The main difference between the two techniques is how the water content is determined: volumetric KF titration measures the volume of titrant used, while coulometric KF titration measures the amount of electricity used.

In volumetric KF titration, a known volume of titrant solution is added to the sample until the water content is completely reacted. The amount of titrant used is then determined, and the water content is calculated from the reaction stoichiometry. Volumetric KF titration is suitable for water content determination in samples with relatively high water content (e.g., >0.1%).

In coulometric KF titration, an electric current is passed through the sample, and the water is electrolytically generated. The amount of electricity required to generate all the water in the sample is measured, and the water content is calculated using Faraday's law. Coulometric KF titration is suitable for water content determination in samples with low water content (e.g., <0.1%).

Accurate KF coulometric titration has two primary prerequisites. First, the system used for the titration must be closed to limit the incursion of humidity into the vessel (which must be mitigated and quantified to achieve accuracy). Second, the water from the sample must be completely releasable.

Coulometric water determination is based on the standard reaction equation for the KF reaction noted above. However, iodine is generated electrochemically by anodic oxidation in the coulometric cell according to the following Karl Fischer titration formula half-reaction:

2 I^- -> I₂ + 2 e

Current is applied to the solution until the species is either oxidized or reduced to a new state as indicated by a dramatic shift in potential. This shift indicates the coulometric titration endpoint. When solution volume is known, current magnitude in amperes and current duration in seconds can be used to determine the molarity of an unknown species.

Benefits include easier control of the titrant (current) when compared to adding titrant volumetrically using a motorized burette drive. The preparation of standard solutions is also unnecessary.

The more widely used volumetric water determination technique employs a standard solution (a solution of a known concentration) which is then titrated against portions of an unknown concentration until the reaction is complete. In Karl Fischer titration, these reactions either use a one-component KF reagent or a two-component KF reagent.

In a modern, one-component (composite) KF reagent, the titrant contains all chemicals required for the Karl Fischer determination. Generally, this is iodine, sulfur dioxide, and the base (usually imidazole) dissolved in a suitable alcohol. The solvent is typically methanol or a methanolic solvent mixture adapted to the sample. One-component reagents tend to be easier to handle and less costly, so they tend to be used more often.

In a two-component volumetric KF determination, the titrant contains only iodine and methanol. The other reaction components are used as the working medium in the titration cell. Two-component reagents have better long-term stability and offer faster titration times; however, they are usually more expensive and have a lower solvent capacity.

Aldehydes (R-CHO) and ketones (R-CO-R) form acetals and ketals if titrated with standard methanol-containing reagents. As a result, additional water is produced and titrated simultaneously, leading to higher water contents and a vanishing end-point.

Special methanol-free one-component KF reagents are commercially available to prevent this problem. These reagents contain iodine, imidazole, sulfur dioxide, and 2-methoxyethanol. A two-reagent solvent contains 2-chloroethanol and trichloromethane.

Titration takes slightly longer using these specialized reagents. It may be necessary to adapt an endpoint value to the reagent used. These reagents are also suitable for substances that react with methanol, such as amines.

Some Karl Fischer reagents replace methanol with ethanol. These reagents also allow titration of several ketones, which form ketals considerably more slowly in ethanol than in methanol. The titrant contains iodine and ethanol, and the solvent contains sulfur dioxide, imidazole, diethanolamine, and ethanol.

Automated Karl Fischer titration has the added benefit of reducing operator exposure to KF reagents, which enhances safety.

For practical tips on volumetric Karl Fischer titration, the guide can be downloaded

In a modern, one-component (composite) KF reagent, the titrant contains all chemicals required for the Karl Fischer determination. Generally, this is iodine, sulfur dioxide, and the base (usually imidazole) dissolved in a suitable alcohol. The solvent is typically methanol or a methanolic solvent mixture adapted to the sample. One-component reagents are easier to handle and less costly, so they tend to be used more often.

In a two-component volumetric KF determination, the titrant contains only iodine and methanol. The other reaction components are used as the working medium in the titration cell. Two-component reagents have better long-term stability and offer faster titration times; however, they are usually more expensive.

