NaCl water electrolysis without membrane, how measure the free chlorine concentration?
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 $$\ce{ClO^- + 2 H^+ + 2 I^- -&gt; I_2 + H_2O}$$ Then the $\ce{I_2}$ so produced is titrated by a standard solution of sodium thiosulfate $\ce{Na_2S_2O_3}$, according to : $$\ce{I_2 + 2S_2O_3^{2-} -&gt; 2 I^- + S_4O6^2-}$$

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 $\Delta T$