What Is Specific Surface Area?

Specific surface area is a measure of a solid object. It compares the surface area of the object to either its mass, and is particularly relevant in materials such as soil, or with materials that are theoretically a smooth surface but actually have defects such as scratches. There are several measurement techniques, which can produce varying results, with each method suited to particular types of material.

From a mathematical perspective, this is a very simple concept. For example, a 4 inch (10 cm) cube has a total surface area of 6 x 4 inches x 4 inches, which equals 96 square inches (660 square centimeters). If the cube has a mass of 7 ounces (approx 200g), the specific surface area is 13.7 square inches per ounce (approx 3.3 square centimeters per gram).

This formula can also be used to measure deficiencies and inconsistencies. For example, a set of dice will have a slightly difference surface area and mass to a pure cube because of the dimples indicating the numbers. Whether this leads to a higher or lower specific surface area depends on the size and depth of the dimples. In theory all the dice will have the same specific surface area as one another, but there may be variations if they are not made consistently. This concept can apply on a far finer scale, for example in scratches on an otherwise smooth piece of metal.

There are three main ways of measuring specific surface area. The first is through adsorption, which is where particles of a gas, liquid or dissolved solid stick to the material being measured; a common example of the process is when moisture is "soaked up" by silica gel. A complex formula known as the BET equation can calculate the surface area by using data observed during this process. However, the result varies depending on what material is used in the adsorption.

A simpler measure, most appropriate to materials such as soil, is particle distribution. This involves using a variety of methods to sort the individual particles of the material by size. This can be done as simply as using a range of sizes, or in a manner as complicated as using laser beams.

The third method is used for materials in powder form. It involve forcing a gas such as air through a bed of the powder and measuring the resistance caused by the particles. One common use of this method is in assessing the quality of powdered cement, the idea being that the specific surface area will influence how quickly it sets.