Sulfuric acid (\(H_2SO_4\)) is one of the most widely produced industrial chemicals globally. This dense, corrosive, and highly reactive liquid is used in everything from fertilizer production to petroleum refining. Because sulfuric acid is typically sold and transported by volume, knowing the precise mass of a standard unit, like a US gallon, is important for commerce and safety. Determining this weight depends entirely on the acid’s purity and concentration.
Determining the Weight of Sulfuric Acid
The weight of a US gallon of sulfuric acid varies significantly, ranging from approximately 8.5 pounds to over 15 pounds. This broad range is due to the varying amounts of water mixed with the pure acid, as a gallon of pure water weighs about 8.34 pounds. Highly concentrated industrial grades are nearly twice as dense as water.
For example, common battery acid is a dilute form, typically around 37% concentration by weight, and weighs approximately 10.7 pounds per gallon. A 50% concentration weighs roughly 11.7 pounds per gallon. These lower concentrations contain a higher proportion of water, lowering the overall density.
The weight approaches its maximum at the highest commercial purities. The standard 93% industrial grade acid weighs about 15.33 pounds per gallon, while the 98% grade measures around 15.3 pounds per gallon. This narrow range highlights that the weight is not a simple linear function of purity, due to complex molecular interactions between the acid and the small amount of water remaining in the solution.
The Role of Concentration in Acid Density
The weight of a given volume of sulfuric acid is a direct result of its density, which is defined as mass per unit volume. In commercial solutions, concentration is expressed as the weight percentage of pure \(H_2SO_4\) dissolved in water. The addition of the heavy sulfuric acid molecule to water increases the overall mass of the solution, causing the density to rise as concentration increases.
However, this density increase is not constant across the entire concentration spectrum. As the percentage of acid increases, the density initially rises sharply due to the increasing number of \(H_2SO_4\) molecules. The density reaches its peak between 93% and 96% concentration, which is the point of maximum molecular compactness.
Beyond this peak, as the solution moves toward 100% pure \(H_2SO_4\), the density actually decreases slightly. This non-linear relationship means that simple density measurement is insufficient to accurately determine the exact purity of highly concentrated acid.
Specific Gravity and Commercial Measurement
To standardize the measurement and sale of sulfuric acid, industry professionals rely on Specific Gravity (SG). SG is a unitless ratio that compares the density of sulfuric acid to the density of water at a specific reference temperature, often \(60^\circ F\). For example, if a sulfuric acid solution has an SG of 1.84, it means that the acid is 1.84 times denser than water.
This ratio provides a quick, practical way for suppliers and purchasers to determine the acid’s concentration and convert it into a precise weight per gallon. Commercial tables are widely used to correlate a measured SG value with a known concentration percentage. Measuring the SG of a batch allows a supplier to quickly confirm its grade and calculate its exact weight for transport and billing.
SG is essential because temperature also affects the weight of a gallon. As the acid’s temperature increases, its volume expands, and its density and SG decrease. Therefore, the SG measurement must be corrected to the standard reference temperature to ensure accurate mass calculations.