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Author: Subject: Why does hypochlorite smell like chlorine?
mycotheologist
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[*] posted on 7-4-2012 at 05:54
Why does hypochlorite smell like chlorine?


I'm familiar with the smell of Cl2 gas and bleach reminds me of that smell. Is it actually the hypochlorite that you smell when you open a bottle of bleach? The wiki page for NaClO lists its boiling point as 100C but it has (decomposes) beside it so I'm not sure if that means it is actually volatile or just decomposes at low temperatures. I strongly suspect its the former since 100C is an insanely low BP for a salt.
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woelen
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[*] posted on 7-4-2012 at 06:32


You certainly do not smell NaClO. No sodium ions can escape from the solution. There will be an equilibrium though in which HOCl is formed and the smell certainly may be from HOCl.

The smell of Cl2 is not exactly the same as the smell of bleach (at least not for me), although the smells are related. So, I think that the smell of bleach is due to the presence of HOCl and the smell of chlorine is really due to the presence of Cl2.

I think, however, that one of the components of the smell of bleach also may be Cl2, due to other equilibria in which Cl2 is formed. Due to the mildly alkaline nature of ordinary household bleach the amount of Cl2 will be low, very low. But chlorine has a very strong odour and one can easily smell it, even at very low concentrations.

A nice experiment is to add a small amount of NaOH to bleach, allow that to dissolve and then smell the liquid again. The more NaOH is added, the weaker the smell, but also the type of smell shifts.





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DougTheMapper
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[*] posted on 7-4-2012 at 08:53


Wikipedia says that

Cl2 + H2O <---> HClO + HCl

So I'd venture to guess that any solution with ClO- is going to have Cl2 in equilibrium somewhere, albeit very small since bleach lacks the HCl component and sets this equilibrium hard to the right. Since this equation can be driven to the left with excess H+, this explains why acidifying bleach releases chlorine.

We know that Cl2 should smell like HClO, HCl, and Cl2 since they should exist in equillibrium in the nose. So what is it about bleach that is unique? Perhaps the lack of formed HCl since you'd mostly get HOCl and Cl2, and the OH- gives it that characteristic odor?




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woelen
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[*] posted on 7-4-2012 at 11:02


OH(-) does not give any odor, this cannot escape from the liquid (no ionic species can escape, only covalent species).

HCl has a smell, very different from Cl2 and HOCl. If only HCl is present, such as when smelling concentrated hydrochloric acid, then there is no equilibrium in which Cl2 is formed. This explains the totally different odor of HCl.

Bleach indeed has a typical rather unique smell. But this smell also is produced by a solution of Ca(ClO)2 and solutions of TCCA and Na-DCCA also have a similar smell. All these components form some free HOCl in equilibrium with other compounds. So, it indeed must be HOCl (and possibly subsequent formation of traces of Cl2 due to a secondary equilibrium) which produces the typical smell.




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AndersHoveland
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[*] posted on 7-4-2012 at 15:15


There is some small equilibrium in hypochlorite solutions.

2 ClO[-] + H2O <==> 2 OH[-] + Cl2

This is why commercial hyochlorite bleach contains a mixture of sodium hypochlorite, sodium chloride, and some sodium hydroxide. This helps keep all the chlorine dissolved.

(Note that generally this equilibrium stays to the left, hypochlorite is more facorable under alkaline conditions)
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mycotheologist
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[*] posted on 7-4-2012 at 15:51


Quote: Originally posted by woelen  

HCl has a smell, very different from Cl2 and HOCl. If only HCl is present, such as when smelling concentrated hydrochloric acid, then there is no equilibrium in which Cl2 is formed. This explains the totally different odor of HCl.

Yeah I know the smell of HCl well and it doesn't remind me of Cl2 or HClO at all.
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[*] posted on 7-4-2012 at 16:23


And ClO2 has a completely different smell. Chlorine dioxide has a slightly sweet spicy odor to it. This is probably because ClO2 dissolves at a much slower rate in water than Cl2. Molecules that are more hydrophobic often smell sweeter or more fragrant.

