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Water in petroleum products, such as lubricating oils, jet fuel, or other similar products can have deleterious effects. Moisture is often associated with corrosion and engine wear. Knowing the water content of petroleum products can prevent damage to costly infrastructure and ensure safer operations.

ASTM D6304 «Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration» is a standard that is often cited for moisture determination in the specifications of various petroleum products. It was updated in January 2021 and now offers three procedures for accurate moisture determination.

The direct sample injection into the titration cell (Procedure A) is recommended for low viscosity samples without expected interferences. An oven (Procedure B) or water evaporator accessory (Procedure C) can be used to analyze samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

In this blog post I want to introduce these three procedures, and then discuss when it is appropriate to use each of them. Click to go directly to each topic:

A coulometric Karl Fischer Titrator such as the 851 Titrando from Metrohm is the basis for all three procedures of ASTM D6304.
The generator electrode with diaphragm is recommended for water determination according to ASTM D6304 Procedure A.
The generator electrode with diaphragm is recommended for water determination according to ASTM D6304 Procedure A.

Direct injection (Procedure A)

The direct sample injection into the titration cell is recommended for low viscosity samples without expected interferences. An aliquot of known mass or volume is injected into the conditioned titration cell of a coulometric Karl Fischer apparatus, where it is titrated automatically, and the results calculated.

Method D6304 permits the use of coulometric generator electrodes with and without diaphragm. We recommend the use of the generator electrode with diaphragm, due to the low water content of the samples.

Not all petroleum products are soluble in Karl Fischer reagent and phase separation can occur when using Procedure A. If phase separation occurs, the reagents need to be replaced. The number of samples which can be analyzed without phase separation depends on the volume and type of sample, the volume of reagent, and the sample solubility in the reagent.

However, for these kinds of samples, Procedures B or C are often the better solution. The same is the case if your sample contains interfering substances.

For more information about ASTM D6304 Procedure A, download our free Application Bulletin (AB-209). For more tips and tricks about how to improve your Karl Fischer titration, have a look at our blog series: «Frequently asked questions in Karl Fischer titration».

AB-209: Water in insulating oils, hydrocarbons, and their products

«Analyze This»: Frequently asked questions in Karl Fischer titration

Schematic drawing of the Karl Fischer oven method.
Schematic drawing of the Karl Fischer oven method.

Water extraction using an oven (Procedure B)

An oven (Procedure B) can be used to analyze samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

For the analysis, a representative sample is weighed into a glass vial, which is sealed immediately. The vial is then heated in an oven to extract any water. The vaporized water is carried into the conditioned Karl Fischer titration cell by means of a dry carrier gas where it is titrated.

The ideal temperature used for the evaporation depends on the sample. The 874 Oven Sample Processor can perform a temperature gradient test to determine the optimal temperature for removing water without degrading the sample.

Water content determination in petrochemical products as per ASTM D6304

To learn more about the oven method, its working principle and its advantages, check out our related blog article below.

«Analyze This»: Oven method for sample preparation in KF titration

For more information about using the KF oven method for ASTM D6304 Procedure B, download our free Application Bulletin (AB-209) or free Application Note (AN-K-070).

AB-209: Water in insulating oils, hydrocarbons, and their products

AN-K-070: Fully automated water analysis in petroleum products according to ASTM D6304

Just want the highlights? Have a look at our short flyer!

ASTM D6304 oven method makes coulometric KFT easier than ever

Schematic drawing of the evaporator method.
Schematic drawing of the evaporator method.

Water extraction using an evaporator (Procedure C)

Instead of using an oven, Procedure C explains how a water evaporator can be used for the water extraction of samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

In this procedure, an aliquot of sample is transferred into a heated chamber containing a suitable solvent (most often, toluene). The temperature of the heated chamber depends on the solvent used. The water vaporizes along with the solvent in an azeotrope distillation. The azeotrope is then transferred into the conditioned Karl Fischer titration cell via a dry non-reactive carrier gas.

If you wish to read more about the three procedures and their advantages and disadvantages, download our free White Paper below.

Moisture in petroleum products according to ASTM D6304

The 874 Karl Fischer Oven Processor with an 851 Titrando for a fully automated analysis according to ASTM D6304 Procedure B.
The 874 Karl Fischer Oven Processor with an 851 Titrando for a fully automated analysis according to ASTM D6304 Procedure B.

When to use which procedure

Procedure A is mainly suited for liquid samples with a low viscosity, such as diesel fuel, jet fuel, or aromatics. A low viscosity is required in order to be able to add the sample easily into the Karl Fischer titration cell. Furthermore, the samples require a good solubility in Karl Fischer reagent. Otherwise phase separation will occur, which requires the replacement of the Karl Fischer reagents. While the reagent exchange can be automated, time is still required until the reagents reach dryness again.

Even if samples are soluble in Karl Fischer reagents, there might still be issues with using Procedure A due to the sample matrix creating side reactions and thus false results. In this case Procedure B or C is the better option.

Procedure B is suitable for all kinds of samples, regardless of their viscosity or matrix composition. It is only the evaporated water that is transferred into the titration cell, leaving the sample as well as interfering matrix components remaining in the sealed vial, which can be simply disposed of after the analysis. For this reason, the reagent exchange frequency is greatly reduced, saving costs as less reagent is required. Depending on the workload in your lab, it is even possible to fully automate the analysis including reagent exchange using an automated Karl Fischer oven.

Procedure C, like Procedure B, is suitable for all kinds of samples, regardless of their viscosity or matrix constitution. It is only the evaporated water in an azeotrope with the solvent that is transferred into the titration cell. The sample, as well as interfering matrix components, remain in the evaporation chamber. However, it is necessary to manually empty and refill the evaporation chamber from time to time, which is time consuming, as the chamber needs to cool down before the content can be exchanged. Furthermore, walk-away automation is not possible with this method.

For a more detailed comparison of the various factors for each procedure, download our free White Paper.

White Paper: Moisture in petroleum products according to ASTM D6304

Author
Meier

Lucia Meier

Technical Editor
Metrohm International Headquarters, Herisau, Switzerland

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