FOS/TAC Titration

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FOS/TAC Titration

The FOS/TAC ratio is a commonly used indicator for assessing the fermentation process within biogas reactors, where FOS is a measure of the volatile organic acids (measured in mg CH3COOH/l) and is TAC is a measure of the total inorganic carbon i.e. the alkaline buffer capacity of the system (measured in mg CaCO3/l). In simple terms, this ratio looks at acid vs alkaline in the system which helps to assess the risk of acidification in the biogas reactor. Monitoring this ratio regularly allows responsiveness to the system and keeps the reactor running efficiently.



Using the Norman method, the TAC value is determined by using 20mL of a centrifuged sample and titrating to pH 5.0 with sulfuric acid (0.1N / 0.05mol/L) . The FOS value is determined using the same sample and titrating to pH 4.4. Both of the amounts of acid titrated allow calculation of the ratio value using the following equation:


The formula is specifically designed for the above mentioned conditions and reagents and must be altered if things were to change for example concentration of sulfuric acid used. Many reactor operators require a quick and easy method for FOS/TAC determination and this can be achieved by the use of an automatic titrator. This unit adds the titrant into the sample and by use of a pH electrode will measure the pH to the end-point.  We have a number of options available to perform this and have selected to showcase the models with FOS/TAC programmes, which removes the need for the above calculation to be done manually.

Titroline 5000 by SI Analytics 


  • Pre-loaded method for FOS/TAC determination.
  • This titrator has a large, full colour display allowing for easily visible results even at distance.
  • Comes complete with pH electrode + buffer set (other models are available in the titroline 5000 range without the pH electrode + buffer set included)
  • Magnetic stirrer attached.
  • 50ml dosing syringe (20ml models are also available)
  • GLP compliant results with titration curve can be documented on a connected printer or USB-memory stick.
  • To get started, you will also need to purchase 0.1N/0.05M sulfuric acid.
  • Available here

AT1000 Series by Hach


  • This system works a little differently to the Titroline 5000. You must purchase the both the automatic titrator and an application package. In this case, a FOS/TAC application package is available and can be used in combination with the AT1102 titrator.
  • Large, clear display
  • Integrated magnetic stirrer
  • Contains single 2.5/5/10/25 mL dosing burette
  • Results can have full traceability. It will produce a trend curve of key parameters and data can be exported by USB in an excel format. PC software is also available.
  • To get started, you will also need to purchase a set of pH buffers to perform regular calibrations of your electrode along with the 0.1N/0.05M sulfuric acid titrant.
  • Available here.

If you would like to know more about the applications of titration you can check out our other blog post on the use of titration to determine Sulphur Dioxide in wine. For more information on our titrators or on FOS/TAC testing, please get in touch using the contact form below. Alternatively you can call us on 01954 233120.

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