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The chlorinated isocyanurates produce free available chlorine, in the form of hypochlorous acid (HOCl) as they dissolve in water. A series of rapid equilibria occur involving hypochlorous acid, hypochlorite ion, six chlorinated compounds (Cl3CY, HCl2CY, Cl2CY, H2ClCY, HClCY, and ClCY–2) and four non-chlorinated isocyanurate compounds (H3CY, H2CY-, HCY-2, and CY-3), where CY-3is the abbreviation for C3N3O3-3and H3CY is cyanuric acid (Brady, et al., 1963; O’Brien, et al., 1974; D. Matte, et al., 1989; Kuechler, 2004). As the equilibria involve all of the possible chlorinated isocyanurates, the toxicity of trichloroisocyanuric acid (TCCA), sodium dichloroisocyanurate (NaDCC) and sodium dichloroisocyanurate dihydrate (NaDCC.2H2O) will be virtually equivalent at the same available chlorine concentration.

Table 1:

Equilibrium* and Reaction Rate Constants** for hydrolysis of various chlorine compounds (at 25C, M= mole/l)



forward rate constant (sec–1)


Cl2+ H2O«H++ Cl+ HOCl


1 x 10–3M2




Cl3CY + H2O«HCl2CY + HOCl


1.6 x 10–2




HCl2CY + H2O«H2ClCY + HOCl


1.2 x 10–3






3.1 x 10–5




H2ClCY + H2O« H3CY + HOCl


8.5 x 10–5






2.4 x 10–6




ClCY–2+ H2O«HCY–2+ HOCl


1.3 x 10–7




NH2Cl + H2O«NH3+ HOCl


2.6 x 10–11


1.9 x 10–5

* Equilibrium constants from O’Brien J, Morris J, Butler J (1974) Equilibria in aqueous solutions of chlorinated isocyanurates. Pages 333 -358 in Chemical Water Supply Treatment District Symposium (A. Rubin, Ed.) Ann Arbor Sciences 1974. Margerum D, Schurter L, Hobson J, Moore E (1994). Water chlorination chemistry: nonmetal redox kinetics of chloramine and nitrite ion. Environmental Science and Technology, 28, 331 -337

** Rate constants from Matte D et al (1990) Kinetic study of N-chlorination of CYA in the aqueous phase. Canadian Journal of Chemistry, 68, 307 -313 or Gray Jr. E, Margerum D, Huffman R (1978) Chloramine equilibrium and the kinetics of disproportionation in aqueous solution. Pages 264 -277 in Organometals and Organometalloids, Brinckman F and Bellama J, eds., American Chemical Society Symposium Series 82.

As the free available chlorine is reduced by reaction with various impurities in the water, it is converted into chloride ion and additional free available chlorine is released from the chlorinated isocyanurates in solution. Once all of the available chlorine has been reduced, the stable reaction products are cyanuric acid (CYA or isocyanuric acid) or its salts (e.g. sodium isocyanurate) and chloride salts.

As the equilibria are so fast, it is virtually impossible to distinguish analytically between free available chlorine (HOCl and OCl-) and the chlorinated isocyanurates (i.e. reservoir chlorine). Activity of the chlorinated isocyanurates is therefore measured in terms of the available chlorine (Cl in the +1 oxidation state) content:

Trichloroisocyanuric acid (TCCA); ca 90% available chlroine

Sodium dichloroisocyanurate (NaDCC); ca 62% available chlorine

Sodium dichloroisocyanurate dihydrate (NaDCC dihydrate); ca 56% available chlorine.

The parent compound for all chlorinated isocyanurates is isocyanuric acid (cyanuric acid). All of the chlorinated isocyanurates are essentially equivalent, once they are dissolved in water at the low concentrations at which they are used.

Based upon the available chlorine content and the dissociation constants for the chlorinated isocyanurate species, TCCA is considered to be the most toxic, or reactive form. Therefore test results for this species will be considered as the "worst-case" for the chlorinated isocyanurates allowing read-across for the less reactive dichlorinated forms. The released free chlorine follows the same fate as any other source of free chlorine, which has well-known and established health and environmental effects at common use levels.

The chlorinated reaction products are short-lived when organic matter is available to oxidize and consume the halogen species, leaving the cyanurate carrier. By using CYA as the test substance, the effects of the triazinetrione (cyanuric acid) moiety could be distinguished from those of chlorine. Sodium isocyanurate was considered to be toxicologically equivalent to isocyanuric acid and, as such, was selected as a suitable test substance for the development of toxicity data.

It is therefore considered that long-term toxicity and ecotoxicity studies on TCCA or the dichlorinated isocyanurates would be scientifically unjustified in view of the behaviour of the active substances to form halogenated species of known toxicity and the stable cyanuric acid molecule. The species of interest for environmental fate is therefore cyanuric acid.