# Converting Chlorine Dioxide to Free Residual Chlorine

Our water treatment facility used Chlorine Dioxide to disinfect the produced drinking water. The analysis results indicate concentrations of 0.2 mg/l of chlorine dioxide at the source. Can we analys residual chlorine in the distribution network? We dont have water quality standards for chlorine dioxide, only residual chlorine. Can I calculate the concentration of chlorine in the water from the concentration of chlorine dioxide?

Some chemistry, first add H2O2 and NaOH:

$$\ce{2 ClO2 + H2O2 + 2 NaOH -> 2 NaClO2 + O2 (g) + 2 H2O }$$

$$\ce{2 NaClO2 + 2 H2O2 -> 2 NaClO + 2 O2 (g) + 2 H2O }$$

$$\ce{2 NaClO + 2 H2O2 -> 2 NaCl + 2 O2 (g) + 2 H2O }$$

which implies a net reaction of:

$$\ce{2 ClO2 + 5 H2O2 + 2 NaOH -> 2 NaCl + 5 O2 (g) + 6 H2O }$$

So, slowly add H2O2 until the bubbling ceases. Collect the volume of oxygen and convert to moles. Divide by 2.5 to determine moles of ClO2 and your chlorine therefrom.

Here is a related reference, Titrimetric and photometric methods for determination of hypochlorite in commercial bleaches, which cites the use of hydrogen peroxide and acidic permanganate.

• – andselisk Apr 8 at 21:48
• The action of H2O2 on NaOCl has to be one of the better-known reactions. The reaction mechanics is actually more complex than many are aware, but the final products are not in dispute. – AJKOER Apr 10 at 0:20
• Also, do not employ concentrated H2O2, dilute solutions are safer especially in forming normal dioxygen (not singlet oxygen). – AJKOER Apr 10 at 0:27