How to Determine Water Levels in Biodiesel

What is Biodiesel?

With ever-increasing energy demands and diminishing natural resources, society needs to find new solutions to fill those demands. Biodiesel, a fuel derived from vegetable oils or animal fats, is one of many renewable energy solutions that are now available on the market. The use of biodiesel has increased heavily since 2001.

Due to the similarity in chemical characteristics between regular petroleum diesel and biodiesel, you can directly supplement one for the other without sacrificing fuel economy. While being a renewable resource, biodiesel also provides the advantage of producing less pollution than petroleum diesel fuel. The primary materials used to produce biodiesel are soybeans, canola, and corn oil, along with other plant-based oils and animal fats.

With the primary source for biofuels being carbon dioxide (CO2) absorbing plants, the US government has deemed biodiesel as carbon neutral (Biofuels Explained Ethanol and Biodiesel, 2018).

Biodiesel is manufactured by facilitating a reaction between either vegetable oil or animal fat with alcohol to form esters and glycerin. The final product is regulated by ASTM International, a regulatory body that provides specification standards on a range of products, including biodiesel, to ensure consistency in the quality of the product. Denoted in ASTM D6751, water concentration is under stringent regulation and is limited to below 500 mg/L (ppm) (ASTM D6751, 2019). If the concentration exceeds the regulated amount, this will increase the chances of corrosion occurring in the engine’s fuel components. Also, excess water can propagate microbial growth, which stimulates the development of biofilm in fuel lines. Due to the limit requirements, precision in measurements and documentation is critical for ensuring the quality of the biodiesel produced.

Methods of Analysis

Karl Fischer titrations are the preferred methods used to measure the water concentration in biodiesel samples.

The decision between Volumetric and Coulometric Karl Fischer methods will depend on the water concentration in your sample.

  • Volumetric Karl Fischer analysis can measure water concentrations from 200 µg to 50 mg, typically recommended for samples that have water concentrations higher than 5 mg.
  • Coulometric Karl Fischer can measure water concentrations from 10 µg to 5 mg, usually recommended for samples that have a water concentration of less than 400 µg.

Both methods provide different advantages, heavily weighing on the sample you are measuring. Since the Volumetric Karl Fischer method of measuring biodiesel only requires the single component reagent, the solvent is more adaptable to suit the solubility of the sample being measured. Coulometry has its advantages in the ease of operation and the determination of very small amounts of water, while volumetry is much more customizable for the sample you are measuring.

In direct comparison, the coulometric Karl Fischer titration is simpler; it has automatic background compensation for drift. With volumetric titration, background compensation for drift is performed prior to analysis of each sample. Both compensation components are a function of keeping the titration cell dry.

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
Operation
  • Very fast
  • Very simple
  • Fast
  • Simple
Working range
  • µg range
  • 10 µg 5 mg water
  • Mg range
  • 200 µg to 50 mg water
Correctness
  • 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)
Reproducibility
  • >400 µg water, typical RSA approximately 1%
  • >5 mg water, typical RSA approximately 1%


Figure 99 from The Titration Handbook

Keeping the titration cell dry is vital for both methods due to external water being the most common error source in KF titration. External water can reach the titration cell through various means. First, the solvent or the presentation component must be dry-titrated. This procedure is known as conditioning. If external water still gets into the titration cell, the water is then assigned to the sample as external water.

External water can reach the titration cell in the following manners:

  1. The titration cell is not tight
  2. Foreign particles can hang between the ground joints
  3. Defective O-rings on the screw connections of the titration cell
  4. While the titration cell is opened to add the sample, humid air enters
  5. The septum for the addition of liquid samples is leaking and worn
  6. The molecular sieve in the drying tube for pressure equalization is used up and must be dried
  7. Humid air is present in the pump system; the air for adding the solvent must also be dried with the molecular sieve

Download the comprehensive guide to the fundamentals of titration and common applications, including Karl Fischer or learn more about Top Tips for More Accurate Titrations.

Titrations | Titrators | Karl Fisher Titration | Autotitrators

(Download Water Determination in Biodiesel by Karl Fischer Titration application note). 

YSI offers both Karl Fischer methods with our TitroLine 7500 KF series titrators. Both titration instruments are compatible with our TitriSoft software, increasing the efficiency in reporting while following standard Good Laboratory Practice (GLP) documentation requirements. This software provides a laboratory with full automation in titration. From the physical act to result calculations, this software can be automated to your specific method requirements.

 Karl Fischer Titrators | YSI

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


References:

Biofuels Explained Ethanol and Biodiesel. (2018). U.S. Energy Information Administration (EIA). Retrieved from https://www.eia.gov/energyexplained/biofuels/biodiesel.php

ASTM D6751, Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels. (2019). ASTM International. Retrieved from www.astm.org


Additional Blog Posts of Interest

Top Tips for More Accurate Titrations

Anatomy of pH Electrodes

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