Anal. Chem. 2004, 76, 528-535
Determination of Primary and Secondary Standards and Characterization of Appropriate Salt Bridges for pH Measurements in Formamide Luigi Falciola, Patrizia R. Mussini,* Torquato Mussini, and Pasquale Pelle Department of Physical Chemistry and Electrochemistry, University of Milano, Via Golgi 19, I-200133 Milano, Italy
For the first time, the standardization for pH measurements is here implemented in the domain of superpermittive media. The nonaqueous solvent studied is formamide (E ) 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, NH4Cl, NH4Br, and NH4I, 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. The present study is a development of the program of work undertaken some years ago along with projects approved by the Electroanalytical Chemistry Commission of IUPAC. As a matter of fact, quite recently, the IUPAC has implemented a restructuring of the important sector of pH scales and standardizations.1 The new recommendations contain a deep redefinition of (a) the basic rationale of pH scales and (b) the classification of the related pH standards. Point (a) concerns the rejection of the previously accepted system2 which hinged on one single, arbitrarily selected, pH standard (the so-called Reference Value Standard, pHRVS) together with the theoretical Nernstian slope factor k ) (ln 10)RT/F, to be now replaced by a system of several equally ranked standards producing a practical slope factor k′ * k. Point (b) concerns cancellation of the previous pyramid made of one RVS standard, several primary standards, and a multiplicity of operational standards, with promulgation of the new two-level classification articulated into primary and secondary standards. The assignment of the qualification of “primary” or “secondary” to a standard buffer solution is now strictly linked with the “primary method” or “secondary method” of determining it. Thus, * To whom correspondence should be addressed. Phone: +39 02 50314211. Fax: +39 02 50314300. E-mail:
[email protected]. (1) Buck, R. P.; Rondinini, S.; Covington, A. K.; Baucke, F. G. K.; Brett, C. M. A.; Camoes, M. F.; Milton, M. J. T.; Mussini, T.; Naumann, R.; Pratt, K. W.; Spitzer, P.; Wilson, G. S. Pure Appl. Chem. 2002, 74, 2169-2200. (2) Covington, A. K.; Bates, R. G.; Durst, R. A. Pure Appl. Chem. 1985, 57, 531-542.
528 Analytical Chemistry, Vol. 76, No. 3, February 1, 2004
the primary pH standards (pHPS) must be determined from potential difference (pd) measurements of a thermodynamically reversible cell, namely, Harned’s cell.
Pt|H2 (101325 Pa)|pHPS buffer + KCl|AgCl|Ag|Pt (1)
In turn, the secondary standards (pHSS) are those derived from pd measurements of nonreversible cells (viz., those including liquid junction potentials or membrane potentials), such as
Pt|H2 (101325 Pa)|pHPS buffer|salt bridge|pHSS buffer|H2 (101325 Pa)|Pt (2)
The resulting present state of art is as follows: In pure water solvent, the primary standardization is now completely achieved, with a system of seven recommended and internally consistent primary standards (determined by different authoritative schools), which includes the previous pHRVS on a par with the other six pHPS , and there is no problem for acquisition of as many secondary standards as necessary, by means of cell 2. In the domain of aqueous-organic solvent mixtures, an IUPAC technical report of 19973 approved the acquisition of pHRVS standards based on the potassium hydrogen orthophthalate buffer in 30 solvent mixtures belonging to 8 (water + organic cosolvent) systems plus a handful of pHPS standards based on acetate, barbiturate, citrate, oxalate, phosphate, salicylate, TRIS, and TES buffers. Needless to say, both those pHRVS’s and pHPS’s were obtained by cell 1, and therefore, they are technically all “primary” standards in the present redefinition and terminology. After the IUPAC report of 1997, further pH standards have recently appeared in the literature.4-6 However, the situation of pH standards in aqueous-organic mixtures is still far from being satisfactory. Finally, to describe how worse hitherto the situation was for the domain of pure nonaqueous solvent media, it can be concisely put in terms of a totally lacking pH standardization (excepting a (3) Mussini, P. R.; Mussini, T.; Rondinini, S. Pure Appl. Chem. 1997, 69, 10071014. (4) Falciola, L.; Mussini, P. R.; Mussini, T. J. Solution Chem. 2000, 29, 11991210. (5) Antonini, D.; Falciola, L.; Mussini, P. R.; Mussini, T. J. Electroanal. Chem. 2001, 503, 153-158. (6) Barbosa, J.; Barro`n, D.; Butı`, S.; Marque´s, I. Polyhedron 1999, 18, 33613367. 10.1021/ac034774k CCC: $27.50
