The pH scale is a logarithmic measure used to quantify the acidity or alkalinity of an aqueous solution, ranging typically from 0 to 14. Values below 7 indicate acidity, 7 is neutral, and values above 7 indicate alkalinity. Sulfuric acid (\(\text{H}_2\text{SO}_4\)) is one of the strongest mineral acids known. Its strength is directly related to its ability to release hydrogen ions (\(\text{H}^+\)) when dissolved in water, which defines an acid. This property results in an extremely low pH, making it a highly corrosive and reactive substance foundational to many industrial processes.
Defining Sulfuric Acid’s Strength
The strength of an acid is determined by its ability to dissociate into hydrogen ions when dissolved in water. Strong acids, like sulfuric acid, undergo nearly complete dissociation, releasing a large concentration of \(\text{H}^+\) ions, which drives the pH value down.
Sulfuric acid is classified as a diprotic acid, meaning each molecule can donate two hydrogen ions. The dissociation occurs in two stages. The first stage involves the complete breakdown into a hydrogen ion and a hydrogen sulfate ion (\(\text{HSO}_4^-\)). This initial, complete dissociation is why sulfuric acid is categorized as a strong acid.
The second stage involves the hydrogen sulfate ion further dissociating into a second hydrogen ion and a sulfate ion (\(\text{SO}_4^{2-}\)). Although this second step is weaker and less complete, it still contributes significantly to the solution’s overall high acidity.
The Role of Concentration in Determining pH
The pH of sulfuric acid does not have a single numerical answer because it is entirely dependent on its concentration in water. The pH is a direct measure of the hydrogen ion concentration, described by the formula \(\text{pH} = -\text{log}[\text{H}^+]\).
For highly concentrated industrial sulfuric acid, such as the 98% concentration commonly used, the theoretical pH approaches 0 or can even be a negative value. A negative pH indicates an extremely high concentration of hydrogen ions that exceeds the molarity of a standard 1.0 M acid solution. For example, a 10% solution may have a pH as low as -0.6.
When the acid is diluted to common laboratory or commercial strengths, the pH increases but remains very low. A dilute 0.1 M solution, which is still highly corrosive, has a calculated pH of approximately 1.01. The sulfuric acid used in lead-acid car batteries is typically a 33.5% solution, registering a pH around 0.5.
Safety and Handling Considerations
The extremely low pH of sulfuric acid is a direct indicator of its severe hazard profile due to its highly corrosive nature. The acid rapidly attacks biological tissues, including skin and eyes, by dehydrating and chemically burning them. It also reacts with most metals, especially when diluted below \(90\%\), sometimes releasing flammable hydrogen gas.
A significant safety concern is the “heat of dilution” phenomenon. When concentrated sulfuric acid is mixed with water, the reaction is highly exothermic, releasing substantial heat energy. This rapid heat generation can cause the water to boil instantly, potentially leading to violent splattering of the hot, corrosive acid.
To mitigate this danger, always add the acid slowly to the water, never the reverse. Adding the concentrated acid to the larger volume of water allows the heat to be distributed and absorbed more effectively. Proper personal protective equipment (PPE), including gloves, goggles, and adequate ventilation, is mandatory when handling sulfuric acid.
Common Applications of Sulfuric Acid
Sulfuric acid is one of the most produced chemicals globally. Its properties, stemming from its low pH and high reactivity, make it indispensable across a vast range of industries. The single largest application is in the production of fertilizers, utilizing over \(80\%\) of the world’s output.
The acid reacts with phosphate rock to create phosphoric acid and various phosphate fertilizers, essential for crop production. It is also a component in the electrolyte solution for lead-acid batteries, facilitating the electrochemical reactions necessary to store and release power.
Beyond agriculture and batteries, sulfuric acid is heavily used in metal processing, particularly for “steel pickling,” which cleans and removes rust from iron and steel surfaces. Furthermore, it serves as a foundational reagent in chemical manufacturing:
- Synthesis of detergents.
- Synthesis of pigments.
- Synthesis of dyes.
- Synthesis of other acids like nitric acid.