Karl Fischer Titration – Measuring Principle

This section contains some information about the basic principles of Karl Fischer moisture determination. The information is divided in several chapters. Please click on the desired section in the menu.

• What is Karl Fischer Titration?
• Which method should be used, coulometric or volumetric?
• Which sample size should be used?
• Which type of Karl Fischer reagent should be used?
• Coulometric cell with or without diaphragm?

What is Karl Fischer Titration?

The Karl Fischer Titration is a titration method for measuring water content in basically all types of substances. It was invented in 1935 by the German chemist Karl Fischer.

The Karl Fischer Titration is based on an iodine / iodid reaction: The water reacts with iodine. The endpoint of the titration is reached when all the water is consumed

SO2 + CH3OH + B ↔ CH3SO3-+ HB+ CH3SO3-+ H2O + I2 + 2B → CH3SO4-+ 2HB++ 2I-

The process uses an organic base (B), sulphur dioxide, iodine and an alcohol. The original Karl Fischer method used pyridine as organic base and methanol as alcohol. Since then the reagents have been improved. Nowadays Karl Fischer reagents are available which are less toxic (no more pyridine but imidazole as organic base, ethanol instead of methanol) and which provide a faster reaction.

During the titration, iodine is added to sample and the amount of iodine used to consume all the water contained in the sample is measured. There are two possibilities to add the iodide to the sample:

In the so called Volumetric Karl Fischer Titration, a solution with an exactly known concentration of iodine is added to sample by means of an electric burette. The amount of iodine added to the sample is calculated from the volume of iodine solution used.

In the Coulometric Karl Fischer Titration the iodine is electrolytically generated. The amount of idodine added to the sample is determined by measuring the current needed for the electrochemical generation of the iodine. When reacting with water, the brown iodine is reduced to the colourless iodide.

The Hybrid Karl Fischer titration is the most advanced and at the same time the most versatile technique for Karl Fischer moisture titrations. It is basically a combination of both, the coulometric and the volumetric method: The iodine is electrolytically generated and –if the moisture content of the sample exceeds a certain level – a solution with an exactly know concentration of iodine is added at the same time.

Endpoint detection

When reacting with water, the brown iodine is reduced to the colourless iodide. At the endpoint of the titration when all the water is consumed the colour of the solution turns increasingly from yellow to brown. As there is no sharp colour change and the coloration differs in nonpolar solvents (such as DMF) and polar solvents (as e.g. methanol) , it is not easy to determine the endpoint of the titration visually.

For this reason, the endpoint of the titration is usually determined electrometrically with a double platinum wire electrode.

There are two ways to electrometrically detect the endpoint:

• Biamperometric indication
• Bivoltametric indication

Biamperometric indication

A constant voltage of approximately 500 mV is applied to the wires of the electrode and the resulting current is measured. As long as there is water in the sample, no free iodine is present in the solution. When the endpoint of the titration has been reached, the following reactions occur at the wires of the electrode:

Cathode: I2 + 2e- → 2I-

Anode: 2I– 2e- → I2

When the endpoint has been reached the current thus increases from almost nil to a few μA.

Bivoltametric indication

A small current (normally in the range of 1 … 50 μA) is applied between the electrodes and the voltage required to maintain this current is measured. Normally alternating current is used (AC) as it yields a higher sensitivity of the electrode than direct current (DC). The voltage required to maintain the current is in the range of several 100 mV as long as an excess of water is present in the sample. When the endpoint of the titration is reached, free iodine is available in the solution and the voltage drops to 100 mV or less. Normally, the endpoint potential level (switch off voltage) must be selected according to the type of solvent being used and/or the type of sample being tirated. The ideal switch off voltage depends on the type of sample and solvent used. With normal Karl Fischer Titrators it must be determined experimentally:

• If the switch off voltage is too low, too much iodine is added before the endpoint is detected, the water contents yielded are too high.
• If the switch off voltage is too high, the titration does not start automatically as no free iodine is required for this voltage to be achieved.

All Karl Fischer Titrators from KEM feature an automatic solution impedance compensation function (Japanese Patent No. 1896338 ). Advantage: The operator does not have to worry about the correct switch off voltage as this function automatically determines the ideal endpoint potential. This ensures reliable results regardless of the type of sample and solvent.

Which method should be used, coulometric or volumetric?

In general it can be said that the method has to be chosen depending on the water content of the samples to be measured:

• the coulometric method is suitable for samples with a low water content (10 μg … 100 mg)
• the volumetric method is suitable for samples with a higher water content (0.1 … 500 mg).

