Water hardness, practical applications

Transmission of ultrasonic waves in air is difficult (the intensities received are low) and to overcome this the material to be examined must be immersed in water. A further limitation follows from the frequencies that may be used with plastics, and in practice application is mainly to hard and to uniformly dense products in which the defects are not more than one millimetre in diameter. 

The first natural microporous aluminosilicate, i.e., natural zeolite, was discovered more than 200 years ago, and after long-term practical applications, the intrinsic properties of natural zeolites such as reversible water-adsorption capacity were fully recognized.13 41 By the end of the 19th century, during exploitation of ion-exchange capacity of some soils, it was found that natural zeolites exhibited similar properties some cations in natural zeolites could be ion-exchanged by other metal cations. Meanwhile, natural chabazite could adsorb water, methanol, ethanol, and formic acid vapor, but could hardly adsorb acetone, diethyl ether, or benzene. Soon afterwards, scientists began to realize the importance of such features, and use these materials as adsorbents and desiccants. Later, natural zeolites were also used widely in the field of separation and purification of air. 

Completely semipermeable membranes do hardly exist. Most of the membranes used in practical applications can generally be passed by substances with a low molar mass, whereas large molecules are rejected. In most of the cases, the solvent is water, an example for a membrane is cellophane. 

Originally, that is in 1795, the gram was chosen as mass standard. It was to represent the mass of 1 cm H2O at its freezing temperature. In 1799 the mass standard was changed to 1000 cm water at its maximum of density at 277.13 K, since the larger mass could be measured more precisely. Today this connection is only approximate, but the difference from the old size is hardly noticeable for practical applications. The mass standard is today independent of the volume of water. 

In treated water for high-pressure boilers or where radiation effects are important, as in some nuclear projects, impurities are measured in very small units (e.g. g/litre or p.p. 10 ), but for most purposes it is convenient to express results in mg/litre. In water analysis, determinations (except occasionally for dissolved gases) are made on a weight/volume basis but some analysts still express results in terms of parts per million (p.p.m.). The difference between mg/litre and p.p.m. is small and for practical purposes the two units are interchtmgeable. For some calculations, the use of milli-equivalents per litre or equivalents per million (e.p.m.) has advantages but has not found much application. Hardness, whatever the constituent salts, is usually expressed as p.p.m. CaCOs (see Table 2.10). 

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