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Today, I made a cup of lemon tea in a different way than usual. Instead of pouring hot water in to a mug containing lemon juice and black tea (my normal routine), I let the tea steep for a little while before squeezing the lemon. Before adding the lemon, I noticed that the tea was much darker than I am used to. After squeezing the lemon, the tea changed color to a lighter brown and became more translucent.

I could see only a half inch down the side of the cup before squeezing the lemon, but afterwards I could see three inches down, all the way to the bottom. I did not notice any precipitated solids on the bottom of the cup. What happened?

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  • $\begingroup$ Something in black tea changes its colour from brownish to less brownish at low pH. An indicator, if you want. $\endgroup$
    – Karl
    Oct 14 '19 at 19:26
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The colour of black tea is given by a complex mixture of theaflavins and thearubigins.

These compounds are formed by enzymatic oxidation and condensation of polyphenols of green tea, typically of gallocatechin and epigallocatechin gallate ( EGCG )

Basic dissociated forms of these compounds have strong absorption of visible light due to delocalization of phenolate electrons. This absorption decreases a lot when phenolates are protonized in acidic environment.

So yes, in some sense, black tea infusion is a $\mathrm{pH}$ indicator, similar to litmus or juice from red beet roots. Many natural colourful compounds have $\mathrm{pH}$ dependent colour, as their molecules undergo acid-basic reactions.

But synthetic indicators are better due stability, sharper and better defined colour transition.

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