Karl Fischer Hybrid Titrators add the iodine coulometrically and volumetrically and are thus suitable for samples with a water content in the range from 10 μg to 500 mg.

Coulometric Titrators

Coulometric Titrators are primarily used to analyse samples with a low water content: KEM Karl Fischer Coulometers can measure samples with a water content in the range of 10 μg … 100 mg with a resolution of 0.1 μg of H2O. The measurement of samples which contain larger amounts of water require a lot of time and may exceed the capacity of the Karl Fischer reagents. Normal Coulomeric Karl Fischer titrators can titrate approximately 2 mg of H2O per minute. The MKC-610 features an accelerated electrolysis which allows to titrate up to 2.5 mg H2O per minute.

Coulometric titrators offer one decisive advantage compared to volumetric titrators: No titer has to be determined. The coulometric method is an absolute method, there is a strictly quantitative relationship between the amount of electric charge and the quantity of iodine generated.

Volumetric Titrators

With volumetric Karl Fischer Titrators samples which contain between 0.1 … 500 mg of water can be measured. The Karl Fischer reagent should be selected according to the water content of the samples. Karl Fischer Reagents are available with different water equivalents (or titers). The water equivalent (WE) is the amount of water in mg which can be titrated with 1 mL of reagent:

WE = weight of water in mg / consumption of reagent in mL

Commercially available Karl Fischer reagents have water equivalents in the range of 2 … 6 mg H2O/mL

The exact water equivalent (or titer) of the Karl Fischer reagent changes over time. It must thus be determined periodically. Certified water standards are normally used for this purpose. It is possible to determine the water equivalent with pure water, but this requires a lot of experience to avoid weighing and handling errors, as the amount of water required for the determination is extremely small (only 10 … 15 mg of water for a Karl Fischer Reagent with a water equivalent of 2).

Hybrid Titrators

With Hybrid Karl Fischer Titrators samples with a water content in the range of 10 μg … 500 mg can be measured. The instrument automatically detects the moisture concentration in the titration cell and always applies the most suitable procedure to perform the titration:

• If the initial water concentration in the titration cell is above a certain level (“change level potential”), the titrator runs both methods to add iodine (volumetrically and coulometrically) in parallel. Once the water concentration is below the change level potential, the addition of Karl Fischer reagent is stopped and the titration is finished coulometrically.
• If the initial water concentration in the titration cell is below the change level potential, the whole titration is performed coulometrically.

As mentioned above, the exact water equivalent of the volumetric Karl Fischer reagent changes over time and must therefore be determined periodically. Hybrid Karl Fischer Titrators can perform such water equivalent determinations fully automatically without requiring any water standards:

• The titrator accurately doses certain amount of the Karl Fischer reagent.
• The instrument determines the amount of Iodine contained in the reagent by coulometric back titration (absolute method, see above).
• The procedure is automatically repeated several times. The factor determined is only accepted and stored, if the relative standard deviation of the results yielded is does not exceed a certain value (normally 1%).

Hybrid Karl Fischer Titrators thus offer the strengths of both, the coulometric and the volumetric methods:

• Titrations of samples with low or high water contents are performed with the shortest measuring time and the highest possible accuracy.
• Water equivalent determinations of the Karl Fischer reagent can be performed with the absolute coulometric method. As no water standards are required for such determinations, weighing errors can be safely ruled out.

Which sample size should be used?

The suitable sample size depends on the

• desired degree of accuracy
• the method used (coulometric, volumetric or hybrid)
• the titer (water equivalent) or the Karl Fischer Reagent (for volumetric and hybrid titrations)

The table below gives a rough indication for the sample size to be used in volumetric and coulometric titrations based on its estimated water content.

When working with a Hybrid Titrator, the recommended sample sizes are

• the same as for coulometric titrations for low water contents (0.001 … 0.1 %)
• the same as for volumetric titraitons for higher water contents (> 0.01%), as it is advisable to work with bigger sample sizes in order to avoid weighing errors.

‘not recommended’ in the table above does not mean that it is impossible to perform the corresponding titration. It just indicates that the analysis lasts long and/or that the accuracy of the results could be affected due to weighing errors, etc.

Volumetric titrations

To ensure maximum accuracy of the results the amount of titrant used should (ideally) be between 20% and 90% of the burette volume. All volumetric Karl Fischer Titrators from KEM are equipped with 10 mL burettes.

The samples should thus ideally contain between 4 and 18 mg of H2O if the titer of the reagent is 2 mg/mL or between 10 and 45 mg of water it the titer of the reagent is 5 mg/mL.

Coulometric titrations

The amount of water contained in the samples should be small for two reasons:

• Many samples can be titrated without needing to replace the Karl Fischer reagent.
• Short analysis times.

