Ammonia (NH3) and ammonium hydroxide (NH4OH) are fundamentally related but distinct chemical compounds. The core difference lies in their physical state: ammonia is a gas, while ammonium hydroxide, also frequently referred to as aqueous ammonia, is a liquid solution of ammonia dissolved in water. This distinction governs their properties, handling requirements, and practical applications in industry and the home.
Ammonia: The Gaseous Starting Point
The chemical compound ammonia (NH3) exists as a colorless gas at standard room temperature and pressure, characterized by its sharp, pungent odor. The term “anhydrous ammonia” specifies this pure form, emphasizing that it is entirely without water. This pure gas is highly corrosive and toxic, posing a significant risk of respiratory injury upon exposure.
Ammonia molecules are highly polar, giving them a high affinity for water. Anhydrous ammonia is a cornerstone of the chemical industry, primarily used in the manufacturing of fertilizers, supplying the nitrogen needed for crop growth. It is also widely used as an industrial refrigerant in large-scale cooling systems.
Ammonium Hydroxide: Ammonia in Solution
Ammonium hydroxide is the common name for the solution formed when ammonia gas (NH3) is dissolved in water (H2O). Chemists often refer to this as aqueous ammonia, as the compound NH4OH does not exist as a stable molecule. Dissolving the gas in water establishes a dynamic chemical equilibrium.
A small fraction of the dissolved ammonia reacts with water, forming ammonium ions (NH4+) and hydroxide ions (OH-). This reaction makes the solution basic or alkaline, classifying ammonium hydroxide as a weak base with a pH above 7. The vast majority of the ammonia, however, remains in the solution as un-ionized NH3 molecules simply dissolved in the water.
Key Physical Differences and Volatility
Anhydrous ammonia is a pressurized gas, while ammonium hydroxide is a liquid solution. Anhydrous ammonia is nearly 100% NH3, whereas commercial ammonium hydroxide solutions typically range from 5% to 30% ammonia concentration in water. The pure gas must be stored under high pressure, often in its liquefied form.
Ammonium hydroxide is a highly volatile liquid that readily releases ammonia gas into the air. This volatility is why household cleaners smell strongly, as dissolved NH3 molecules escape from the solution. The amount of ammonia gas escaping increases significantly as the temperature rises. The presence of water dramatically changes the handling profile, as the liquid solution does not require the specialized high-pressure containment necessary for the pure gas.
Real-World Uses and Safety Considerations
The distinct physical properties of each form dictate their practical uses and safety protocols. Anhydrous ammonia is used in large-scale industrial and agricultural settings where a high concentration of nitrogen is needed. It is injected directly into soil as a fertilizer and utilized in industrial refrigeration systems. Handling the pure gas requires extensive safety training and specialized, steel-constructed equipment due to the hazards of extreme pressure and the risk of severe chemical and frostbite burns.
Ammonium hydroxide is the form encountered in many common consumer products, such as window cleaners. Its basic nature helps to dissolve grease and dirt, making it an effective, though diluted, cleaning agent. While capable of causing irritation to the eyes and respiratory system, the presence of water significantly reduces the hazard compared to the pure gas. Safety for the aqueous solution involves ensuring proper ventilation to prevent the buildup of the pungent, off-gassing ammonia vapor.