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I've come across the claim a few times that Diet Coke is 99% water.

Examples:

  1. RealAge - 8 Diet Coke Claims You'll Want to Know About
  2. Freakonomics - Diet Coke is 99% Water (And That Is Now a Good Thing)

Partial support for the claim seems to stem from the following advertisement, which seems to imply there is no major difference between Diet Coke and water.

Is this claim accurate?

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    I would think that just about any soft drink is at least 98% water by volume, but the 2% makes a difference— just like the 2% of DNA that separates humans and chimpanzees. – choster Nov 16 '12 at 5:25
  • @Choster I don't find it hard to believe that they are mostly water, but I've encountered a lot of people who disagree, so am looking for some solid proof either way. – Sonny Ordell Nov 16 '12 at 5:44
  • @Sklivvz What was wrong with the image? – Sonny Ordell Nov 16 '12 at 16:13
  • @SonnyOrdell it was an ad, but it didn't add to the question because it was not the ad in question! :-P There's a "99%" ad which would be relevant, but the ad you put in was just a random Coca-Cola ad. – Sklivvz Nov 16 '12 at 16:16
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    Maybe we could do a new question: Does the 99% water ad exist? Because - I can't find it, either. – Sklivvz Nov 16 '12 at 19:25
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TL,DR: Diet cola is 99.8% water. Normal cola is 90% water and (basically) 10% sugar.

The claim is not correct, however in my opinion when someone claims "A is 99% B" they may not mean that literally. In particular the Freakanomics article works with any other ratio in mind - the article is about how claiming that something is mostly water has actually become a good thing and that relates to the price of water.

According to the USDA data:

Some notes:

  1. The Coca-Cola company sells syrup which is then diluted in order to make the beverage. The dilution varies on a case-by-case basis, within some acceptable parameters. See for example here for a suggested soda fountain setting. This means that the percentage of water is variable, depending on the sample.

  2. There is a big difference between 99% and 99.8% of water. There is five times more syrup (or "taste") in a 99% dilution than in a 99.8% dilution . 99% means 1g every 100g of liquid; 99.8% means 1g every 500g.

  3. The USDA data is meant to be a standard reference - you can check the links above - given they provide 4 significant digits in the measurement, I doubt that there's more than 1% error in the values. This would include making sure the sample is representative.

  • As a side note (or homework left to the reader), you can also do your chemistry calculations based on syrup composition and dilution ratio. I trust the chemists of USDA to give a more precise result, though, hence the comment. – Sklivvz Nov 16 '12 at 9:23
  • Diet cola is 99.8% water would indicate that the claim is indeed correct, right? – Stefan Nov 16 '12 at 15:29
  • @ste It would indicate that it's off by 0.8%. – Sklivvz Nov 16 '12 at 16:12
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    I understand that as meaning at least 99% but not 100%. – Stefan Nov 16 '12 at 16:19
  • @Stefan I agree, and in fact I specifically say "in my opinion when someone claims "A is 99% B" they may not mean that literally" – Sklivvz Nov 16 '12 at 16:23

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