For certain materials and powders, porosity is a characteristic of great importance. Porosity is a measure of the voids or pores in a solid substance, and is defined as the ratio of the sample’s void volume and total volume.

There are a number of ways to measure the void volume, and mercury porosimetry is one widely used technique. Mercury is introduced under pressure into a sample container. As the pressure is increased, the mercury is forced into smaller and smaller voids in the sample. The volume of the filled voids is thus equal to the volume of intruded mercury, which the porosimeter measures. Because this volume depends on the applied pressure, the instrument builds up a curve of pore volume versus pressure. Making use of a mathematical relation between the actual size of an idealized cylindrical pore and the pressure, porosimetry can provide measurements of the actual size and distribution of the voids in the sample.

The pore size is inversely proportional to the pressure, so that smaller pores require higher pressures to reach. Thus, one way to differentiate mercury porosimeters is by the maximum pressure reached by the system. Typical high-pressure systems reach pressures thousands of times greater than atmospheric pressure, corresponding to pore sizes with diameters of about a nanometer.

While mercury has advantages for this technique, since it is a nonwetting fluid, there are also drawbacks. With some metal samples, the mercury will amalgamate under pressure. Mercury can also be physically left behind in voids, requiring elaborate recovery methods or difficulties in disposal. To eliminate these difficulties, the same method of intrusion porosimetry can be used with other fluids. Nevertheless, mercury porosimetry faces competition from completely different methods, such as those involving gas exchange or direct microscopic inspection of the sample.

There are many practical applications for porosimetry in the chemical and material sciences. Porosity is related to chemical activity, as the voids provide additional surface area where chemical reactions can take place. Thus porosity is important in optimizing industrial reactions, catalysts and the dissolution behavior of pharmaceutical preparations. Similarly, porosity also bears on the properties of chromatography column packings. Other sample types include soils, ceramics, glasses, refractories, textiles, paper and building materials. The porosity of construction materials is related to compressive strength, while the porosity of soils and rocks can yield information about seismic behavior and water or oil reservoirs.

The market for mercury porosimetry was about $10 million in 2009. Relatively few vendors participate in this market. Among them are Micromeritics, Quantachrome, Thermo Fisher Scientific, and Porous Materials. Vinci Technologies is a niche supplier of systems for use with rock samples generated in petroleum exploration.

Mercury Porosimetry at a Glance:

Leading Suppliers

• Micromeritics

• Quantachrome

• Thermo Fisher Scientific

Largest Markets

• Chemicals

• Pharmaceuticals

• Construction Materials

Instrument Cost

• $15,000–$50,000