The concept of pH is very special in the field of physicochemical quantities, because of its widespread use as an important control property and process descriptor and because of the difficulty of defining its theoretical meaning. Despite recent efforts to assess both the theoretical rationale and the experimental strategy for assignment of primary pH values, these have not yet been accomplished satisfactorily, since the definition of pH involves the activity of a single ion [1], which, according to the rules of thermodynamics, cannot be inferred alone in practice. To bypass this point, several technical details for the measurement of pH have been developed, along with theoretical considerations of thermodynamics. Currently, primary measurements are available with the Harned cell, but their traceability and comparability is achievable only within the constraints of international conventions and predefined conditions that cannot account for all possible situations when pH is measured. Under these circumstances, it has been shown that comparison with primary results can be contradictory when secondary measurements (e.g. with glass electrodes) are obtained by considering the concentration scale of the hydrogen ion, for example when acid–base titration, aimed at formation constant refinement, is appreciated through potentiometric data. It is thought that the overall accuracy suffers because of the rough nature of some general assumptions about the chemical nature of the solution. Critical parameters have been identified in the ionic strength (I) and the ionic activity coefficients (i), usually approximated with the extended Debye–Hückel equation. In particular, difficulties arise when properties of the solutions, such as ionic strength and ionic medium, are varied, thus introducing significant noise able to reduce the accuracy of a comparison, since the quoted features, ionic medium included, may significantly affect a pH values. We propose the analysis of a series of comparative exercises supported by modifications of the theoretical approach to activity coefficients. Primary measurements (Harned cell) of the pH of a phosphate buffer have been considered and the results have been compared with secondary (glass electrode) measurements, executed according to either activity (paH) or concentration (pcH) scales. The emerging discrepancies have been mainly assigned to incomplete inferences of i from the chemical features of the testing solution. More comparable results are attainable if evaluation of i is performed using better estimates of the ionic strength, through an enhanced application of the Debye–Hückel theory [2, 3]. The final aims are the achievement of better harmonization for this fundamental quantity of chemistry and the improvement of the agreement between primary and secondary measurements of pH.

Quality control of pH measurements: uncertainty budget comparison of primary and secondary apparatuses

PRENESTI, Enrico;BERTO, Silvia
2007-01-01

Abstract

The concept of pH is very special in the field of physicochemical quantities, because of its widespread use as an important control property and process descriptor and because of the difficulty of defining its theoretical meaning. Despite recent efforts to assess both the theoretical rationale and the experimental strategy for assignment of primary pH values, these have not yet been accomplished satisfactorily, since the definition of pH involves the activity of a single ion [1], which, according to the rules of thermodynamics, cannot be inferred alone in practice. To bypass this point, several technical details for the measurement of pH have been developed, along with theoretical considerations of thermodynamics. Currently, primary measurements are available with the Harned cell, but their traceability and comparability is achievable only within the constraints of international conventions and predefined conditions that cannot account for all possible situations when pH is measured. Under these circumstances, it has been shown that comparison with primary results can be contradictory when secondary measurements (e.g. with glass electrodes) are obtained by considering the concentration scale of the hydrogen ion, for example when acid–base titration, aimed at formation constant refinement, is appreciated through potentiometric data. It is thought that the overall accuracy suffers because of the rough nature of some general assumptions about the chemical nature of the solution. Critical parameters have been identified in the ionic strength (I) and the ionic activity coefficients (i), usually approximated with the extended Debye–Hückel equation. In particular, difficulties arise when properties of the solutions, such as ionic strength and ionic medium, are varied, thus introducing significant noise able to reduce the accuracy of a comparison, since the quoted features, ionic medium included, may significantly affect a pH values. We propose the analysis of a series of comparative exercises supported by modifications of the theoretical approach to activity coefficients. Primary measurements (Harned cell) of the pH of a phosphate buffer have been considered and the results have been compared with secondary (glass electrode) measurements, executed according to either activity (paH) or concentration (pcH) scales. The emerging discrepancies have been mainly assigned to incomplete inferences of i from the chemical features of the testing solution. More comparable results are attainable if evaluation of i is performed using better estimates of the ionic strength, through an enhanced application of the Debye–Hückel theory [2, 3]. The final aims are the achievement of better harmonization for this fundamental quantity of chemistry and the improvement of the agreement between primary and secondary measurements of pH.
2007
Combinig and reporting analytical results. The role of (metrological) traceability and (measurement) uncertainty for comparing analytical results.
Roma
6-8 Marzo 2006
Royal Society of Chemistry
A. Fajgelj, M. Belli, U. Sansone
96
103
Paola Fisicaro; Enzo Ferrara; Enrico Prenesti; Silvia Berto
File in questo prodotto:
Non ci sono file associati a questo prodotto.

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/79768
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact