What is Karl Fischer titration? How does it work?

Karl Fischer (KF) titration is a widely used technique for the determination of water or water in solid, liquid, and gaseous samples. It has many applications in industrial quality control.

Karl Fischer titration principle.

The KF reaction is based on iodine and will be sulphur dioxide.

oxidize and buffer the solution.

Medium water consumption: I2 + 2H2O + SO2- 2Hi + H2SO4

The titration reaches the end point when the titrant reaches a volume sufficient for the total amount of water in the sample to react, and water and iodine are consumed at equimolar ratios.

The color change indicating the end point of the reaction is caused by the detection of excess iodine on the double platinum electrode acting as an indicator after all the water has been consumed in the sample, causing the reaction to stop. The water content is then calculated from the amount of reagent added.

The titration cup must be kept away from atmospheric moisture, and the sample must not react with the KF reagent, e.g., ketones and aldehydes must be methanol-free.

of solvents titrated because otherwise they would react with methanol to form water, which would lead to false high water concentrations and disappearing endpoints.

The reaction is sensitive to pH. Therefore, if you want to titrate strongly acidic or alkaline samples, you must use a buffer.

Alcohol (methanol, ethanol or propylene glycol.

mixture) is the solvent commonly used in samples and titration cells, where the reaction has reached its endpoint when there is an excess of iodine, and state-of-the-art Karl Fischer titrator titration technology uses dual platinum electrodes to electrochemical the endpoint.

indications, but visual and photometric indicators are also used.

Method. There are two main methods for KF titration, the volumetric method and the Coulometric method, KF titration is very fast and specific, and only the free and bound forms of water can be determined, and special Karl Fischer titrators (also called micromoisture analyzers) are available, as well as ready-to-use KF reagents.

Now a third method, mixed Karl Fischer titration, is used, which is more flexible and advanced than the other two methods and combines them mainly by coulometric titration, but if the water content exceeds the required volumetric titration value, a solution containing a preset amount of iodine is added at the same time.

Sample preparation. Samples must be prepared differently depending on their state. The water bound by the solids is either absorbed or retained as crystalline water.

or trapped in the molecule. Therefore, they must be completely dissolved in a suitable solvent. The solids should be powdered to aid in dissolving. Sometimes external extractions are made.

The liquid is injected into the titration cell through a diaphragm without disturbing the anhydrous conditions. Gases are best titrated by coulometric KF titration, as the water content is usually very small.

Insoluble or highly reactive samples are suitable for the KF oven method, which removes water by evaporating into the titration chamber to react with the KF reagent.

Application of Karl Fischer titration.

The water content in various materials can be determined using KF titration. These include food, chemical reagents, pharmaceuticals, and plastics. The amount of water titrated can be in the range of 1 ppm to 100 water. This includes surface water and water bound in crystals.

Respondent: Huatian Power.

The KF reaction is based on iodine oxidation of sulfur dioxide and the consumption of water in a buffer solution:

The titration reaches the endpoint when the titrant reaches a volume sufficient to reflect the total amount of water in the sample. Water and iodine are used up in molar ratios.

The color change that signals the end point of the reaction is due to the detection of excess iodine on the double platinum electrode that acts as an indicator, and the subsequent consumption of all the water in the sample, causing the reaction to stop. The water content is then calculated based on the amount of reagent added.

The titration cell must be kept away from atmospheric moisture and the sample must not react with the KF reagent. For example, ketones and aldehydes must be titrated in a solvent that does not contain methanol, because otherwise they react with methanol to form water, which leads to a false high water concentration and a disappearing endpoint.

The reaction is pH sensitive, so buffers must be used if strongly acidic or basic samples are to be titrated.

Alcohol (methanol, ethanol, or propylene glycol mixture) is the solvent commonly used for samples and titration cells. When there is an excess of iodine, the reaction reaches its endpoint. State-of-the-art KF titration technology uses dual platinum electrodes to electrochemically indicate the endpoint, but also visual and photometric indicators.

There are two main methods of KF titration, namely volumetric titration and coulometric titration. KF titration is very fast and specific, and only free and bound forms of water are determined. Dedicated KF titrators are available as well as off-the-shelf KF reagents.

