Estimation of low-level carbonate alkalinity from single endpoint acidimetric titration to ph = 4.5

An established method for determining alkalinity is acidimetric titration to a single endpoint of pH = 4.5, which for dilute carbonate buffersystems is below the equivalence pH. Here we demonstrate that a formula commonly used to adjust for this over-titration itself leads to a significant overestimation of the alkalinity. Applied on a large data set from southwestern Norway, this formula caused an alkalinity overestimation of 10 %. Calculations of CO2(aq) based on these alkalinity data yielded similar overestimation. We propose an adjustment of the alkalinity formula to minimize this error.