In electrochemistry, which acid is used as the electrolyte in lead storage cells (lead–acid batteries)?

Introduction / Context:
This question deals with the chemistry of lead storage cells, commonly known as lead–acid batteries, which are used in automobiles and backup power systems. The electrolyte in these batteries is a dilute acid that allows ions to move between the electrodes during charging and discharging. Knowing which acid is used is important in both basic electrochemistry and practical safety, because it explains why battery electrolyte is corrosive and why proper handling is necessary. The standard acid used in lead storage cells is dilute sulphuric acid.

Given Data / Assumptions:
- The device mentioned is a lead storage cell or lead–acid battery.
- The question asks which acid serves as the electrolyte in this battery.
- Options include sulphuric, nitric, hydrochloric, and phosphoric acids.
- It is assumed that you are familiar with the basic construction of a lead–acid battery: lead and lead dioxide plates immersed in an acidic electrolyte.

Concept / Approach:
In a lead–acid battery, the electrolyte is dilute sulphuric acid (H2SO4). The overall reactions during discharge involve the conversion of lead (Pb) and lead dioxide (PbO2) at the electrodes into lead sulfate (PbSO4), with sulphuric acid participating in the reaction and water being formed. The concentration of sulphuric acid decreases as the battery discharges and increases again during charging. Nitric acid, hydrochloric acid, and phosphoric acid are not used as electrolytes in standard lead storage cells; they would lead to different and often undesirable reactions and corrosion patterns. Therefore, the correct answer is sulphuric acid.

Step-by-Step Solution:
Step 1: Recall that the common car battery is a lead–acid battery containing lead and lead dioxide plates. Step 2: Remember that the liquid inside such a battery is corrosive and is described as dilute sulphuric acid. Step 3: Understand that during discharge, Pb and PbO2 react with sulphate ions from H2SO4 to form PbSO4 on both plates, and water is produced. Step 4: Recognise that the reversible electrochemical reactions rely specifically on sulphuric acid because it supplies sulphate ions and hydrogen ions in the correct stoichiometry. Step 5: Compare this with other acids: nitric acid and hydrochloric acid would lead to different side reactions, and phosphoric acid is not used in the classic lead–acid system. Step 6: Conclude that sulphuric acid is the acid used in lead storage cells.

Verification / Alternative check:
Automotive manuals and battery specifications describe the electrolyte as diluted sulphuric acid, often around 30–40 percent by mass. Electrochemistry textbooks that discuss the lead–acid cell explicitly write the cell notation with H2SO4 as the electrolyte. They explain that the state of charge of the battery can be monitored by measuring the specific gravity of the sulphuric acid solution. No standard reference suggests that nitric, hydrochloric, or phosphoric acid is used in conventional lead storage batteries, confirming that sulphuric acid is the correct answer.

Why Other Options Are Wrong:
Nitric acid is a strong oxidising acid and would rapidly corrode the lead plates in an uncontrolled way, making it unsuitable as a stable battery electrolyte. Hydrochloric acid forms soluble chlorides and presents corrosion issues with metals, again not matching the well known chemistry of lead–acid batteries. Phosphoric acid is used in some industrial processes and rust treatments but is not the electrolyte in typical automotive batteries. Therefore, options B, C, and D do not accurately describe the acid used in lead storage cells.

Common Pitfalls:
Students sometimes confuse sulphuric acid with other strong acids simply because all are corrosive. Others may only remember that the electrolyte is "acid" without recalling which specific acid is involved. To avoid such mistakes, it helps to associate the name lead–acid battery directly with sulphuric acid, and to remember that the formation of lead sulfate (PbSO4) during discharge is directly linked to the presence of sulphate ions from H2SO4 in the electrolyte.

Final Answer:
The acid used as the electrolyte in lead storage cells is sulphuric acid (H2SO4).