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We did a titration experiment at school and I did not understand one part of it.

The scenario:

  • Titrate : White wine (ethanoic acid)
  • Titrant: $\ce{NaOH}$
  • Indicator: Phenolphthalein

What happened: We carried out the titration and the end point was a brown colour!

How does that happen? Why is the color brown and not pink?

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    $\begingroup$ I strongly recommend AGAINST closure of this question. It may have come from a school demonstration, but it's a curious observation. Why indeed would phenolphthalein turn brown instead of pink? A Google search yields no obvious explanation, and brown appears nowhere on the phenolphthalein Wikipedia page. $\endgroup$
    – hBy2Py
    Apr 4, 2016 at 13:26
  • $\begingroup$ I don't think this can be untangled. (1) There shouldn't be acetic acid in white wine. (2) When titrating did the solution turn pink where the drops of NaOH went into the solution? (3) I assume that the color of the "wine" was yellow. What happens if you titrate with NaOH using no phenolphthalein? $\endgroup$
    – MaxW
    Apr 4, 2016 at 17:52
  • $\begingroup$ I went looking for a titration of apple cider vinegar (which would be yellow) with NaOH and phenolphthalein on youtube but didn't find one. It could be that the yellow plus pink gives some sort of "dirty orange" that looks brownish right at the end point. $\endgroup$
    – MaxW
    Apr 4, 2016 at 18:17
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    $\begingroup$ @MaxW "I don't think this can be untangled." Certainly possible, with how complex a solution the white wine is. Still, worth keeping open, I think. $\endgroup$
    – hBy2Py
    Apr 5, 2016 at 18:15

2 Answers 2

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Assuming there are no major color-influencing side reactions, the subtractive color mixing as follows:

#FFA8D9 (Pink, phenolphtaleine) + #DAA520 (Goldenrod, white wine) = #D2691E (Chocolate, result)

actually already gives brown(ish) color. It's hard to pick the right color codes, especially for the white whine, but OP can probably try to perform the same operation with the color he remembered/picked from the photo.

enter image description here

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It happened to me as well when I tried to titrate a sulfuric acid based process solution. The brown color at end-point is normally not observed during this very standard test. A pink color appears when some fresh phenolphthalein is being added to the (brown) solution. I believe that in our case, organic contamination was the cause. Since the tested sample in the question is wine, my guess is that the reason is similar. Organic degradation by-products of spoiled or contaminated wine (a bottle used for quite a while in school lab, right?) probably reacted with phenolphthalein. Try using a fresh wine sample.

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