# How much ascorbate to neutralize one liter of chlorinated tap water?

In Las Palmas de Gran Canaria, tap water is chlorinated. In an Internet forum, someone claims that in 2008 the water had a chlorine concentration of 0,5 - 1mg/l. On the official site of Emalsa, the water provider, I couldn't find any information about chlorine concentration. I tried contacting them online, but the contact form is broken.

In another forum, I read that ascorbic acid (vitamin C) can be used to neutralize the chlorine: My taste buds say it works!

How much ascorbate/l do I need to neutralize the above amount of chlorine?

• The first result from a Google search yielded the following website: fs.fed.us/t-d/pubs/html/05231301/05231301.html, which appears to have precisely the information you're searching for. May 9 '13 at 0:59
• @GregE. I suggest you make an answer out of this. In the article, they write: "Approximately 2.5 parts of ascorbic acid are required for neutralizing 1 part chlorine." May 9 '13 at 1:46
• I'm not sure that's enough meaningful content for a full answer. But if you'd care to answer your own question using data from that website, I'd have no issue with that. May 9 '13 at 1:55
• @GregE. I don't feel qualified to answer this question, and of course, I prefer a detailed answer over just a link to an article. May 9 '13 at 2:00
• Have you tried calling your water provider about the average chlorine content? Sometimes picking up the phone is much more likely to get you what you want. May 9 '13 at 11:23

The reaction between ascorbic acid and hypochlorous acid is as follows: $$\ce{C5H5O5CH2OH + HOCl → C5H3O5CH2OH + HCl + H2O}$$ so for 1 mole of hypochlorous acid you need 1 mole of ascorbic acid. This can be recalculated into a mass ratio by the use of their molecular masses:
$$M(\ce{C5H5O5CH2OH})=176\;\text{g/mole}$$ $$M(\ce{HOCl})=52.5\;\text{g/mole}$$ From this we can see that if the molar ratio is $1:1$, the mass ratio is $176:52.5$ which is $3.3:1$ somewhat higher than the article claims.
• In addition to the first oxidation, $\ce{HOCl}$ can oxidize the alcohol side chain of ascorbic acid to an aldehyde and then an acid. These reactions are slower than the first oxidation, and so the 2.5 mass equivalents probably comes from the extent of overoxidation in a reasonable time. $$\ce{C5H3O5CH2OH + HOCl -> C5H3O5CHO + HCl + H2O}$$ and $$\ce{C5H3O5CHO + HOCl -> C5H3O5CO2H + HCl}$$ May 9 '13 at 11:27