Volumetric Determination of an Ammonium Salt

Ammonium salts in solution can be determined by adding a known excess of a standard alkali. After boiling the solution until all the ammonia has been lost, the excess alkali is 

Pipette 25.0 cm of 0.1 M NH4CI solution into a 250 cm conical flask, add 50 cm of the standardised 0.1 M solution of NaOH (use a pipette for the addition) and place a small funnel in the neck of the flask. Boil the solution carefully until a Hg2(N03) paper is no longer turned black, indicating that no more NH3 evolved. (The black colour is due to fmely divided mercury formed by the reaction between Hg2 ions in solution and NH3). Cool the flask, add a few drops of methyl orange indicator and a little distilled water down the sides of the flask. Titrate with standard 0.1 M HCl in the burette to the fust sudden change of colour. Duplicate determinations are required. Calculate the concentration of NH4CI in g dm of the solution. 

Nessler s solution or reagent contains potassium tetraiodomercurate(II) and is available commercially although it can be prepared in the laboratory easily. When the reagent is added to a dilute ammonium salt solution, the liberated ammonia reacts forming an orange brown colloid  

In dilute solution, the absorbance of the coloured compound can be measured in a spectrophotometer. 

---