Moisture content
From Wikipedia, the free encyclopedia
Moisture content (MC) is a term used in a wide range of scientific and technical areas to express the quantity of water that a material contains.
For rigid materials such as ceramics the moisture content w is expressed as the ratio of the mass of moisture to the dry volume of the material:
For materials that change in volume with moisture content, such as wood, the moisture content u is expressed in terms of the mass of moisture per unit dry mass of the specimen:
Moisture may be present as adsorbed moisture at internal surfaces and as capillary condensed moisture in small pores. At low relative humidities, moisture consists mainly of adsorbed water. At higher relative humidities, liquid moisture becomes more and more important, depending on the pore size. In wood-based materials, however, almost all moisture is adsorbed at humidities below 98% RH.
In biological applications there can also be a distinction between physisorbed water and free water — the physisorbed water being that closely associated with and relatively difficult to remove from a biological material. The method used to determine water content may affect whether water present in this form is accounted for.
Water molecules may also be present in materials closely associated with individual molecules, as "water of crystallization", or as water molecules which are static components of protein structure.
Detemining moisture content can be achieved in many ways, depending on the material being examined (see electronic moisture analysers) — chemical titrations (for example the Karl Fischer titration), determining mass loss on heating (perhaps in the presence of an inert gas), or after freeze drying. In the food industry the Dean-Stark method is also commonly used.
