Water, water everywhere, but not a drop that doesn't stink...

June 4, 2007 8:47 PM

If I leave a glass of tap-water out for a few hours, then drink it, I get a strong chlorine taste. Why?

I just read that leaving water out for a few hours can actually cause any chlorine present to dissipate...but I swear the chlorine taste is substantially stronger.
Bonus points: I just tested it, and it almost seems that the chlorine taste is concentrated in the water at the bottom of the glass. Is it possible for the chlorine to dissipate, but only from, say, the water in the top half of the glass or so?

posted by Ziggurat to Science & Nature (15 answers total)

As we learnt from the goldfish post, some places now treat their water with chloramine, which doesn't dissipate like chlorine.
posted by zamboni at 9:19 PM on June 4, 2007

Is it possible for the chlorine to dissipate, but only from, say, the water in the top half of the glass or so?

Yes.

What's more likely is that chlorine that comes out is still hanging around in the glass, and you're smelling it.
posted by frogan at 9:21 PM on June 4, 2007

If as zamboni mentioned, it is chloramine and it doesn't dissipate like chlorine, perhaps the reason why the chlorine like taste is stronger than first out of the tap, the water may be evaporating but not the chloramine. The chloramine becomes more concentrated since it's now in a small volume of water. Just a guess.
posted by GlowWyrm at 9:24 PM on June 4, 2007

And when you drink it cold from the tap, the chill kills the taste a bit.
posted by peep at 9:25 PM on June 4, 2007

Well chemically, its possible that your water treatment plant is using chloramine which doesn't dissipate, although I don't believe it shares the same chlorine taste (is slightly green as opposed to chlorinated water which is blue). From looking at the city of Vancouver's water treatment plant, it looks like they are using a combination of ozone and chlorine. Both should dissipate however.

I did come across a forum that mentioned that leaving water out overnight no longer works as chlorine compounds are stirred into the water rather than bubbled through it as a gas. This could explain why it is a higher concentration at the bottom of your glass.

Unfortunately my google-fu isn't too good on this one, but from my initial interpretation it looks like the only way to remove the chlorine is to send the water through a carbon filter or reverse osmosis. Leaving water stand appears to be a method for lessening the taste nowadays rather than removing the gas.
posted by samsara at 9:40 PM on June 4, 2007

Water fresh from the faucet is aerated. If you leave it to sit for a while, it flattens and loses some of it's mineral flavor. I suspect the chlorine flavor comes through more strongly as other tastes disappear, but even if you couldn't taste the chlorine, the taste of the water would suffer from leaving it out. If you want to neutralize the chlorine you'll need sodium thiosulfate. Incidentally, the best home baked bread supposedly uses bottled water because the chlorine interferes with the yeast
posted by BrotherCaine at 10:04 PM on June 4, 2007

Sorry, amend the post to say "letting it flatten" rather than "leaving it out" which I just realized is ambiguous.
posted by BrotherCaine at 10:05 PM on June 4, 2007

On the off chance that it is something in your pipes, you could let the water run for a minute or so, first thing in the morning. They say to do that if you have lead pipes, because when the water sits in the pipes overnight, the lead leeches into the water and is in higher concentration.
posted by IndigoRain at 10:15 PM on June 4, 2007

I wonder if the chloramine diffuses as a vapor and sort of sits in the part of the glass above the water, waiting to be smelled.
posted by hodyoaten at 10:28 PM on June 4, 2007

Get a Brita or PUR water filter -- it will remove substantial amounts of the chlorine and several other things that affect the taste as well. I've used one for my straight drinking water for years. (I'm not a chemical superfreak, I don't worry that much about anything else, like ice or bath water. It's just for clarity of flavor.)

If you suspect you have unusual concentration of chlorine or a substitute, get a test kit at your pet store.

If this is just "Mr. Science", hey kid, do your own homework. ;-)
posted by dhartung at 11:52 PM on June 4, 2007

dhartung: The post tags suggest ziggurat is vehemently against water filters.
posted by zamboni at 12:25 AM on June 5, 2007

I wonder if the chloramine diffuses as a vapor and sort of sits in the part of the glass above the water, waiting to be smelled.

Chlorine gas is heavier than air so that's a possibility. As I mentioned before though I don't believe chloromine is being used. Check this link on the water treatment process for the Greater Vancouver Regional District.

(ps. I know Vancouver isn't a city as I stated in my first comment...realized my mistake right as I hit submit, wishing there was an edit option..doh')
posted by samsara at 7:03 AM on June 5, 2007

Just FWIW, as I dislike the damn filters, too (I think they're a waste of money), my SO uses a Brita pitcher because she hates the taste of good tap water, much less bad tap water. She has found that the filters last two to three times what Brita claims they should, so replaces them only as needed. It makes it much cheaper.

Of course, even using the normal replacement schedule it would be cheaper than her former bottled water habit!
posted by wierdo at 8:06 AM on June 5, 2007

From what I understand of the chemistry, water left standing in an open glass should have a low rate of convection. At the top of the glass, evaporation and dissipation of absorbed gasses should cool the water and condense the water, making it heavier than the layer immediately below. This will cause the top layer to slowly fall down to the bottom of the glass, replacing the upper layer with warmer, chlkorinated water. Eventually, most of the chlorine will have dissipated, leaving a small cloud of chlorine gas above the container to interact with the upper layer (equilibrium). The more empty the container the less air currents will displace the chlorine and the less true dissipation you should see (though there isn't THAT much chlorine in tap water, so the equilibrium is probably fairly neglible).

This equilibrium won't be reached quickly though, as the difference in temperature/density should be small. Water also likes to stratify based on salinity and temperature, a tendency that will work against any real convection currents. Stratification may be why you taste more chlorine in the bottom inch or so of the glass, and is almost certainly why the goldfish thread contained instructions to leave the bucket out overnight.
posted by Orange Pamplemousse at 9:19 AM on June 5, 2007

Oh, and from the wikipedia article on chloroamine:

Furthermore, water treated with chloramine lacks the distinct chlorine odour of the gaseous treatment and so has improved taste.
posted by Orange Pamplemousse at 9:20 AM on June 5, 2007