Ammonium may be determined by
precipitation with sodium tetraphenylborate as the sparingly soluble ammonium
tetraphenylborate NH4[B(C6H5)4],
using a similar procedure to that described for potassium; it is dried at 100
°C.
Weigh out accurately about 0.15 g ammonium chloride and dissolve it in 50 mL distilled water. Add 10mL of 0.1M hydrochloric acid. Then introduce from a burette 40 mL of the sodium TPB reagent (Note 1) slowly (5-10 minutes) and stir continuously. The temperature throughout must be below 20 °C. Allow the precipitate to settle during 1 hour. Collect the precipitate on a sintered-glass filtering crucible (porosity No. 4), wash the precipitate with a small volume (5-10 mL in small portions) of saturated ammonium TPB solution (Note 2), and finally with 1-2 mL ice-cold distilled water. Dry at 120 °C and cool the covered crucible in a desiccator. Weigh as NH4[B(C6H5)4].
Notes: (1) Prepare the sodium TPB reagent by dissolving 3.0 g of the solid reagent in 500 mL distilled water in a glass-stoppered bottle. Add about 1 g moist aluminium hydroxide gel, break up the gel if necessary, and shake the suspension for 15 minutes. Filter through a Whatman No. 40 filter paper. Re-filter the first part of the filtrate, if necessary, to ensure a clear filtrate.
Precipitate about 0.1 g ammonium
(present as ammonium chloride in 50 mL water) with 40 mL of the sodium TPB
solution added slowly and with constant stirring. Allow to stand for 30
minutes, filter through a sintered-glass filtering crucible, wash with
distilled water, and dry for 1 hour at 120 °C. Shake 20-25 mg of the dry
precipitate with 200 mL distilled water in a stoppered bottle at 5-minute
intervals during 1 hour. Filter through a Whatman No. 40 filter paper, and use
the filtrate as the wash liquid.
If the ammonium salt is present with other cations and anions, a titrimetric procedure is usually employed.