Determination of Primary and Secondary Standards for pH Measurements in N-Methylacetamide and Its 0.50 Mass Fraction in Admixture with Water, with Characterization of Appropriate Salt Bridges

Abstract

Following the IUPAC criteria, four primary standards (pHPS) and three secondary standards (pHSS) have been determined at various temperatures using Harned's reversible cell and Baucke's nonreversible cell, respectively, the solvent media Z studied being N-methylacetamide and its mixture with water at 0.50 mass fraction. Their internal consistency was ascertained by the linear pH dependence of the emf of the cell (Pt|H2|pH standards in Z||NH4Cl bridge in Z|AgCl|Ag|Pt). Essential thermodynamic functions (standard emfs E° of cell and ionization constants of orthophthalic acid) hitherto unknown in the pertinent solvent Z had also to be determined from the emfs of the appropriate reversible cells. These pHPS and pHSS acquisitions enable the user to perform routine pHX measurements by the regular operational cell. A supplementary systematic search has been performed according to the method of Helmholtz transference cells both in N-methylacetamide and in its w = 0.50 mixture with water, to single out appropriately equitransferent salt bridges for the abatement of liquid junction potentials in the operational pH cells. The intercomparability of the pH scales in the above solvents Z with that in pure water is discussed in terms of the solvent autoprotolysis constants in conjunction with the primary medium effects upon the H+ ion.