Sodium hypochlorite

Endpoint summary

Additional information

In concentrated 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

It is the resultant of two reactions: a slow one with formation of chlorite and a fast one with formation of chlorate by reaction between chlorite and hypochlorite.

2 NaClO => NaClO2 + NaCl (slow reaction)

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

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

Time Dependence

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

pH Dependence

Hypochlorite should not be added to a unbuffered medium, because at low pH, the following secondary reactions could occur:

In acid media under pH 4 hypochlorite will be transformed to gaseous chlorine.

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

Between pH 4 and 11, both ClO- and HOCl are present with the latter being much more active. This pH will be obtained when all the sodium hydroxide present in the

hypochlorite solution has been carbonated (see chapter 1.2). 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

Hypochlorous acid (HClO) is very unstable and it suddenly decomposes with formation of oxygen:

2 HOCl => 2 HCl + O2

Dependence upon Impurities

Sodium hypochlorite can decompose to oxygen according to the following reaction:

2 NaClO => 2 NaCl + O2

The decomposition reaction is a bimolecular one and requires an activation energy of 113.3 kJ/mol (26.6 kcal/mol). Although it is slower than the chlorate formation

reaction, it is catalysed by trace amounts of metallic impurities.

The strongest decomposition catalysts to oxygen are: Co, Ni and Cu; whereas Fe and Mn are weaker catalysts. To avoid decomposition of commercial hypochlorite

solutions, these metals must be reduced as much as possible. Generally, their elimination occurs mechanically by filtering as their solubility is reduced during the

hypochlorite production step.

Salts such as sodium chloride, sodium carbonate and sodium chlorate have only a very low influence on reaction rate within the range of concentration where they are

normally present. Their influence on reaction rate is only remarkable in some particular cases (e.g. diluted High Grade Sodium Hypochlorite where NaCl content is highly reduced because of the specific production process).

Sodium hydroxide does not influence the reaction rate if its concentration is greater than 10-3 M (0.04 g/l).

Light Dependence

The sodium hypochlorite solution is very sensitive to light. Direct sunlight may cause rearrangement and decomposition resulting in the formation of chlorateand oxygen. The presence of isocyanuric acid in solution reduces this sensitivity to a great extent.

Temperature Dependence

The influence of temperature is very high: the decomposition rate doubles if the temperature increases by ~ 5.5°C. If temperature is more than 35°C, the

decomposition reactions are very rapid:

3NaClO => NaClO3 + 2NaCl

In every case, the temperature of the solution must be below 55°C in order to prevent a sudden decomposition of the hypochlorite.

The more stable solutions are those of low hypochlorite concentration, with a pH of 11 and low iron, copper, and nickel content, stored in the dark at low temperature.

Photolysis in water

The photolysis half-life of aqueous chlorine in, exposed to summer noon sunlit with clear sky (47°N) at a pH 8 is 12 min when measured at the surface. The half-life increases with decreasing pH due to the decreasing ratio of OCl^-/HOCl to 60 min at pH 5. The pseudo-first-order rate constant for the photolysis of HOCl becomes 2 x 10^-4s^-1and that of OCl^-1.2 x 10^-3s^-1The variation of the rate of photolysis with depth was calculated for water columns exhibiting different light absorption coefficients by taking into account that, for both HOCl and OCl^-, the most effective wavelength for photolysis in sunlight is approx. 330 nm. These results show that in water treatment, chlorine photolysis should be minimized whenever possible by operating at low pH, sun shielding or night-time addition of chlorine or avoiding storage in shallow reservoirs. The rate of chlorine photolysis controls the formation of OH radical which acts as a secondary highly reactive photooxidant.

On UV (255 nm) irradiation both HOCl and OCl- photolyze at comparable rates and slowly enough that chlorine depletion will not occur during the time of irradiation typical in UV disinfection.

Photolysis can also contribute to the depletion of chlorine in atmospheric waters whenever chlorine is formed by (slow) ozonation of chloride.