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For some time I've been buying a brand 'A' of (non-sparkling) bottled water.
It wasn't easy to find one that I liked, because I make tea with it, and most other brands I tried made murky, smelly, dark tea, with a disgusting film on top, whereas this one was perfect.

Unfortunately, I just found that the supermarket doesn't stock brand 'A' anymore (I know, it's not a big problem, by any standards; just discussing a bit of science, for fun).

So I picked the closest one I could find (brand 'B'), naively thinking that the main 'factor' to look at was pH.
Brand 'A' had a pH pf 6.7; brand 'B' was 6.9. I thought: close enough.

Actually: wrong.

I made tea with brand 'B' water, and while it wasn't as horrible as with higher-pH ones I had tried before, it was nowhere as good, it made that slimy film, and smelled a bit off.

So I looked at the composition of the two brands side by side, converting the concentration from mg/L to mmol/L:

$$ \begin{array}{lcc} \hline \text{Component} & C_\mathrm{A}/\pu{mmol L^-1} & C_\mathrm{B}/\pu{mmol L^-1} \\ \hline \ce{HCO3-} & 0.38 & 4.72 \\ \ce{SiO2} & 0.32 & 0.00 \\ \ce{Na+} & 0.13 & 0.18 \\ \ce{K+} & 0.02 & 0.16 \\ \ce{Mg^2+} & 0.04 & 1.60 \\ \ce{Ca^2+} & 0.10 & 1.07 \\ \ce{F-} & 0.01 & 0.00 \\ \ce{Cl-} & 0.02 & 0.06 \\ \ce{NO3-} & 0.03 & 0.00 \\ \hline \end{array} $$

Clearly there is much more 'stuff' in water 'B' than in 'A', and in fact the residue is 289 mg/L in the former, 43.6 mg/L in the latter.

One thing that caught my eye was the charge balance, which seems a bit off for 'B' (+0.91 mmol/L), whereas it's a bit closer to 0 for 'A' (-0.02 mmol/L).

But OK, my main question at this point is: can I add something to water 'B' so that it 'behaves' a bit more like 'A', for the use that I mentioned?

My hypotheses so far are that:

  • the pH should be lowered
  • $\ce{Ca^2+}$ and $\ce{Mg^2+}$ are responsible for the formation of that 'film', and I might prevent that by adding something that forms soluble coordination compounds with these ions (and more stable than the ones they form with whatever it is in tea that reacts with them)

Obviously I am not going to use stuff that is not edible, like EDTA; and for the pH correction, I'll have to rely on calculations, not going to stick a pH-meter into it.

So my best bet at the moment is: citric acid.
Though I am not sure how I should calculate the correct quantity to add to the 5 L bottle I buy (surely I can't aim to convert most $\ce{HCO3-}$ to $\ce{H2CO3}$, or the final pH would have to be too low).

What do you think?
Any ideas/comments/suggestions?

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    $\begingroup$ Where do you live that you cannot take tap water to make tea? Most bottled water is high on minerals. $\endgroup$
    – Karl
    Commented Sep 30, 2020 at 19:16
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    $\begingroup$ Search for a brand C,closer to A in the bicarbonate content. The bicarbonate content of B is nowhere close to make a good tea from it. You may try a Brita ionex based filter or similar. $\endgroup$
    – Poutnik
    Commented Oct 1, 2020 at 1:58
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    $\begingroup$ BTW, to the 4 people who voted to close my question without actually commenting or explaining what they don't like about it: well done! We're never going to get any shortage of sad, cowardly backstabbers in this world! $\endgroup$ Commented Oct 1, 2020 at 7:31
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    $\begingroup$ Remember bicarbonate content, aside of heavy metals like iron and manganese, is probably the most critical parameter. Boiling converts it to rather alkaline CaCO3 or Mg(OH)2, what leads to scale deposites and adsorption/oxidation/condensation/darkening of various tea flavonoids and unpleasant sensoric perception, together with eventual precipitation of hydrated oxides of Fe or Mn. $\endgroup$
    – Poutnik
    Commented Oct 1, 2020 at 8:05
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    $\begingroup$ @user6376297 You shouldn’t jump to assumptions about the close votes. I can see them; three are on needs more focus, the fourth is on we do not give personal medical advice. Personally, I disagree with both but neither would immediately suggest adding a comment. (Indeed, unless a custom off-topic reason is used, most close votes here come without comment and are considered self-explanatory.) $\endgroup$
    – Jan
    Commented Oct 1, 2020 at 9:28

