Recording Microbalance Applications
Water Content and Vapour Pressure
CI Vacuum Microbalance
A CI Vacuum Microbalance has been used to monitor continuously the weight of a sample, while changing in steps the humidity of the atmosphere surrounding it. Thus a convenient method of measuring the water content of the sample at different humidities is provided.
The sample of materials to be tested is placed in the left hand pan, which hangs on a wire several inches long. This wire is passed through a vertical tube, which is set into a hole in the centre of a lid of a glass jar, which surrounds the sample. The wire is free to move in the tube, and the lid of the jar is securely clamped. A number of jars is provided, each containing a saturated solution of a salt, to provide a known humidity.
The humidity surrounding the sample can be changed simply by replacing one jar with another. The weight of the sample at each humidity level can be read from the balance, and the percentage weight gain due to water absorption calculated.
Determination of Vapour Pressure
The Vapour Pressure of a substance is defined as the saturation pressure above a solid or liquid substance. At thermodynamic equilibrium, the vapour pressure of a pure substance is a function of temperature only. The balance head, removed from the standard glassware, is placed inside a large vacuum chamber. The vacuum chamber contains a micro-furnace and a means of cooling the balance pan.
The vapour pressure can be determined in two ways. Directly, from the force of the vapour pressure on the pan, or by measurements of the rate of evaporation of the substance under test.
The first method is dependent on the area of the hole through which the vapour exits from the sample chamber, but it is independent of the molecular mass. It is therefore suitable for determining the vapour pressure of mixtures where impurities may be present. The force of the vapour striking the pan can be read from a chart recorder trace as a negative change in weight of the balance pan.
The second rate of calculating the vapour pressure involves measuring the rate of condensation of the vapour onto the balance pan, which can be used to calculate the evaporation rate of the substance.
