# NaCl water electrolysis without membrane, how measure the free chlorine concentration?

I'm trying to made electrolysis using water and NaCl. I need at the end a concentration of 2 g/l. How can I measure the free/active chlorine? Do you know a way to "measure" the chlorine during the reaction without chlorine sensor? Using for example the measure of conductivity of the water, the electrode current and voltage and the temperature or the salt remaining?

The electrolysis is 'open' without membrane, a titan electrode is directly put in the water + NaCl.

• There are not many solutions, I've tried a lot of ISE sensors (amperometric) but it seems that the hydrogen generated influences a lot the measurements, the only functional (but not cost effective!) are colorimetric measurements. I've used Swan Analytics devices. Actually I am looking for a cheaper solution. – arianmares Apr 7 at 9:13

The best way for determining the active chlorine amount ($$\ce{ClO^-}$$) in a bleach is to dilute it with water, then to add Potassium iodide $$\ce{KI}$$, and enough HCl solution to make it acidic. It produces $$\ce{I_2}$$ by the equation $$\ce{ClO^- + 2 H^+ + 2 I^- -> I_2 + H_2O}$$ Then the $$\ce{I_2}$$ so produced is titrated by a standard solution of sodium thiosulfate $$\ce{Na_2S_2O_3}$$, according to : $$\ce{I_2 + 2S_2O_3^{2-} -> 2 I^- + S_4O6^2-}$$
A quicker (but not so precise) way is to add 2 mL acetone to 50 mL bleach in a insulated flask, and to measure the change of temperature due to the reaction. The reaction produces chloroform $$\ce{CHCl_3}$$ and acetate. After one minute, the temperature increases to a stable final value. The change of temperature is proportional to the amount of chlorine. If $$\Delta T$$= 10°C, the concentration of active Cl is 5.6%. Generally speaking, the concentration of active Chlorine is : 0,56% x $$\Delta T$$