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Removing Chlorine with Ascorbic Acid

Removing Chlorine with Ascorbic Acid tile

Recently we got the unwelcome news that all council water supplies in New Zealand will be chlorinated going forward. While this is understandable from a public health lens, chorine is sometimes undesirable. It tastes weird, and for some it can aggravate skin disorders. In this article, I investigated removing chlorine with ascorbic acid. Can you really remove chlorine from water using plain old vitamin C? Let’s find out.

A bag of Sana Direct Ascorbic Acid on the side of a bath

Does Ascorbic Acid Really Remove Chlorine?

When I first heard that ascorbic acid neutralises chlorine in water systems, I was skeptical. How often is something that simple? It sounded suss. However, after one particularly swimming-pool-flavoured cup of tea, I was motivated to investigate further.

The first article I found was guidance for employees of the USDA around using ascorbic acid and sodium ascorbate to neutralise chlorine in aquatic systems. The article notes that both effectively remove chlorine from water. Neither leaves harmful residues or lowers the oxygen content of the water. All of these things are important for aquatic species.

Then a friend mentioned they have a showerhead filter with an ascorbic acid puck to remove chlorine. Ok, so it works, but how? Ahaha, I’m no organic chemist but let’s give this a go.

How Does Ascorbic Acid Remove Chlorine from NZ Water?

While the USDA article gave a chemical equation for the reaction of ascorbic acid (C6H8O6) with hypochlorous acid (HOCL), I needed more info. Specifically, is it important what forms of chlorine are added to our water (?). I discovered that NZ uses three forms of chlorine: liquified chlorine gas (Cl2), sodium hypochlorite (NaOCl a.k.a bleach) which is produced here, and calcium hypochlorite Ca(OCl)2), which is imported. have a good summary of the situation in this document from 2020.

Which type of chlorine each district uses is unclear. If you have any information on that, please put it in the comments. Anyhoo, it’s not too important because ascorbic acid will work in all cases.

The chemical reactions are below. Firstly, what happens with chlorine gas? This took me a while to figure out, but initially, Cl2 reacts with water to become hypochlorous acid (HOCL). Then we add our ascorbic acid and have the reaction from the USDA document.

The products of this reaction are dehydroascorbic acid and hydrogen chloride. HCl can exist as a gas or aqueous solution and I’m not sure which applies here, but in the context of normal water chlorination it is very weak, changing the pH of the water very little. Dehydroascorbic acid is the oxidised form of vitamin C. Within your body this form exists and carries out similar functions to ascorbic acid. Glutathione reduces Dehdroascorbic acid back to ascorbic acid – there’s your biochemistry lesson for the day.

The next reaction is what happens when adding ascorbic acid (C6H8O6) to sodium hypochlorite.

The products of this reaction are dehydroascorbic acid, salt and water.

With calcium hypochlorite, the reaction looks like this:

This reaction forms dehydroascorbic acid, water and calcium chloride, which is harmless.

All of these reactions were found on I also found that there are circumstances where the reaction of ascorbic acid and either form of chlorine can release chlorine gas (Cl2). From what I’ve read, this appears to happen when there are high concentrations of chlorine. For those in NZ, you may remember that there was a dosing error when chlorine was first added to the Christchurch water supply. Rather than the goal of 1.0ppm, the dose was at around 5 ppm, more than you’d normally have in a swimming pool. At that point, I ran a bath, added my ascorbic acid and then had to evacuate the bathroom as the off-gassing made my eyes burn. That doesn’t happen at the usual chlorine levels.

Disclaimer: I’ve done my best with this section, however this type of chemistry is not my field of expertise. If you spot any errors or can clarify anything, please feel free to shame me in the comments.

How Much Ascorbic Acid Do You Need to Remove Chlorine from Water?

Going back to our original USDA article, the amount of ascorbic acid you need to neutralise chlorine is approximately 2.5 parts ascorbic acid to 1 part chlorine. A quick google around the various city councils suggests that in NZ, chlorine dosage is around 0.2 ppm (0.2mg/L). The Christchurch City Council reports chorine dose being 0.2 ppm generally, but 0.5-1.0 ppm where the wells are insecure.

So, assuming the high end of 1 ppm (1mg/L), we would need just 2.5mg/L of ascorbic acid to neutralise it. That is a very, very small amount. In a whole bath of around 80 litres, you need just 200mg, or around 1/20 of a teaspoon.

Searching around the interwebs, the most often cited source for this calculation is the ‘Guidance Manual for the Disposal of Chlorinated Water‘. The quantity mentioned there is 1 gram (1/4 tsp) of ascorbic acid to neutralise chlorine at 1 ppm in 100 gallons (380L) of water. Therefore, to summarise the situation, you probably need a whole lot less than you think, and any amount added to water is likely more than enough.

Personally, my highly scientific method for drinking water is to add just enough that you can’t taste it, or barely taste it. When I take a bath, I just tip some in straight from the bag. I’ve noticed that this helps keep the bath clean too, so I’m almost certainly using way too much. Obviously, you’d want to be careful if you have sensitive skin, err on the side of caution until you’ve found your level. I read that level of up to 50mg per litre can be used in fish tanks to help heal wounds on fish, so perhaps those vitamin C baths are keeping me youthful.

Hopefully, this article has given you some peace of mind around using ascorbic acid to remove chlorine from tap water. It looks like the chlorination is here to stay in NZ, so it’s nice to know there’s an easy and safe way to remove it where it’s not wanted.

Have you used ascorbic acid for removing chlorine? If so, I’m keen to know how much you used and if you were happy with the result. Comments are always welcome (and replied to) ☺️