# Any idea why tap water goes stale overnight? [duplicate]

We have great tap water here in Hungary. My only problem that it goes stale overnight sometimes in some glasses or sometimes even the 2L water boiler has this problem too. But microbes cannot resist 100°C or 20% vinegar, so I thought it might be rather a chemical compound what causes it. It is really hard to get rid of it. When I use boiling water or vinegar it does not remove it. When I use soap and water several times, it removes it most of the times, but only for a few days. The only working solution was not using the affected glass for a month. It is hard to descibe the taste, but it is something like how new Chinese clothes or paint thinner smell so it is sort of strong plastic taste. The longer I use a glass without washing it every day, the bigger the risk it develops this feature (2 days are enough for certain glasses, while others never develop it), so maybe microbes are producing it after all, but the compound is long lasting even if they die off. Any idea what causes it and more importantly how to get rid of it permanently without having to wash my glasses, boiler, water jug each day?

• You should get your stale water tested from a laboratory. Normal water will not change its taste overnight in normal containers nor should it develop plastic taste. Are your glasses food grade glasses? How do you store it? In fridge, water can absorb food odors. – M. Farooq Dec 12 '20 at 1:30
• Just so I get this right, you don't want to wash your dishes? – Karl Dec 12 '20 at 8:28
• @Karl Haha, lol. I don't want to wash them this often and washing them does not seem to solve it. Even after washing them with boiling water and soap I can taste it a few hours later. So far I tried it with glass and stainless steel. I'll try it with ceramic too. – inf3rno Dec 12 '20 at 8:34
• You're contradicting yourself. You said the taste occurs sometimes if you leave a glass with water out overnight. And that it only occurs if the glass hasn't been washed for 2+ days. – Karl Dec 12 '20 at 8:40
• There is a nozzle on most faucets, whose purpose most people don't realise is to aerate the water as it comes out. Without any other information or testing I would have assumed this is the important effect. – Tim Seguine Dec 12 '20 at 15:31

Exclude reasons one by one

Surface biological contamination: use two vessels, wipe the one of them with H2O2, hydrogen peroxide solution, leave another untreated. Check if time till smell appears is different by a few days. Bacterias can survive 20% vinegar and 100*C for a short time, some of them, if they are in a sleeping stage, it is not quite reliable method i would say.

Tap water biological contamination: use two vessels with water, add a few tablespoons H2O2 with water in one, and just water in other. Check if time till smell appears is different by a few days.

Tap water chemical contamination: buy some bottled water, use two vessels, one with tap water, another with bottled water. Check if it will be different in a few days.

Bulk material of the vessel releasing chemicals: leave water in different material vessels, one is glass, another one is your preferred one. Check if glass vessel has less smell adter a few days. Glass releases the least amount of chemicals into the water.

Dust accumulation: use two vessels, cover one and leave another one open, check if smell will appear faster in one.

Psychological factor: use two identical vessels, ask your friend to give you one or another, randomly, blindfold yourself, do a test 10 times, check if your score is above 8 correct guesses out of 10 tries.

After completing this tests you will narrow down your search significantly to give you some idea about what is going on.

• Thanks! I'll do the experiments. I'll rather use KMnO4, I find it safer for drinking water not sure how much contaminants the H2O2 would contain. It is hard to get decent chemicals in my country. – inf3rno Dec 12 '20 at 2:12
• KMnO4 also works, just as good, you can use it too. I offered H2O2 because its usually easier to get, used in medicine for small wounds treatment. And H2O2 is exceptionally sensitive to contaminants, it decomposes if any dirt is present, unlike KMnO4. And KMnO4 is prohibited in some countries because something related to drugs. And H2O2 decomposes into safe products, water and oxygen. – Surprised Seagull Dec 12 '20 at 2:20
• It was prohibited here for a few years to have big quantities of KMnO4, but currently it isn't. Wineries use it a lot afaik. I have a little, maybe 5-10g crystals from an organic chemistry research lab, so I am pretty sure it is pure asf. I really liked to work in labs, but I ended up in IT somehow. I still miss it, I would do both if I could. – inf3rno Dec 12 '20 at 4:16
• KMnO4 is fine from toxicology point of view, but maybe not for this experiment since it has a distinct taste. – cbeleites unhappy with SX Dec 12 '20 at 12:04
• @cbeleitesunhappywithSX In theory it is sweet. I don't think it will be hard to distinguish it from the other taste. – inf3rno Dec 12 '20 at 21:38

