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26 December 2003

Natural Acidity or Alkalinity Indicators

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published September, 1968.

The pigments in flower petals and green leaves are sensitive to the ratio of hydronium to hydroxyl ions in water, as can be demonstrated by a simple experiment. Crush the petals from a red rose to a fine pulp with a mortar and pestle. Add to the pulp about five milliliters of tap water and continue grinding for a minute or so. Filter the fluid through a sheet of paper toweling into a clean glass container. Discard the pulp. Similarly, filter the juice of a lemon into a clean glass container and discard the pulp.

Place five milliliters of tap water in a clean glass container and make a saturated solution of baking soda by adding soda until no more will dissolve. In another container of clean, clear glass (preferably a small test tube) place 10 milliliters of tap water and to it add 10 drops of the filtered extract from the rose petals. The extract will impart a pink tint to the tap water.

Now add five drops of the concentrated soda solution to the pink water. Swirl the test tube for a few seconds to mix the fluids. The color of the mixture will soon turn light blue. Add 10 drops of filtered lemon juice to the mixture and swirl the tube as before. The pink tint will reappear.

The color can be cycled from pink to blue and back again many times by alternately increasing the concentration of hydronium ions with lemon juice and decreasing it with soda solution. Deeper colors can be developed by using stronger reagents. The addition of a single drop of concentrated lye solution (sodium hydroxide) will change the pink color to a rich brown. Two drops of hydrochloric acid will restore the pink hue.