Chemistry Question #376

Michael Baird, a 45 year old male from the Internet asks on May 5, 1998,

Can you help me with a wonderful magic trick I performed as a boy? I made a silk rose change magically from white to violet as follows:
  1. I placed a few drops of ammonia in the base of a stand where it was absorbed by some felt.
  2. I treated a white silk rose with a chemical.
  3. I sprayed the rose with 99% rubbing alcohol, then placed it upright on the base stand.
  4. I put a glass cover over everything.
The white rose then amazingly changed to violet in color. Here's my question: prior to spraying the rose with alcohol, I treated it with another chemical which I cannot remember. It dried on the silk rose and lasted for many color changes. What was that chemical?

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The answer

Reg Mitchell answered on May 5, 1998

I checked around the University of Victoria chemistry department, and most people suggested phenolphthalein - however that is really more pink than violet. One person suggested copper sulphate solution, which is pale blue but turns a deep violet blue with ammonia. The Merck Index in its Miscellaneous Tables section has a huge table of indicators. Here are some more suggestions:

  • Methyl violet is yellow in acid and violet in base (ammonia)
  • Bromophenol blue is same
  • alizarin sodium sulphonate is same
  • quinoline blue is colourless in acid/neutral and violet in base
  • m-cresol purple is yellow in neutral/acid purple in base
  • cresolphthalein is colourless in neutral/acid purple in base as opposed to phenolphthalein which we mentioned is pink in base

The principle is that the ammonia creates a basic (i.e. not acidic) environment and therefore the indicator changes colour. Good luck!

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