The third method now used is hybrid Karl Fischer titration. This method is more flexible and advanced than the other two methods and is mainly combined by coulometric titration, but if the water content exceeds the specified limit of the required volumetric titration, a solution containing a predetermined amount of iodine can be added at the same time.

The KF titration method was first devised by the German chemist Carl Fischer to determine the water content of liquid sulfur dioxide. Since other commonly used methods were not suitable, he used the Bunsen reaction shown below.

He used new reagents including sulfur dioxide, iodine, and pyridine in the methanol solution to get the following equation:

Finally, the color changes from yellow to brown. So he performed the first volume of KF titration. Later in 1939, American researchers improved the method and modified both steps by changing the molar ratio of water to iodine:

The final version appeared in 1984, when it was known that pyridine was not part of the reactant, but only a base (denoted as b).

Determination principle:

In the presence of methanol and organic amines, a molecule of water can react quantitatively with a molecule of iodine. When the water is completely consumed, the excess iodine can undergo a redox reaction at the double platinum indicator electrode, causing a drop in the polarization voltage and thus indicating the end point of the reaction.

The Karl Fischer method is divided into the Coulomb method and the volume method.

The principle of the Coulometric method is that Karl Fischer solution is a mixture of dominant iodine and sufficient sulfur dioxide pyridine, methanol, etc. Based on the fact that when water is present, iodine is reduced by sulfur dioxide, and in the presence of pyridine and methanol, pyridine hydroiodate and pyridine methyl sulfate are generated.

The endpoint of the reaction is indicated by a detection unit consisting of a pair of platinum electrodes. According to Faraday's law of electrolysis, the number of iodine molecules involved in the reaction is equal to the number of molecules of water, and the electric charge is proportional. In layman's terms, the number of electron moles consumed by electrolysis is equal to the number of electron moles obtained by the decomposition of water molecules into hydrogen and oxides.

The amount of water electrolyzed is determined according to the amount of electrons consumed.

The principle of volumetric method is to titrate a sample containing water with a reagent of known concentration, and when the titration reaches the end point, the amount of water in the sample is judged according to the amount of reagent consumed.

The advantage of the coulometric method is that the test accuracy is extremely high, generally reaching microgram accuracy. It is suitable for the determination of samples with very low water content, such as oils, alcohols, esters, benzene, ethers, resins and various chemical raw materials. And Karl Fischer is used in very little amount.

At present, the market price ranges from more than 10,000 to 30,000 yuan according to the grade. The instrument features a main unit with an electrolytic cell bottle with two pure platinum electrodes.

At present, the instrument test accuracy of the domestic capacity method is low, and it belongs to the manual test device. Of course, if we increase the research and development of instruments, we can also achieve high precision, such as Metrohm. At present, the domestic volumetric moisture meter can roughly measure the sample with large water content, and the test results are not credible when measuring the ppm level sample.

The advantage is that it is cheap, and the market price is about a few thousand yuan now. The instrument features two graduated glass burettes and an indicator pointer in the main unit.

It is important to note that the Karl Fischer reagent of the coulometric method needs to be shaded, sealed, and refrigerated. Otherwise, the life will be shortened. Volumetric reagents should pay special attention to sealing, as poor sealing directly affects the accuracy of test results.

My unit is the production of Karl Fischer trace moisture analyzer, Zibo Shengkang Electric.

1. Karl Fischer reagent: known accurate concentration of I2, excess SO2 than I2 and an appropriate amount of pyridine, a mixture of methanol as a solvent (I2 + SO2 + PY + CH3OH).

2. Principle: I2 needs a certain amount of water for SO2 oxidation.

In the presence of pyridine, the methanol reaction is added before quantifying to the right.

i2 + so2 + ch3oh + py + h2o → 2pyh+i- +pyh+ch3oso3-

3. Application: It is used for the determination of water content in organic substances.

In order for the Karl Fischer (kf) reaction to proceed reliably, the basic conditions for the Karl Fischer (kf) reaction must be met. The basic condition is to select the appropriate solvent for the sample as the solvent for the Karl Fischer (kF) reaction and set the pH to the optimal range for the Karl Fischer (kF) reaction (5-7).

