ammonium hydroxide

Ammonium hydroxide (NH₃[aq]), also known as ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, or aqueous ammonia, is a solution of ammonia in water. Although its name suggests a salt of the formula NH₄OH, it is not actually possible to generate samples of NH₄OH - it exists only in dilute aqueous solutions.^[1]

Basicity of ammonia in water

In aqueous solution, ammonia deprotonates a small fraction of the water to give ammonium and hydroxide according to the following equilibrium:

NH₃ + H₂O NH₄⁺ + OH⁻.

In a 1M ammonia solution, about 0.42% of the ammonia is converted to ammonium, equivalent to a pH of 11.63. The base ionization constant is

K_b = [NH₄⁺][OH^-]/[NH₃] = 1.8×10⁻⁵ M

Saturated Solutions

Like other gases, ammonia exhibits decreasing solubility in solvent liquids as the temperature of the solvent increases. "Ammonium hydroxide" solutions decrease in density as the concentration of dissolved ammonia increases. At 15.5556 °C, the density of a saturated solution is 0.88 g/mL and contain 35% ammonia by mass, 308g/L w/v, (308 grams of ammonia per litre of solution) and have a molarity of approximately 18 mole/L. At higher temperatures, the molarity of the saturated solution decreases and the density increases.

When solutions that are saturated at cold temperatures are sealed in containers and subsequently warmed, the concentration of the solution decreases and the vapor pressure of ammonia gas increases. Unsealing such containers can lead to a burst of ammonia gas. In extreme cases, the containers could rupture.

From a laboratory perspective, one should be aware that the concentration of a saturated solution is continually dropping as the container is handled in a warmer environment. Thus, old samples of ammonium hydroxide will deviate from 18 M, as can be verified by titration.

Applications

Household ammonia is dilute ammonium hydroxide, which is also an ingredient of numerous other cleaning agents.

Prepare paintwork for newly painting an already painted surface by cleaning the painted surface with household ammonia or ammonium hydroxide.

In industry, ammonium hydroxide is used as a precursor to some alkyl amines, although anhydrous ammonia is usually preferred. Hexamethylenetetramine forms readily from aqueous ammonia and formaldehyde. Ethylenediamine forms from 1,2-dichloroethane and aqueous ammonia.^[2]

Ammonium hydroxide is used in the meat packing industry. Some companies add ammonium hydroxide to their beef.^[3]

Laboratory use

Aqueous ammonia is used in traditional qualitative inorganic analysis as a complexant and base. Like many amines, it gives a deep blue coloration with copper(II) solutions. Ammonia solution can dissolve silver residues, such as that formed from Tollens' reagent.

When ammonium hydroxide is mixed with dilute hydrogen peroxide in the presence of a metal ion, such as Cu²⁺, the peroxide will undergo rapid decomposition.

See also

• Common chemicals
• Conjugate acid
• Windex

External links

• Ammonium hydroxide physical properties

References

1. ^ Housecroft, Catherine E.; Alan G. Sharpe (2008). Inorganic Chemistry (3rd ed.). Prentice Hall. p. 187. ISBN 978-0-13-175553-6. 
2. ^ Karsten Eller, Erhard Henkes, Roland Rossbacher, Hartmut Höke "Amines, Aliphatic" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a02_001
3. ^ http://www.beefproducts.com/the_process/index.cfm

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