The importance of traceability in relation to pH and conductivity

For laboratories concerned by certification or accreditation, the use of certified reference materials traceable to their national standards is highly recommended. Patricia Dalmas reports.

Today, analytical laboratories are obliged to make use of metrological tools in order to ensure they obtain totally reliable and easily traceable results. The pH and conductivity measuring systems they use are calibrated using reference materials, which are fully traceable to primary standards. This article contains advice on how to be sure of the quality and traceability of your pH and conductivity measurements.

ph measurements

A pH measuring system consists of three elements: the pH meter, the sensors and the standard solutions used to calibrate the entire system.

Most manufacturers are able to supply a verification certificate guaranteeing that the meter meets its specifications and a calibration certificate including the measured values after checking. The same applies to the temperature sensor. Electrode specifications (ie sensitivity and pH0) tend to vary over time whether you are using a combined pH or separate electrodes, making it necessary to calibrate regularly using standard solutions. As all the measurements that follow are based on the calibration result, the choice of standards is crucial. Many manufacturers offer standard solutions, but they do not all have the same degree of traceability. The following guidelines can help you evaluate their quality:

* The formulations of the standards should comply with NIST/IUPAC or DIN19266 specifications.

* The manufacturer should state the nominal pH value with an uncertainty at a given temperature. For standards following IUPAC/NIST formulation, the uncertainty is 10 mpH.

* Standards should be delivered with a certificate of conformity and traceability giving details including the batch number of the product and Reference Material, the certified value and its uncertainty, the length of time for which the certified value is guaranteed and measurement and control procedures.

* Traceability to the Standard Hydrogen Electrode should be clearly defined (see Figure 1). Do not hesitate to contact your supplier and ask for a certificate proving this traceability. This usually takes the form of an accreditation certificate established either by your national authority or by a body with a mutual accreditation agreement. Note that very few laboratories* are accredited for pH reference materials.

Conductivity measurements

The principle is similar to that of pH. The conductivity meter and temperature sensor can be calibrated by the manufacturers. The conductivity cell constant has to be checked regularly so theuse of certified and traceable conductivity standards is required. The standards you choose should be:

* Traceable to SI via Standard Reference Materials manufactured by NIST.

* Manufactured and calibrated according to the internationally recognised Demal scale laid down by the OIML** (Recommendation No.56).

* Delivered with a certificate of conformity and traceability.

* Supplied with an accreditation certificate to prove traceability to your national standards. Note that very few laboratories are accredited* for conductivity reference materials.

By using these reference materials, you can take advantage of full and recognised traceability to your national standards, calculated uncertainties and certificates of proof, giving you complete confidence in your results.

* Radiometer Analytical is one of the only laboratories in Europe accredited for pH and conductivity reference materials.

** OIML= International Organisation of Legal Metrology.

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Patricia Dalmas, MeterLab Product Specialist, is with Radiometer Analytical SAS, Villeurbanne, France. www.radiometer-analytical.com

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