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R9 Laboratory SOP 1135
TOTAL AND FREE RESIDUAL CHLORINE - HACH DPD
Summary
These methods are based on Hach Methods 8167 (total) and 8021 (free), and are equivalent to EPA Method 330.5 for wastewater and Standard Method 4500-Cl G for drinking water. These methods are applicable to the measurement of total and free residual chlorine in any drinking water, wastewater, ground or surface water. The practical detection range of the determination is from 0.2 - 2 mg/L.
Chlorine (hypochlorite ion, hypochlorous acid) and chloramines stoichiometrically liberate iodine from potassium iodide at pH 4 or less. The liberated iodine reacts with N,N-diethyl-pphenylene diamine (DPD) to produce a red colored solution. The solution is measured spectrophotometrically and compared by the instrument to a standard response curve. The results are read directly as mg/L chlorine.