DIFFERENTIAL-POTENTIOMETRIC CELL FOR THE RESTANDARDIZATION OF PH REFERENCE MATERIALS

An economical and accurate differential-potentiometric cell for restandardizing pH reference materials is described, which eliminates the time consuming and costly primary standardization of single charges of standard reference materials. The quasi-identical buffer solutions, whose pH values are compared by means of identical Pt, H-2 electrodes, form a common liquid junction across a sintered glass disk. For buffers with pH values between 3 and 11, and with pH differences of less than +/- 0.02, the liquid junction potential depends mainly on the ions other than hydrogen and hydroxyl, and is smaller than 10% of the measured electromotive force (e.m.f), as shown experimentally and by calculations for numerous buffer pairs with defined pH differences. The measurements yield restandardized pH(RS) values with uncertainties which are larger than those of the standard pH(S) values by only +/- 0.001 to +/- 0.003, depending on the pH difference of the solutions compared. Restandardization of buffers with pH values less than three or greater than 11 necessitates DELTA(pH) < +/- 0.02, the ideal case for any buffer being identical solutions, i.e. DELTA(pH) = 0. Also, operation of the cell is simpler and more economical than that of the related cell with a KCl bridge solution. The cell has been used for restandardizing commercialized DIN standard buffer materials since 1975 and was recommended for this purpose by IUPAC in 1985. Its application by national standards organizations and metrological laboratories is recommended.