Ammonium Hydroxide: Formula, Strengths, Uses and Handling

The short answer

Ammonium hydroxide is ammonia gas dissolved in water. It is written NH₄OH, CAS 1336-21-6 for the solution, and traded as ammonia solution or aqueous ammonia at strengths of 20, 25 or 30 % ammonia by weight.

It is a weak base, which is exactly why it is useful: it lifts pH into the 10 to 11.5 range without the aggression of caustic soda, and it buffers there instead of overshooting. The cost of that convenience is volatility — an open or warm container loses ammonia continuously, and the strength on the certificate is not the strength in the drum three months later.

Formula, strengths and identifiers

Does NH4OH actually exist?

Not really, and the point is worth understanding because it explains the chemistry. When ammonia dissolves, most of it stays as neutral NH₃ molecules hydrogen-bonded to water. Only a small fraction reacts to give ammonium and hydroxide ions:

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

At 25 °C the base dissociation constant pK_b is about 4.75, so in a 1 M solution only around 0.4 % of the ammonia is ionised. That is what makes it a weak base: a 1 M solution reaches about pH 11.6, whereas 1 M sodium hydroxide reaches pH 14.

Two consequences follow. Ammonia solution buffers — with ammonium chloride or sulphate it holds pH steadily around 9 to 10, which is why it is the standard buffer for complexometric titration and for many plating baths. And it will not attack skin, glass or plastics the way caustic soda does, though it still causes burns on prolonged contact.

Why stronger ammonia is lighter

This catches people out. For almost every acid or salt, a stronger solution is a denser one. Ammonia is the reverse: ammonia is lighter than water, so the more of it you dissolve, the less dense the solution becomes.

That last line is the origin of the old laboratory term “point eight eight zero ammonia”, still heard for the strongest commonly available solution. A hydrometer is the quickest field check on a delivery, but read it the right way round: a lower reading means a stronger acid solution would be weaker — here it means stronger ammonia.

Physical properties

Volatility, and why the drum loses strength

Ammonia's solubility in water falls sharply as temperature rises. A solution that is comfortably stable in a cool warehouse will build real pressure in a drum standing in the sun, and every time a container is opened some ammonia leaves and does not come back.

Practical consequences that belong in a purchase and storage procedure:

• Buy to consumption. Specify a pack size the plant will finish. A part-used drum drifts down in strength week by week.
• Keep it cool and shaded. Below 25 °C wherever possible, never in direct sun, never near a steam line.
• Pressure relief. Bulk tanks need a vent or relief device sized for the ammonia, and the vent should go to a water scrubber rather than to the workshop roof.
• Re-assay before critical use. If the dose matters, titrate the drum rather than trusting the certificate. It is a five-minute test.
• Dose into the liquid. Adding ammonia solution to the surface of a warm tank simply flashes it off; dip the addition line below the liquid level.

What ammonium hydroxide is used for

• Water treatment. Chloramination, where ammonia is dosed with chlorine to form monochloramine, giving a longer-lasting residual disinfectant with fewer disinfection by-products than free chlorine.
• Flue gas treatment. The reagent in selective catalytic and non-catalytic reduction of nitrogen oxides in power and cement plants.
• pH control. Neutralisation in effluent treatment, boiler water conditioning, and pH adjustment in processes where sodium would be a contaminant — a real advantage over caustic soda in electronics and some pharma work.
• Rubber. Preservative and stabiliser for natural rubber latex from the moment it leaves the tree.
• Electronics. Component of the standard SC-1 wafer clean with hydrogen peroxide, at semiconductor-grade purity.
• Cleaning products. Glass, oven and industrial degreasing formulations.
• Food processing. As E527, a processing aid in cocoa alkalisation, caramel colour manufacture and pH adjustment, on food grade material.
• Textiles, printing and photography. pH control in dyeing and finishing, and in developer chemistry.

Scapex Exports supplies ammonium hydroxide 25 % solution and ammonia solution 20 % technical.

Materials of construction and incompatibilities

The copper rule is the one that causes real damage, because brass fittings, bronze valves and copper instrument tubing are everywhere in older plants and none of them belong anywhere near ammonia service.

Two mixing rules are absolute. Never mix with hypochlorite bleach — the reaction produces chloramine gases that have caused fatalities in cleaning work. Never mix with acid — neutralisation is violently exothermic and drives off ammonia gas. Also segregate from halogens, calcium hypochlorite and strong oxidisers generally.

Safety, handling and export documents

• Classification. UN 2672, Class 8, packing group III for solutions between roughly 10 and 35 %. Above 35 % a different, more restrictive entry applies.
• Health. Causes burns to skin and serious eye damage; the vapour is severely irritating to the respiratory tract. The odour is detectable far below the harmful level, which is a useful warning but not a substitute for monitoring the workplace exposure limit.
• Environment. Very toxic to aquatic life. Bund the storage and keep spills out of drains.
• PPE. Face shield with goggles, nitrile or butyl gloves, apron; respiratory protection for transfers and for any work in a confined space.
• Ventilation. Ammonia vapour is lighter than air and collects at high level — extract from the top of the room, the opposite of what an acid store needs.
• Packing. 30 to 50 kg HDPE carboys, 200 to 250 kg HDPE drums, 1000 litre IBCs, and stainless or lined road tankers.

An export consignment should carry a certificate of analysis stating ammonia content, specific gravity, residue on evaporation, chloride, sulphate, iron and heavy metals; a 16-section GHS safety data sheet; a dangerous goods declaration with UN 2672, Class 8, PG III; commercial invoice and packing list; certificate of origin; and for food grade the E527 or FCC reference with any Halal, Kosher or food-contact declarations required.

Enquire with a specification

Ammonium hydroxide and ammonia solution are supplied by Scapex Exports as a manufacturer and supplier from qualified Indian producers, at 20, 25 and 30 % in technical, food and analytical grades, with certificate of analysis, safety data sheet and full export and dangerous goods documentation.

Send the ammonia percentage, grade, impurity limits, packing, quantity and destination port, and you will get a quotation written against that specification rather than a generic one.

Frequently asked questions