Special methanol-free one-component KF reagents are commercially available to prevent this problem. These reagents contain iodine, imidazole, sulfur dioxide, and 2-methoxyethanol. A two-reagent solvent contains 2-chloroethanol and trichloromethane.

Titration takes slightly longer using these specialized reagents. Adapting an end-point value to the reagent used may be necessary. These reagents are also suitable for substances that react with methanol, such as amines.

Some Karl Fischer reagents replace methanol with ethanol. These reagents also allow titration of several ketones, which form ketals considerably more slowly in ethanol than in methanol. The titrant contains iodine and ethanol, and the solvent contains sulfur dioxide, imidazole, diethanolamine, and ethanol.

Titrant concentration in the Karl Fischer method is typically expressed as milligram water per milliliter titrant. One-component reagents are typically available in three different concentrations:

• 5 mg/mL for samples with a water content of 1000 ppm to 100%
• 2 mg/mL for samples with a water content of less than 1000 ppm
• 1 mg/mL for samples with a water content of less than 200 ppm

One-component reagents can usually be stored for about two years. The drop in titer, i.e., the decrease in concentration, is approximately 0.5 mg/mL per year in a sealed bottle.

For two-component reagents, titrant contains both iodine and methanol. The solvent contains sulfur dioxide, imidazole, and methanol. A titration speed two or three times as high can be achieved with the two-component reagent.

Both components are stable in storage, provided the bottle is tightly sealed. They are available in two different concentrations:

• 5 mg/mL for samples with a water content of 1000 ppm to 100%
• 2 mg/mL for samples with a water content of less than 1000 ppm

Since the KF titrants used for volumetric determinations are not stable over longer time periods, manufacturers usually supply a concentration that is 5-10% higher than the nominal concentration on the bottle. This means that, to guarantee the concentration of a titrant upon opening or after storage, a concentration determination should be performed.

A concentration or titrant determination is performed using a primary standard with a defined water content. Different standards are available for concentration determination:

• Di-sodium-tartrate dihydrate
• Liquid water standard (certified)
• Water tablet (defined concentration per tablet)
• Pure water

Liquid standards are the easiest to use. Solids require sufficient pre-mixing to dissolve the standard completely.

It should be noted that Di-sodium-tartrate has a limited solubility in methanol. Therefore, the solvent (i.e. methanol) must be exchanged after no more than three determinations.

To obtain accurate results of water content, the sample must release its water completely. Only freely available water reacts with a Karl Fischer reagent.

Additional solvents can be used as co-solvents to achieve complete sample dissolution. In such cases, a large part of the solvent mixture, at least 30%, must always be an alcohol (preferably methanol) to ensure that the Karl Fischer reaction is strictly stoichiometric.

Many liquid samples dissolve easily in the classical KF solvent methanol, and thus the titration can be performed after direct injection of the liquid sample in the vessel. Many samples cannot be easily dissolved or are very difficult to be completely dissolved due to their non-polar nature. Mixtures of organic solvents are used to improve the dissolution process. This is achieved either by adding auxiliary solvents to Methanol, or by using dedicated KF solvent mixtures. On the other hand, viscous oils, pastes, or solid samples require additional preparation steps to release water from the sample matrix.

For more information on how to prepare samples for Karl Fischer titration, download this guide.

Gas phase extraction is used for samples that are unable to be directly added to a titration vessel. This method is suitable for solids and liquids that cause side reactions with the Karl Fischer reagent or that release water very slowly.

In gas phase extraction, samples are placed into a sample boat or vial and moved to a Karl Fischer oven. When heated to the sample-specific temperature, the water vaporizes and is purged to the titration cell on a current of dry air or inert gas (usually nitrogen) where the water determination is made.

An InMotion Autosampler KF Oven allows for up to 26 samples to be analyzed using gas-phase extraction at any one time. Sample preparation is simplified by the innovative one-piece cap, which does not require extra tools or adhesive membranes. Gas flow is controlled electronically, allowing an operator to monitor the amount of dried gas entering the titration cell. Operators simply press one button to start the KF water determination.

Download the guide for automation in Karl Fischer Titration