EVALUATION OF ACID NEUTRALIZING CAPACITY DATA FOR SOLUTIONS CONTAINING NATURAL ORGANIC-ACIDS

When the acid neutralizing capacity (ANC) of an aqueous solution is measured by an acidimetric titration, the measured parameter (ANCt) is generally assumed to correspond to the ANC of the sample (ANCi). This paper demonstrates that ANCt actually corresponds to ANCi - ANCe, where ANCe is the ANC of the sample at the titration endpoint. This relationship applies whether the endpoint occurs at a fixed pH or is determined by a Gran plot. When the sample contains organic acids, ANCe is generally non-zero and its neglect can introduce serious errors into the interpretation of ANC titration data. This paper presents computer simulations of ANC titrations in systems containing both carbonate and organic alkalinity, using a realistic thermodynamic model of the acidities of natural organic acids. The necessity of considering ANCe is demonstrated both computationally and by analysis of published results in which discrepancies between Gran ANC values and anion deficits could not previously be explained. © 1990.