Most natural waters, domestic wastewaters, and many industrial wastes are buffered principally by a carbon dioxide-bicarbonate system. By reference to Fig. 4.8, which shows titration curves for several weak acids, it will be noted from the curve for carbonic acid that the stoichiometric end point is not reached until the pH has been raised to about 8.5. On the basis of this information, it is customary to consider that ahwaters having a pH lower than 8.5 contain acidity. Usually the phenolphthalein end point at pH 8.2 to 8.4 is taken as the reference point. Inspection of the curve for carbonic acid in Fig. 4.8 shows that at pH 7.0 considerable carbon dioxide remains to be neutralized. It also shows that carbon dioxide (carbonic acid) alone will not depress the pH below a value of about 4.
Figure 4.7 shows a titration curve for a strong acid, and from the nature of the curve, it may be concluded that neutralization of the acid is essentially complete at pH 4. Thus, from the nature of the titration curves for carbonic acid and for strong acids, it becomes obvious that the acidity of natural waters is caused by carbon dioxide or by strong mineral acids, the former being the effective agent in waters having pH values greater than 4 and the tet^Jjre j^ectij^ with pH
values less than 4, as shown in Fig. 17.1;
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