Anal Bioanal Chem (2002) 374 : 765–766 DOI 10.1007/s00216-002-1585-3

E D I TO R I A L

Petra Spitzer · Günther Meinrath

Importance of Traceable pH Measurements

Published online: 3 October 2002 © Springer-Verlag 2002

There is hardly a chemical quantity in science, medicine, and industry more frequently measured than pH. The measurement of pH is performed routinely and the values assigned to this quantity are almost always reported without any indication of the associated measurement uncertainty. There is not always immediate evidence of the relationship between the measured quantity and the definition of pH. Even chemists react with surprise when they learn that pH is the only quantity in IUPAC’s Green Book that is, in principle, immeasurable according to its definition. Thus, there is a casual understanding of the meaning of the quantity pH that is not matched by the complexity of the facts. There has been, and still is, on the other hand, an ongoing discussion about what these facts actually are. The answer is not easily given. There is, however, the need of society and industry to ensure the comparability of the results of measurements that often play crucial roles in economic exchange, industrial production, medical treatment, legal disputes, and many other aspects of daily life. Chemical measurements make up an important part of a world-wide web of measurements that ensure the quality of products and information. The 20th century has established chemical measurement methods to ensure the identity of molecules that must be considered masterpieces of human ingenuity. To ensure chemical identity is of vital interest in applying chemicals in pharmacy, medicine, and biotechnology. To appreciate these achievements consider palytoxin (C129H223N3O54), possibly the most poisonous substance

P. Spitzer (✉) Physikalisch-Technische Bundesanstalt 3.202, Metrologie in der Elektrochemie, Bundesallee 100, 38116 Braunschweig, Germany e-mail: [email protected] G. Meinrath RER Consultants, Schießstattweg 3a, 94032 Passau, Germany e-mail: [email protected]

known. There are 72 asymmetry centers giving rise to about 5×1021 different stereoisomers. Chemists may take considerable professional pride in the fact that it has not only been possible to characterize the stereoisomer palytoxin but also to synthesize the one stereoisomer that is identical with the natural product palytoxin. Such successes might, however, obscure the fact that quantifying the amounts of identical atoms, isotopes, or compounds still is a difficult task in chemistry. As a matter of fact, almost all methods of analytical chemistry are relative measurements – because to measure means to compare, there is nothing wrong with such relative comparisons. To fulfil the request that measurement results be mutually accepted, and therefore comparable irrespective of where and when the measurement process is performed, however, all results must be linked to internationally recognized “stated” references. In 1993, by issuing the “Guide to the Expression of Uncertainty in Measurements” (GUM), the International Standards Organization (ISO) in cooperation with other international organizations, including IUPAC, set a framework for the evaluation of the measurement uncertainty with which comparable measurement results must comply. The Eurachem/CITAC Guide “Quantifying Uncertainty in Analytical Measurement” (second edition issued in 2000) shows how the concepts in the GUM can be applied in chemical measurement. Complete knowledge of the chain of traceability linking the measured value in the sample to a stated reference is a prerequisite for evaluation of the uncertainty in a measurement result. To traditionally trained chemists these concepts are not familiar – but compliance with these concepts will be crucial in exploiting the full benefits of chemical measurements to our globalizing societies. Statements of the measurement uncertainty on the basis of traceability to internationally recognized references are also required by regulatory bodies and by the international quality assurance standard ISO EN 17025 in general. If applied reasonably, these concepts will ensure transparency of measurement results, promote competition, and create trust.

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Metrological concepts must work for pH measurements also. The (still provisional) recommendation of the IUPAC Working Party on pH measurement explores the benefits of the concept of metrology in chemistry. For analytical data to be internationally accepted it is necessary to demonstrate the equivalence of national traceability structures, including national measurement standards. The equivalence of the primary standards for pH measured at different national metrology institutes (e.g. NIST in the United States, PTB in Germany) is established by the Mutual Recognition Arrangement (MRA) for national measurement standards and for calibration and measurement certificates issued by national metrology institutes in 1999. To support the MRA, key comparisons on pH (CCQM-K9, CCQM-K17) organized under the auspices of the highest metrological authority, the BIPM, have successfully been performed on two primary pH standards. Some details of the traceability chain from primary pH standards down to routine pH measurements in laboratory and field studies must still be decided. These details encompass, among others, statistical questions about the calibration procedure and possible alternatives to the Bates–Guggenheim convention. Partially because of the pioneering nature of establishing a traceability chain for measurement values of the quantity pH, the options and possible fallacies must be considered carefully. For that purpose metrologists and chemists interested in metrological issues from 13 countries met at the German national metrological institution, the Physikalisch-Technische Bundesanstalt in Braunschweig on September 24th and 25th, 2001, to discuss theoretical and practical aspects of future developments in the measurement of the quantity pH during the 166th PTB seminar. A selection of topics presented during this semi-

nar, partly augmented by contributions from experts with special interests, is collected in this special issue of Analytical and Bioanalytical Chemistry.

PETRA SPITZER is currently Head of the project “Metrology in Electrochemistry” at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig/ Germany. Her research is focused on the development of primary electrochemical standards for pH, electrolytic conductivity, coulometry, and ion activity. Embedded in international research networks her interests are focused on developing metrologically reliable calibration methods for electroanalytical methods.

GÜNTHER MEINRATH directs RER Consultants/Passau, and teaches regularly at the Technical University of Freiberg and as guest lecturer at several other institutions in conjunction with national and international research cooperation. His research interests include safety assessment of nuclear waste repositories, reliability issues of nuclear chemistry data, algorithms for probabilistic predictive geochemical modeling, chemometrics, and metrological issues of chemical data.