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Calibration of pH meters and glass electrodes at the National Physical - page 4 / 7

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stated uncertainties. The unbroken chain of the comparisons (calibrations) is called a traceability chain [3].

    In Israel, the National Physical Laboratory (INPL) is the national metrological institute. The national primary standard of pH value developed at INPL consists of a precise pH/ion-meter and the following electrochemical cell:

                     Ag/AgCl│3 M KCl║3 M KCl║Test solution│H2 Pt/Pt,                          (5)

where the pH-indicator electrode is the Pt/Pt hydrogen gas electrode; the reference electrode is Ag/AgCl│3 M KCl; the bridge solution is 3 M KCl. The primary standard is the standard that is designated or widely acknowledged as having the highest metrological qualities, and whose value is accepted without reference to other standards of the same quantity [3]. The emf of cell (5) depends on the activity of hydrogen ions according to the Nernst equation:

                                                 E = EAg/AgCl - k·lg aH,                                                     (6)

where EAg/AgCl is potential of the Ag/AgCl electrode, V.

    Calibration of cell (5) is carried out using six primary pH buffers at 25°C [7]. These buffers are prepared from the NIST standard reference materials (RMs) with expanded uncertainties of ± 0.005 pH, which are minimally known today.

    The national standard of pH value has successfully participated in two interlaboratory international key comparisons of pH measurement results in 2002 and 2004 organized by the Consultative Committee for Amount of Substance (CCQM) at

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