Measuring Water Levels in Biodiesel

With continued increase in energy demands and lessening natural resources, society must find new solutions to these issues. Biodiesel is a fuel that is derived from animal fats or vegetable oils, and is one of several renewable energy solutions that are currently available on the market. Since 2001, the use of biodiesel has increased significantly.

YSI Karl Fisher Titrators: TL 7500 KF and TL 7500 K Trace.

YSI Karl Fisher Titrators: TL 7500 KF and TL 7500 K Trace. Source: YSI

Regular petroleum diesel and biodiesel have similar chemical characteristics, so it is possible to directly supplement one for the other without forgoing fuel economy. As well as being a renewable resource, biodiesel also has the benefit of creating less pollution than petroleum diesel fuel.

The main materials used to produce biodiesel are soybeans and canola and corn oil, in addition to other plant-based oils and animal fats. The primary source for biofuels is carbon dioxide (CO2) absorbing plants, so the government of the United States has deemed biodiesel as carbon neutral.

Biodiesel is produced through a reaction that is created between either vegetable oil or animal fat and alcohol to form esters and glycerin. The end product is controlled by ASTM International, a regulatory body that gives specification standards on a variety of products, inclusive of biodiesel, to guarantee consistency in the quality of the product.

Indicated in ASTM D6751, water concentration is under strict regulation and is limited to 500 mg/L (ppm) and under. If this regulated amount of concentration is exceeded, the chances of an increase in corrosion within in the engine’s fuel components will increase.

In addition, surplus water can propagate microbial growth leading to stimulation of the development of biofilm in fuel lines. Precision in measurements and documentation is imperative for guaranteeing the quality of the biodiesel produced due to the limit requirements.

Analysis Methods

To measure the concentration of water in biodiesel samples, Karl Fischer titrations are the preferred methods. It will depend on the water concentration in the sample as to which method to use: Volumetric Karl Fischer or Coulometric Karl Fischer.

  • Volumetric Karl Fischer analysis: this gives measurements of water concentrations from 200 µg to 50 mg, and is usually recommended for samples that have water concentrations over 5 mg.
  • Coulometric Karl Fischer analysis: this gives measurements of water concentrations from 10 µg to 5 mg, and is typically recommended for samples that have a water concentration of 400 µg or less.

Both of these methods provide different benefits, heavily weighing on the sample being measured. The Volumetric Karl Fischer method of measuring biodiesel only needs a single component reagent, so the solvent is more flexible to adapt to the solubility of the sample being measured. Advantages of Coulometry are its easy use and the determination of very small amounts of water. However, Volumetry is significantly more customizable for the sample you are measuring.

Comparatively, the Coulometric Karl Fischer titration is simpler. With volumetric titration, background compensation for drift is completed before analysis of each sample. Coulometric Karl Fischer titration has automatic background compensation for drift, providing ease of use. Both compensation components are a function of keeping the titration cell dry.

Table. Source: Figure 99 taken from The Titration Handbook.

Property Coulemetry Volumetry
Water content and sample amount
  • Small water contents
  • Small sample amounts
  • Medium and large water contents
  • Adapted sample amounts
Sample type
  • Liquid
  • Gaseous (e.g. oven)
  • Fixed samples with oven
  • Solid
  • Liquid
Sample addition
  • Direct with the syringe
  • Gas introduction with oven
  • External extraction
  • Heat solid samples in the oven
  • Solid direct
  • Sample crushing with homogenizer
  • Work with increased temperature
  • With syringe directly
  • Very fast
  • Very simple
  • Fast
  • Simple
Working range
  • µg range
  • 10 µg 5 mg water
  • Mg range
  • 200 µg to 50 mg water
  • Very good for small water amounts >400 µg water (+/- 0.5%)
  • Very good for water amounts >5 mg water (+/- 0.5% current titer determination necessary)
  • >400 µg water, typical RSA approximately 1%
  • >5 mg water, typical RSA approximately 1%


External water is the most common error source in KF titration, and so keeping the titration cell dry is imperative in both methods. External water is able to reach the titration cell through numerous means.

First, the solvent or the presentation component needs to be dry-titrated, a process known as conditioning. If external water continues to get into the titration cell, the water is then assigned to the sample as external water.

External water is able to reach the titration cell in many ways:

  1. If the titration cell is not tight
  2. If foreign particles are able to hang between the ground joints
  3. If there are defective O-rings on the screw connections of the titration cell
  4. When the titration cell is opened to add the sample, humid air can enter
  5. If there is leakage or wearing on the septum for the addition of liquid samples
  6. If the molecular sieve in the drying tube for pressure equalization is used up, it is important that it is dried
  7. If humid air is present in the pump system, the air for the addition of the solvent must also be dried using the molecular sieve

YSI offers users both Karl Fischer methods, in addition to TitroLine 7500 KF series titrators. Both titration instruments are compatible with TitriSoft software, enhancing the efficiency in reporting while following standard Good Laboratory Practice (GLP) documentation requirements. The TitriSoft software allows full automation in titration in a laboratory. It can be automated to the user’s specific method requirements, from the physical act to result calculations.

This information has been sourced, reviewed and adapted from materials provided by YSI.

For more information on this source, please visit YSI.


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