Floral litmus solutions

Laboratory thirteen in physical science had the students collect flowers from around campus, produce floral solutions by boiling in water, and then test to see which floral solutions changed color for both a known acid and a known base.

The ideal flower is one that turns different distinct colors from the original floral solution color for both acids and bases. A less ideal situation is in which one floral solution changes in a distinct way only for acids while another floral solution changes only for bases.

Jeremy, Tracy, Joyleen

The laboratory uses a variety of local key limes as the known acid. Baking soda is the known base. Summer term is a quick run from proton and electrons, hydrogen to oxygen, and then the structure of hydrogen and hydronium ions, providing an attempt at a seque into this laboratory. This material always reminds me that physical science is a bizarre course, an assembled beast that has way too much material in it.

Lewis and Jeremy

Flowers that work as floral litmus solutions tend to turn magenta in the presence of acids. In the presence of bases the solution may turn blue, steel blue, green, dark blue-green-gray, or almost black.

This summer I did not bring in Alpinia purpurata which had worked so well spring 2011. Experience had suggested that "waxy" flowers did not usually perform well in this laboratory. Although I was aware that the key compound which makes many of the flowers useful as litmus indicators are the anthocyanins, and that these are unrelated to the outer coating, my experience had been that waxy flowers on island either did not contain anthocyanins or they were hard to extract simply by boiling. Thus I was pleasantly surprised when the Alpinia yielded a full two-color changing litmus solution.


Once a functional floral solution is found, the students test of a variety of household compounds such as Pine Sol (yellow variant), ammonia, diluted bleach, drain openers, hand soap, and rubbing alcohol. The bleach has to be diluted or the color change is followed so quickly by a loss of color (the fluid is bleach after all) that the color change is hard to catch with the human eye.

Solenostemon scutellarioides (Coleus) was used by me as a demonstration of a functional floral solution. I noted that technically the solution was not a floral solution, but rather a leaf solution. The summer lab is at 11:00. 

The late lab start this summer (following MS 101 and the SC 130 lecture)  means I can also have the students collect Hibiscus tiliaceus. Prior experience has taught me that at 8:00 the blooms are neither fully opened nor have the necessary litmus compounds developed to act as effective floral litmus solutions. Although the flower is bright yellow, the flower produces a dark purple solution - the same color the flowers turn after they fall off the tree, and the same color seen on the inside of the very base of the petals when the flower is on the tree.
Angie's floral solution displays a lack of color change, indicating a neutral pH.

 Floral litmus solutions do not always act in the same way to different bases. In general acids tend to change what are likely to be anthocyanin compounds pink, hot pink, fuchsia, or magenta colors. Response to bases can be much less consistent with colors ranging from olive greens, dark greens, steel blues, blues, and variations on black. In some instances even yellows are generated.

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