HARDNESS - Hardness was originally defined as a measure of the ability of water to precipitate soaps made from fatty carboxylic acids. These "soaps" precipitated in the presence of calcium and/or magnesium ions. Today, hardness is used to describe the total concentration of calcium and magnesium, expressed as mg/L of calcium carbonate. It is generally calculated from measurement of calcium and magnesium in ion concentrations using:

ION - A charged particle of matter. In each system of matter, the number of positive ions (cations, such as sodium) is equal to the number of negative ions

ION EXCHANGE - Ion exchange is based on the principle of electroneutrality, that is, charged species are stable only when they exist as balanced pairs of positive and negative charges. Ion exchange resins, the materials used to carry out the process of ion exchange, are particles which contain fixed charges on their surface. To maintain electroneutrality, each of these charges has an ion of equal and opposite charge held to it; these ions are called counter ions. The counter ions are mobile and can leave the fixed charge if some other counter ion is available to replace it.

MEMBRANES - Membranes are thin films made with structures designed to provide selective transport of solutes. In general, the selectivity of a membrane is based on its ability to pass or exclude species according to their size. Membrane structures may be homogeneous or asymmetric. Homogeneous membranes have structures which are uniform in cross-section, at least to a magnification of 100x (Figure M.1a). Most homogeneous membranes have been developed for Microfiltration and hemodialysis.