Mull, coal

Like conductivity, dielectric constant is strongly dependent on water content. Indeed, the dielectric constant can even be used as a measure of moisture in coal (Chapter 3). Meaningful dielectric constant measurements of coal require drying to a constant dielectric constant, and several forms of coal are used for dielectric constant measurements. These include precisely shaped blocks of coal, mulls of coal in solvents of low dielectric constant, or blocks of powdered coal in a paraffin matrix. 

The problems associated with water in alkali halide preparations suggest that other preparative techniques are worth pursuing. Mulling techniques were originally used in coal studies but require long grinding times. Furthermore, quantitative studies are difficult if not impossible on the resulting smears that are usually used for analysis. 

The example shown in Figure 13.29 is the spectrum of a high-volatUity coal. The sample was prepared by grinding as described under the discussion of mulling at a concentration of about 10% with KBr powder. Coal is a difficult sample to prepare for an IR spectrum, and diffuse reflectance provides a relatively easier way to produce good data. Quantitative analysis has been performed on coal samples using this technique (see Fredericks et al.). 

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