Determination of low citric acid concentrations in a mixture of weak acid/bases

A titration approach was developed to measure low concentrations of citric acid (C6H8O7) in a mixture of other weak acid/bases. Two methods were tested. The first and more practical method (a 4-point titration procedure) is applicable in conditions where volatile fatty acids (VFAs) are not normally present. The second method (a 5-point titration procedure) was developed for anaerobic environments where VFAs may be encountered. Generally, fairly accurate and repetitive results (precision >95%) were obtained for both situations although stability and accuracy were better in the absence of VFA. Both methods can be used for routine monitoring of biological reactors where citric acid is added as a carbon source and electron donor. Mg2+ and Ca2+ form complexes with citric acid and thus inhibit the use of the method. To overcome this, a sodium-saturated cationic ion exchanger was used to exchange these cations with Na+. Following cation exchange, citric acid concentration was determined accurately using the method. Keywords: citric acid measurement, acid titration, mixed weak-acid system, VFA Water SA Vol. 31(4) 2005: 497-502