How do you test for acids and bases?

1 Answer

Well chemists initially used their tongues to test for acidity (sharpness) versus basicity (bitterness), but I don't advize that you continue that tradition.


In aqueous solution, acidity is characterized by the presence (over and above the equilibrium level) of #[H_3O^+]#. If there are more #H_3O^+# ions than #HO^-# ions, the solution is acidic; if not the solution is basic. If #[H_3O^+]# #=# #[HO^-]# the solution is neutral.

So measurement of #pH# is an option (with a #pH# meter, a glass electrode sensitive to #H_3O^+#). Failing that, a simple indicator could be added to the solution; this is a large organic molecule, whose acidic and basic forms have distinct and distinguishable colours.

I have written before here that most of the things that we eat that taste good (i.e. wine, beer, oranges, lemons, cheese, butter) are slightly acidic. The most basic thing that we eat are egg whites (#pH# approx #8-9#); their soapiness and bitterness are characteristic of a base.

The video below shows an experiment using an indicator derived from boiling red cabbage. A pigment from the cabbage called anthocyanin is what causes all of the different colors you see. Different pH levels of solutions tested produce all the different colors you see.

Other common indicators include:
bromothymol blue
thymol blue
methyl orange
bromocresol green
methyl red
phenol red

Hope this helps!