Ammonium Hydroxide
Properties
| State | Liquid (aqueous solution of NH3) |
| Color | Colorless |
| Solubility | Miscible with water at all proportions |
| Melting Point | -57.5°C (for 25% solution) |
| Boiling Point | 37.7°C (for 25% solution, releases NH3) |
About Ammonium Hydroxide
Ammonium hydroxide is a chemistry-textbook fiction that's nonetheless useful — there is no isolable molecule called NH4OH, but the formula is shorthand for the equilibrium that exists when ammonia dissolves in water (NH3 + H2O ⇌ NH4⁺ + OH⁻). At room temperature only about 1–2% of the dissolved ammonia is ionized at typical concentrations, so the solution is mostly molecular NH3 in solvated form, with the small ionized fraction giving the basic pH. Calling the species NH4OH is a convenient way of writing reactions where ammonia behaves stoichiometrically as a single base. Commercial concentrated ammonia is sold at about 28–30% NH3 by mass (specific gravity 0.90, roughly 14.8 M as NH3), and that's what shows up as the '14 N ammonium hydroxide' or 'concentrated ammonia' bottle on the reagent shelf. The solution is one of the most useful things in qualitative analysis: a few drops dropped into a copper(II) solution drops Cu(OH)2 first, then dissolves it as the deep royal-blue [Cu(NH3)4]²⁺ tetraamminecopper complex when more ammonia is added — the textbook demonstration of ammonia's dual role as both a Brønsted base and a Lewis-base ligand. Silver, zinc, nickel, and cobalt cations all show similar dissolution-on-excess behavior, which forms the basis of the classical Group 2 separation scheme. In semiconductor manufacturing, dilute ammonia (mixed with hydrogen peroxide) is the SC-1 step of the RCA wet-clean sequence on silicon wafers — it removes organic contamination and grows a thin, controlled native oxide before the wafer goes downstream.
Where you'll encounter it
If you've used the strong ammonia bottle in a glass-cleaning kit (the kind that comes in a brown bottle with the eye-watering odor), you've used aqueous ammonia at around 5–10%. Lab-grade is much stronger — 28–30% in glass bottles labeled ammonium hydroxide concentrated — and opening one in an enclosed space without a fume hood is a memorable experience. The sharp irritation from a fume of NH3 vapor is what triggers the upper airway response that makes the compound useful as a smelling salt in a different formulation. In analytical chemistry teaching labs, the [Cu(NH3)4]²⁺ formation is one of the earliest visual demonstrations of ligand-field chemistry — students drip ammonia into a pale-blue copper sulfate solution and watch it transition through a turquoise hydroxide precipitate to a deep royal-blue solution as the complex forms.
Common Uses
- Group-2 metal-cation separation reagent in qualitative analysis
- Ammine-complex formation for Cu, Ag, Ni, Zn, Co cations
- SC-1 silicon-wafer cleaning step in semiconductor manufacturing
- pH adjustment in chemical synthesis and water treatment
- Glass-and-surface cleaner active in dilute formulation
Safety Information
Concentrated solution is corrosive and the released ammonia vapor is acutely irritating — uncapping a bottle outside a fume hood produces eye watering and immediate airway response. Mixing with hypochlorite bleach generates chloramine gases (NH2Cl, NHCl2), which are toxic to the lungs at low concentrations; this is one of the most common dangerous accidental mixtures in household cleaning. GHS H290, H302, H314, H335. Always work under ventilation with splash goggles.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.