For these reasons, the sample should ideally contain between 100 and 1000 μg of H2O. To ensure reliable results, the samples should contain at least 50 μg of H2O as always small traces of water can enter into the titration cell.

Hybrid titrations

The sample size should be such that the samples contain between

• 50 μg and 18 mg of H2O if the titer of the reagent is 2 mg/L
• 50 μg and 45 mg of H2O if the titer of the reagent is 5 mg/L

When working with a Hybrid titration, there is thus much less risk to work with a sample size which is too big or too small. It is always advisable, however, to work whenever possible with a sample size > 0.1 g in order to avoid weighing errors.

Which type of Karl Fischer reagent should be used?

Today a wide range of different Karl Fischer Reagents are available from various manufacturers. Many of them do not contain any toxic methanol anymore. Their handling is less dangerous for users.

Reagents for volumetric titrations

Single component reagents

This type of reagent contains all reactants (iodine, sulfur dioxide, imidazole and diethyleneglycol monoethyl ether). Since methanol has been replaced by diethyleneglycol monoethyl ether, the titer of one component reagents became quite stable. The loss of titer is approximately 5% per year. This type of reagent is suitable for most volumetric Karl Fischer titrations, including the analysis of ketones.

Two component reagents

The reactants are in two separate solutions. The solvent is normally a solution of sulphur dioxide and imidazole in methanol. The titrant is a solution of iodine dissolved in a suitable solvent. Two component reagents offer several advantages compared to one component reagents:

• Faster titration speed.
• More accurate results for samples with low water contents.
• Exact titre with a high stability.
• Higher buffer capacity.

Two component reagents contain methanol and are thus not suitable to analyse samples which contain aldehydes and ketones with short chain lengths. Aldehydes and Ketones react with the methanol contained in the Karl Fischer reagents: They form acetals and water which leads to erroneously high results.

Special reagents for aldehydes and ketones

During the titration of aldehydes, a side reaction, the so called bisulfite addition can occur. This side reaction consumes water and leads to erroneously low results. For the determination of water in aldhydes and in certain reactive ketones it is thus required to use this type of reagent.

Reagents for coulometric titrations

There are two types of coulometric Karl Fischer reagents: Anolytes and Catholytes. When working with a coulometer equipped with a generator electrode with diaphragm (1) , the Anolyte (3) is filled into the titration vessel whereas the Catholyte (2) is filled into the cathode compartment of the generator electrode (see picture). For coulometers with a diaphragmless generator electrode, only one single reagent is required. It is important to make sure that only reagents intended for diaphragmless electrodes are used. For coulometric titrations there are as well special reagents available for the analysis of aldehydes and ketones. Such reagents are only available for generator electrodes with diaphragms.

Reagents for hybrid titrations

For hybrid titrations, three reagents are required:

• Anolyte (solvent in the titration cell)
• Catholyte (cathode compartement of the generator electrode)
• Volumetric titration reagent

It is important to use a combination of this three reagents, which is suitable for the type of samples to be measured.

The tables below contain the recommend combinations of reagents from three different suppliers. All these reagents do not contain any pyridine.

Kyoto Electronics Manufacturing Co., Ltd.

Fluka (Riedel-de Haën)

Mitsubishi Chemical

Coulometric cell with or without diaphragm?

During coulometric Karl Fischer titrations, iodine is formed at the anode. The diaphragm of the generator electrode prevents the iodine from being immediately reduced at the cathode. Generator electrodes without diaphragm have a specially modified cathode which prevents the problem mentioned above.

Generator electrodes without diaphragm

• are easy to clean and easy to handle
• require only one single Karl Fischer Reagent
• require less time to yield a stable drift as no water can adhere to the diaphragm.

For most samples generator electrodes without diaphragm offer thus more convenience.

For certain types of samples, however, generator electrodes with diaphragm must be used:

• Samples which contain aldehydes or ketones. Such samples require the use of special reagents (see section above) in order to prevent side reactions which cause erroneous results. This type of reagents is only available for generator electrodes with diaphragms.
• Samples with a low conductivity such as hydrocarbons, insulating oils, turbine oils etc. where nonpolar solvents like chloroform trichlorthylene must be used.
• Samples which have a very low water content (water content < 50 μg / sample).

Hybrid Karl Fischer titrators are always equipped with a coulometric cell with diaphragm. This is required because of the automatic factor determination of the Karl Fischer reagent by coulometric backtitration (see above). In order to facilitate the cleaning, the coulometric cell of the MKH-700 Hybrid Karl Fischer Titrator is equipped with a membrane, which can easily be replaced.