Free chlorine is the chlorine level shown by the DPD1 test – i.e. the chlorine that is freely present in the water after it has been added (whether liquid, gaseous, in tablet form or by electrolysis). It is often assumed that this reading corresponds to the effective disinfectant. In fact, only the share present as hypochlorous acid (HOCl) disinfects; the DPD1 value also includes the much weaker hypochlorite (OCl⁻) and – in stabilized water – the chlorine bound to cyanuric acid. What really matters for disinfecting pool water is therefore how much HOCl is actually formed from the free chlorine.
How added chlorine relates to free chlorine
The key factors determining how much HOCl (and therefore disinfectant) can be formed from the added chlorine are the pH value and the cyanuric acid content of the pool water.
1. pH value
At a pH of 6.5, about 85% HOCl is formed; the remaining 15% forms OCl⁻, which has hardly any disinfecting power. At a pH of 7.0, about 74% HOCl is formed, at 7.5 only about 50% HOCl, and at a pH of 8.0 less than 20% HOCl (what we generally call free chlorine). This dissociation equilibrium between HOCl and OCl⁻ is reversible. This means that if you change the pH of water that already contains HOCl/OCl⁻, the share of HOCl in the water increases or decreases accordingly.
2. Cyanuric acid
Almost all slow-dissolving chlorine tablets and granules consist of dichloro- or trichloroisocyanurates. These products, known as organic chlorine, contain cyanuric acid, which acts as a “stabiliser”. This means that a large part of the chlorine added to the pool water binds to the cyanuric acid present and is therefore initially not available to form HOCl (active chlorine). It is completely “inactive” because it has initially entered into a different chemical compound. A hydrolysis equilibrium forms between chlorine bound to cyanuric acid and HOCl (active chlorine). In this case, the total amount of chlorine in pool water containing cyanuric acid (DPD1 test) should therefore rather be called “total available chlorine” than “free chlorine”.
Even a content of 10 mg/l cyanuric acid in the water binds about 40% of the added chlorine, leaving only about 60% of the added amount available to form free chlorine. 20 mg/l cyanuric acid binds about 50–55%; after that the curve slowly flattens, and above a cyanuric acid content of 50 mg/l more than 65% of the chlorine is already bound to the cyanuric acid. Even higher cyanuric acid levels hardly affect the result any more. The following chart illustrates this:
Combined with a pH that is too high, this sometimes results in far too low levels of actually active chlorine in the pool water.
If, for example, you add a theoretical amount of 1.0 mg/l chlorine to pool water that contains 50 mg/l cyanuric acid and has a pH of 7.4, about 65–70% of the added amount is bound to the cyanuric acid, and because of the elevated pH, the remaining 30–35% (0.3–0.35 mg/l) forms only about 55–60% HOCl. The actual share of free or actively working chlorine is therefore only about 0.17–0.2 mg/l.
We provide an online calculator here that lets you approximately calculate the share of actively working chlorine and the dosages needed to raise the chlorine level to a specific value: Online calculator.
DPD1 measurement – why the test gives misleading results under these conditions
When you determine “free chlorine” with a standard tablet tester or photometer using DPD1, the DPD1 indicator initially only reacts to the hypochlorous acid (HOCl) and hypochlorite anion (OCl⁻) present. The discoloration of the sample results from the oxidation of the indicator by the HOCl, which is completely consumed in the sample water. Because of the hydrolysis equilibrium between the chlorine bound to cyanuric acid and the now consumed HOCl, the cyanuric acid releases the remaining chlorine bound to it, and new hypochlorous acid forms in the sample chamber, colouring the DPD1 indicator further. In addition, the DPD1 tablet buffers the sample solution to a pH between 6.2 and 6.5. This, too, produces more HOCl than is actually the case in our pool water from the example above with pH 7.4. The DPD1 test will therefore show almost the expected 1.0 mg/l of free chlorine – even though in fact no more than about 0.2 mg/l HOCl is ever present and effective in the pool water.
Even in water without cyanuric acid that merely has an elevated pH, the DPD1 test (because the sample is buffered to pH 6.2 to 6.5) gives a seemingly sufficient reading. In the pool water itself, however, only a much smaller share is actually present as effective disinfectant (HOCl).