From the potential difference measurements of the reversible buffered cells Pt \ H-2 \KHPh + KCl \ AgCl \ Ag \ Pt and Pt \ H-2 \ KHPh + KCl \ Hg2Cl2 \ Hg \ Pt in (propylene carbonate + water) and (ethylene carbonate + water) solvent mixtures, respectively, primary pH-metric standards pH(PS) based on the potassium hydrogen phthalate buffer (KHPh) have been determined. This is the first time that this type of standard is acquired for water mixtures with aprotic organic cosolvents nearly isodielectric with water. The pH(PS) results at the Various solvent compositions are interpreted through the function (pH(PS) + E-o/k), where E-o = standard potential difference, and k = 1n 10 RT/F, in terms of the primary medium effects upon HCl and the single ions H+ and Cl-.