Mary Stevenson

epic adventures in organic sciences

Pomegranate pH February 15, 2009

Filed under: biology,botany,chemistry,science — marystevenson @ 4:28 pm

Pomegranates are pretty cool fruits.  I received a package of pomegranate green tea for Valentines Day.   I heated water, dropped in the teabag, and began to poke it around with a spoon.  With most teas, the color seeps out like solar prominences and spreads as one stirs the water.  In this case, however, I observed the red seeping from the teabag… and disappearing as I stirred.  This seemed very familiar- where had I seen it before?  Then I recalled my analytical lab last Thursday.

We were performing acid-base titrations using phenolpthalein as an indicator.  As the NaOH was added to the analyte (KHP in this instance), a pink color would appear suddenly in the water and disappear as I swirled the flask.  As I got closer to the endpoint, the color remained visible for longer until I reached the endpoint; then it abruptly became a faint pink that would not disappear no matter how much I swirled.

Performing my own titration in the dorm with my teabag, I watched the red.  The red must be, of course, an indicator (I have since learned it is in the class of anthocyanins), and as I nudge the teabag with my spoon, I am essentially titrating the water with acid that is being released from the teabag.  It must be a weak acid because water is a weak buffer (and because we couldn’t eat a pomegranate otherwise), and because the appearance of the red color was more gradual than the sudden change I observed in lab. (Reviewing my notes, I see that weaker acids produce “flatter” titration curves).

Now I’m curious to perform my own experiment: is it possible to brew the tea, remove the teabag, and then use a simple back-titration with a base to determine the pH of pomegranate tea, using pomegranate’s own natural anthocyanins as an indicator?  I realize a few simple clicks through Google would tell me everything I want to know… and I realize that the tea is more than dried pomegranate juice and so I’ll have to take the fact that it is a mixture into account, which could (will) make things more complicated.  But it’s the practical application of what I thought (until yesterday) was a useless piece of class information that excites me.  I can be told all day long, all semester long, why acid-base titrations are important, but when I saw it happening in my own mug- THEN it became important to me.

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