Medium effects, comparability and predictability of pH-standards in aqueous-organic solvents isodielectric with water: behaviours of the (ethylene carbonate + water) and (propylene carbonate + water) systems

From the potential difference measurements of the reversible buffered cells Pt mid H2 mid KHPh+KCl mid AgCl mid Ag mid Pt and Pt mid H2 mid KHPh+KCl mid Hg2Cl2 mid Hg mid Pt in (propylene carbonate+water) and (ethylene carbonate+water) solvent mixtures, respectively, primary pH-metric standards pHPS 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 pHPS results at the various solvent compositions are interpreted through the function (pHPS+E°/k), where E°=standard potential difference, and k=ln 10RT/F, in terms of the primary medium effects upon HCl and the single ions H+ and Cl−.