Karl Fischer titration is defined as a titration method that uses either volumetric or coulometric titration to determine the water quantity present in a given analyte. This method, which is used for quantitative chemical analysis, was developed in 1935 by the German chemist named “Karl Fischer.” Today, the specialized titrators (which are called Karl Fischer titrators) have been made available to carry out such titrations.
Let us look at the Karl Fischer principle in detail.
The principle of Karl Fischer titration is completely based on the oxidation reaction between sulphur dioxide and iodine. Water reacts with sulphur dioxide and iodine to form hydrogen iodide and sulphur trioxide. When all the water is consumed, it reaches an endpoint. The chemical equation that takes place for the reaction between sulphur dioxide, iodine, and water (which is employed during the Karl Fischer titration) is given below.
I₂ + SO₂ + H₂O → 2HI + SO₃
Sample injection cap,
A cathode chamber,
[Image will be Uploaded Soon]
The experiment of Karl Fischer titration can be performed in two different methods. They are listed below.
This particular technique is suitable in determining water content down to 1 percent of water. The respective sample is dissolved in KF methanol, and then iodine is added to KF Reagent. Here, the endpoint is detected potentiometrically.
Here, the endpoint is electrochemically detected in this experiment. Iodine needed for the KF reaction is obtained by anodic oxidation of iodide from the solution.
It can be used in technical products such as plastics, oils, gases,
It is used in cosmetic products,
It is used in pharmaceutical products,
It is used in the industry.
Prepare a solution by mixing 670 mL of methanol and 170 mL of pyridine. Now, add 125 g of iodine to the solution and cool it. Then, take a 250 mL graduated cylinder and add 100 mL of pyridine to it. Keep the solution in an ice bath. Pass in dry sulphur dioxide till its volume reaches 200 mL.
The popularity of Karl Fischer titration (henceforth called KF) is due in large part to many practical advantages that it holds over other moisture determination methods, like speed, accuracy, and selectivity.
KF is selective for water since the titration reaction itself consumes water. In contrast, the mass loss measurement on drying will help to detect the loss of any volatile substance. However, the strong redox chemistry (SO2 or I2) means that the redox-active sample constituents can react with the reagents. Because of this reason, KF is not suitable for the solutions containing dimethyl sulfoxide (for example).
KF has high precision and accuracy, typically within 1% of the available water. For example, 3.00% appears as 2.97 - 3.03%. Although this KF is a destructive analysis, the sample quantity is very small and typically limited by the weighing accuracy. For instance, to obtain 1% of accuracy using a scale with a typical accuracy of 0.2 mg, the sample solution must contain 20 mg water, which is 200 mg (for example) for a sample with 10% water.
Whereas, for coulometers, the measuring range falls from 1–5 ppm to around 5%. Volumetric KF readily measures the sample solution up to 100%, but it requires impractically large amounts of sample for analytes which are less than 0.05% water. The response of KF is linear, and therefore, single-point calibration using a calibrated 1% water standard becomes sufficient, and there is no need for calibration curves.
KF is suitable to measure the liquids, and also with special equipment, gases. The major disadvantage with solids is: water has to be accessible and brought easily into the methanol solution. Several common substances, especially the foods like chocolate, release water slowly and with difficulty, claiming for additional efforts to bring the total water content reliably into contact with the KF reagents. For example, a high-shear mixer can be installed in the cell to break the sample. KF has problems with strong binding to water compounds, as in the water of hydration. For example, lithium chloride. So, KF is unsuitable for the special solvent like LiCl or DMAc.
KF is also suitable for automation. In general, KF is conducted using a separate KF titrator, or for the volumetric titration, a KF titration cell will be installed into a general-purpose titrator.
It is also possible to use the volumetric titration with visual detection of a titration endpoint with the coloured samples by UV or VIS spectrophotometric detection.
Q1. What is Meant by Coulometric Karl Fischer?
Ans. Karl Fischer titration is merely described as a means of measuring the sample water content. A few of the modern instruments, like the Aquamax KF, apply the coulometric principle, whereby the water that exists in the sample is titrated coulometrically to a predefined endpoint, where free iodine exceeds a minute.
Q2. Why We Use the Sodium Tartrate in Karl Fischer?
Ans. The volumetric standard for the Karl Fischer titration is the sodium tartrate dihydrate. It is non-hygroscopic and stable under normal conditions. The sodium tartrate dihydrate contains a 15.66 percent stoichiometric water content and is primarily used in volumetry in measuring titer.
Q3. Mention the Electrode of KF Titration?
Ans. Because the iodine is in abundance, the reaction has reached its termination point. For an electrochemical indication of the endpoint, most of the complex KF titration technology uses a double platinum electrode, whereas the photometric and visual indications are still used.
Q4. How Do We Make a Karl Fischer Reagent?
Ans. A Solvent, alcohol (ROH), a base (RN), sulphur dioxide (SO₂), an established concentration of iodine (I₂) are the reagents. The Bunsen reaction between sulphur dioxide and iodine can be given as the Karl Fischer Reagent’s basis for the reactions in an aqueous environment.