Karl Fischer Coulometric Titration Explained and Illustrated

Water contamination is a cause for major concern in a large number of applications. In some industries and environments, water is a far more damaging contaminant than solid particles and is often overlooked as a primary cause of component failure. For certain applications, even a small amount of water may have damaging effects on production or equipment.

Water can exist in three states or phases. Dissolved water is characterized by individual water molecules dispersed throughout the oil. Similar to humidity in the air, dissolved water cannot be seen in the oil. If too much water is present in the oil, the dispersed water molecules begin to saturate; this is similar to the formation of fog. When this occurs, the water is considered emulsified. Free water is formed when the addition of water leads to a phase separation of both liquid components producing a layer of water.

Several methods are available to determine the water contamination level in fluids. A Karl Fischer (KF) coulometric titrator is one of the most accurate methods. Unlike other techniques, it can trace low levels of free, emulsified and dissolved (which cannot be detected with other methods such as a crackle test). When used correctly, the test is capable of measuring water levels as low as 1 ppm or 0.0001 percent.

Coulometric vs. Volumetric

Titration is a chemical analysis that determines the content of a substance, such as water, by adding a reagent of known concentration in carefully measured amounts until a chemical reaction is complete. There are two types of Karl Fischer titrators: volumetric and coulometric titrators. The main difference between the two is that with the volumetric method, the titrant is added directly to the sample by a burette. Conversely, with the coulometric method, the titrant is generated electrochemically in the titration cell. The coulometric method measures water levels much lower than the volumetric method.

Coulometric Titration Chemistry Fundamentals

The following reaction scheme has been proposed for the Karl Fischer titration:

ROH represents an alcohol like methanol or ethanol.

In the coulometric method, the titration cell consists of two parts: an anodic and a cathodic compartment. Figure 1 shows both compartments separated by a ceramic diaphragm. The anodic compartment contains the anolyte solution which includes sulfur dioxide (SO₂), iodide (I^–) and imidazole needed in the chemical reaction. Methanol or ethanol (ROH) is usually used as a solvent.

Figure 1. Titration Cell (Courtesy of Mettler-Toledo)

In coulometric Karl Ficher titration, iodine (I₂) is generated electrochemically from iodide (I^–). When iodine (I₂) comes in contact with the water in the sample, water is titrated according to the above mentioned reaction scheme (equations No. 1 and No. 2). Once all of the water available has reacted, the reaction is complete. The amount of water in the sample is calculated by measuring the current needed for the electrochemical generation of iodine (I₂) from iodide (I^–) according to the following reaction (equation No. 3):

Coulometric Titrator Features

There are several features to consider when selecting a Karl Fischer coulometric titrator. Table 1 lists a number of manufacturers that offer Karl Fischer coulometric titrators. Each manufacturer offers different features.

Sensitivity to Humidity
Humidity is probably the largest source of error during the titration. Special precautions should be taken during setup and testing, especially in coastal or tropical regions. The air conditioning system should be equipped with a moisture condenser. Also, a Karl Fischer titrator should not be installed near an air conditioner vent.

The titration cells are enclosed to help ensure that water does not enter from the atmosphere; however, a very small amount of water almost always makes it into the titration cell. The amount of water that enters over a period of time is known as the drift. Many manufacturers will give specifications on drift values and maximum allowable air humidity.

Sensitivity to pH
The chemical reaction is sensitive to the solution’s acidity or alkalinity. The optimum pH range of the sample solution for efficient Karl Fischer titration is between pH 5.5 and 8. When the pH is greater than 8.5, the reaction rate increases due to chemical side reactions. This results in a more sluggish endpoint and higher iodine consumption, which will affect results. Buffering agents are available for acidic or basic samples to keep an ideal pH between 5.5 and 8.

Diaphragm vs. Diaphragmless Titration
The diaphragm separates the anodic and cathodic compartments. Its purpose is to prevent the electrochemically generated iodine from reversing back to iodide at the cathode instead of reacting with the water. The diaphragmless titrator uses a different geometric construction to prevent the generated iodine from reversing back to iodide (Figure 2). As hydrogen gas is generated in the cathodic compartment, it creates a layer of gas bubbles on the surface of the cathode. This layer of gas prevents the iodine from being reduced at the cathode. However, it is still possible for small amounts of iodine to be reversed to iodide when reaching the cathode.

