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Electrochemical analysis

Electrochemical analysis - Primary reference bench for pH measurement

Measurement of pH, a routine procedure

Apart from the chemical balance and the thermometer, the pH meter is the most common measuring instrument in chemical analysis laboratories (95% of laboratories possess one). Measurement of pH is a routine procedure in a wide range of sectors. pH is a key factor of numerous reactions in biology, it can indicate the presence of pollution in the environment, and can reveal a metabolic disorder in medicine.

A traceability chain for pH measurements

In order to set up a traceability chain for pH measurements in France, LNE has developed a pH-metry primary reference bench.

Two ways of transferring references to testing and calibration laboratories are envisaged:

  • The first consists in certifying primary and secondary reference materials in order to ensure measurements are traceable to international SI units and to certify commercial pH buffer solutions.
  • The second consists in performing specific studies using a junction-free cell in sectors such as the environment (acid rain, surface water, sea water) or medicine (pH of blood and plasma) in order to define new calibration protocols and/or develop new reference materials.

A secondary reference bench is currently being developed. It will be used for certification of secondary materials or commercial solutions.

LNE plays an active part in French and European working groups to establish protocols for on-site assessment of pH measuring instruments.

A primary reference bench for pH-metry

This bench (figure 1) is the starting point for the national pH measurement traceability chain. It is based on measurement of an electromotive force between a hydrogen electrode and a silver/silver chloride reference electrode in a junction-free cell containing the solution to be measured (figure 2).

Figure 2: Junction-free cell

Figure 2: Junction-free cell

Figure 1: pH-metry primary reference bench

Figure 1: pH-metry primary reference bench

An international comparison of laboratories has confirmed LNE's calibration capacities between 15° and 37°C with an average expanded uncertainty of 0.004 pH unit. This places LNE among the leading National Metrology Institutes for primary pH measurements (see fig. 3).

Figure 3: Results of an international comparison on a carbonate buffer solution

Figure 3 : Results of an international comparison on a carbonate buffer solution

Extending the pH range

LNE's methods have been validated for phthalate, phosphate, borate and carbonate buffer solutions (pH range 4 to 10). The Laboratory's present calibration possibilities cover a limited pH range. In order to meet the needs of all industrial sectors, work is in progress to expand the pH range covered to values between 1 and 12.

Publications

R. Philippe, C. Rivier, I. Maksimov, "Banc de référence de pH-métrie", Congrès international de métrologie, Toulon, October 2003

R. Champion, C. Rivier, "Etat de l'art de la mesure primaire du pH", Revue Française de Métrologie, April 2006

[Download these documents] (in french)

Accreditation

LNE is accreditated by French accreditation committee COFRAC for calibration of primary reference materials for pH between 4 and 9.2.

[For more information see our COFRAC accreditation database] (in French)

Contact

Rachel Champion

00 (33) 1 40 43 38 77

 
 

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