Rossander’s Security Reader

Today’s post doesn’t have anything to do with information security but it was just too cool and I had to write about it.

Last Friday, I got to work with my son’s elementary school’s Science Day and led the lab on Cabbage Chemistry – using red cabbage juice as a pH indicator. To make the cabbage juice:

1. Cut up a red cabbage.
2. Cover with water and boil for 10-15 min.
3. Strain, cool and bottle.

The resultant juice contains a high proportion of flavins. The mixture starts purple (or sometimes blue). In combination with acid, it changes to pink. In combination with a base, it turns green. The best write-up of the reaction and a good chart of the full range of possible colors is here.

The kids then put an eyedropper’s worth of their test substance into a disposable cup, added about the same amount of cabbage juice and stirred. They recorded their results and then analyzed the kinds of chemicals which were acidic, basic and neutral. They discovered, for example, that acids generally taste sour (at least, all the ones that are safe to eat) and bases generally taste bitter. They also discovered that while adding lemon juice directly to the laundry detergent neutralizes the mixture’s pH, it also turns the result into a gelatinous goop. (I have not yet been able to replicate that result – it may be specific to the detergent we used.)

At home, we took the experiment one step further. We took the juice and poured it on several kinds of paper, then let it dry on a cookie sheet for about two days. We cut the result into small strips and made our own litmus paper. The litmus paper had the advantage of showing a clear color change even when the test substance was colored.

Paper towels and toilet paper both absorbed and retained enough of the juice to be useful indicators. Of those two, the paper towels were the easier to use since they were stronger and the waffle-pattern of the material helped keep it from sticking so tightly to the pan. The kraft paper (from some pale cardboard) failed to absorb enough juice. The construction paper turned a pretty shade of light blue but bound it too tightly to the paper and did not show a clear color change.

Oddly, our homemade juice and our homemade litmus paper showed the same strong pink color change for acids as we’d seen in the class but showed much weaker color changes at the base end of the spectrum. No matter how strongly we mixed the baking soda, we can’t get much past a blue color. We might investigate that more next weekend…