MadSci Network: Chemistry

Re: What does light do to chlorine bleach?

Date: Fri Dec 22 10:17:23 2000
Posted By: Gareth Evans, Senior Research Associate
Area of science: Chemistry
ID: 976743716.Ch

I was tempted to defer this question because I'm not expert in chlorine photochemistry. However, a quick search using my favourite engine, Google (using "chlorine + photochemistry" and "hypochlorite + photochemistry") I thought therefore I would give response if only to pass this on and a couple of specific references.

There is a lot of activity in water purification using sunlight and chlorine compounds, apparently synergistically. includes a lot of other references on the subject. Also, of course there has been a huge amount of work on atmospheric chlorine and chlorine compound photochemistry and is a good source of information about ozone layer depletion, the main reason for the interest in this field. I didn't find much on chlorine photochemistry as such. Please let me know if you'd like me to dig a little deeper.

Domestic bleach is a little different but I imagine, some of the same reactions to light occur with bleach as with dissolved chlorine. The difference is domestic bleach uses sodium hypochlorite, NaOCl. This is used rather than hypochorous acid, HClO because it is milder and more suitable for home use.

There is an old paper on the photochemistry of sodium hypochlorite but it is in German and probably not easy for you to access. I havn't looked it up because my German is very rusty. ( Allmand, A.J., Webb, W.W. 1928. "The photolysis of sodium hypochlorite solutions." Zeitschrift fuer Physikalische Chemie, 131A, 189-204 ). It would probably tell you what the products of the effect of light are but I think it is not too difficult to guess.

The OCl- ion is probably the coloured species. Bleach is very pale yellow so its main absorption is in the near UV (It is possible that there is a very small amount of chlorine, Cl2 which is pale yellow as a gas). If we consider that we have NaOCl in solution in water, the absorption of light produces an excited state of the OCl- ion. This ion is not too stable at the best of times which is why it is a useful bleach, ie an substance which turns coloured species ( in dirt ) colourless, usually by oxidation. The excited state is probably less stable and given that we have Na, Cl, and O present it seems likely that we would finish up with Na+, Cl-, and O2. The pathway may include hydrogen peroxide, H2O2 which itself is a bleach but probably decomposes to oxygen gas and water fairly quickly. What we finish up with is common salt in water - not a useful bleach.

How about an experiment to test this hypothesis. You could irradiate a volume of bleach and collect the gas which came off. If a gas is produced and is oxygen, it should be easily collected and identified, e.g. by its effect on combustion.

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