Chlorine

Hydrolysis

Description of key information

Waiving of Study:
In accordance with column 2 of REACH Annex VIII, the hydrolysis test (required in section 9.2.2.1) does not need to be conducted as the reaction of chlorine with water and the speciation of the resulting products hypochlorus acid and hypochlorite anion were well investigated and published several times. There is no need to perform an additional guideline study as the solution of chlorine in water is a well known and characterised process.
In the pH range relevant for environmental biota, the relevant chlorine species are hypochlorous acid and hypochlorite anion. Hypochlorite is very sensitive to light. The half life of a 10 - 15 % available chlorine solution at 25 °C is 220 days.

Additional information

In water, chlorine is transformed to free available chlorine (gaseous chlorine), hypochlorous acid and hypochlorite ions, whose relative amounts depend on the pH and other physicochemical properties of the water. At environmental pH, only hypochlorous acid and hypochlorite will be present. The behaviour of these species is described below. The available knowledge about sodium hypochlorite solutions is summarised in the following:

Species in aqueous solution as a function of pH

There are three species of chlorine in equilibrium in water: gaseous chlorine, HOCl (also a gas at room temperature and pressure), and ClO-. An example of the distribution between these species as a function of pH is shown in the figure below. For example, at pH 7.5 half of the chlorine is available as HOCl and half is available as ClO-. The pH of commercial solutions is above 11 (due to sodium hydroxide addition) and the only species effectively present is ClO-.

In sodium hypochlorite solutions, the content of available chlorine decreases because NaClO tends to disproportionate to chloride and chlorate ions:

The reaction is:

3 NaClO => 2 NaCl + NaClO3 Keq = 10²⁷

This reaction is the sum of two reactions: a slow one producing chlorite and a fast one producing chlorate by reaction between chlorite and hypochlorite:

2 NaClO => NaClO2 + NaCl (slow reaction)

NaClO + NaClO2 => NaClO3 + NaCl (fast reaction)

The first reaction (which produces chlorite) controls the reaction rate leading to chlorate. The formation rate of chlorate, at room temperature and pH = 11, is very slow. The process is dependant on time, temperature, impurities, pH and concentration of the sodium hypochlorite solution. Also light can decompose hypochlorite solutions.

Time dependency

At constant temperature, the inverse of the active substance concentration is a linear function of the time. A solution (concentration 150 g/l available chlorine) which is stored protected from sunlight and at 15°C, loses 1/6 of its concentration within less than 3 months. In diluted hypochlorite solutions the losses are smaller.

pH dependency

In acid media, below pH 4 hypochlorite is transformed to gaseous chlorine:

HOCl + H+ + Cl- => Cl2 + H2O

Between pH 4 and 11, both ClO- and HOCl are present. This pH will be obtained when all the sodium hydroxide has been carbonated. Degradation of HOCl is more rapid than the degradation of ClO-.

if pH <6, the main reaction is: 2HClO => 2HCl + O2

if pH >6, the main reaction is: 3 NaClO => NaClO3 + 2 NaCl