The LITMUS TEST

Litmus is a weakly acidic, colored organic dye. As its environment changes from acid (pH < 7) to base (pH > 7), the molecule changes from the protonated acid to the ionized salt. Its color also changes from red to blue. (The actual pH range for this color change is from about 4.5 to 8.3.) Because of this, litmus paper is widely used to distinguish between acids and bases.

So widespread is the use of litmus in testing whether a solution or gas is acidic or basic, that the concept of a "litmus test" has spread to include any simple but definitive test. Politicians are always calling political issues a "litmus test".

However, chemically the  litmus test  is a test for the acidity of a substance and is defined by the following rules:

  • red litmus turns   blue in base
  • blue litmus turns   red in acid

The key word is turns. If red litmus is put in a solution and remains red it does not mean the solution is acidic. It may be acidic but it also could be neutral. However, it definately is not basic.

The following pictures represent some litmus tests.
In each case the litmus papaer was put in a sample tube containing the gas.

A. Moist red litmus placed in the vapor of ammonia, NH3

B. Moist red litmus placed in the vapors of chlorine, Cl2

C. Moist red litmus in nitrogen, N2, but touching NaOH solution which wet the sides.

D. Moist red litmus placed in nitrogen, N2

A

B

C

D

...
NOTE:
  • Gasses react more rapidly with moist rather than dry litmus. Why?
  • Gasses change the color of litmus paper over its entire exposed surface, not just at the edges because they make contact over the entire surface.
  • The neutral gasses  nitrogen and oxygen do not affect the color of either red or blue litmus. (Otherwise how could you keep litmus paper from turning in air which is 79% nitrogen and 20% oxygen?)
  • The reaction of chlorine does not indicate acidity or basicity, but bleaching.
Also Remember:
  • Litmus changed from red to blue by a gas, can be reused as blue litmus paper and vice versa.
  • When you are through with this experiment, you should have used only a few pieces of litmus paper.

RWK 1997