Does PHREEQC calculate carbonate alkalinity after it corrects for silica and other ions that act as alkalinity in a titration?
Yes. The alkalinity that is entered as an input parameter is assumed to include carbonate species and all other species that would be titrated in an alkalinity titration. PHREEQC calculates alkalinity by a sum(alk(i)*m(i)), that is sum of alkalinity contribution of each aqueous species times its molality. For example, H3SiO4- has an alkalinity contribution of 1. So alkalinity is not simply carbonate alkalinity, but the sum of all aqueous species that will accept a proton in an alkalinity titration. One possibility that sometimes occurs is that the alkalinity you specify is less than the alkalinity of silica, borate, or other noncarbonate species. The program is trying to adjust the carbon species to sum account for the difference between the noncarbonate alkalinity and the total alkalinity. If this difference is negative, no mathematical solution exists in which carbon concentrations are positive. You’ll get a nasty-gram about Noncarbonate alkalinity exceeding total alkalinity. • ALKAL
