The protonation constants of glycine in artificial seawater at 25 °C

The ionization constants of glycine in artificial seawater containing NaCl, KCl, CaCl2, MgCl2 and Na2SO4 were potentiometrically determined at 25 ° C by using a commercial glass electrode. Measurements were carried out at different salinities, and thus ionic strengths, to simulate seawater and estuarine waters. The experimental data were fitted using different equations as functions of the salinity based on Millero (1979). Comparisons were made between the parameters obtained by changing the variables and also the parameters yielded by the Pitzer interaction model, which was previously applied by Millero to seawater systems. After considering all the proposed models we found the function in S1/2 to give the best fit and the smallest standard deviation of the parameters, although the extrapolated pK is not as good as the one obtained by the use of Pitzerʹs model. Some of the models we used yielded extrapolated pKs closer to the values in the literature (2.36 and 9.78) but higher standard deviations in the parameters.