Difference Between Karl Fischer Coulometry and Volumetric Method Compare the Difference Between Similar Terms
Karl Fischer (KF) titration is a specific moisture determination technique widely used by industry and scientists. The different ways of producing iodine can be divided into the coulometric method and the volumetric method.
Karl Fischer Titration Principle
The KF reaction is based on an earlier reaction called the Bunsen reaction, in which sulfur dioxide is oxidized by iodine and consumes water in the oxidation process. The initial response is as follows:
The currently accepted KF titration reaction is as follows, which was achieved through an understanding of the reaction mechanism and modification of the original reagents:
Once the reaction between iodine and the sulfite alkyl intermediate generated after the half-reaction stops, resulting in excess iodine in the solution and a color change, the titration reaction reaches the end. This indicates that there is no more water to use up, so the reaction stops.
As shown in the equation, the molar ratio of water and iodine is 1:1. When current is passed through the platinum electrode, the drop in voltage indicates the end point of the titration. The voltage required for this current is initially high but drops dramatically when iodine is in excess.
The water content can be calculated from the reacted reagent volume. In KF titration, samples can be titrated from 100 to 1x106 ppm (water concentration from 0.01 to 100%). It has high sensitivity and can therefore measure even trace amounts of water. This is valid regardless of whether the water in the substance is free, emulsified, or dissolved.
Difference Between Volumetric and Coulometry Compare the Difference Between Similar Terms
KF titration, volumetric and coulometric. In short, the two methods can be compared as follows:
Volumetric Method
Reagent Type
The volumetric method uses a KF reagent containing sulfur dioxide and iodine, which also contains a hydroxyl group and a base. Before formally testing samples, a pre-titration is required to ensure that any contaminating cells or dissolved water from the air are removed by the reagents. The sample is then dissolved in a solvent. The iodine reacts with the sample water in a two-step reaction.
Reagent Addition
Add the reagent drop by drop with a buret until the solution changes color. The discoloration of the solution proves that the water in the titration cell has reacted. At this time, the iodine in the sample is excessive, so the color will be produced.
Samples containing 0.1 to 500 mg of water can be measured with this method using the correct KF reagent. The titers of these reagents change over time, so they must be calibrated with standard water before each use. This method uses more reagents but can handle larger sample volumes.
Endpoint Detection
The endpoint signal is detected by a voltammetric indicator electrode. Due to the presence of excess iodine, which causes a polarizing current to flow at a defined value between the electrodes of the titration cell, the required voltage drops sharply at the endpoint. The titer of the reagent added before this was used to obtain the amount of water in the sample.
Coulometric Method
Reagent Type
The coulometric method is a high-precision water determination method. A Coulomb cell consists of two cells, an anode, and a cathode. It is extremely sensitive to the presence of water, so the electrolytic cell must be free from any external moisture.
Therefore, in coulometric KF titration, pre-titration is a longer process. The sample is injected into the electrolytic cell, which is sealed from the outside.
Reagent Addition
The coulometric method generates a titrant through an electrochemical reaction in a titration cell. The iodide in the reagent is oxidized to the iodine element at the anode, and then the iodine element reacts with the sample water until all the water is used up. Once the endpoint is reached, the water content is calculated from the amount of current required. Coulometric methods are generally used for samples with little water content, and features include:
ühigher sensitivity
üfaster titration
üReagents do not need to be calibrated each time as iodine is generated in situ
ü No need to change solvent every time
ü Multiple samples can be tested without reloading reagents
ü High economic cost
Endpoint Detection
A very sensitive platinum electrode indicator using alternating current was used. The amount of current passed to achieve sufficient iodine electrolysis was measured and the corresponding water content was calculated.
Sample Volume
Coulometry can detect even small amounts of water present in the gas. Typically recommended sample water content may be as high as around 2% of the sample, or 200 micrograms of water, which means a total volume of 10 mL for a liquid sample.
The ideal sample is one that contains less than 1% water, has a liquid sample volume of 2 mL or less, and produces less than 20 µg of water. Larger samples fill the cell too quickly, causing the cell to need to be cleaned and re-filled with reagents, which increases downtime. In contrast, the volumetric method is suitable for samples over 10 mL and 2% water.
Solvent Range
Coulometric assays are limited by the range of solvents, and if suitable solvents are not available, samples must be oven-conditioned.
In general, the volumetric method is preferred when the sample is soluble in available solvents and is likely to contain less than 1% water.
Reference：
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3.Anthony Lucio. (2013) Available at: https://chem.uiowa.edu/sites/chem.uiowa.edu/files/people/shaw/LUCIO GM KF-Titration March-2013.pdf. (Accessed: 13, April, 2023)
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