Sciencemadness Discussion Board

Mixing Acid to NaOCl

QueueC1 - 8-9-2006 at 08:17

What type of reaction comes with combining Sodium Hypochlorite and Sulfuric Acid?

Thanks for your help?

QC1

azaleaemerson - 8-9-2006 at 08:26

Search functions are a gift from the gods? Use them?

Please move to Detritus?

Your very welcome?

Nicodem - 8-9-2006 at 08:56

Sodium hypochlorite (NaClO) gives hypochlorous acid (HClO) with acids. Since HClO is a weak acid (pKa=7.5) it even acetic acid (pKa=4.7) is strong enough to favor the hypochlorous acid over the hypochlorite equilibrium. If you use sulfuric acid on a solution of NaClO you get a solution of sodium cations, sulfate anions, hypochlorous acid and some chlorine(I) oxide (Cl2O). Cl2O can be considered the anhydride form of hypochlorous acid and forms readily at high acidity:

2NaClO(aq) + H2SO4(aq) => Na2SO4(aq) + 2HClO(aq)

or more correctly in a ionic form:

ClO(-) + H(+) <=> HClO

And the formation of Cl2O:

2HClO <=> Cl2O + H2O

Of course there are some other reactions going on, mainly in equilibrium, but they are less important.

If however you add an acid to a "chlorine bleach” (a mixture of NaClO and NaCl) or you acidify NaClO with HCl, then the formed hypochlorous acid, which is a very strong oxidant, oxidizes the chloride anions forming chlorine (Cl2):

NaClO(aq) + NaCl(aq) + H2SO4(aq) => Na2SO4(aq) + H2O + Cl2(g)

or more correctly in a ionic form:

ClO(-) + Cl(-) + 2H(+) <=> Cl2 + H2O

chloric1 - 8-9-2006 at 15:26

Your going to sear the inside of your lungs with chlorine if you don't watch it!

AJKOER - 5-1-2013 at 13:55

Quote: Originally posted by Nicodem  
Sodium hypochlorite (NaClO) gives hypochlorous acid (HClO) with acids. Since HClO is a weak acid (pKa=7.5) it even acetic acid (pKa=4.7) is strong enough to favor the hypochlorous acid over the hypochlorite equilibrium. If you use sulfuric acid on a solution of NaClO you get a solution of sodium cations, sulfate anions, hypochlorous acid and some chlorine(I) oxide (Cl2O). Cl2O can be considered the anhydride form of hypochlorous acid and forms readily at high acidity:

2NaClO(aq) + H2SO4(aq) => Na2SO4(aq) + 2HClO(aq)

or more correctly in a ionic form:

ClO(-) + H(+) <=> HClO

And the formation of Cl2O:

2HClO <=> Cl2O + H2O

Of course there are some other reactions going on, mainly in equilibrium, but they are less important.

If however you add an acid to a "chlorine bleach” (a mixture of NaClO and NaCl) or you acidify NaClO with HCl, then the formed hypochlorous acid, which is a very strong oxidant, oxidizes the chloride anions forming chlorine (Cl2):

NaClO(aq) + NaCl(aq) + H2SO4(aq) => Na2SO4(aq) + H2O + Cl2(g)

or more correctly in a ionic form:

ClO(-) + Cl(-) + 2H(+) <=> Cl2 + H2O


I think your comment on the production of Cl2O is not likely as aqueous NaOCl, even absence any NaCl presence, is usually dilute due to stability issues. Hence, the likelihood of producing significant Cl2O by adding conc H2SO4, which serves as a dehydrating agent on the HOCl via:

2 HOCl <---> H2O + Cl2O

is unlikely as dilute HOCl does not have significant amounts of Cl2O in solution (Intuitively, more water means that the equilibrium in the above reaction is moved to the left). One source notes to quote: "As [Cl2] is proportional to [Cl–] and [Cl2O] is proportional to [HOCl]2", as is expected (see http://pubs.acs.org/doi/abs/10.1021/es203094z?mi=0&af=R&... ), and the reaction with conc H2SO4 is expected to be highly exothermic, at best. Note, the use of dry Ca(NO3)2 to dehydrate conc HOCl (30%) is a referenced path to Cl2O (see http://www.academia.edu/1557547/Chlorine_oxoacids_and_struct... ).

The reason that I am adding my 2 cents here is that on another forum (http://www.chemicalforums.com/index.php?topic=64532.0 ), your comment is adding confusion as to the likely products on adding H2SO4 to Bleach.


[Edited on 5-1-2013 by AJKOER]

AndersHoveland - 5-1-2013 at 23:05

Adding acid to bleach is a good (or hazardous, depending on your perspective :o ) method of generating chlorine gas. Hydrochloric acid is usually used, but sodium bisulfate works well also.

The reaction of sodium hypochlorite with acid very much depends on whether there are chloride ions present, but this is normally not considered since hypochlorite solutions free from chloride are virtually never encountered.

