Measuring the IR transmission through a loosely packed powder layer offers a cheap and experimentally simple method for the quantitative and qualitative analysis of surface-treated powders. It is, however, practically impossible to obtain a perfectly smooth and even powder layer—a fact that introduces large errors in the quantitative determination. To cope with this problem, one derives an equation, by means of which it is possible to carry out the determination irrespective of the layer-thickness variations. The equation is based on the use of an "internal thickness standard." The theory is illustrated with, and supported by, a series of determinations of a silane coupling agent on a phlogopite powder. As predicted by the theory, a linear relationship is obtained when the absorbance ratio of the silane and the standard is plotted against the relative amount of silane. The quantification limit of the method lies in the monolayer range, depending on the powder surface area and the specific silane absorptivity. The method is thought to work equally well with powder mixtures.

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