Anhydrous ammonia (NH₃) is a highly volatile chemical compound used extensively in agriculture and industry. It is named “anhydrous” because it contains no water and is typically stored and transported as a pressurized liquid. Since ammonia readily boils above -28 degrees Fahrenheit, containment vessels are designed to keep it in a liquid state under significant pressure. The weight of anhydrous ammonia is not fixed; its density—the weight per unit volume—changes dramatically based on its physical state and environmental conditions. Understanding these density variations is important for accurate measurement, transportation, and safety.
Standard Density Measurements
The weight of anhydrous ammonia depends heavily on its physical state and the temperature at which it is measured. As a liquid, anhydrous ammonia is lighter than water, which weighs about 8.34 pounds per gallon. At the standard reference temperature of 60 degrees Fahrenheit, one US gallon of liquid anhydrous ammonia weighs approximately 5.15 pounds. This figure is commonly used in commercial transactions and calculations for bulk liquid storage. The liquid’s density is higher at colder temperatures, reaching about 5.69 pounds per gallon at its boiling point of -28 degrees Fahrenheit.
When ammonia is a gas at standard atmospheric pressure and 32 degrees Fahrenheit, its density is significantly lower, measuring about 0.0597 pounds per cubic foot. The gaseous state is far less dense than the liquid. A single pound of liquid ammonia is capable of generating over 22 standard cubic feet of ammonia vapor.
Factors That Alter Weight
The variability in weight per volume stems from how liquids and gases respond to temperature and pressure changes. Anhydrous ammonia liquid has a notably high coefficient of thermal expansion compared to many other liquids. This means that as the temperature of the liquid increases, its volume expands considerably. This expansion directly lowers the density, causing the weight per gallon to decrease as the liquid warms. For example, a 20-degree Fahrenheit temperature increase can cause the volume of the liquid to rise by nearly five percent.
In a closed storage vessel, rising temperature also increases the vapor pressure exerted by the ammonia gas above the liquid. The pressure inside the tank is directly related to the liquid’s temperature, which is a unique characteristic of a liquefied compressed gas.
Commercial Measurement and Conversion
Since the density of liquid anhydrous ammonia constantly changes with temperature, commercial measurements cannot rely on simple volume readings alone. Industry practices use specific gravity tables and volume correction factors to accurately determine the actual mass of the product transferred. These tables correlate the measured temperature of the liquid with its true density, allowing for a precise calculation of the weight in the container.
For bulk transactions, ammonia is often measured in tons, representing a fixed mass of 2,000 pounds. The conversion from the measured volume in gallons to the sold mass in tons is performed using the temperature-corrected weight per gallon. At the standard 60 degrees Fahrenheit, one ton of anhydrous ammonia is equivalent to approximately 388.5 gallons.
Safety Related to Volume and Storage
The variability of density is a major safety consideration in the storage of anhydrous ammonia. The high rate of thermal expansion means that if a tank is filled too full, even a small increase in ambient temperature can lead to a dangerous situation. As the liquid expands, it fills the necessary vapor space within the container, which is intended to buffer pressure changes. Safety regulations mandate that storage tanks must never be filled beyond 85% of their total water capacity by volume.
This 15% vapor space is required to accommodate the expansion of the liquid as its temperature rises, preventing the tank from becoming liquid-full. If the tank becomes completely full, the liquid can no longer compress. This leads to a rapid and massive increase in hydrostatic pressure that can cause the vessel to rupture.