4 Answers 4

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Long time ago, I supervised an undergraduate on a summer tea project (for fun). In South Asia there is a natural strawberry colored tea made from slightly matured green tea. You boil the leaves in water with a small amount of sodium bicarbonate. Ice cool it, and boil again and aerate it with a ladle. The tea becomes reddish on aeration and after adding milk, it becomes pinkish. So I can recall your experiences because I tried this recipe in another country. No matter what I did, the right color would not develop. It remained murky brown. It turned out that the chlorinated water was ruining the natural colors, it was particularly destroying the reddish pigment. Passing the tap through a carbon filter+ion-exchanger did not work as calcium also seemed important for the right color.

Finally a mineral water bottle worked and the undergrad saved a project for his poster presentation.

Anyway, it seems your tap water is of bad quality or it is hard. This tea "scum" are Ca/Mg salts. You guessed it right. My suggestion is do not add any acid such as citric acid. Tea is not meant to be made in the presence of citrate salts especially if you add milk to it. I do not know what the taste be like. Most likely horrible!

I do not know how easy it is find potable distilled water in stores. Ask the manufacturer if it is fit for human consumption. Distilled water is not ultrapure water as it may have organic impurities or volatile impurities. It is only free from water soluble salts. All these tea scum problems should vanish.

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  • $\begingroup$ Thanks; I think distilled water is positively NOT fit for human consumption, at least that's what I am told. Unfortunately before I saw your comment I already tried adding citric acid. I assumed 1 mol citric acid per mol of bivalent ions would do, so 2.67 mmol/L, which worked out to about 2.8 g citric acid monohydrate to add to the 5 L bottle. So... hmm... let's say that I now have 5 L of rather sour water to go through :( Not going to back-correct it with bicarbonate, I messed it up enough as it is :) $\endgroup$ Commented Oct 1, 2020 at 7:35
  • $\begingroup$ 1) both waters are bottled, according to the question. 2) Traditional Earl Grey Tea drinkers may heavily object to not adding citric acid to their tea but they would also not add milk. Some Germans took that idea too far and tend to add a slice of lemon to whatever black tea they make until I request they stop … $\endgroup$
    – Jan
    Commented Oct 1, 2020 at 9:31
  • $\begingroup$ Adding a few drops of lemon juice to green tea or black tea (without milk) is very common throughout the world. This is done after boiling the tea. If we boil the tea with a few drops of lemon juice, it will be a horrible taste. But lemon juice or slice is not equivalent to citric acid alone-hence my suggestion. $\endgroup$
    – AChem
    Commented Oct 1, 2020 at 13:25
  • $\begingroup$ @user6376297, Drinking distilled indeed not fit for consumption because of the lack of ions, however when you boil tea it is no longer distilled water and it will have ions. $\endgroup$
    – AChem
    Commented Oct 1, 2020 at 13:27
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The ones you want to look out for are calcium and magnesium. Either try finding a water that has lower content of those two ions or try finding a water filter that can soften the water.

I grew up in an area with very hard (but potable) tap water and such a water filter was constantly used before boiling our tea water. It had the nice added side-effect that there was significantly less limestone deposition on our kettles.

I would not recommend trying to fudge around the pH by yourself. The pH values of both bottled waters are very close to neutral meaning that additions of minute quantities of acid will likely have a large effect. But that notwithstanding, both the process of brewing tea and potentially adding milk (or lemon, if you are into that) to a black tea will alter it anyway.

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Your brand B mineral water is pretty hard water. Boil it for 5 minutes, cool, decant the water. Do your tea with that.

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Distilled water unfit for human consumption should be identified on the label with giant red letters; otherwise it should be mineral-free and quite pure. Getting an accurate pH will be difficult since there will be such low ionic conductivity. Possible contaminants like low vapor pressure organic compounds? Like ethanol, methanol, benzene? Highly unlikely.

Distilled water is bland - perhaps too bland for drinking straight. And by the time you boil out the little bit of $CO_2$ present, it will be blander than bland! Therefore perfect for your tea. If by chance, the distilled water is of low quality, try another brand. I suspect that distilled water is made in truckload quantities and sold to bottlers who do a good job and to bottlers who are sloppy.

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