Curious case rule violation testing aldehydes, ketones

Curious case of rule violation while testing aldehydes, ketones

9:45 AM, 22nd February 2016
Curious case of rule violation while testing aldehydes, ketones
© loraks - Fotolia.com

BRUSSELS, BELGIUM: Joint Research Centre science hub (JRC), a European Commission’s in-house science service scientists said that they have investigated an apparent violation of the well-established rule that chemical reactions proceed faster at a higher temperature (Arrhenius’ law).

This investigation was triggered by observations during the regular stability monitoring programme of certified reference materials (CRMs) by JRC-IRMM. It was found that one material (BCR-551) was stable when stored at -20 °C, but was degrading at -70 °C. This effect was reproducible, as samples moved from storage at -20 °C to -70 °C started also to show degradation peaks.

In collaboration with the Flemish research institute VITO, scientists from the JRC-Institute for Reference Materials and Measurements (IRMM) discovered that the degradation product of the CRM BCR-551 (a solution of four 2,4 dinitrophenylhydrazine derivatives) is most likely a reaction product of the starting material, dinitrophenylhydrazone and the excess reagent from the synthesis of this CRM produced in 1993. Cryo-concentration seemingly causes the reaction, as the solvent (acetonitrile) is liquid at -20 °C but solid at -70 °C.

It was suggested that the higher concentration of the substances in the remaining liquid at -70 °C has a larger influence on the reaction rate than the lower temperature, thus leading to the seeming violation of the Arrhenius’ law.

This finding has implications for the stability testing of reference materials in general: It is sometimes assumed by some researchers that stability under specifically tested conditions can be extrapolated to untested conditions. For example, if a material is found to be stable at 4 °C for x years, it shall be stable at -20 °C for 4 times x years. This publication shows that this may not be true even for simple systems like organic substances in solution and, therefore, acts as a strong warning: Do not extrapolate stability data beyond the tested conditions!

The CRM BCR-551 is an important tool for the quality assurance of measurements of the concentration of aldehydes and ketones eg. in indoor-air to understand workplace exposure or emissions from furniture and construction materials.

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