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I was (probably foolishly) reading the side of a box of herbal tea-bags, and noticed that they recommended always using freshly boiled water, because "Freshly boiled water contains more oxygen."

As I understand it, boiling water knocks a fair amount of the oxygen out, so I can't see that there would be much difference between freshly boiled and reboiled water.

Is there any validity to this claim?

Oddthinking
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Avrohom Yisroel
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  • I don't know whether the claim on the package means more oxygen than unboiled water, than water that was boiled a while ago or than "reboiled water" (as suggested by the question). – Oddthinking Jul 15 '15 at 13:22
  • @Oddthinking If it means "more than unboiled water" it's definitely wrong (because gas solubility decrease with temperature), if it means more "than water boiled a while ago" it's definitely wrong (because the dissolved oxygen concentration re-approaches the room temperature equilibrium value over time), but if it means briefly boiled or boiled for the first time it has some truth. – DavePhD Jul 15 '15 at 13:33
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    @Oddthinking "Pour just-boiled water over the tea. (Water that's been boiling awhile loses its oxygen and gives tea a flat flavour.)" http://www.canada.com/story.html?id=9278ff66-f222-4ee2-b87f-5ce1c94adcfc Basically, it should mean briefly boiled including consideration of any previous boilings to be true. – DavePhD Jul 15 '15 at 13:42
  • @DavePhD: Ah, so it means "water, just brought to boil, and not allowed to continue to boil." That seems to make sense. – Oddthinking Jul 15 '15 at 17:25
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    Does it make a difference if there is more or less oxygen in your tea-water? Does it taste different? Is it healthier? Did it say on the packet? I must test this at home. – RedSonja Jul 16 '15 at 08:48
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    @RedSonja In Japan [they add oxygen to bottled water](http://www.wsj.com/articles/SB116595432100447996). See also: *http://skeptics.stackexchange.com/q/28466/5337* – gerrit Jul 16 '15 at 13:53
  • @gerrit Thanks for the links. My guess is that this is all a marketing ploy, and is absolute rubbish! The odd thing is that the comment on the box of herbal bags wasn't obvious marketing, as it wasn't claiming the product was better because it contained more oxygen. It just recommended using freshly boiled water. Could be subtle marketing, in that they want you to think the company is very health-concious. Dunno, as I said, I drink them because they are about the only hot drink I like :) – Avrohom Yisroel Jul 16 '15 at 14:19

1 Answers1

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In considering the concentration of a dissolved gas (such as oxygen) in liquid (such as water) there are two separate concepts, the equilibrium concentration and the rate at which equilibrium is approached.

The solubility of gases is higher at low temperature and lower at high temperature. However, there is still dissolved oxygen in water no matter how long you boil it in the presence of air (which contains oxygen). See tables 2 and 3 of The Solubility of Oxygen and Ozone in Liquids. In fact from room temperature (293K) to boiling temperature, equilibrium dissolved oxygen only decreases from 2.5 units to 1.4 units according to the tables.

As reported in Removal of dissolved oxygen from water: A comparison of four common techniques Talanta vol. 41 pages 211-215

Boiling at 1 atm was found to be the least effective. None of the techniques evaluated here lead to complete removal of oxygen. The concentration of residual dissolved oxygen after purging for 20-40 minutes with nitrogen is 0.2-0.4 ppm.

This reference, beyond confirming that oxygen still remains after boiling, gives an idea of the timescale for the approach of equilibrium. The faster you heat and use the water, the more oxygen will remain, but no matter how long you heat the water (in the presence of air) there will still be significant dissolved oxygen.

DavePhD
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    Thanks for that, very interesting. However, the original point was that the packet claimed that freshly boiled water contained more oxygen than reboiled water. I'm interested to know if there's any basis to that claim. As a side-issue, I'd like to know if there's any benefit either way, as I would have thought we wouldn't use the oxygen in the water, as we'd get plenty from breathing. Thanks again. – Avrohom Yisroel Jul 15 '15 at 11:51
  • @AvrohomYisroel the reboiling can be similar to boiling for a long time. It depends how long you wait between boilings and how exposed the water is to air (including the ratio of air/water interface surface area to water volume). If you wait a long time between boilings, the concentration of oxygen will go back to the room temperature equilibrium value. – DavePhD Jul 15 '15 at 12:08
  • @DavePhD Do you have a source for that claim? – March Ho Jul 15 '15 at 12:46
  • @MarchHo http://www.seas.ucla.edu/stenstro/GasTransfer.pdf – DavePhD Jul 15 '15 at 12:52
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    @DavePhD Thanks for the reply. Do you have any numbers? Suppose you had water in a kettle, and you left it for a few hours between boilings. Would there be a noticeable difference in the amount of oxygen when you boiled it for the second time, compared to the amount there would be boiling fresh water? And of course, we still have the question of whether or not it makes the slightest bit of difference! Thanks again for your great replies. – Avrohom Yisroel Jul 15 '15 at 13:07
  • @AvrohomYisroel I can't answer whether more oxygen or less oxygen is good or bad, it depends upon the person's goal, for example more oxygen could be bad if it reacts with a compound in the herbal beverage that someone considers desirable. No, I don't have any numbers beyond what I put in the answer. One you turn off the kettle, it starts to cool, sucks in air as the water vapor in the kettle contracts and condenses, then the oxygen in the air in the kettle starts to dissolve in the water that has been partially depleted of oxygen. (continued below) – DavePhD Jul 15 '15 at 13:24
  • @AvrohomYisroel (continued from above) The deeper the water is in the kettle, the longer it would take for the oxygen concentration in the water to return to the room temperature equilibrium value. Then when you boil again, it will start to approach the boiling point equilibrium value again. – DavePhD Jul 15 '15 at 13:26
  • @DavePhD OK, thanks. I think the bottom line is that it's almost certainly a load of rubbish! The drink is very nice, but the blurb on the box is junk :) – Avrohom Yisroel Jul 15 '15 at 13:50
  • @AvrohomYisroel Reboiled water will have a higher consentration of metals and salts because more water will have evaporated. I have no idea about how significant this effect is for the quality of the tea though. – Taemyr Jul 15 '15 at 14:24
  • I think the answer could do with a simple summary making clear that freshly boiled water contains less not more oxygen, opposite to the claim in the OP. Also thanks for the great answer. – Murphy Jul 15 '15 at 15:13
  • So I guess the argument goes like this. When you heat the water, it loses oxygen. Once you reach the right temperature and stop heating, the water keeps on losing oxygen until it reaches the equilibrium concentration. So make the tea as soon as possible to avoid that extra oxygen loss. (Sounds fishy to me.) – GEdgar Jul 15 '15 at 15:49