1) Solvent for a Karl Fischer titrator (also known as a trace moisture analyzer).

The most important condition is to dissolve the solvent of the Karl Fischer (kf) reaction, i.e., the sample, and to select the solvent or solvent mixture for the Karl Fischer (kf) reaction according to the sample.

Methanol is well used as a solvent for Karl Fischer (kF) reactions. Methanol easily dissolves most samples and makes the endpoint highly reliable. Sometimes methanol is mixed with other solvents. However, methanol should be maintained at least 25 by volume.

Since propanol dissolves organic compounds whose chains are longer than methanol, it can be used to measure long-chain organic compounds, either as a mixed solvent with methanol or as a single solvent.

Methoxyethanol (ethylene glycol monomethyl ether) is another alcohol solvent. This solvent can be used well when a side reaction (thundering, ketoylation) occurs in the presence of methanol. Measurements in this solvent are slower than in methanol.

Chloroethanol can inhibit side reactions and make Karl Fischer (kF) react quickly.

Chloroform is a solvent that dissolves oil well and can be mixed with methanol. In this case, at least 25 methanol should be added by volume. It is usually used in a mixed ratio of 50.

Pure chloroform is not suitable for use as a solvent for Karl Fischer (kf) reactions because it changes the stoichiometry of Karl Fischer (kF) reactions.

Formamide increases the solubility of polar substances. It can be used in combination with methanol to quantify the moisture of proteins. When mixed with methanol, the amount of formyl glue should not exceed 50 (vol). Formyl glue is usually mixed in a ratio of 30 percent.

2) pH conditions of the Karl Fischer titrator.

The optimal pH range for Karl Fischer (kF) titration is 5-7, and the Karl Fischer (Kf) reaction proceeds rapidly in stoichiometric ratios within this pH range.

When the pH is high, side reactions occur and iodine is slowly depleted. It can be seen that side reactions occur due to the absence of an endpoint.

Under strong acidic conditions, the pH value decreases, the reaction constant of the Karl Fischer (kF) reaction decreases, and the reaction process is slow. In fact, Karl Fischer (KF) titration is performed in the pH range of 4-7.

When titrating water with a Karl Fischer titrator, the acid must be neutralized. After the addition of imidazole, the pH value stabilized, reaching the pH range of 5.

Respondent: Huatian Power.

You can take a look at the introduction of ZDY-504 on the Internet.

Moisture is expressed as a percentage of the water content in solids, liquids, and gases.

The moisture determination methods include Karl Fischer method, drying method, infrared absorption method, and dielectric constant method, among which the Karl Fischer method and drying method are mainly used in the laboratory.

We use a Karl Fischer titrator (also known as a micromoisture analyzer) volumetric titration.

The Karl Fischer method for determining water content takes advantage of the fact that the Karl Fischer reagent containing iodine, sulfur dioxide and pyridine (hereinafter referred to as the KF reagent) reacts specifically with water in the presence of methanol, which is divided into volumetric and coulometric methods according to the titration procedure.

The capacitance method can theoretically measure from a few ppm up to 100 h2o

Principle of volumetric titration of a Karl Fischer titrator.

Volumetric titration involves placing a titration solvent into a titration flask, dissolving the sample in the titration solvent to extract the water from the sample, and then titrating using Karl Fischer reagents with iodine, sulfur dioxide, and alkali as the main components to determine the water content.

In the presence of alkali and alcohol, water reacts with iodine and sulfur dioxide.

h 2 0 + i 2 + so 2 + ch 3 oh + 3rn→2rn·hi + rn· hso 4 ch 3

According to the above formula, H20 reacts with I2 at a ratio of 1:1, therefore, the number of milligrams per 1 ml of water (titrant) of Karl Fischer reagent is found in advance with water or a water reference substance. The water content (mg) is calculated from the required Karl Fischer reagent titer (ml).

Moisture (mg) = Karl Fischer reagent titer (ml) x titer (mg H2O ml).

Respondent: Huatian Power.

Karl Fischer Method.

Advantages: wide measurement range, many test varieties, and mature various schemes.

Disadvantages: Karl Fischer reagents need to be consumed, and some samples need to be equipped with special testing equipment.