ANALYTICAL EDITION
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&??pem?w@ BfHwi?L Figure 1-Correction Chart for Effect of Temperature on B. and B. Soap Test
below 10’ C. is the only part of the curve having significance for work in the field, but here the correction assumes real importance. For instance, in Chicago, where the winter temperature of the water is 1.5” to 2.0” C., the error amounts to 4 drops of soap. Since softener runs are often terminated when the effluent requires 5 drops, this test would indicate that the softener was incapable of delivering water softer than 1 grain per gallon, whereas in reality the water would be of “zero” hardness, requiring 1, or at most, 2 drops of soap. The effect of temperature is independent of the true hardness of the water up to 3 or 4 grains per gallon, the increase in soap requirement being identical for a “2-drop” water and a “15-drop” water. Like low temperature, free carbon dioxide is a very disturbing factor. I n the field its influence appears to be exceedingly erratic, owing in part to its fugacity, in part to the poor end point that it causes, and in part to the fact that its effect varies with the alkalinity of the water, the hardness, and other factors. Thus a simple correction chart for its influence cannot be constructed. However, its effect can be stated in more general terms, and a typical example is illustrated by Figure 2. For instance, the increase in soap required for a
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carbon dioxide will cause a slightly greater increase in the soap requirement of a very soft water, say a “2-drop” water, than in the requirement of a harder water of 10 or 15 drops. Further, it may be seen from the chart that the influence of carbon dioxide decreases very markedly with increasing alkalinity of the water. This is owing to the repression of the hydrogen-ion concentration by the buffer action of the bicarbonate. The lower chart on Figure 2 illustrates the relationship between hydrogen-ion concentration, alkalinity, and carbon dioxide content. It shows how a given carbon dioxide concentration produces a greater depression of the pH in waters of low alkalinity. Furthermore, a comparison of the two charts demonstrates that the depression of the pH and increase in soap requirement go hand in hand-that is, the lower the pH, the greater the soap requirement. This is the significant point, for it indicates that the effect is a function of hydrogen-ion concentration rather than of the carbon dioxide itself. It is substantiated by the fact that a similar increase in soap requirement is obtained with distilled water, free of carbon dioxide, to which minute quantities of mineral acid have been added. In such waters the increase in soap requirement i%directly proportional to the quantity of acid added. The action is apparently a decomposition of the soap with liberation of fatty acid, The end point with mineral acid is very sharp, a copious lather being produced immediately when neutralization of the acid is complete. With carbon dioxide, however, as might be expected, the progressive shift of the equilibrium of the slightly ionized acids with successive additions of soap produces a poor lather and an uncertain end point. The difficulty caused by the presence of carbon dioxide and mineral acid may be completely removed by adding to the soap solution sufficient sodium hydroxide to neutralize the acidity of the water, which reestablishes very satisfactorily the proportionality between true hardness and drops of soap up to 3 or 4 grains per gallon. This expedient is very convenient for zeolite work where the waters tested lie within this hardness range, but the use of such alkaline soap for general field work is not recommended, because with larger amounts of soap solution the caustic concentration may become sufficient to effect some softening of magnesium waters and waters containing carbonate hardness. It has been indicated on Figure 2 that increased alkalinity of the water is without effect on the soap test. This was ascertained by adding different amounts of sodium bicarbonate to a water of known hardness. Sodium chloride and sodium sulfate likewise have no effect up to concentrations of 2500 p‘ p. m. (150 grains per gallon). The only action to be expected from these neutral salts would be a salting-out of the soap but, within the limits of concentration encountered in water treatment, this does not occur.
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