Hard water, definition

The so-caUed hardness of water is sometimes mentioned in connection with calcium and magnesium contents, but it is not unambiguously defined in the literature as it is based on either technology or analysis. As the term hardness does not correspond to the actual behaviour of water ( hard water = ice) and because there are difficulties with the exact definition of various types of hardness, this term is gradually disappearing from the literature on hydrochemistry. 

Topics under the practical importance of the alkaline-earth metals include their significance in living systems, the incidence of beryllium disease, radiochemical applications as components and residues of nuclear power, metallurgical uses in preparing pure metals and alloys, use in fireworks and X-ray technology, the definition and properties of hard water, and the role of calcium phosphate in bone and teeth structure. 

Description of the Method. The operational definition of water hardness is the total concentration of cations in a sample capable of forming insoluble complexes with soap. Although most divalent and trivalent metal ions contribute to hardness, the most important are Ca + and Mg +. Hardness is determined by titrating with EDTA at a buffered pH of 10. Eriochrome Black T or calmagite is used as a visual indicator. Hardness is reported in parts per million CaCOs. 

Before proceeding further it is well to consider the term cement, for its definition can be the source of some confusion. Both the Oxford English Dictionary and Webster give two alternative definitions. One defines a cement as a paste, prepared by mixing a powder with water, that sets to a hard mass. In the other a cement is described as a bonding agent. These two definitions are quite different. The first leads to a classification of cements in terms of the setting process, while the second lays emphasis on a property. In this book the term cement follows the sense of the first of these definitions. 

As mentioned earlier, the hardness of a water sample is often reported as parts per million of calcium carbonate. Using our definition of parts per million for liquid solutions (Table 5.4), we have... 

It must bo remembered that this mode effects the bicarbonate of lime, not the more troublesome earthy salts, each as the sulphates and chlorides, on which the hardness of spring waters mainly depends. The diffi culty of mixing lime and watar, in definite proportions, on the large scale, must he obvious to every one. If toe much be employed, the. companies would supply their customers with litne- icater If too little, the bicarbonate of limo would not be completely destroyed, and the process would be a Failure. Altogether, the difficulties of carrying out the process Will ever prevent its adoption on a large scale. 

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