AMMONIUM BROMIDE
Thursday, October 2nd, 2008Formula: NH4Br; MW 97.94; ionic salt, cubic crystal
Uses
Ammonium bromide is used for photography in films, plates, and papers; in fireproofing of wood; in lithography and process engraving; in corrosion inhibitors; and in pharmaceutical preparations.
Physical Properties
White crystal or granule; strong saline taste; no odor; slightly hygroscopic; density 2.429 g/cm3 at 25°C; refractive index 1.712; sublimes at elevated temperatures: vapor pressure 54.75 torr at 300°C and 758.2 torr at 395°C; highly soluble in water: 60.6 g and 75.5 g/100 mL at 0° and 20°C, respectively - solubility increasing approximately 16 to 18 g/100 mL for every 20°C increase in temperature.
Thermochemical Properties
ΔHƒ°(s) –64.73 kcal/mol
ΔGƒ°(s) –41.9 kcal/mol
S° 27 cal/degree mol
Cρ 23 cal/degree mol
Preparation
Ammonium bromide is prepared by treating excess ammonia with bromine:
8NH3 + 3Br2 → 6NH4Br + N2
It may be also prepared by the reaction of ammonia and hydrobromic acid:
NH3 + HBr → NH4Br
NH4Br is also made by the reaction of ammonia with ferrous and ferric bromide, which may be obtained by passing aqueous bromine solution over iron filings.
2NH3 + FeBr2 + H2O → 2NH4Br + FeO
6NH3 + 2FeBr3 + 3H2O → 6NH4Br + Fe2O3
Reactions
Ammonium bromide exhibits acid reaction in aqueous solution; acts as an excellent acid in liquid NH3, undergoing neutralization reactions:
NH Br + NaNH3→NaBr + 2NH3
Contact with metal surfaces produces bromides of the metals; similarly reactions with metal hydroxide bases yield corresponding bromides:
2NH4Br + Ca(OH)2 →CaBr2 + 2NH3 + 2H2O
Ammonium bromide decomposes to ammonia and hydrogen bromide when heated at elevated temperatures:
NH4Br→ NH3+HBr
Chemical Analysis
Elemental composition: Br 81.58%, H 4.12%, N 14.30% Mixed with NaOH solution and distilled; distillate analyzed for ammonia by titration, colorimetry, or electrode method (see Ammonia and Ammonium chloride). Bromide portion of NH4Br in aqueous solution may be analyzed by ion chromatography, or by the colorimetry method in which red to violet color is produced upon treatment with chloramine–T, and phenol red at pH 4.5. The colorimetry test for bromide is subject to interference from oxidizing and reducing agents, chloride, and bicarbonate. NH4Br may then be determined stoichiometrically.