© 2004 American Chemical Society Published on Web 12/11/2003
recent introductory short contribution7), and implementation of the latter is now definitely overdue. It is precisely to meet this situation that the present research work has been undertaken. Its aim is to implement the above pH standardization covering all the operative aspects. These imply satisfaction of some essential requirements: (i) Characterization of several primary or secondary pH standards in order to enable the users to check the functionality of the chosen H+-sensor electrode by the bracketting standards technique; (ii) Verification of the internal consistency and Nernstian compatibility of the standards found above, by means of the cell
Pt|H2 (101325 Pa)|pHPS and pHSS|NH4Cl bridge|AgCl|Ag|Pt (3) where the NH4Cl bridge is here quoted in anticipation of the present results; (iii) Checking the scale width in the nonaqueous solvent and comparing it with the aqueous one by accounting for the primary medium effect; and (iv) Characterization of and providing appropriate salt bridges for the abatement of the liquid junction potentials intervening in the regular pHX measuring cell
reference electrode|salt bridge|sample at pHX or standard at pHPS|H+-sensor|Pt (4) because, of course, a meaningful pH measurement requires that both electrodes and the salt bridge in cell 4 be in the same solvent. The solvent chosen for the present study is formamide, a superpermittive one ( ) 109.5 at 298.15 K), also because of the interest it arouses as a possible solvent for support electrolytes in high-performance cells. In fact, formamide has a high ionizing and solubilizing power on a number of electrolytes and also has some physicochemical features similar to those of water. As a result, three primary standards pHPS (based on phthalate, equimolal phosphate, and unequimolal phosphate buffers, respectively) and two secondary standards, pHSS (based on the tetraoxalate and the citrate buffers), have been acquired. Furthermore, three salt bridges (NH4Cl, NH4Br, and NH4I, respectively) have been characterized by the method of the concentration cell with transference
Ag|AgX|MX(m1)||MX(m2)|AgX|Ag
(5)
a method that has recently been restructured critically by our research group.8,9 Finally, a comparison of the pH scale in formamide with that in water has been established in terms of Owen’s primary medium effect3,5,10-12 on the H+ ion. (7) Falciola, L.; Mussini, P. R.; Mussini, T.; Pelle, P. Electrochem. Commun. 2002, 4, 146-150. (8) Mussini, P. R.; Mussini, T. J. Appl. Electrochem. 1998, 28, 1305-1311. (9) Mussini, P. R.; Mussini, T. Electrochem. Commun. 2000, 2, 108-111. (10) Owen, B. B. J. Am. Chem. Soc. 1932, 54, 1758-1769. (11) Bates, R. G. Determination of pH - Theory and Practice, 2nd ed.; Wiley: New York, 1973; pp 180-187, 211-228.
EXPERIMENTAL SECTION The hydrogen gas electrodes in cells 1 and 2 were prepared as in our previous works in aqueous-organic media.4,5 The silver/ silver halide electrodes in cells 1, 3, and 5 were prepared according to the bielectrolytic type.13 The solutions were prepared by mass from the following chemicals: ultrapure disodium hydrogen phosphate (Merck), potassium dihydrogen phosphate (Merck), potassium hydrogen phthalate (Merck), potassium tetraoxalate (Fluka Microselect >99.5%), potassium dihydrogencitrate (Fluka purum p.a. >98%), potassium chloride (RPE Carlo Erba), ammonium chloride, bromide and iodide (Fluka Biochemica Microselect), and formamide (Merck, having residual water
