Sodium hypochlorite

[Ingeniarius]

Dear friends,
Can you explain to me one thing that has been bothering me for a long time. If household bleach like Domestos is made as solution of sodium hypochlorite in water then, according to my knowledge sodium hypochlorite cant be its ingredient, since it decomposes almost immediatelly into hypochlorous acid HOCL and caustic soda NaOH. Nevertheless, on all household bleaches they put sodium chlorite as main ingredient.
Thanks in advance,

 

[25362]

To my grasping sodium hypochlorite aqueous solutions are stable at ambient temperatures and in the presence of alkalinity.

 

[moltenmetal]

You're misunderstanding the chemistry.

Chlorine dissolves in water to form hyopochlorous acid (H+ and ClO-).

Cl2 +2H2O <==> 4H+ + 2 ClO-

The equilibrium is therefore dependent on pH: less H+ around in the water to begin with (i.e. high pH, i.e. the presence of OH- such as in sodium hydroxide solution) tends to shift the equililbrium away from free Cl2 and toward -OCl ion. Add acid to bleach (i.e. by diluting the base with a lot of water, such as when you add bleach to your washing machine) and you shift the equilibrium back toward chlorine.

It's the same deal with CO2.

 

[Ingeniarius]

Thank you for your answers.
You helped me a lot. After surfing the web, consulting some books and carefully reading what you wrote, I came to conclusion:
Sodium hypochlorite disociate to Na+ and OCl- ions in water. If solution is acidic (ph<7) folowing reaction starts: Ocl- + H2O = HOCl + OH-. Futher decomposition happens:
3HOCl = HClO3 + 2HCl
and finally HOCl + HCl = H2O + Cl2
So finally we have highly corrosive Cl2 needed for disinfection.
In order to start that chain of reactions, the solution has to be acidic. Acidity of solution we achieve by mixing bleach with lot of water. In order to prevent this from happening during shelf life of bleach, sodium hydroxide NaOH is added to bleach, which increases pH value of bleach thus preventing those reactions.

 

[dcasto]

and the irony is that the electrochemical plant makes Cl and NaOH from salt water. its full circle.

 

[moltenmetal]

If you electrolyze NaCl in an open (mixed) cell, you get NaOH, NaOCl (hypochlorite), and then NaO3Cl (chlorate) and NaO4Cl (perchlorate). If you separate the anode and cathode with a membrane, you can obtain the chlorine gas separately.

 

[786392]

Lovely discussion focused on Chemistry/electro chemistry

Best Regards
Qalander(Chem)