Ammonia solution, also known as
ammonium hydroxide,
ammonia water,
ammonical liquor,
ammonia liquor,
aqua ammonia,
aqueous ammonia, or simply
ammonia, is a solution of
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
in water. It can be denoted by the symbols NH
3(aq). Although the name ammonium hydroxide suggests a base with
compositionA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
[NH
4+][OH
−], it is actually impossible to isolate samples of NH
4OH, as these ions only comprise a significant fraction of the total amount of ammonia in extremely dilute solutions.
Basicity of ammonia in water
In aqueous solution, ammonia
deprotonatesDeprotonation is the removal of a proton from a molecule, forming the conjugate base.The relative ability of a molecule to give up a proton is measured by its pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base...
a small fraction of the water to give
ammoniumThe ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
and
hydroxideHydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
according to the following
equilibriumIn a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
:
- NH3 + H2O NH4+ + OH−.
In a 1M
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
solution, about 0.42% of the ammonia is converted to ammonium, equivalent to a pH of 11.63. The base ionization constant is
- Kb = [NH4+][OH-]/[NH3] = 1.8×10−5
Saturated solutions
Like other gases, ammonia exhibits decreasing solubility in solvent liquids as the temperature of the solvent increases. Ammonia solutions decrease in density as the concentration of dissolved ammonia increases. At 15.6 °C (60.1 °F), the density of a saturated solution is 0.88 g/ml and contains 35% ammonia by mass, 308 g/l w/v, (308 grams of ammonia per litre of solution) and has a molarity of approximately 18 mol L
−1. 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 chemist's 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
titrationTitration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
.
Applications
Household ammonia is dilute ammonium hydroxide, which is also an ingredient of numerous other
cleaning agentCleaning agents are substances, usually liquids, that are used to remove dirt, including dust, stains, bad smells, and clutter on surfaces. Purposes of cleaning agents include health, beauty, absence of offensive odor, avoidance of shame, and avoidance of spreading of dirt and contaminants to...
s, including many window cleaning formulas. In addition to use as an ingredient in cleansers with other cleansing ingredients, ammonium hydroxide in water is also sold as a cleaning agent by itself, usually labelled as simply "ammonia". It may be sold plain, lemon-scented (and typically colored yellow), or pine-scented (green).
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
formaldehydeFormaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
. Ethylenediamine forms from
1,2-dichloroethaneThe chemical compound 1,2-dichloroethane, commonly known by its old name of ethylene dichloride , is a chlorinated hydrocarbon, mainly used to produce vinyl chloride monomer , the major precursor for PVC production. It is a colourless liquid with a chloroform-like odour...
and aqueous ammonia.
In furniture-making, ammonium hydroxide was traditionally used to darken or stain wood containing
tannic acidTannic acid is a specific commercial form of tannin, a type of polyphenol. Its weak acidity is due to the numerous phenol groups in the structure...
. After being sealed inside a container with the wood, fumes from the ammonium hydroxide react with the tannic acid and iron salts naturally found in wood, creating a rich, dark stained look to the wood. This was commonly used during the arts and crafts movement in furniture- a furniture style which was primarily constructed of oak and stained using these methods.
Ammonium hydroxide is also used in the meat packing industry. Some companies treat their beef "with a pH enhancement process that forms ammonium hydroxide in the finished product."
It is also known to be used by aquarists for the purposes of
fishless cyclingFishless cycling is a form of "maturing" an aquarium. In this process, ammonia is provided to allow beneficial bacteria to colonize. Fishless cycling can reduce the chance of fish loss resulting from insufficient populations of these bacteria.- Process :...
, but this requires that there are no surfactants or scents, it should be colorless and shouldn't foam when shaken up.
Laboratory use
Aqueous ammonia is used in traditional
qualitative inorganic analysisClassical qualitative inorganic analysis is a method of analytical chemistry which seeks to find elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, so that materials in other forms may need to be brought into this state before using standard...
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' reagentTollens' reagent is a chemical reagent most commonly used to determine whether a known carbonyl-containing compound is an aldehyde or a ketone. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous diamminesilver complex is present...
.
When ammonium hydroxide is mixed with dilute
hydrogen peroxideHydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
in the presence of a metal ion, such as Cu
2+, the peroxide will undergo rapid decomposition.