Micro-chemical test for ammonia

Author: Christine Bläuer

back to Micro-chemical testing

For more information on the required equipment and materials see the Micro-chemistry article.

Principle

When dissolving a salt containing ammonium in an alkaline 2M NaOH solution, gaseous ammonia (NH₃) is liberated which has a typical smell and will change the color of a previously wetted yellow pH indicator paper to green or blue.

Procedure

Add a drop of 2M sodium hydroxide, i.e., caustic soda, to a drop of the sample solution and immediately hold a wetted pH indicator paper directly above it into the released vapors.

or

put drop of sodium hydroxide solution (NaOH 2M ) on a microscope slide and then put one grain of the salt into it. Hold a wetted pH indicator paper directly above the preparation into the liberated vapors, while the salt is still dissolving.

Indications from the pH-value of the salt solution The pH value of an aqueous solution of nitrammite [NH₄,K (NO₃)], (also known as gwihabaite) is slightly acidic (circa pH 5). If the sample solution is acidic this is no proof for the presence of ammonium nitrate but it is a first indication.