Overview
Hydrogen sulfate, commonly called bisulfate in older texts, is the anion HSO4− produced when sulfuric acid (H2SO4) loses one proton. It is the intermediate species between sulfuric acid and the fully deprotonated sulfate ion (SO4 2−). The name "hydrogen sulfate" emphasizes that one hydrogen atom remains associated with the sulfate moiety.
Structure and acid–base behavior
The hydrogen sulfate ion has a central sulfur atom surrounded by four oxygen atoms in a roughly tetrahedral arrangement; one of those oxygens carries the hydrogen. Chemically it is the conjugate base of sulfuric acid and a conjugate acid of the sulfate ion. In aqueous solution it can donate a proton to form SO4 2− or accept a proton to regenerate H2SO4, so it behaves as an amphiprotic species in many reactions.
Salts and common examples
Compounds that contain HSO4− are usually called hydrogen sulfates or bisulfates. Typical examples include sodium bisulfate and potassium bisulfate. These salts are often isolated as crystalline solids and are more stable and less corrosive than concentrated sulfuric acid.
- Sodium bisulfate (NaHSO4) — used as a mild acidifying agent.
- Potassium bisulfate (KHSO4) — used in some industrial processes.
- Ammonium bisulfate (NH4HSO4) — occurs as a decomposition product in some reactions.
Practical uses and importance
Hydrogen sulfate salts serve as convenient, solid sources of acidity where concentrated sulfuric acid would be impractical or unsafe. They are employed in pH adjustment, cleaning formulations, and as intermediates in chemical manufacturing. Because the HSO4− ion is less strongly acidic than undissociated sulfuric acid, bisulfate salts offer more controllable acidity for many applications.
Distinctions and notable facts
It is important to distinguish hydrogen sulfate (HSO4−) from sulfate (SO4 2−) and sulfuric acid (H2SO4): these three differ by the number of protons attached. The term "bisulfate" remains common in industry and older literature. For brief definitions and general reference see basic reference or a chemistry database entry at additional resource. Understanding the role of HSO4− clarifies reaction pathways in acid–base chemistry and environmental processes such as aqueous aerosol formation.