There are three types of chlorine, each with a different makeup. Following is an overview of what constitutes each type.
Free chlorine is defined as the concentration of residual chlorine in water which is present as dissolved gas (Cl2), hypochlorous acid (HOCl), and/or hypochlorite ion (OCl-). The three forms of free chlorine exist together in equilibrium and their relative proportions are determined by the pH value and temperature.
The effect of pH value on the composition of free chlorine
Between a pH of 2 to 7.4, the predominant form of chlorine is HOCl. When the pH is between 2 and 7, the equilibrium is in favor of HOCl. As the pH falls below 2, the predominant form of the chlorine is Cl2. At a pH of 7.4, HOCl and OCl – are about equal, and as the pH goes above 7.4, increasing proportions of OCl – are present. A test kit that measures free chlorine will indicate the combined concentrations of HOCl, OCl–, and Cl2.
Combined chlorine is defined as the residual chlorine existing in water in chemical combination with ammonia or organic amines. Organic amines can be found in natural or polluted waters. Ammonia is sometimes deliberately added to chlorinated public water supplies to provide inorganic chloramines.
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 the total chlorine will be higher than free chlorine.
Georg Fischer has released a new chlorine analyser system, which can measure free chlorine for water treatment and food and beverage applications. The Signet 4630 Chlorine Analyser System is an integrated, all-in-one unit designed to measure free chlorine in applications with stable pH, temperature and chlorine levels. It combines sensors, a flow regulator, filter and rotameter in the one unit. The Signet 4630 extends the existing GF range of analytical sensors which includes pH/ORP, Conductivity and Turbidity.
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