Ammonium ions can be determined by their reaction with methanal; the reaction gives the neutral compound hexamethylenetetramine and hydrogen ions which can be titrated with standard sodium hydroxide solution:
6 HCHO + 4NH4+ à C6H12N4 + 4H+ + 6H2O.
The reaction must not be used to determine ammonium chloride, since HCl and methanal react to produce a highly carcinogenic compound.
Method:
Weigh accurately about 1.5g of ammonium sulphate, transfer completely to a 250 cm3 graduated flask and dissolve in pure water. Make to the mark with pure water and mix well.
Pipette 25.0 cm3 of this solution into a 250 cm3 conical flask, and add 5 cm3 (measuring cylinder) of 40% methanal solution (Care! Irritant and toxic). Allow the mixture to stand for 2 – 3 minutes, then add 3 – 4 drops of phenolphthalein indicator.
Titrate the mixture with standard 0.100 mol dm-3 sodium hydroxide solution until the first appearance of a permanent pink colour.
Repeat to obtain three consistent titres.
Results:
Mass of ammonium sulphate used/g:
| Final volume/cm3 | ||||
| Initial volume/cm3 | ||||
| Titre/cm3 |
Mean titre/cm3:
Hence calculate the % purity of the ammonium sulphate.