The uses and applications of Sodium hypochlorite solution require a good knowledge of the substance from the chemical and physical points of view . The information here reported are a simple compendium on sodium hypochlorite solution and they are addressed to end users for a correct use of the product.
The Hypochlorite is a chemical source of “active chlorine “ or available chlorine , in fact when it is dissociated in anionic state as ClO- it makes an oxidant action transforming itself to a reduced state as chloride .
Chemical stability and quality of the product a strong correlation
The sodium hypochlorite solution has chemical stability ( shelf – life ) that depends on many different chemical and physical factors . The free residual NaOH content coming from the production process is very important , as the quality of the raw materials like gas chlorine and sodium hydroxide .In fact, raw materials with high purity will produce a final product of high quality . Caffaro Brescia in its own industrial process uses pure gas chlorine that comes from a two steps chemical process: one of acid generation and the second of purification by stripping that removes all the impurities . The selected and checked sodium hydroxide quality and the use of purified water by reverse osmosis complement the quality of the industrial production process of sodium hypochlorite solution .
In any case the sodium hypochlorite solution can decompose over time even if with slow reaction kinetics according to the two following reactions :
2NaClO → 2NaCl + O2 (reaction A)
3NaClO → 2 NaCl + NaClO3 (reaction B)
It is well known that, in product of quality, the main decomposition reaction is the B one , that accounts for over the 80% .
The causes, that originates this decomposition reaction even in high quality sodium hypochlorite , are often present under not easy control environment. For example direct sunlight promotes these reactions, but the most important variable is the Temperature that, as known, increases the chemical reaction rate .
Taking into account a sodium hypochlorite solution with an active content of 160 g/L ( a product widely available on the chemical market ) the decomposition reaction is already active at 10°C and the rate has been determined to be around minus 0,1 g/L ( time 12 h ) , it can increase up to minus 1,5 g/L ( 12 h ) .
The following graph gives a clear representation of this decomposition process underlining the thermodynamic aspect of the reaction . It means that an higher initial active content corresponds to an higher rate of decomposition reaction .
Registration number ( REACh ) 01-2119488154-34-0028
CAS number 7681-52-9
EINCS number 231-668-3
Applications and uses:
- Industrial use as intermediate
- Industrial use in textile industry
- Industrial use in wastewater treatment, of cooling water and of heating
- Industrial use in paper industry
- Industrial use as cleaning agent
- Professional use as cleaning agent
- End consumers uses
Caffaro Brescia S.r.l. in coordination with the Euro Chlor Sodium Hypochlorite Biocides Registration Group has notified sodium hypochlorite (CAS No. 7681-52-9; EC No. 231-668-3) as an active substance under the BPD as required by the Biocides Directive 98/8/EC ( now substituted by B.P.R. –Biocidal Product Regulations ) for the following PTs ( Product Type ) :
- PT 1: Human hygiene biocidal products
- PT 2: Private area and public health area disinfectants and other biocidal products
- PT 3: Veterinary hygiene biocidal products
- PT 4: Food and feed area disinfectants
- PT 5: Drinking water disinfectants
- PT 11: Preservatives for liquid-cooling and processing systems
- PT 12: Slimicides
The chemical structure of sodium hypochlorite is shown here:
The chlorine chemistry in water, in water treatment application, considering the pH ranges and dosages required, therefore is equivalent to hypochlorite chemistry . In fact hypochlorite in sodium salt aqueous solution state is preferred to chlorine for simplicity and ease of use and to avoid security issues relating to handling, transport and stocking of gas chlorine . Therefore the Hypochlorite is a chemical source of “active chlorine “ or available chlorine , in fact when it is dissociated in anionic state as ClO- it makes an oxidant action transforming itself to a reduced state as chloride . The chemical reaction behavior for this dissociation is in two steps . The chlorine disproportion in water happens with formation of hydrochloric acid and hypochlorous acid (1) :
Cl2 + H2O → HClO + H+ + Cl (1)
In solutions that have pH >3 the absorption and dissociation reaction is completely shifted to the right with a minimum residual concentration of molecular chlorine .
Instead the hypochlorous acid, resulting from hydrolysis of chlorine is a weak acid that dissociates in water according to the following reaction ( 2 ) :
(hypochlorous acid) HClO ↔ H+ + ClO- (hypochlorite anion) ( 2 )
With an equilibrium constant Ka equal a 3,2 *10-8 (a 25 °C).
The pH plays a relevant role and from its value depends the concentration at equilibrium of the species, the dissociated one (hypochlorite ion ) and the not dissociated one (hypochlorous acid ). In water treatment, where the pH is in the typical interval between 5,5 and 8,5, therefore the species HClO e ClO-. are present .
The E° ( normalized potential ) redox values of the species above described (at 25 °C and 1 atm) are the following :
Cl2 + 2 e- –→ 2 Cl- 1,36 V
HClO + H+ + 2 e- → Cl- + H2O 1,49 V
ClO- + H2O + 2 e- → Cl- + 2 OH- 0,90 V
Although in real application conditions for hypochlorite solution, not too concentrated, it makes more sense to evaluate redox potential E, determined on the basis of the Nernst equation :