CERTIFICATION OF REFERENCE MATERIAL WITH SPECIAL EMPHASIS ON POROUS SOLIDS

Certified reference materials (CRMs) are very important for the reliab ility of the results of chemical analyses and, at the same time, const itute the basis of comparability of analytical data. The development o f a new type of CRMs is demonstrated using: the example of porous and finely dispersed solids having specific surface al ea, specific pore v olume, pore volume distribution, and mean pore radius as their charact eristic values. Many of the properties of reference materials to be ce rtified, such as their elemental content, are established by means of measurement procedures relying on different and independent principles of measurement. With porous or finely dispersed solids, however, the characteristic values referring to pores strongly depend on the size o f the probe molecules used, a fact which is due to the fractal charact er oi. these substrates. This is particularly true for the specific su rface area or tile pore volume. As a result, when applying the method of gas adsorption. the certified values refer to a specific probe gas, e.g. to nitrogen. The use of other gases yields different numerical v alues for the characteristic values to be certified. Generally, the po re system of a solid contains pores of irregular shape. Therefore, dep ending on the presence of e.g, cylinder or slit pores, different evalu ation models must be applied. For the verification of model calculatio n of microporous solids, zeolites with their crystallographically defi ned voids are particularly well suited. They can serve as a reference point for the correctness of the model used.