) reacts with iodine (I2) and sulfur dioxide (SO2). This basic reaction uses exactly one molar equivalent of water to iodine.
Iodine is gradually added to the solution until it reaches an excessive level, indicating the end point of the titration, which is detected by potentiometry. The reaction is carried out in an alcoholic solution containing a base, which consumes the iodric acid and sulfur trioxide generated.
Karl Fischer titration of the Bunsen reaction:
The basic principle of Karl Fischer titration is based on the Bunsen reaction between iodine (I2) and sulfur dioxide (SO2) in an aqueous medium. Karl Fischer discovered that this reaction could be modified to determine the amount of water in a non-aqueous method with an excessive concentration of sulfur dioxide. He used pyridine as a buffering agent and a primary alcohol (methanol) as a solvent.
The alcohol reacts with SO2 and a base to form an intermediate alkyl sulphite salt, which is then oxidized to an alkyl sulphate salt by iodine. In this oxidation process, water is consumed.
Different physical and chemical methods are used to determine the water content. Among these, the KF titration method has been recognized as a standard for general use. It is featured for its high precision and specificity working over a wide concentration range from 100% to PPM.
Type of Karl Fischer titration:
Volumetric Karl Fischer titration and Coulometric Karl Fischer titration are the two types of techniques that Karl Fischer uses for water content determination.
Volumetric Karl Fischer titration:
In a volumetric titration, iodine is mechanically introduced into a solvent containing the sample throughout the titration through the burette of the titrator. 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 main 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 in solution provides the iodine needed for the KF reaction.
The colorimetric method measures the water level much less than the volumetric method. The two main types of coulometric KFT reagent systems are conventional or sintered cell coulometric KF and non-sintered cell coulometric KF:
Karl Fischer titration procedure:
There are two ways to perform the Karl Fischer titration experiment: volumetric determination and coulometric determination.
Karl Fisher reagent, dry methanol and Karl Fisher titrator instrument, etc.
The volumetric titrator works according to the following three main steps.
- Using a burette, dispense the iodinated KF titration reagent into the cell.
- The endpoint of the titration is detected by a platinum double pin indicator electrode.
- Calculates the final result based on the amount of Karl Fischer reagent dispensed using the built-in microprocessor.
The coulometric titrator works according to the following three main steps.
- This titration generates iodine at the anode of the titration cell, rather than delivering the reagent as in a volumetric titration.
- The end point of the titration is detected by a platinum double pin electrode.
- Calculates the final result based on the total charge passed, in coulombs, using the on-board microprocessor.
Karl Fischer titration applications:
- It is most commonly used in the pharmaceutical industries to detect water content and maintain product quality and shelf life.
- It is used in the petroleum industry to detect the presence of water in products such as brake oil, transformer oil, kerosene, etc.
- It is used in the food industry, because the observation of water is necessary at the different stages of the products.
- The Karl Fischer titrator is used in the manufacture of cosmetics to determine the amount of water in different products such as lipsticks, shampoos, creams and toothpastes etc.
- It is the most important technique used in different industries such as chemicals, power plants, plastics, gases, paper, wood, silk, wool, paints and adhesives, etc.
Advantages of the Karl Fischer method:
- Provides very accurate and fast results.
- It is suitable 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 determination range from 0.001 to 100% or ppm to ng.
- The availability of the two different methods extends the versatility of the range of applications.
Preparation of the KF reagent:
- Take 670.00 ml of methanol and 170.00 ml of pyridine with a graduated cylinder.
- Next, accurately weigh 125.00 g of iodine and pour it into the solution and cool it.
- Next, pour 100.00 ml of pyridine into a 250.00 m cylinder, hold the solution in an ice bath and pass through dry sulfur dioxide until its volume reaches 200.00 ml.
Instrumentation of the Karl Fisher titrator:
A Karl Fisher apparatus typically consists of an automated burette assembly (a gas-tight syringe with a Teflon-tipped plunger with a Luer fitting), stirrer, titration flask, reagent dropper, electrode detection, a silica tube, a keyboard, a screen and a printer output, etc.
Normal ambient operating conditions are as follows.
Temperature: Ambient to 450C
Relative humidity: 05 to 70%
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 dosing water, due to its precision, specificity and speed of measurement.
Why is methanol used in Karl Fischer titration?
Karl Fischer titrations use methanol as the solvent. Since methanol has higher hydrophilicity than other alcohols and is a more polar solvent, analytes dissolve in it quickly.
How is the Karl Fischer factor calculated?
The water equivalence factor F is calculated using the formula 0.1566 xw/v in mg of H2O per ml of reagent, where W is the weight in mg of sodium tartrate and V is the volume of reagent in ml.
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