Measuring Geometric Density without Mercury
Historically, geometric density was measured using liquid mercury pycnometry. However, using a uniform, small particle size powder, the same principles can be applied with powder pycnometry. As an illustration, powder pycnometry measurements for geometric density have been performed using the Anton Paar Autotap on two samples and compared with results from mercury measurements.
One of the most common density measurements involves the determination of the geometric space occupied within the envelope of a solid material, including any voids, cracks, or pores. This is called geometric, or envelope, density. This value only equals skeletal density when there are no internal openings in the material being measured. If the material has a uniform rectilinear shape, the volume it occupies can be calculated from measurements by caliper or ruler. However, a great majority of measurements involve complex shapes in formed materials such as ceramics, molded polymers, and granulated or pelleted products. In this case, the unknown volume may be determined by immersing it in a liquid and measuring the volume of liquid displaced, which is Archimedes principle (Figure 1).
For decades, liquid mercury has been used to measure geometric density because its non-wetting nature prevents its entry into small voids. However, if there are problems with mercury amalgamation, safe handling, or disposal facilities, liquids such as hot wax, oil, kerosene, and water are used. In the latter case, the sample is usually coated with a polymer spray to seal openings and an appropriate correction is made in each measurement.
As an alternative to mercury and to the other liquids mentioned, powder pycnometry can be performed. This technique consists of filling the space between particles with a sufficiently uniform, small powder and measuring tap density. This procedure is readily performed on the Anton Paar Autotap series.
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