# Intense purple colour of litmus paper in contact with calcium oxide

When conducting an experiment to identify some common chemicals, I tested crystal/powder samples of the chemicals with moist litmus paper. When I put a piece of blue paper on some $$\ce{CaO}$$ powder, much to my surprise, it turned an intense, bright purple (A). When I used a piece of red litmus paper later to test it, it turned a purplish blue as expected (B). Below roughly shows the colours I saw.

I have been unable to figure out what caused the intense purple colour, and why the same does not happen for red litmus paper.

• What is the meaning of the eight long, thin and colored rectangles, which do not seem to be related to the letters R, G and B, printed before each line of rectangles? And what is the meaning of the eight numbers varying between $5$ and $232$ and corresponding to each rectangle ? Sep 25, 2022 at 14:20
• Yes, I left them out in a new edit. The values aren't too important for this question indeed. Sep 25, 2022 at 14:27
• @maurice FYI on those color rectangles. The first three are red, green and blue components in the rgb color solid. If there is 102 blue, let us say, the color would be 40% of the way from a relatively dark reddish or greenish color (0 blue) to a brighter, predominantly blue color (full blue component, which is 255). The last rectangle shows how bright the color is overall, from 0 for black to 100 for a fully brightened hue. Formally this number is the max rgb component divided by 255, as a percentage. Sep 25, 2022 at 19:06
• @ Oscar Lanzi. The colored rectangles have been removed now. Sep 25, 2022 at 19:14
• Still thought you'd like to know. Actually the red, green and blue numbers are still there as the letters and digits you still see, which are hexadecimal representations of the original numbers. Sep 25, 2022 at 19:16

## 1 Answer

You may have overreacted the litmus paper. Such overreactions are possible when indicators are exposed to strongly basic (or strongly acidic) conditions.

The complex organic compounds that make up typical pH indicators may undergo multiple stages of acid-base reaction, which could then impart more than one color change. Perhaps the best known example is with phenolphthalein. We all know what happens when the indicator, typically added inits colorless "weakly acidic" state, is exposed to a basic solution. But:

Between strongly acidic and slightly basic conditions, the lactone form (HIn) is colorless. The doubly deprotonated (In2-) phenolate form (the anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In(OH)3− form, and its pink color undergoes a rather slow fading reaction1 and becomes completely colorless when pH is greater than 13.

So if we were to expose phenolphthalein to let us say, a one molar sodium hydroxide solution to get the beautiful pink color, we would instead see the pink "unexpectedly" fade away. Milk of magnesia is actually better optimized for turning phenolphtahlein pink, because of the lower pH to which magnesium hydroxide solutions are limited.

So it likely is with litmus. Adding a powerful base such as calcium oxide to an already "deprotonated" blue litmus may cause an overreaction similar to what Wikipedia reports for phenolphthalein. Here, too, using the less soluble but still sufficiently basic magnesium oxide or hydroxide should avoid the overreaction.

Cited reference

1. Kunimoto, Ko-Ki (February 2001). "Molecular structure and vibrational spectra of phenolphthalein and its dianion". Spectrochimica Acta Part A. 57 (2): 265–271. Bibcode:2001AcSpA..57..265K. https://doi.org/10.1016/S1386-1425(00)00371-1. PMID 11206560.
• (+1) Nice answer! I used to do the calcium metal in water lecture demo, using a 2 L beaker with about 100 mL of water in it. I had a little phenolphthalein to the water initially and the resulting turbid solution, after the very exuberant reaction of calcium with water, was bright pink. But the color faded away, “softly and suddenly”, over a period of a few minutes. Then I learned about that further reaction you discuss.
– Ed V
Sep 25, 2022 at 12:00
• Thanks Ed. As an example of magnesia being better optimized, magnesium metal will give an enduring pink color, which would be more prominent if warmed. Also be sure to avoid exposure to CO2 from air. Sep 25, 2022 at 12:03
• @EdV see sciencephoto.com/media/568914/view/… for the color imparted by magnesium metal with phenolphthalein. The left one is warm water, the right one cold. Sep 25, 2022 at 19:10