Electrochemical Properties of Water and Selected Organic Solvents

4 ELECTROCHEMICAL PROPERTIES OF WATER AND SELECTED ORGANIC SOLVENTS 

A laboratory engaged in careful electrochemical work with aqueous solutions or in trace analysis will need facilities for the preparation and storage of highly purified water. Water commonly is contaminated with metals in both dissolved cationic form and in the form of colloidal or particulate matter that is not ionized appreciably.70 Frequently it also is contaminated by bacteria and by organic impurities that cannot be removed by ordinary or oxidative distillation because of the steam volatility of the impurities.71 

The purification of water has been treated exhaustively in the literature, and reviews have summarized general conclusions and recommended techniques for the ultrapurification and analysis of water.70,72 There is a general consensus among investigators that (1) water purified in quartz or plastic apparatus contains a lower level of cationic impurities than water purified in borosilicate or metal apparatus (2) storage of purified water for periods exceeding 30 days, even in plastic or Teflon containers, will result in an increase in cationic impurities and (3) resistivity measurements may be used as a survey technique but cannot be relied on for an unequivocal indication of water quality because they do not indicate nonionic impurities. 

Removal of cationic impurities from water. Careful analysis of water purified by various methods (see Table 7.10) indicates that the water that is obtained by passing ordinary distilled water through a small monobed deionizer (contained in polyethylene) and a submicrometer filter is equal or superior (with respect to cations) to water obtained by distillation in conventional quartz stills, and is distinctly superior to the product from systems constructed of metal.70 From the data available in the literature, simple distillation clearly does not produce high-purity water. In practice, two effects cause contamination of the distillate. Entrainment is the major factor that prevents the perfect separation of a volatile substance from nonvolatile solids during distillation. Rising bubbles of vapor break through the surface of the liquid with considerable force and throw a fog of droplets (of colloidal dimensions) into the vapor space 

TABLE 7.10 Metal Content of Water Purified by Various Methods  

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