Sodium hypochlorite (bleach) is a commonly used disinfectant for swimming pools, spas and hot tubs. Bleach purchased for disinfection will usually contain 100 to 150 g/L (10 to 15%) of chlorine. The concentration of bleach will degrade and lose its strength during storage. For bodies of water that are continuously monitored and controlled with process instrumentation this change in concentration can cause issues with instrument settings. Control parameters are set based on the strength of the chemical being dosed. For this reason it is important to monitor the concentration of chlorine that is being purchased for the automatic dosing into the swimming pool, hot tub or spa.
Available chlorine refers to chlorine liberated by the action of dilute acids on hypochlorite:
OCl– + Cl– + 2H+ = Cl2 + H2O
HI38434 uses an iodometric titration method. First, the hypochlorite solution is treated with potassium iodide and acidified with a strong acid. The amount of iodine generated is equivalent to the chlorine in the sample. The next step is to determine the concentration of iodine by the titration with thiosulfate to reduce the iodine back to iodide ions. The amount of thiosulfate added is then used to calculate the amount of chlorine that was in the original sample.