OCl[-] + Cl[-] + 2 H[+] --> H2O + Cl2

Actually, solutions of chlorine in water have an equilibrium also:

2 H2O + Cl2 <==> Cl[-] + H3O[+] + HOCl

There is a small equilibrium in bleach, which is why some additional sodium hydroxide is added to keep the equilibrium to the left.

OCl[-] + H2O <==> HOCl + OH[-]

OCl[-] + Cl[-] + 2 H2O <==> 2 OH[-] + Cl2

This is why bleach has the odor of chlorine.

By adding some dilute acid, we can begin to shift the equilibrium to make hypochlorous acid, which is not very stable, and usually only transiently exists in the solution. By using a very weak acid such as vinegar, we can be sure no chlorine will be produced, since the chloride ions will be left alone and there will be no hydrochloric acid to react with the hypochlorous acid. Hypochlorus acid is a weak acid and so is readily formed:

OCl[-] + H[+](aq) --> HOCl


Now, for some more advanced chemisty...
If we are using commercially available calcium hypochlorite, there is a much lower ratio of chloride ions relative to hypochlorite, and the chemistry will begin to be different. Calcium hypochlorite with a hypochlorite content of 72% is often sold at pool supply stores. Alternatively, if one wants to prepare hypochlorite solutions free from chloride ions:
Quote:

A solution of free hypochlorous acid is obtained by shaking chlorine water with yellow mercuric oxide until the solution no longer smells of chlorine:
2 HgO + 2 Cl2 + H2O = (HgCl)2O + 2 HOCl

Brown, insoluble mercuric basic chloride is formed from the reaction, and the solution contains hypochlorous acid. If the solution is poured off from the basic mercuric salt and distilled, a pure solution of hypochlorous acid will be obtained; which however, cannot be kept long in the light, for it decomposes into hydrochloric acid and oxygen:
2 HOCl --> 2 HCl + O2

Analytical chemistry, Volume 1, Frederick Pearson Treadwell, William Thomas Hall, p314


Aqueous solutions of hypochlorous acid probably have several complex equilibriums, one of them being:

(8)HOCl --> (4)H2O + (2)ClO2 + (3)Cl2

If, however, only a limited ammount of acid was present to react, the reaction would be different, with the formation of chlorate:

(3)OCl[-] + (2)H[+] --> ClO3[-] + H2O + Cl2

These last two equilibriums, of course, will be neglibible if there is hydrochloric acid in the solution also, since hydrochloric acid tends to immediately reduce any chlorine oxides floating around.

[Edited on 6-1-2013 by AndersHoveland]

woelen - 6-1-2013 at 04:20

Quote:
These last two equilibriums, of course, will be neglibible if there is hydrochloric acid in the solution also, since hydrochloric acid tends to immediately reduce any chlorine oxides floating around.
One indeed would expect this, but in my experience, ClO2 is remarkably stable in the presence of chloride ions. I frequently made ClO2 and when this gas is dissolved in dilute hydrochloric acid, then you get a deep yellow/orange solution and it remains like that for hours at least. In the long run, indeed all of the ClO2 reacts with HCl to Cl2 and water, but this reaction takes many hours, or even a day or so.

Chlorate ion/chloric acid also produces a lot of ClO2 (besides Cl2) with conc. HCl and in this case, the deep yellow color of ClO2 remains present for at least several hours. The reaction from chloric acid to ClO2 is immediate, the further reaction from ClO2 to Cl2 and water is slow.

AndersHoveland - 6-1-2013 at 08:16

Quote: Originally posted by woelen  
I frequently made ClO2 and when this gas is dissolved in dilute hydrochloric acid, then you get a deep yellow/orange solution and it remains like that for hours at least.

I would prefer not to speculate here, but I read that ClO2 is slow to hydrolyse in water. It might possibly be that hydrochloric acid cannot directly reduce the dissolved ClO2 until it has hydrolysed to its respective acids. I think I remember reading that chlorine dioxide initially has a low solubility in water until it has time to reach equilibrium with HClO2 and HClO3, at which time it becomes more soluble. This can be problematic when passing chlorine dioxide gas into water and trying to get it to dissolve.

I remember seeing a graph somewhere online showing a solubility table for chlorine dioxide relative to contact time and temperature. This makes chlorine dioxide rather unique, as the solubility of most gases in water have no dependance on time.

Again, all this is just from my memory, perhaps not all of this is accurate. Hopefully someone else can find the information and post a link. I am going to sleep now.

This is the best I was able to find, and does lend some support to the concept I described:
Quote:

"Solutions do not always behave normally upon heating. As a ClO2 solution is heated, ClO2 can remain in solution above the expected solubility at the corresponding temperature and therefore become supersaturated. Impurities can then "seed" the supersaturated solution and allow the ClO2 to evolve from solution unexpectedly. Because of this unpredictability, there can be differences in the behavior of ClO2 solutions."


I seem to remember reading it took some time for ClO2 to reach its equilibrium constant with water. ClO2 is considered the "mixed acid anhydride" of chlorous and chloric acid.

[Edited on 6-1-2013 by AndersHoveland]