I use filtered water to make my coffee.

If I mix a few drops of said coffee with our tap water, it turns a grayish color; it's much darker than the coffee would turn the water by itself (I compared by doing a side by side comparison and adding an equal amount of coffee to filtered water, resulting in a much lighter brown (not gray) color that is closer to what I'd expect from coffee). It's a very pronounced effect (i.e. easily visible when you rinse out a coffee mug that has just a few drops of coffee at the bottom). Because of the distinct color change and the fact that it's significantly darker than it should be from simple mixing, it's obviously a chemical reaction.

What common tap water contaminants would react with coffee in this manner? Iron perhaps?

  • $\begingroup$ Tap water doesn't contain a lot of iron (see auroville.info/ACUR/documents/laboratory/…) however that can vary depending on where you live - wouldn't it be more likely that calcium or magnesium reacted? $\endgroup$
    – user2117
    Nov 18, 2013 at 18:06
  • $\begingroup$ @LievenB That's why I'm asking. ;) I had assumed maybe iron or manganese (manganese in particular can apparently cause the grayish appearance) but both are supposed to react with air. This is reacting with coffee. It may be basic since coffee is acidic, and it doesn't seem to produce any gases. I left a glass of filtered water next to the tap water for several hours and the color of the tap water did change a bit (it got slightly yellow) but iron/manganese are supposed to turn the water red/gray respectively (and also cause deposits at the bottom, which I don't see). $\endgroup$
    – jstm88
    Nov 18, 2013 at 21:31
  • $\begingroup$ this might be strange to think of, but could the coffee solution contain a pH indicating compound? (tea does) (that would clarify the colour change) I would suggest that you check the pH value of the different solutions that you made? $\endgroup$
    – user2117
    Nov 19, 2013 at 17:05
  • $\begingroup$ @LievenB Interesting thought. I don't have any pure acids/bases handy to test it on, but I'll definitely try that once I find some (most likely vinegar/bleach) to test it. If that's true, and if it works like tea, the darker color may indicate alkalinity. $\endgroup$
    – jstm88
    Nov 20, 2013 at 14:30
  • $\begingroup$ yes - vinegar is a good idea to test it. Normally a coffee solution would be slightly acidic - if you want to use milder bases then I suggest you use soap (what is reasonably alkali) $\endgroup$
    – user2117
    Nov 21, 2013 at 16:04

1 Answer 1


The tap water might be slightly hard. Hard water has a high content of minerals such as calcium or magnesium which makes the water more alkaline. Alkaline water will draw tannin out of the coffee much quicker than pure water.

Filtering the water with something like activated carbon filter then an ion exchange filter will lower the mineral content, thus lowering the pH. With a lower pH the dissolution of tannin will be slower.

Slowing the dissolution of tannin relative to more soluble portions like caffeine is part of the reason for steeping tea and coffee for only a limited amount of time. More steep time, or more alkalinity in the steeping solution, releases more tannin. More tannin makes the coffee more anti-nutritional, more bitter, and it darkens the color.

  • 2
    $\begingroup$ I doubt that filtering with activated carbon will lower the mineral content. $\endgroup$
    – aventurin
    Aug 27, 2016 at 23:00

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