Fourth in an occasional series. Links to the first entry, category archive.
In the last two posts of this series, I’ve outlined how you could determine whether a given cup of coffee is caffeinated or decaffeinated. Both procedures require a reasonably well-stocked chemistry lab, including some specialized equipment, so we’re not going to take one of those procedures and make it into a test kit that could be carried in a purse or pocket.
But that doesn’t answer the litmus paper question. I think litmus paper must be slightly mysterious to laypeople. You take a strip of blue paper, dip it into two apparently-identical clear solutions. In the first one, nothing happens. In the second, it suddenly turns BRIGHT RED! Obviously something significant and mysterious is happening.
What’s happening is that a very important and profound bulk property of the liquid — the proportion of free protons or pH — changes the shape of the dye molecules and also changes their color. But it’s easy to find molecules (especially biological ones) that respond to changes in pH. In fact it’s often a challenge to find a biomolecule with a particular function that does not respond to pH in an annoying way (for example by ceasing to have the useful function).
But the concentration of caffeine in coffee is both substantially smaller than the concentration of protons, and substantially less chemically interesting. So it would be very unlikely to find a naturally occurring compound that displays a color change triggered by the concentration — or even the presence — of caffeine.