To select the electrode type for optimal performance, consider the pH and temperature range at which the measurements are to be made. For measurements in alkaline solutions, select a glass electrode with a low Na + sensitivity so that the alkaline error will be as low as possible. For measurements at temperatures above 60 °C, select a reference electrode other than the mercury/ mercurydVchloride (calomel) electrode, which exhibits instability at elevated temperatures. At elevated temperatures the Ag/AgCl electrode is a good choice because it has excellent temperature stability. A detailed discussion of sources of error in potentiometric measurements can be found in Reference 17. The future Because the pH concept is fundamental to many chemical and biochemical processes, it is important to obtain international agreement on the use of pH scales. Currently several different scales are in use. Reevaluation of these international pH scales and, ultimately, selection of one recommended scale would be beneficial.
(17) Durst, R. A. In Ion-Selective Electrodes in Analytical Chemistry; Freiser H., Ed.; Plenum Publishing Corp.: New York, 1978; pp. 311-38.
Hans Bjarne Kristensen is head of the Chemical Reference laboratory of Radi ometer A/S, Denmark. He received his M.Sc. degree in physics and chemistry at the University of Copenhagen in 1982. Since 1982 he has been working with ref erence systems for aqueous electrolyte solu tions of sodium, potassium, calcium, and in particular the reference system for the hydrogen ion activity, pH. He serves as Denmark's representative to the IUPAC commission V.5 (electroanalytical chem istry).
References (1) Sorensen, S.P.L. Comtes-Rendus des Travaux du Laboratoire de Carlsberg 1909, 8, 1. (2) Sdrensen, S.P.L.; Linderstrém-Lang, K. Comtes-Rendus des Travaux du Laboratoire de Carlsberg 1924, 15, 40. (3) Bates, R. G. Determination ofpH, Theory and Practise, 2nd éd.; John Wiley and Sons: New York, 1973. (4) Covington, A. K.; Bates, R. G.; Durst, R. A. Pure. Appl. Chem. 1985, 57, 531-42. (5) Bates, R. G.; Robinson, R. A. / Solution Chem. 1980, 9, 455-56. (6) Maas, A.H.J; Weisberg, H. F.; Burnett, R. W.; Muller-Plathe, O.; Wimberley, P. D.; Zijlstra, W. G.; Durst, R. Α.; Siggaard-Andersen, O. /. Clin. Chem. Clin. Biochem. 1987, 25, 281-89. (7) Durst, R. Α.; Koch, W. F.; Wu, Y. C. Ion-Sel. Electrode Rev. 1987, 9, 173-96. (8) "International Recommendation No. 54, pH Scale for Aqueous Solutions"; re port to OIML, Sixth International Con ference of Legal Metrology, June 1980. (9) Bates, R. G.; Bower, V. E. / Res. Natl. Bur. Stand. 1954, 53, 283-90. (10) Bates, R. G. Crit. Rev. Anal. Chem. 1981, 10, 247-78. (11) Covington, A. K. Anal. Chim. Acta. 1981 127 1—21 (12) Bates,' R. G.; Guggenheim, E. A. Pure. Appl. Chem. 1960, 1, 163-68. (13) Taylor, J. K.; Smith, S. W.J. Res. Natl. Bur. Stand. 1959, 63A, 153-59. (14) Marinenko, G.; Taylor, J. K. Anal. Chem. 1968, 40, 1645-51. (15) Reference Electrodes; Ives, D.J.G.; Janz, G., Eds.; Academic Press: New York, 1961. (16) Pinching, G. D.; Bates, R. G. /. Res. Natl. Bur. Stand. 1946, 37, 311-19.
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 18. SEPTEMBER 15, 1991 · 891 A
