Tuesday, March 13, 2012

Fun with Phenolphthalein

Phenolphthalein is an easily available chemical that is an acid/base indicator that turns purple/pink in the presence of bases. It's commonly used in construction to test whether concrete carbonation has happened and for children's toys to make parts change colour magically when they are sprayed with a special solution.

Another fun thing to do with phenolphthalein and some household chemicals is make disappearing ink: some caustic soda, some phenolphthalein, some ammonia and an acid like lemon juice or clear vinegar.

Note that some of those chemicals are very nasty if you splash them in your eye, inhale them or swallow them. If you do this experiment be careful and protect yourself.

To make disappearing ink you do the following:

1. Take 12g of caustic soda and dilute it in 100ml of water.

Since caustic soda is so nasty do this slowly and carefully and note that it's an exothermic reaction so the water will get hot. Here's my solution reading 50C:

2. Next take 10ml 1% phenolphthalein solution (which is typically dissolved in alcohol) and mix with 90ml of water.

3. Take drops of the caustic soda solution and drop them into the phenolphthalein. It will instantly change colour as the indicator reaction happens. Here's the result of a single drop of the caustic soda solution (top) dropped into the phenolphthalein (bottom).

And here it is with 20 drops of caustic soda.

You now have disappearing 'ink' which you can pour onto a piece of cloth. Here's a big splash of that ink on a piece of white cotton:

About 15 minutes later the ink has disappeared:

What's happened is that the water has reacted with carbon dioxide in the air to form carbonic acid, that in turns reacts with the caustic soda to produce sodium carbonate. The result is that the pH drops and the phenolphthalein returns to transparent.

If you now pour a little ammonia on the cloth the colour will reappear. Another experiment is to take the remaining phenolphthalein solution and add an acid (lemon juice, for example) and see the colour disappear. Add a little caustic soda back and the colour reappears.

PS. If you prefer to make blue 'ink' then use thymolphthalein.

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