Free chlorine versus total chlorine

There are three types of chlorine, each with a different makeup. Following is an overview of what constitutes each type.

Free chlorine

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

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

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.

If you require more information on measuring chlorine in water or need to speak with one of our Chemical engineers please let us know by posting a comment.

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  1. The total chlorine keeps getting higher then 1 ppm the free chlorine in my spa. I have changed the water and it still happens.
    My question is what make the total chlorine go up?
    It’s a public spa

    • There could be a number of reasons for the Total Chlorine going up or down:

      1. Faulty Chlorine Sensor
      2. The addition of Chlorine
      3. Temperature changes

      For this application it would be the best to visit a pool or spa shop.

  2. For an apllication to potable water in food and beverage plant…Which is the appropiated measure for control.. Free Chlorine or total chlorine ?

    • there is a free chlorine (FRC) test chemical in Merck, part number 1,14977.0002 which follows the colorimetric estimation tells the value in seconds


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