Determination of Primary and Secondary Standards and Characterization of Appropriate Salt Bridges for pH Measurements in Formamide

Abstract

For the first time, the standardization for pH measurements is here implemented in the domain of superpermittive media. The nonaqueous solvent studied is formamide (epsilon = 109.5 at 298.15 K) for which three primary standards and two secondary standards have been determined, whose excellent internal consistency has also been demonstrated by specific cell measurements. A comparison of the pH scale in formamide with the aqueous scale has been duly tried by accounting for the primary medium effect on the H+ ion. Furthermore, as a result of an ad hoc supplementary systematic investigation, three salt bridges of appropriate level of equitransference in formamide, that is, NH(4)Cl, NH(4)Br, and NH(4)I, have been characterized for abating the liquid junction potentials intervening in the pH-measuring cell to enable the user to carry out regular pH measurements and related controls.

Capillary zone electrophoresis in non-aqueous solutions: pH of the background electrolyte

“…The following conditions should be fulfilled for correct pH measurements [18]. (i) The same electrode pair should be used for both, calibration and measurements, (ii) temperature should be same, and (iii) the solvent composition should be the same for the calibration solution(s), the sample solution and for the equitransferent reference electrode filling electrolyte (and salt bridge electrolyte).…”

“…Accordingly, it is not a surprise that there are hardly any primary pH standards, which meet the IUPAC criteria, available for organic solvents. One exception has recently been introduced, namely potassium hydrogen phthalate buffer in formamide (FA) [18]. FA is a solvent with high relative permittivity (109.5 at 25.0 • C [20]), in which complete dissociation of the salts takes place; it is thus suitable for the IUPAC primary pH standard measurements using the Harned cell.…”

The principles of migration and dispersion in capillary zone electrophoresis in nonaqueous solvents

“…[60]). Also, potassium hydrogen phthalate buffer in FA has recently been introduced [61]. Maintaining the liquid junction potential constant is possible if the composition of the electrolyte solution is the same for both standard solution and the sample.…”

Formamide as solvent for capillary zone electrophoresis

“…pK a values used for the calculations were 6.86 and 6.31 for acetic acid and benzoic acid, respectively (note that the molal pK a values taken from the literature [41,42] were converted to molar ones). The BGE used for the determination of the limit of detection was 56.4 mmol/L potassium hydrogen phthalate in FA with pH of 6.38 at 257C (corresponds to 50 mmol/kg buffer, pH 6.43 in the molal scale [43]). The water content of FA and the BGEs was measured by the Karl Fischer method (756 KF Coulometer from Metrohm, Herisau, Switzerland).…”

“…In general, pH adjustment in organic solvents is not as straightforward as in aqueous media (for a detailed discussion, see, e.g., [2]). In FA, even though some standard buffers have recently been introduced by Falciola et al [43,72], a proper maintenance of the pH-and reference-electrodes is not an easy task. Accordingly, the concept of reference BGE acids, already used for some other organic solvents (see [1,2] and literature cited therein), can be an appropriate choice for pH tuning in FA.…”