• "What does my chlorine test kit measure?"
• "What is the difference between free and total chlorine?"
• "Which form of chlorine is the best disinfectant?
Free Chlorine
Free chlorine is defined as the concentration of residual chlorine in water present as dissolved gas (Cl2), hypochlorous acid (HOCl), and/or hypochlorite ion (OCl-). The three forms of free chlorine exist together in equilibrium.
Cl2 + H2O HOCl + H+ + Cl-
HOCl H+ + OCl-
Their relative proportions are determined by the pH value and temperature.
The table above shows the effect of pH value on the form of Free Chlorine in water at 25°C. As the pH falls below 2, the predominant form is Cl2. Between pH 2 and 7 the equilibrium is in favor of HOCl. At pH 7.4 HOCl and OCl- are about equal, while above this increasing proportions of OCl- are present. A test kit which measures free chlorine will indicate the combined concentrations of HOCl, OCl-, and Cl2.
Combined Chlorine
Combined chlorine is defined as the residual chlorine existing in water in chemical combination with ammonia or organic amines which can be found in natural or polluted waters. Ammonia is sometimes deliberately added to chlorinated public water supplies to provide inorganic chloramines.
Total Chlorine
Total chlorine is the sum of free and combined chlorine. When chlorinating most potable water supplies, total chlorine is essentially equal to free chlorine since the concentration of ammonia or organic nitrogen compounds (needed to form combined chlorine) will be very low. When chloramines are present in the municipal water supply, then total chlorine will be higher than free chlorine.