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 importan t 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. Thermodynami c data are commonly derived from a set of analytical measurements. Comparab ility among thermodynamic data is an important issue especially for those d ata to be applied in politically sensitive issues of environmental prognosi s, long-term safety assessment of nuclear waste repositories in deep geolog ical formations and assessment of environmental impact of technical interve ntion 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 f or a metrologically acceptable approach is demonstrated. Statistical concep ts improving a reliable assignment of meaningful measurement uncertainty to a thermodynamic datum are presented. Unresolved issues, i.e. measurement u ncertainty of a pH measurement, hampering the construction of a traceabilit y chain are outlined.