Ammonium hydroxide (NH4OH)
Ammonium hydroxide is the common name for aqueous ammonia, a solution of ammonia gas (NH3) in water. It is a weak base, and only a small fraction is present as ammonium (NH4+) and hydroxide (OH−) ions. Concentrated commercial grades typically contain about 28–30% NH3. It is widely used in laboratories and industry for pH adjustment, cleaning, precipitation and preparing volatile buffers.
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Ammonium Hydroxide 35% Solution ACS reagent
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What exactly is ammonium hydroxide?
Ammonium hydroxide (CAS 1336-21-6) is ammonia dissolved in water. The formula NH4OH describes the equilibrium NH3 + H2O ⇌ NH4+ + OH−. A discrete NH4OH molecule has not been isolated, so the solution is more accurately called aqueous ammonia. It is a weak base. The pKb of ammonia is about 4.75. Concentrated reagent solutions have a strong, pungent odour and readily release ammonia gas.
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Which grades and specifications are available?
Suppliers offer several grades:
- ACS reagent grade: typically specified at 28.0–30.0% NH3.
- Pharmacopoeial grades: for example, Strong Ammonia Solution USP and the corresponding EP monographs.
- Specialist grades: LC-MS or HPLC grades with low residue, and electronic or semiconductor grades with tight limits on trace metals.
- Diluted solutions: various dilutions for general use.
Procurement teams should confirm NH3 assay, density, residue on evaporation and heavy-metal limits on the CoA. Because ammonia escapes from solution, the assay can fall over time once a container has been opened.
How should it be handled and stored?
Under GHS, concentrated ammonium hydroxide is classified as causing severe skin burns and eye damage (H314). It may cause respiratory irritation (H335) and is very toxic to aquatic life (H400). Always check the supplier's SDS for the exact classification of the concentration you are buying.
Handling and storage precautions:
- Work in a fume hood and wear suitable eye and skin protection.
- Store containers tightly closed in a cool, well-ventilated area.
- Keep away from acids, oxidising agents and halogens.
Its uses include:
- pH adjustment
- metal hydroxide precipitation
- preparing ammonium acetate and ammonium bicarbonate buffers
- mobile-phase modification in HPLC and LC-MS
- RCA (SC-1) wafer cleaning in semiconductor manufacturing
Related concepts
The following entries cover closely related topics: ammonia (NH3), ammonium chloride, ammonium acetate, ammonium bicarbonate, weak bases and pKb, volatile buffers for LC-MS, sodium hydroxide, GHS hazard classification, SDS and CoA documentation.
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