The measurement of organic acids, by adsorption and elution from a chromatographic column, by gas chromatography, or by distillation, can be used as a control test for anaerobic digestion. The chromatographic separation method is presented for organic acids (5560 B), while a method using distillation (5560 C) and a gas chromatographic method (5560 D) are presented for volatile acids.
Volatile fatty acids are classified as water-soluble fatty acids that can be distilled at atmospheric pressure. These volatile acids can be removed from aqueous solutions by distillation, despite their high boiling points, because of co-distillation with water. This group includes water-soluble fatty acids with up to 6 carbon atoms.
The distillation method is empirical and gives incomplete and somewhat variable recovery. Factors such as heating rate and proportion of sample recovered as distillate affect the result, requiring the determination of a recovery factor for each apparatus and set of operating conditions. However, it is suitable for routine control purposes. Removing sludge solids from the sample reduces the possibility of hydrolysis of complex materials to volatile acids.
The gas chromatographic method determines individual concentrations of many of the fatty acids, giving additional information about the sample.