kThe potential differences E of the cells Pt\H-2\H2Ph(m(1)) + KHPh(m(2)) + KCl(m(3)) in Z\AgCl\Ag and Pt\H-2\H2Ph(m(1)) + KHPh(m(2)) + KCl(m(3)) in Z\Hg2Cl2\Hg have been measured at T = 298.15 K in mixtures Z = (W + S) of water (W) with cosolvents S = propylene carbonate (PC) and S = ethylene carbonate (EC), to determine the first ionization constants K of the o-phthalic acid H2Ph (benzene-1,2-dicarboxylic acid), which are indispensable for the determination of primary pH-metric standards based on the potassium hydrogen phthalate buffer (KHPh) in such solvent mixtures. The value of K is seen to decrease progressively with increasing mass fraction w(s) of the organic cosolvent, as with all of the other cosolvents studied earlier, but no simple relationship with the cosolvent permittivity is discernible. Since the required values of the standard potential difference E-O of the second cell were hitherto missing, they have now been obtained based on potential difference measurements of the cell Pt\H-2\HCl(m) in Z\Hg2Cl2\Hg. The primary medium effect (E-W(O) - E-Z(O), by Owen's definition) upon HCl in water-rich mixtures Z is seen to increase linearly with increasing ws, as in earlier investigations. In this comparative context, the slope of the primary medium effect against ws plots for the aprotic cosolvents increases regularly with decreasing permittivity, whereas for the protic (alcoholic) cosolvents the slope is ill-defined.