The
solution in the first jar turns rose coloured, and in the second, blue,
indicating in each case that the solution is alkaline. If now, however,
carbonic acid be blown into the two solutions, that in the first jar,
containing the phenolphthalein, becomes colourless as soon as the
monocarbonate of soda is converted into bicarbonate, and this
disappearance of the rose colour indicates acidity; the blue solution in
the jar containing litmus, on the other hand, is not altered by blowing
in carbonic acid. Furthermore, if to the colourless solution containing
phenolphthalein, and which is acid towards that reagent, a little
reddened litmus is added, this is still turned blue, and so still
indicates the presence of alkali. We have, therefore, in bicarbonate of
soda a salt which behaves as an acid to phenolphthalein and as an alkali
to litmus. Another extremely sensitive indicator is the coal-tar
dyestuff known as "Congo red"; the colour changes produced by it are
exactly the inverse of those produced in the case of litmus, that is, it
gives a blue colour with acids and a red colour with alkalis.
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