Concentrating sulfuric acid involves removing water to achieve a higher percentage of \(\text{H}_2\text{SO}_4\), moving from common grades (around 33%) toward industrial grades (98% and higher). This process is fundamentally an evaporation challenge, complicated by the acid’s extreme chemical properties. The conditions required are intensely hazardous, involving high temperatures and highly corrosive materials. For nearly all non-industrial applications, attempting concentration is impractical and poses a grave safety risk, making the purchase of pre-concentrated acid the only sensible option.
The Chemical Barrier to Concentration
The primary obstacle to simple concentration is the formation of a constant-boiling mixture, known as an azeotrope, between sulfuric acid and water. An azeotrope is a specific mixture where the composition of the vapor phase is identical to the liquid phase, meaning continued boiling will not change the concentration of the liquid remaining.
For the sulfuric acid-water system at standard atmospheric pressure, this azeotrope forms at approximately 98.3% \(\text{H}_2\text{SO}_4\) by weight. Below this concentration, heating allows water to evaporate preferentially, increasing the acid percentage. Once the mixture hits 98.3%, the concentration cannot be increased further by simple boiling.
This azeotrope corresponds to the maximum atmospheric boiling point for the mixture, around \(338^\circ\text{C}\). The required high temperatures create significant engineering and safety challenges, including rapid corrosion of most standard materials. Furthermore, the acid’s powerful dehydrating properties at high heat cause it to break down organic impurities, which can lead to toxic gas release.
Concentrating Sulfuric Acid Using Heat
The initial stages of concentration, moving from a dilute solution (e.g., 70%) up to the 90% range, can be achieved by applying heat to evaporate the water. As water boils off, the concentration of the remaining acid increases, and the boiling point of the mixture steadily climbs.
As the acid concentration approaches 90%, the temperature required for boiling increases dramatically, reaching over \(300^\circ\text{C}\). At these elevated temperatures, the acid becomes highly corrosive, and a noticeable amount of sulfuric acid vapor begins to escape with the water vapor. This acid loss reduces efficiency and necessitates specialized equipment to capture and neutralize toxic sulfur oxide fumes.
Beyond approximately 97% concentration, the simple heating method becomes impractical due to the azeotrope limit. The high heat also poses risks of acid decomposition.
Industrial Scale Concentration Techniques
Industrial processes are designed to bypass the limitations of the azeotrope and the severe conditions required for simple boiling. One widely used technique is vacuum evaporation, which focuses on lowering the pressure within the reaction vessel. Operating under a vacuum significantly reduces the boiling point of the acid-water mixture.
This allows water to evaporate at much lower temperatures, minimizing acid decomposition and reducing corrosive effects on the equipment. This two-stage process typically involves an initial vacuum stage to reach 88–90% concentration, followed by a second, lower-vacuum stage to achieve the final concentration, often up to 98%. Specialized materials like tantalum or high-silicon iron are used for the heat exchangers to withstand the harsh environment.
Achieving Concentrations Above the Azeotrope
To achieve concentrations above the 98.3% azeotrope, a different chemical approach is used, often involving the production of oleum, or fuming sulfuric acid. In the industrial Contact Process, sulfur trioxide (\(\text{SO}_3\)) gas is dissolved into concentrated sulfuric acid.
The \(\text{SO}_3\) reacts with any remaining water to form additional \(\text{H}_2\text{SO}_4\), effectively pushing the concentration beyond the azeotropic limit. The resulting oleum is a solution of \(\text{SO}_3\) in sulfuric acid, which can be diluted back to precise, high concentrations of \(\text{H}_2\text{SO}_4\) with high purity. This method is the standard for creating the highest grades of sulfuric acid needed for specialized chemical manufacturing. The oleum itself is intensely reactive, releasing significant heat when diluted, requiring careful control.
Essential Safety Protocols
Handling any concentration of sulfuric acid requires strict adherence to safety protocols, as it is a highly corrosive substance with dehydrating and oxidizing properties. Full personal protective equipment (PPE) is mandatory, including a full face shield over safety goggles, impervious gloves (such as butyl or Viton, as standard nitrile degrades quickly), and chemical-resistant clothing. Contact with the skin or eyes can cause severe chemical and thermal burns due to the acid’s dehydrating action.
All work with concentrated acid must be conducted within a chemical fume hood to prevent the inhalation of mists or vapors, which can severely irritate the respiratory tract. Emergency eyewash and safety shower stations must be immediately accessible.
The Dilution Principle
A fundamental rule for handling sulfuric acid is the dilution principle: always add acid to water, never water to acid. The hydration of sulfuric acid is a highly exothermic process, releasing a large amount of heat.
Adding water to concentrated acid can cause the water to flash boil, resulting in a violent reaction that splatters hot, concentrated acid. By slowly pouring the acid into a larger volume of cold water with constant stirring, the heat is dissipated more safely.