Sciencemadness Discussion Board

Colour of HOCl?

Kola - 21-2-2012 at 05:33

I did an electrolysis of NaCl and H2O using two half cells connected with a cotton wool. The Cathode half Cell was NaCl solution and the anode half cell was H2O. I used a 6V battery for the electrolysis. If you want a full report, you can follow this link
http://kolathechemist.blogspot.com/2012/02/is-hocl-really-da...
My idea was to prepare HOCl and HCl at the anode halfcell by Chlorine dissolution(I used graphite electrodes)
what i obtained after running the cell for 25 hours was a dark brown liquid which had the smell of HOCl. Do i assume it's HOCl that gave it that colour? HCl solution is definitely Colourless, is HOCl solution dark brown?
Another observation is that the solution isn't corrosive...it gets me confused. Does the graphite electrode participate in the cell reactions? I don't think so..:p

weiming1998 - 21-2-2012 at 06:24

The dark brown solution is the anodic erosion of your graphite electrode. Basically carbon dust caused the brown colour. HOCl is colourless like HCl. As for the non- corrosion of the HOCl, what did you use to measure corrosion? If you used your hand or an iron nail, then nothing would happen because you have a Cl2 solution, which is only about 1%Cl, which would not corrode much at all. The iron would be corroded, but under the graphite muck, you could see nothing.
A better way to make HOCl is to use a hypochlorite salt and a sufficiently weak acid.

Or just ask the HOCl expert, http://www.sciencemadness.org/talk/member.php?action=viewpro... and he'll tell you lots and lots of ways on making HOCl.

Kola - 21-2-2012 at 06:45

Yeah i think you're right parce qué after a few days i noticed that the brown colour was gone, but the HOCl smell was also gone as well...i think the HOCl must have decomposed to HCl because the HCl concentration increased

Pulverulescent - 21-2-2012 at 08:46

Being a weak acid, HOCl decomposes to; Cl<sub>2</sub>,O<sub>2</sub> and water.
Water containing dissolved Cl is yellow . . .

P

Kola - 21-2-2012 at 11:17

Quote: Originally posted by Pulverulescent  
Being a weak acid, HOCl decomposes to; Cl<sub>2</sub>,O<sub>2</sub> and water.
Water containing dissolved Cl is yellow . . .

P

No
HOCl decomposes to HCl and O2
2HOCl--->2HCl+O2
Notice that the pH of the solution obtained after HOCl decomposition is always lower

Pulverulescent - 21-2-2012 at 12:25

Actually, we're both right!
Think about it?

P

AndersHoveland - 21-2-2012 at 16:20

HOCl mainly decomposes into HCl and O2, but in difuse sunlight some chloric acid can form, and possibly chlorine.
http://www.sciencemadness.org/talk/viewthread.php?tid=13828

Other related threads:
http://www.sciencemadness.org/talk/viewthread.php?tid=14490
http://www.sciencemadness.org/talk/viewthread.php?tid=16378
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

weiming1998 - 22-2-2012 at 00:31

Quote: Originally posted by Kola  
Quote: Originally posted by Pulverulescent  
Being a weak acid, HOCl decomposes to; Cl<sub>2</sub>,O<sub>2</sub> and water.
Water containing dissolved Cl is yellow . . .

P

No
HOCl decomposes to HCl and O2
2HOCl--->2HCl+O2
Notice that the pH of the solution obtained after HOCl decomposition is always lower


Then this happens: HCl+HOCl<====>Cl2+H2O. the equilibrium is shifted to the right when the solution is acidic and HCl, being a strong acid, creates quite dramatic pH changes. The pH being lower is probably because at the end, the chlorine simply dissolves in the water, so it just decomposes into HCl+O2 without any interference.

AJKOER - 27-2-2012 at 06:43

OK, if trace amounts of HClO3, perhaps some ClO2 as well?

Now activated Carbon and ClO2, per the referenced article ("Reactions of chlorine dioxide with hydrocarbons: effects of activated carbon", Environmental Science & Technology, 1982, 16 (5), pp 268–273), are not exactly inactive with tap water.

http://pubs.acs.org/doi/abs/10.1021/es00099a007

The article also mentions studies of the reaction of free Chlorine (HOCl and ClO-) and granular activated carbon. If the solution does ever produce higher concentrations of HOCl, then the availability of Cl2O increases. DiChlorine Mono-oxide is a thousand times more reactive than HOCl, and upon mixing with Carbon and any organic matter plus energy, I would advise to avoid the fumes as you cannot be exactly sure what you have.

There are indeed better ways to prepare pure HOCl, but your experiment is interesting. Actually, I found it also scary as C + O (from the decomposing HOCl ) + Cl2, happens to spell a highly toxic compound (level at .1 ppm). That's only spelling, but I am uncertain as the electrochemical corrosion of graphite electrodes is usually attributed to a form of oxidation, usually to CO2 (see link: http://www.electrochem.org/dl/ma/200/pdfs/0640.pdf ).

Another paper: "Hypochlorous Acid-Activated Carbon: An Oxidizing Agent Capable of Producing Hydroxylated Polychlorinated Biphenyls", by Evangelos A. Voudrias, Richard A. Larson, Vernon L. Snoeyink, Abraham S. C. Chen and Pat L. Stapleton, at Environmental Health Perspectives
Vol. 69, (Nov., 1986), pp. 97-100, LINK:

http://www.jstor.org/pss/3430376
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On the Color of HOCl:

"Dilute HOCl solutions are colorless; at higher concentrations the color ranges from yellow to yellow-orange due to small equilibrium amounts of Cl2O." LINK:

http://www.scribd.com/doc/30121142/Dichlorine-Monoxide-Hypoc...

At 0 C, a 5% solution of HOCl contains about 0.03% Cl2O and a 25% solution is about 1% Cl2O. This is important as it relates to the different in reactivity between HOCl and Cl2O (much more).


[Edited on 27-2-2012 by AJKOER]