| Summary: | The practice common in desalination plants of determining carbonate by titration of a sample of water with acid to pH 8.3 or 8.1, and total alkalinity by titration to pH 4.5, leads to an underestimation of the amount of carbonate present in high ionic-strength samples, which in turn may lead to erroneous assessments of scaling potential. Well-established empirical equations for the dissociation constants of carbonic acid in seawater as a function of salinity and temperature were used to estimate the proportion of each carbonate species present in samples of Arabian Gulf seawater, desalination concentrate, and nanofiltration reject. Samples were then titrated and inflection points identified associated with the protonation of the carbonate ion and the protonation of the hydrogen carbonate ion. Colorimetric titration gave end-points consistent with these inflection points and with the equations of Millero et al. Standard test method ASTM D3875-15 gives values of close to zero for the carbonate concentration in samples which were found to contain up to 40% of their inorganic carbon as carbonate according to the other methods of estimation. Thus ASTM D3875-15 is not suitable for seawater or desalination brines.
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