A few more thoughts:

• fresh water taste is AFAIK associated with come $$\ce{CO_2}$$ being dissolved in the water. This can get lost, and make the water taste stale.
Here's an easy experiment to check this: compare the taste of:

• fresh tap water,
• water gone stale "your way" (cooled to the temp of the fresh tap water), and
• de-gassed fresh tap water: take water, boil it to remove dissolved $$\ce{CO_2}$$, cool to same temp as the other 2.

If the boiled water tastes stale like the stale water, loss of $$\ce{CO_2}$$ is likely the cause.

I wouldn't describe this as anything like paint-thinner or plastic or the like. But for me it is the textbook meaning of "stale" for water.

• Sidethought about "chlorine" smell. What we usually describe as chlorine smell in water are chloramines (mono-, di-, tri-). In first approximation, we can say that they form from e.g. from chlorine used for water treatment with amino groups from organic matter. AFAIK, our nose is more sensitive to them than to chlorine itself. I.e. the smell indicates that some organic contamination occured, but also that chlorine was there and did work - but it does not indicate whether the water is now safe from a microbiological point of view, the amount of chlorine may not have been sufficient.
So if your tap water arrives fine and with some residual chlorine, but you then get organic matter into the water, chloramine smell may develop later on.

In my supported opinion, it’s first chemistry (actually Fenton-type chemistry and contributing photocatalytic sunlight ). I am assuming one has copper (or iron) metal pipes bringing in the water containing dissolved oxygen into their home. Further, it has been treated with say Chlorine (creating residual Hypochlorous acid, HOCl) or even Chlorine dioxide (where .ClO2 is stable free radical) and left sitting out in some light.

The second piece of the puzzle is the introduction, at some point, of organisms that will flourish upon removal of said chlorinating agent or Chlorine dioxide.

The chemistry-based explanation, in words, I would describe as, for example, a Fenton (or Fenton-type and photo-Fenton) reaction occurring with transition metal ions in the presence of dissolved oxygen creating the so-called superoxide radical anion. The later radical can directly react with Chlorine dioxide creating the Chlorite anion (ClO2-), which is unstable and far less effective in disinfecting water than Chlorine dioxide. The superoxide can also react with Hypochlorous acid in a decomposition reaction forming powerful, but unfortunately, very transient radicals.

Some background material:

Ozone treatment while effective produces no residual continuing disinfecting action, but can reduce the subsequent amount of chlorine, for example, required, so no further mention of ozone is needed.

$$\ce{ClO_2}$$ is a gas that is highly soluble in water (some 8 g/L at 20 °C). However, per Wikipedia , to quote:

It does not hydrolyze when it enters water, and is usually handled as a dissolved gas in solution in water.

Also, from a government source makes the statement:

In air, chlorine dioxide readily dissociates both thermally and photochemically and may form chlorine, oxygen, hydrogen chloride, HClO3, HClO4.ClO, chlorine peroxide, and/or chlorine trioxide, dependent on temperature and humidity. Chlorine dioxide dissociates in water into chlorite and chloride, and to a lesser extent into chlorate (Budavari et al. 1996).

Further, a key supporting Dutch study: Chlorine Dioxide as a Post-Disinfectant for Dutch Drinking Water, to quote:

In this paper, results are presented of experiments into the consumption and reaction kinetics of chlorine dioxide in a number of (drinking) waters in The Netherlands. It was found that chlorine dioxide consumption is related to the dissolved oxygen content (DOC) of the water and the reaction time.

