Certification of Reference Material with Special Emphasis on Porous Solids

Abstract

Certified reference materials (CRMs) are very important for the reliability of the results of chemical analyses and, at the same time, constitute the basis of comparability of analytical data. The development of a new type of CRMs is demonstrated using the example of porous and finely dispersed solids having specific surface area, specific pore volume, pore volume distribution, and mean pore radius as their characteristic values.Many of the properties of reference materials to be certified, 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 of the probe molecules used, a fact which is due to the fractal character of these substrates. This is particularly true for the specific surface area or the 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 values for the characteristic values to be certified.Generally, the pore system of a solid contains pores of irregular shape. Therefore, depending on the presence of e.g. cylinder or slit pores, different evaluation models must be applied. For the verification of model calculation of microporous solids, zeolites with their crystallographically defined voids are particularly well suited. They can serve as a reference point for the correctness of the model used. Zertifizierte Referenzmaterialien (ZRM's) sind fur die Sicherheit und Zuverllissigkeit von Analysenergebnissen von groder Bedeutung und gleichzeitig Voraussetzung fur die Vergleichbarkeit von Analysendaten. Am Beispiel der fiir porose und feindisperse Festkorper typischen physikalischen Kenngrohn spezifische Oberflache, spezifMhes Porenvolumen, Porenvolumenverteilung und mittlerer Porenradius wird die Entwicklung eines ZRM dargestellt. Gegenuber der Zertifizierung der meisten Eigenschaften von Referenzmaterialien, etwa der Bestimmung der Elementgehalte, die mit moglichst verschiedenen, auf unabhiingigen MeSprinzipien beruhenden Verfahren vorgenommen wird, sind die Meflwerte porenbezogener KenngroDen von por& sen bzw. feindispersen Feststoffen infolge ihres Fraktalcharakters von der G r o k des eingesetzten Sondenmolekuls abhangig, was beispielsweise fur die spezifische Oberflache oder das Porenvolumen zutrifft . Daraus resultierend beziehen sich die zertifizierten Werte bei Anwendung der Methode der Gasadsorption auf ein spezifisches Medgas, z. B. Nz. Die Verwendung anderer Gase fiihrt zu anderen Zahlenwerten fur die zu zertifizierenden Kenngrohn. Im allgemeinen weist das Porensystem eines Festkorpers Poren von unregelmaiger Gestalt auf. Es bedarf: daher der Anwendung unterschiedlicher Modelle, z. B. fiir Zylinder-oder Schlitzporen. Fur die Uberprufung von Modellrechnungen fiir mikroporose Festkorper eignen sich Zeolithe mit ihren kristallografisch definierten Hohl...

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