|My textbook says that lichens on trees and rocks
can be used as indicators for acids and bases.
It said that the lichens in the acid (apple cider
vinegar) turns red, and it turns blue in the base
(household ammonia). I tried 3 different
varieties of lichens, but nothing happened. The
other science teachers say that they've tried
this experiment too over the years, and it's
never worked for them either. Why not? (It's
frustrating the text recommends a lab that
|Question Date: 2002-11-12|
I understand your frustration, and like many
experiments, "the magic is in the details".
the experiment you described does in fact work, a
little critical information was inadvertently left
out. To my knowledge, this experiment will only
work with Lecanora tartarea and Roccella
(only two of the more that 16,000 species of
lichens found worldwide), both of which are
extremely abundant in the Netherlands (the major
source of litmus).
In litmus production, the
lichens are pulverized, treated with potassium
carbonate and ammonia and allowed to ferment.
Mixed with colorless compounds such as chalk and
gypsum, it is marketed as blue lumps, masses, or
tablets. Litmus paper is paper impregnated with
this substance. If you are still interested in
attempting this experiment, you might try
contacting your local biological supply house and
inquiring about the availability of these two
species. Also, as a general rule of thumb, it is
always a good idea to try experiments such as
these first before attempting to repeat them as a
classroom laboratory exercise. Good luck.
The reason of your failure is that you are using
the lichens as such, and in order to observe the
color change you are looking for, you need to
prepare the substance called litmus, that is
extracted from the lichens, and that is the one
that has the indicator properties (blue in base,
red in acid).
Here is a recipe to make litmus out
of the lichens:
Details are difficult to find
because the processes were kept secret.
summary of a modern manufacturing procedure is
from The vanishing lichens, D H S Richardson,
The lichens are ground in a
solution of sodium carbonate and ammonia.
the lichens from time to time and the color
changes from red to purple and finally blue after
about four weeks. The lichens are then dried and
powdered. At this stage the lichens contain partly
litmus and partly orcein pigments. The
removed by extraction with alcohol, leaving the
pure blue litmus. This and a lot more information
about the manufacturing of litmus can be found in
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