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  #1  
Old 08-20-2011, 09:11 PM
Johanna Johanna is offline
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Why does red wine turn blue when diluted?

A glass with 2 or 3 drops of red wine in it. Fill it with water. No more red—it's blue! How on earth?
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  #2  
Old 08-20-2011, 09:35 PM
MikeS MikeS is offline
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I don't have a bottle of red wine around to investigate this myself (more's the pity), but my first guess would be Rayleigh scattering.
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Old 08-20-2011, 09:52 PM
kittenblue kittenblue is offline
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I was just about to crack open a bottle of wine.....lol...oh, wait...there are five drops left in the bottle in the recycling bin! Let me go test this out...

....
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Old 08-20-2011, 09:59 PM
kittenblue kittenblue is offline
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No blue. Now, it wasn't a deep red wine....just some cheap soft red. But nothing went blue.
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Old 08-20-2011, 10:28 PM
Sigene Sigene is offline
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I'm prettysure it isn't Rayleigh scattering.

I'm inclined to think its due to a change in pH. It is a fairly common occurance that vegetable dyes change color due to pH changes. I've seen this in wine myself.

I have NOT tried to specifically add a base or acid to the wine to intentionally make it change color. I'm going to guess that the wine is slightly acidic and red, but turns blue when pH is increased towards the basic range.

I have some wine and some sodium hydroxide......TO THE LAB!!!!
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  #6  
Old 08-20-2011, 10:35 PM
MikeS MikeS is offline
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Quote:
Originally Posted by Sigene View Post
I'm prettysure it isn't Rayleigh scattering.

I'm inclined to think its due to a change in pH. It is a fairly common occurance that vegetable dyes change color due to pH changes. I've seen this in wine myself.
You're probably right — and this madsci.org post seems to back you up. The moral, I guess, is never listen to a physicist when the question deals with chemistry.
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  #7  
Old 08-20-2011, 10:38 PM
Sigene Sigene is offline
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RESULTS:

A little red wine (Cabernet in a box) diluted a bit....because the first time it was too intense to make out a change.
Added NaOH. the crystals fell to the bottom of the glass and appeared to give off a dark blackish blue trail as they fell.
swirl to dissolve crystals
Glass of wine turned a VERY intense VERY dark blue/black.

Conclusion:
Hypothesis supported. the color change seems to be dependent on a change in pH. Specifically when the wine is turned basic, it's color changes to dark blue or even black
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  #8  
Old 08-20-2011, 10:41 PM
Sigene Sigene is offline
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Proposed new studies

Well now that I've cracked open the red wine......

I'll have to get a fresh glass.
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  #9  
Old 08-20-2011, 10:43 PM
bump bump is offline
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Anthocyanin pigments are purple/red in acidic solutions, and blue in basic ones. The same thing happens with red cabbage, by the way.

Your water must be pretty hard if it'll turn red wine blue.
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  #10  
Old 08-20-2011, 11:39 PM
user_hostile user_hostile is offline
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Aha! I notice when I rinsed the Merlot in a white sink the color of the wine turned blue.

The water softener used is a potassium salt.

Better living through chemistry1
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  #11  
Old 08-21-2011, 12:24 AM
Johanna Johanna is offline
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Quote:
Originally Posted by kittenblue View Post
No blue. Now, it wasn't a deep red wine....just some cheap soft red. But nothing went blue.
I'm referring to merlot, specifically.

Quote:
Originally Posted by bump View Post
Your water must be pretty hard if it'll turn red wine blue.
<shrug> It's from the Potomac.

Last edited by Johanna; 08-21-2011 at 12:26 AM..
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  #12  
Old 08-21-2011, 08:20 AM
kittenblue kittenblue is offline
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Well, great. Now I'm going to have to go buy some merlot. Thanks a lot!
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  #13  
Old 08-21-2011, 09:44 AM
LSLGuy LSLGuy is offline
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This happens with pretty much any red wine. And pretty much any water. I'd always wondered about the the mechanism, but suspected that, like litmus paper, it was pH related. Good to see that confirmed.

If you let the red wine evaporate the effect seems stronger. e.g. Drink a glass of wine. leave the glass sitting on the counter overnight with that last couple of drops in the bottom. The next day (once fully awake), put a 1/2" or so of water in the glass & swirl. the result is light blue water. If you fill the glass full of water the color will be too light to distinguish from ordinary water.
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  #14  
Old 08-21-2011, 01:31 PM
Johanna Johanna is offline
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Quote:
Originally Posted by LSLGuy View Post
If you let the red wine evaporate the effect seems stronger. e.g. Drink a glass of wine. leave the glass sitting on the counter overnight with that last couple of drops in the bottom. The next day (once fully awake), put a 1/2" or so of water in the glass & swirl. the result is light blue water. If you fill the glass full of water the color will be too light to distinguish from ordinary water.
Yeah. Thank you. That was the detail I omitted. Wineglasses left out all night and washed the following day is where I always observe the phenomenon. Now what could be the reason for the color getting stronger that way?
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  #15  
Old 08-21-2011, 01:51 PM
ultrafilter ultrafilter is offline
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Quote:
Originally Posted by Johanna View Post
Yeah. Thank you. That was the detail I omitted. Wineglasses left out all night and washed the following day is where I always observe the phenomenon. Now what could be the reason for the color getting stronger that way?
Water evaporating?
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  #16  
Old 08-22-2011, 12:48 AM
Johanna Johanna is offline
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Quote:
Originally Posted by ultrafilter View Post
Water evaporating?
The phenomenon happens with dilution, not evaporation. I don't get why first evaporating the few drops of wine would make a stronger color when later diluted.
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