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Zusammenfassung DerpH-WertisteinewichtigeMessgröße zur Charakterisierung von wässrigen Lösungen und damit auch von biologischen Systemen. Von wenigen Ausnahmen der spektroskopischen Bestimmung abgesehen, wird der pH-Wert einer Lösung potenziometrisch, durch Messen der Kettenspannung einer pH-Messkette mit Glaselektrode mit Hilfe einer geeigneten pH-Messeinrichtung bestimmt. Eine Aussage darüber, ob der gemessene pH-Wert den Ansprüchen an die jeweilige Anwendung genügt, ist nur möglich, wenn die Zuverlässigkeit des Messergebnisses durch die Angabe einer Messunsicherheit belegt werden kann. In dem Beitrag wird die Rückführungs-kette für die Messgröße pH-Wert beschrieben sowie einzelne Unsicherheitsbeiträge betrachtet, die der Anwender bei der Einführung und bei der Bestimmung der pH-Werte berücksichtigen sollte. Das schließt auch die technischen Parameter, die an das Produkt gestellt werden, ein.Summary The pH value is an important quantity used to characterize aqueous solutions including biological media. There are only few fields of application for spectroscopic pH measurements. pH is usually measured electrochemically by potentiometry. The cell voltage of a pH measuring chain with glass electrode is measured by means of an appropriate pH measuring device. To be able to assess whether the result of a pH measurement meets all requirements a statement of the measurement uncertainty is mandatory. In the following the metrological traceability chain for pH is described. The paper will focus on important uncertainty contributions to be taken into account by the user. This especially includes technical product parameters.Schlagwörter pH, Kalibrierung, Rückführung

Results of the CCQM-K91 key comparison on pH of an unknown phthalate buffer with a nominal pH value of pH ~ 4.01 at 25 °C are reported. Measurements are performed at 15 °C, 25 °C and 37 °C and optional also at 5 °C and 50 °C.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The subsequent key comparison CCQM-K19.1 was carried out in 2010 as a follow-up comparison to CCQM-K19. Three laboratories (NMIJ, PTB and SMU) took part in both comparisons. Their results are consistent. The assigned uncertainties are reliable and of the same order. These results provide the link to CCQM-K19.As well as the original comparison the subsequent key comparison was organized to demonstrate the capability of the interested national metrology institutes to measure the pH value of an unknown borate buffer by a primary method at 15 °C, 25 °C and 37 °C. Eight laboratories took part in CCQM-K19.1. The participants either were not able to participate in the original comparison or participate only in the Pilot Study CCQM-P82 running in parallel to CCQM-K19. In one case the laboratory expressed doubts on the integrity of the sample provided in CCQM-K19.The result in CCQM-K19.1 is the acidity function at zero chloride molality of the unknown borate buffer solution.Most participants in the key comparison CCQM-K19.1 demonstrated improved capabilities. This especially applies to the laboratories that had originally taken part in the pilot study CCQM-P82. In some cases previous results could be confirmed. The hidden reasons for that have still to be clarified.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

A new setup for high precision, automated secondary pH measurements together with a reference measurement procedure has been developed and tested in interlaboratory comparisons using buffers pH 4.005, pH 7.000, and pH 10.012 at 25 °C and 37 °C. Using primary buffers as standards, a standard uncertainty in pH better than 0.005 can be reached. The central measuring device is a one piece, thermostatted cell of PFA (perfluoroalkoxy) with a built-in Hamilton(®) Single Pore™ Glass electrode. Due to its flow-through principle this device allows pH measurements with low consumption of measurement solutions. The very hydrophobic and smooth PFA as construction material facilitates complete emptying of the cell. Furthermore, the tempering unit affords very precise temperature control and hence contributes to the low target uncertainty of the produced secondary buffer solutions. Use of a symmetric measurement sequence and the two point calibration was sufficient to reach high precision and accuracy.