Now, relating to the superoxide radical anion, it can be created by a so-called metal auto-oxidation reaction with say ferrous (as in $$\ce{Fe(HCO3)_2}$$) or cuprous presence:

$$\ce{Fe(II)/Cu(I) + O2 <=> Fe(III)/Cu(II) + .O2-}$$ (See also, Figure 1 here)

Further, an implied radical reaction to account for the loss of Chlorine dioxide:

$$\ce{.O2- + .ClO2 -> O2 + ClO2-}$$

In summary, the presence of generated superoxide from various sources in natural oxygen-rich waters (from sunlight and transition metals,..., interacting with O2), which has been treated with say chlorine dioxide, where superoxide concentration is connected to dissolved oxygen presence, is one of the more likely pathways to account for the reported removal of $$\ce{ClO_2}$$.

On references citing superoxide presence in natural waters, I refer those interested to Measurement of Antioxidant Activity toward Superoxide in Natural Waters, which mentions a photochemical pathway for superoxide, which is likely the largest source for sunlit waters. Also, ferrous concentration in natural waters is discussed in this paper, where, it should be noted, that absence light, transition metals and dissolved oxygen could be sources of superoxide.

Bottom line, oxygenated water, transition metal ions (newly introduced from household plumbing), and associated water disinfecting agents can interact resulting in the eventual removal of said agent. This leaves the water vulnerable to colonization by microbes, algae, or whatever opportunistic organism finds the aqueous environment favorable.

Note, my proposed explanation accounts for how the treated tap water after entering the home and exposed to light, could soon lose its disinfecting power, and in time, can become readily contaminated, therein altering the water's taste.

[EDIT] As a side comment, the mechanism at work here (radical chemistry assisted by UV light exposure and biological) may similarly be at play in the observed off-taste that develops in previously open water bottles left in cars subject to UV light, added air and bacteria (from partial consumption) and/or warming. My personal experience strongly suggests, at times, a UV-assisted hydroxyl radical attack on the plastic vessel itself introducing problematic organic breakdown products.

• This is good. I mean we can add another factor to the experiment "Surprised Seagull" suggested: sunlight or artificial light. I am not sure about the exact material of the pipes, but they are made of metal. – inf3rno Dec 12 '20 at 7:26
• Tap water in Germany is by law absolutely free of any disinfectants. I have never heard about anybody having a "paint thinner" taste in the morning. – Karl Dec 12 '20 at 8:32
• @Karl: also in Germany, drinking water is treated routinely, and chlorine treatment is one way. We may have two advantages over Hungary, though: I know from some water engineer friend that many German regions have soil temperatures that are just sufficiently low to make drinking water microbiological safety easier than it would be at only a few degrees warmer. Hungary may need a bit more chlorine (and it may be unable to go for e.g. UV disinfection only for the same reason) in order to have micobiological safety all the way to the tap. The second aspect is the state of the supply network:... – cbeleites unhappy with SX Dec 12 '20 at 12:37
• ... how likely is microbiological/organic contamination to happen after the treatment point of the local drinking water supply. Not sure how Germany vs. Hungary compare in that respect, but I'd anyways expect substantial regional differences. All this is not paint-thinner like, though. – cbeleites unhappy with SX Dec 12 '20 at 12:40
• @cbeleitesunhappywithSX There are big differences between regions here. I live in a mountain region, where water quality is better than in lowlands. – inf3rno Dec 12 '20 at 21:42

My daughters all kept their fresh drinking water in the fridge and never complained about the taste of water that had been kept there in a covered plastic jug even after several days. When I was a child living in Malawi in the 1950s, all our water was drawn from the river. My mother kept our boiled filtered drinking water in a charcoal cooler. That primitive device would never have reached a temperature anywhere near that of any modern fridge, but I still found that a glass of water straight from the cooler was very refreshing and tasted sweet, even if it had been kept there for a day or so. Cool water always tastes fresher than water at room temperature. I suggest you do a simple before and after comparison. Take the temperature of your fresh tap water and check its temperature again before you taste it the next morning.