What Is Battery Acid? Sulfuric Acid Facts

Battery acid could refer to any acid used in a chemical cell or battery, but usually, this term describes the acid used in a lead-acid battery, such as those found in motor vehicles. 

Car or automotive battery acid is 30-50% sulfuric acid (H₂SO₄) in water. Usually, the acid has a mole fraction of 29%-32% sulfuric acid, a density of 1.25–1.28 kg/L and concentration of 4.2–5 mol/L. Battery acid has a pH of approximately 0.8.​

Construction and Chemical Reaction

A lead-acid battery consists of two lead plates separated by a liquid or gel containing sulfuric acid in water. The battery is rechargeable, with charging and discharging chemical reactions. When the battery is being used (discharged), electrons move from the negatively-charged lead plate to the positively-charged plate.

The negative plate reaction is:

Pb(s) + HSO₄^-(aq) → PbSO₄(s) + H⁺(aq) + 2 e^-

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The positive plate reaction is:

PbO₂(s) + HSO₄^- + 3H⁺(aq) + 2 e^- → PbSO₄(s) + 2 H₂O(l)

Which may be combined to write the overall chemical reaction:

Pb(s) + PbO₂(s) + 2 H₂SO₄(aq) → 2 PbSO₄(s) + 2 H₂O(l)

Charging and Discharging

When the battery is fully charged, the negative plate is lead, the electrolyte is concentrated sulfuric acid, and the positive plate is lead dioxide. If the battery is overcharged, electrolysis of water produces hydrogen gas and oxygen gas, which are lost. Some types of batteries allow water to be added to make up for the loss.

When the battery is discharged, the reverse reaction forms lead sulfate on both plates. If the battery is fully discharged, the result is two identical lead sulfate plates, separated by water. At this point, the battery is considered completely dead and can't recover or charged again.

Sulfuric Acid Names

Calling sulfuric acid "battery acid" gives an indication of the acid concentration. There are, in fact, several different names for sulfuric acid that typically reflect its usage.

• Concentration less than 29% or 4.2 mol/L: The common name is dilute sulfuric acid.
• 29-32% or 4.2-5.0 mol/L: This is the concentration of battery acid found in lead-acid batteries.
• 62%-70% or 9.2-11.5 mol/L: This is chamber acid or fertilizer acid. This is the acid concentration made using the lead chamber process.
• 78%-80% or 13.5-14.0 mol/L: This is tower acid or Glover acid. It is the concentration of acid recovered from the bottom of the Glover tower.
• 93.2% or 17.4 mol/L: The common name for this concentration of sulfuric acid is 66 °Bé ("66-degree Baumé") acid. It reflects the density of the acid using a hydrometer.
• 98.3% or 18.4 mol/L: This is concentrated sulfuric acid. While it's possible to make almost 100% sulfuric acid, the chemical loses SO3 near its boiling point and subsequently becomes 98.3%.
  

Battery Acid Properties

• Battery acid is highly corrosive. It reacts vigorously with skin and mucous membranes, releasing a lot of heat.
• It is a polar liquid.
• Battery acid has a high electrical conductivity.
• Pure battery acid is colorless, but the acid readily picks up impurities and becomes discolored.
• It is not flammable.
• Battery acid is odorless.
• Its density is nearly twice that of water, at 1.83 g/cm³.

Sources

• Davenport, William George; King, Matthew J. (2006). Sulfuric acid manufacture: analysis, control and optimization. Elsevier. ISBN 978-0-08-044428-4.
• Haynes, William M. (2014). CRC Handbook of Chemistry and Physics (95th ed.). CRC Press. pp. 4–92. ISBN 9781482208689. 
• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Jones, Edward M. (1950). "Chamber Process Manufacture of Sulfuric Acid". Industrial and Engineering Chemistry. 42 (11): 2208–2210. doi:10.1021/ie50491a016
• Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). Houghton Mifflin Company. p. A23. ISBN 978-0-618-94690-7.