Question

In an experiment that uses an ammonia-selective electrode, the electrode is immersed in an NH4Cl solution, and the solution is stirred slowly and covered with parafilm before NaOH is added. Why does it need to be stirred slowly?

Answer 1

In water, ammonia exists simultaneously as two forms: as free unionized gaseous ammonia (NH3) and as the ionized ammonium ion (NH4 ). The ratio of NH3 to NH4 that exists in water depends mostly on the pH and to a lesser degree the temperature;

At 25°C a pH 7.25 sample is almost all NH4
At 25°C a pH 9.25 sample is equally NH3 & NH4
At 25°C a pH 11.25 sample is almost all NH3

Dissolved NH3 is gaseous and passes thru the hydrophobic membrane of the ammonia ISE. The pH change of the electrolyte solution on the other side of the membrane caused by the diffused gas is sensed by the inner body of the ammonia electrodea pH electrode. And a chance to escape the ammonia gas when the speed stirring occur.