Problem 9.4: A cooling tower operates at pH 83 with a free

From Chapter 9 Problem Set. Level 1: Conceptual Mastery.

Question

A cooling tower operates at pH 8.3 with a free chlorine residual of 0.5 mg/L. Using the chlorine/pH table, estimate the percentage of chlorine present as HOCl. Is this adequate for biological control?

Answer

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At pH 8.3, interpolating from the chlorine/pH table, approximately 13–14% of the chlorine is present as HOCl, with 85–88% as the far less effective OCl⁻. At 0.5 mg/L total residual, only about 0.06–0.08 mg/L is present as HOCl. This is marginal at best for effective biological control. The tower needs either pH reduction to shift the equilibrium toward HOCl, a switch to a more pH-stable oxidant such as bromine, or an increase in total residual to compensate for the reduced effectiveness at this pH.