The rate at which either Cl2 or ClO2 dissolves in water is mostly not related to the actual solubilities, but rather the chemistry of each. ClO2 only slowly hydrolyses in water to HClO2 and HClO3. This is probably because of steric effects. But the Cl—Cl bond is relatively weak, so chlorine can much more quickly hydrolyse in water to HCl and HOCl.

[Edited on 8-4-2012 by AndersHoveland]
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AJKOER
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[*] posted on 10-4-2012 at 19:25


The smell of Bleach is from dissolved Chlorine, HOCl and traces of Cl2O (the gaseous anhydride of HOCl). The Cl2 is from the creation of the Bleach upon treating aqueous NaOH:

Cl2 + H2O <--> HOCl + HCl

NaOH + HOCl <--> NaOCl + H2O

NaOH + HCl ---> NaCl + H2O

So the net reaction with cooling:

Cl2 + H2O + 2 NaOH --> NaOCl + NaCl (s) + 2 H2O

Upon cooling, most of the NaCl is usually removed as NaOCl is more soluble. Note, to increase stability of the Bleach, some NaOH is usually added.

Also, depending on the concentration of the Bleach (higher), pH (lower) and temperature (higher), the available DiChlorine monoxide in solution also increases:

HOCl <----> Cl2O + H2O

Exposure to air containing CO2 with age can also increase the concentration of HOCl:

NaOCl + H2CO3 --> NaHCO3 + HOCl

Light can also decompose the HOCl:

2 HOCl --> 2 HCl + O2

and the HCl will react with more HOCl to increase available Chlorine. Heat and light can also increase the rate of disproportionation into HCl and HClO3:

2 HOCl --> HCl + HClO2

HOCl + HClO2 --> HCl + HClO3

with the HCl further adding to Cl2 creation.
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[*] posted on 13-4-2012 at 16:41


Although the equilibrium concentration of Cl2 (aq) is small, the evaporation of tiny amounts from the surface of the liquid will cause more Cl2 to form, according to LeChatelier's principle. This is what gradually reduces the concentration of a bleach solution.

Cl2 (g) has a very low odor threshold. It doesn't take much to impart that chlorine smell. As others have said, it seems possible it could be a trace of something other than "just" chlorine.

As AJKOER pointed out, the interface with air probably has a chemical effect. It's probably at the localized level of the surface. The formation of H2CO3 would yield HOCl, and the very slight dissociation into H+ ions could provide some Cl2 (g) by that route, too. With that localized surface effect of CO2, it seems possible that Cl2O would be forming in traces.

A *trace* of the re-condensed vapors from the decomposition of HCl by MnO2 caused my fingers to smell like "bleach", but there was definitely that trace of sour odor from the acid. This odor was picked up merely by removing the stopper - not even directly touching the area that had contacted the vapor. I thought that was kind of interesting.

What's really interesting is how such a simple, commonplace household product can have such complex equilibria. That's part of why I love chemistry.
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barley81
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[*] posted on 13-4-2012 at 17:00


Actually, bleach solutions degrade because hypochlorite disproportionates into chloride and chlorate, and also reacts to form chloride and oxygen. The amount of Cl<sub>2</sub> is likely too small to cause any appreciable decrease in the concentration of the solution.
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Pyridinium
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[*] posted on 13-4-2012 at 17:36


Quote: Originally posted by barley81  
Actually, bleach solutions degrade because hypochlorite disproportionates into chloride and chlorate, and also reacts to form chloride and oxygen. The amount of Cl<sub>2</sub> is likely too small to cause any appreciable decrease in the concentration of the solution.


I wasn't sure the extent to which that disproportionation happens, since Cl2 dissolved in NaOH soln. gives pretty much all hypochlorite and chloride ion. However, you make a good point, because the loss of oxygen is another one-way equilibrium shift. Didn't think of that one.

By the way, I just opened up an old inorganic chem textbook (1940) and it says the unique odor of bleaching powder actually comes from HOCl.





[Edited on 14-4-2012 by Pyridinium]

[Edited on 14-4-2012 by Pyridinium]
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