Jar Tests For Optimum pH Determination

We may summarize the optimum pH's of the coagulants obtained in the previous examples: alum _ 5.32, ferrous _ 11.95, and ferric _ 8.2. The problem with these values is that they only apply at a temperature of 25°C. If the formulas for the determination of these pH's are reviewed, they will be found to be functions of equilibrium constants. By the use of the Van't Hoff equation, values at other temperatures for the equilibrium constants can be found. These, however, as mentioned before, also need the value of the standard enthalpy change, AH298, as discussed in the chapter on water stabilization. For the aforementioned coagulants, no values of the enthalpy change are available. Thus, until studies are done to determine these values, optimum pH values must be determined using the jar test.

In addition, the optimum pH's of 5.32, 11.95, and 8.2 were obtained at a dissolved solids of 140 mg/L. The value of the dissolved solids predicts the values of the activity coefficients of the various ions in solution, which, in turn, determine the activities of the ions, including that of the hydrogen ion. It follows that, if the dissolved solids concentration is varied, other values of optimum pH's will also be obtained not only the respective values of 5.32, 11.95, and 8.2. This is worth repeating: the values of 5.32, 11.95, and 8.2 apply only at a dissolved solids concentration of 140 mg/L. In addition, they only apply provided the temperature is 25°C. In subsequent discussions, mention of these optimum pH values would mean values at the conditions of 25°C of temperature and a solids concentration of 140 mg/L.

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