pH measurements are used to assess both the free acidity of aqueous solutions and the concentrations, m(i), of chemical species i related to hydrogen ion, H+, through chemical equilibrium, provided that measurements in reference solutions with well-known compositions are also carried out. Seawater is a highly complex electrolytic matrix of variable composition, rich in NaCl, where the determination of pH = Ig a(H+) (a(H+) = m(H+)gamma(H+) is the activity of the specified ion, H+ with the concentration m(H+) and activity coefficient gamma(H+) not equal 1) is an issue of controversy. Electroneutrality prevents measurement of individual ion activity coefficients, which are affected and depend on the other ions in presence, namely the counter ions. Nevertheless, mean activity coefficients, gamma(+/-) = root gamma(+)gamma(-), in this case gamma(+/-) = root gamma(H+)gamma(Cl-), can be experimentally assessed from the application of the Nernst equation to Harned cell (Pt vertical bar H-2 vs. Ag vertical bar AgCl electrodes, without transference) in solutions of known mH(+) and m(Cl-). In this work measurements of electrical potential, E/V, have first been taken in strategically planned solutions, ranging from 0.01 mol kg(-1) HCl + (NaCl) to 0.01 mol kg(-1) HCl + (NaCl + KCl + CaCl2 + MgCl2), of ionic strength, I approximate to 0.67 mol kg(-1) at 15 degrees C, 25 degrees C and 35 degrees C with experimental pressure corrected to reference pressure of 1 atm. Mean activity coefficients were experimentally assessed with associated uncertainty. Further addition of sodium sulfate, Na2SO4, targeting artificial seawater (ASW), originates acid base equilibria that affect the free hydrogen ion concentration, m(H+). The changes introduced by the addition of sulfate in the measured Flamed cell potential have been used to assess the free proton concentration and, based on the obtained results, to propose for the first time a procedure aiming to calculate values of bisulfate association constants with the corresponding uncertainty at 25 degrees C for both studied media. Further, the effective ionic strength of artificial seawater (ASW) was calculated and compared with the value obtained using the salinity based equation largely conveyed in the literature. (C) 2016 Elsevier B.V. All rights reserved.