Pigment of purple violets as pH indicator?

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Offline valeg96

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Pigment of purple violets as pH indicator?
« on: 26/03/2014 19:46:30 »
What is the pigment in purple violets? Does it decompose over short (hours) periods of time?
I've prepared a solution of 120 mashed purple violets in 100mL ethanol, as a pH indicator. The solution was purple/blue when I was preparing it, but after a day in the dark it became olive-yellow/green. It now turns bright yellow in acids and bright pink (like phenolphtalein) in bases (now that I think about it, it may be the opposite).
http://en.wikipedia.org/wiki/Viola_odorata [nofollow]
Anyway, why did it decompose? Do you have suggestions on possible natural pH indicators? When June/July comes I'll try with sambucus berries, they have an interesting deep black/violet color.

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Offline evan_au

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Re: Pigment of purple violets as pH indicator?
« Reply #1 on: 26/03/2014 20:45:00 »
The greenish colour may be chlorophyl from the plant?

A plant has many pigments & enzymes in its cells, carefully orchestrated. However, when you mash the plant, all of the contents go into an uncontrolled soup, which is then exposed to air. These will all continue to react together, and evidently the purple pigment has decomposed. But it certainly sounds like a colorful chemical test when it is fresh!

Another "herbal" pH indicator is red cabbage - boil it up, and use the water as a chemical indicator.

Not nearly as intense as violet mash, but still very educational.

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Offline chiralSPO

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Re: Pigment of purple violets as pH indicator?
« Reply #2 on: 26/03/2014 23:28:34 »
I can vouch for the cabbage juice--excellent stuff.

As for the color change. Organic pigments are often quite susceptible to oxidation, which alters the color of the compound. I am not surprised that a solution of flower pigments faded or changed color on standing in air over night.