Comparability of thermodynamic data--a metrological point of view.

In order to compare and to interpret chemical measurements, compliance with general rules of metrology is compulsory. Such rules are the more important the more the chemical measurements are applied under circumstances where material assets and goods or immaterial values like health may be affected. Metrology of chemical measurements attempts to define rules for achieving comparability and for guaranteeing quality of analytical data. Thermodynamic data are commonly derived from a set of analytical measurements. Comparability among thermodynamic data is an important issue especially for those data to be applied in politically sensitive issues of environmental prognosis, long-term safety assessment of nuclear waste repositories in deep geological formations and assessment of environmental impact of technical intervention in the geosphere. Taking the data evaluation step in the traceability chain of thermodynamic data as an example, the existing thermodynamic data is shown to be affected by deficiencies in comparability and quality that may severely limit its dependability in environmental prognosis. The need for a metrologically acceptable approach is demonstrated. Statistical concepts improving a reliable assignment of meaningful measurement uncertainty to a thermodynamic datum are presented. Unresolved issues, i.e. measurement uncertainty of a pH measurement, hampering the construction of a traceability chain are outlined.