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Wolman and aiman3 were the first to call attention to the value of hydrogen ion concentration in water treatment. Since the appearance of their article the influence of hydrogen ion concentration on the chemical reactions involved in the treatment of public water supplies has been well established, and no water purification plant using a coagulant before filtration can be operated at its greatest efficiency without a fairly good knowledge of such influence. When a chemical compound, such as aluminum sulfate or lime, is added to the water there is always produced a change in the hydrogen ion concentration. In certain cases the measurement of this change will actually indicate the amount of chemical applied. The application of lime is generally made for the direct purpose of causing a change in hydrogen ion concentration or pH. Thus, in the "iron and lime" method, the purpose of the lime is to bring the water to approximately that pH at which maximum precipitation of iron hydroxide will take place. In those cases where lime is added for softening purposes, or to prevent corrosion, the purpose is the same namely, to alter the hydrogen ion concentration. In the application of alum, however, the change in pH is usually incidental and mayor may not be in such direction as to aid in the clarification. It is quite generally recognized that, with alum, the isoelectric point and the maximum precipitation and clarification occur at a pH of between 5.5 and 6.O.4 Several workers,5 however, have found that |
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