Figure 2. Diaphrapm and Diaphragmless Configurations
(Courtesy of Mettler-Toledo)

Diaphragmless titration is advantageous because there is no diaphragm to become contaminated; it is easier to clean; and a lower drift can be used (this relates to how quickly the reaction completes).

A diaphragmless cell is accurate enough for many applications. However, check with your Karl Fischer Titrator supplier to verify whether your applications require a diaphragm.

Codistillation Method
Some samples may release water slowly or have side reactions with the reagents. In this case, the ASTM D6304 with codistillation is a more reliable technique and will mitigate the chances of this occurring. With this method, the oil sample is heated under a vacuum so that any water in the sample will evaporate. Water vapors are condensed and dissolved in toluene.

This is then titrated using the ASTM D6304 procedure. Because additives and other interfering contaminants that may be present in a used oil sample remain dissolved or suspended in the oil, the condensed water in the toluene is free from interference effects and is a true count of water in the sample. Many companies offer optional drying ovens for this purpose.

Programmable or Built-in Methods
The Karl Fischer coulometric titrator method can be adjusted depending on the application and accuracy needed. Adjustment requires some understanding of the titrator’s working and controlling principles. To help ensure accurate adjustment, many titrators have methods built-in that can be used for more common applications. This more user-friendly method makes it possible for a person who doesn’t have a lot of experience to use the instrument.

For special applications, a particular method may not be built-in. In this case, many titrators allow the user to program a method. Once all of the settings are in place, that specific method can be selected each time the application is needed, and the titrator will run on its own.

The number of built-in and programmable methods vary among instruments. It is useful to determine approximately how many applications are available, and to become familiar with each application’s specialties.

Built-in Pumps
It is important that the reagents do not become contaminated. Some coulometric titrators come with a built-in pump that can fill and drain reagents. This helps eliminate reagent contamination and reduces the number of steps required for the procedure.

Titration Speeds
Titration speeds vary for each unit. While speed may be important, the faster the unit titrates, the less accurate it will be. For samples with low levels of water (less than 50 µg), the titration should take place slowly. It is also best to titrate the sample slowly if the water level accuracy is critical. If high levels of water (greater than 1,000 µg per sample) are being measured, the titration can be quicker.

Memory Storage Capacity
Most units come with a connection for a computer or printer and some have built-in printers. Each unit has a different memory storing capacity. If it will not be attached to a personal computer, then its memory storage capacity may be an important consideration.

References

1. ASTM D6304: Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration.
2. (2003, July-August). Water - Oil Analysis 101. Practicing Oil Analysis.
3. (2001, July-August). Water - The Forgotten Contaminant. Practicing Oil Analysis.
4. Mettler-Toledo Applications Brochure 32, Fundamentals of the Coulometric Karl Fischer Titration with Selected Applications. Mettler-Toledo Inc. Retrieved August 1, 2003 from www.mt.com.

Sidebar 1

Karl Fischer Method Made Easy Through Auto Sampling Titration

Automated titration is an accessory offered by some Karl Fischer manufacturers that allows the user to perform multiple titrations without stopping to prepare a sample. It is combined with either a coulometric or volumetric Karl Fischer titrator to determine the amount of water in the sample efficiently and unmonitored, allowing the analyst to focus on other important jobs while it runs.

How Does it Work?

The samples are placed in the rotating tray and sealed. Once the instrument starts, the first sample is moved into a temperature-controlled oven. Then, the sample is pierced so that as the water is converted into a vapor, it is transferred into either a coulometric or volumetric titrator which will determine the water content of the sample. By controlling the temperature, only the water evaporates while the remaining sample stays in the sample bottle. Once all of the water is removed and measured, the sample bottle is put back into its original position on the tray, and the tray rotates to the next sample to be performed.

Advantages

• Multiple samples can be analyzed at once without stopping to prepare each sample.
• There is no contamination in the oven or the titration cell because the remaining liquid or solid sample remains in the sample bottle.
• There’s no need to worry about any side reactions that may occur between the sample and the titrating reagents.
• Much less reagent is used in the process.

The Mettler-Toledo DL39
Coulometer with Stromboli and Optional Air Pump

The Metrohm 774 KF Oven
Sample Processor with 756
Karl Fischer Coulometer