Project Details Comparable pH measurements by metrological traceability

Project No.: 
Start Date: 
01 January 2005
End Date: 
31 March 2013


Series Titles: pH Measurements in Complex Matrices

  • Part I – pH Measurements in water quality monitoring and assessment
  • Part II – pH Measurements of clinical, biochemical and environmental relevance

– To implement traceability chains for pH values in routine measurements in order to achieve target uncertainties for specific applications.

– To develop educational and quality control tools for reference and testing laboratories under the observation of chemical and metrological principles.

– To improve the comparability and the assessment of pH values.


The most often measured chemical parameter, pH, has driven the awareness of scientists and decision makers to the need for reliable analytical results. Its unique feature of being defined in terms of a single ion activity, the multiplicity of practical steps and assumptions involved in the assessment of pH values for primary reference buffer solutions and the diversity of secondary methods, give a slight overview of the multidisciplinarity of the subject with multifold implications. After the production of the IUPAC paper “The Measurement of pH. Definitions, Standards, and Procedures (IUPAC Recommendations 2002)”, Pure Appl. Chem. 74, 2169-2200 (2002), a workshop “Importance of Traceable pH Measurements in Science and Technology conducted at PTB/Braunschweig, Germany, in September 2001, organized and promoted by members of this Project’s Task Group, with a wide range of participants, revealed priorities and showed a strong request from the concerned community for continuing action of the task group, on

– Educational efforts on the calculation of the uncertainty of pH values;

– Elaboration of recommended protocols for specific applications (e.g. quality monitoring and assessment in the different applications of water and in physiological media) by round robin studies, observing the traceability chain, calculating the uncertainty of the sample pH;

– Critical assessment of the existing methods to calculate the hydrogen ion activities and concentrations, allowing extension of the presently adopted model for a wider range of applications.

The proposal aims at covering these objectives. Work is progressing in the frame of the group’s research activities and will be submitted to IUPAC for endorsement.

> link to former project of the on the Definition of pH Scales (#551/1/97)


> project announcement published in Chem. Int. May/Jun 2005

The Task Group Chair has published in February 2005 the following paper of relevance to this project: ‘Reassessment of pH Reference Values with Improved Methodology for the Evaluation of Ionic Strength’, M. J. Guiomar H.M.Lito and M. Filomena G.F.C. Camoes, Anal. Chim. Acta, 531, 2005,141-146 [doi:10.1016/j.aca.2004.09.048]

The task group met in October 2005 and in April 2006.

> May 2006 report update (pdf file – 36KB)

> Jan 2007 – completion of related project 2004-016-2-500, titled ‘Guidelines for potentiometric measurements in suspensions’

> July 2007 report update (pdf file – 45KB)

> October 2008 report update (pdf file – 18KB)

> January 2012 update – project completed

A publication titled pH of seawater by G.M. Marion, F.J. Millero,
M.F. Camoes,
P. Spitzer,
R. Feistel,
and C.-T.A. Chen has been published in Marine Chemistry Volume 126, Issues 1-4, 20 September 2011, Pages 89-96;
[…] The main conclusions of this review are: (1) pH definitions and conventions are highly variable, which leads to highly variable estimates of pH. For example, for seawater at SA = 35.165 g/(kg soln), t = 25 °C, P = 1.0 atm, and fCO2 = 3.33E-4 atm, model calculated pH values varied from 8.08 to 8.33 on the various pH scales; (2) An acceptable nomenclature is needed to keep pH variability unambiguous, due to alternative definitions and conventions. A nomenclature example is given in this paper. It is the (still unsolved) task of international bodies such as IUPAC or IOC to develop and promote such widely recognized conventions; (3) pH can be accurately estimated based on measurement (potentiometric, spectrophotometric) and modeling approaches. Accuracy via different definitions and conventions clearly requires consistency with respect to experimental measurements, equilibrium constants, activity coefficients, and buffer solutions that are used for specific approaches; (4) “Total” pH accuracy that includes the Bates-Guggenheim convention is ± 0.01 pH units. Removing the Bates-Guggenheim convention from the accuracy calculation can lead to “conventional” accuracies of ± 0.004 pH units; (5) pH extensions to high solution concentrations are capable using the Pitzer modeling approach. Modeling can, in principle, lead to pH estimates that are more accurate than measurements, which is illustrated with two Pitzer models for natural waters made up of the major components of seawater. But this principle still needs to be proven; (6) It is recommended that ocean scientists use the free concentration or activity of the proton to examine the effect of pH on processes in the oceans.

Outcoming activities will proceed in recently approved European Project- ENV05 JRP- EURAMET- Metrology for ocean salinity and acidity (OCEAN), which counts with worldwide collaboration at the level of the pertaining experts and institutions (pdf summary report issued Nov 2011)

See next project titled “pH Measurement in Seawater” (project 2013-013-1-500)