Question

PH meter has

• A. one cell
• B. two cells
• C. three cells
• D. no cell

Hint:

To measure any electrical potential difference (voltage), you need two reference points.
Similarly, to measure electrochemical potential in pH sensing, you must have two half-cells: a reference half-cell and an indicator half-cell.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks about the structural/electrochemical configuration of a standard pH meter system, specifically regarding the number of electrochemical cells or half-cells used in its operation.

Step 2: Detailed Explanation:

• A pH meter is an electronic instrument used to measure the hydrogen-ion activity (acidity or alkalinity) in a solution.

• The electrochemical measurement of pH requires a complete electrical circuit, which in turn necessitates a potential difference measurement between two points.

• To achieve this, a standard pH probe consists of two distinct electrodes (which act as two electrochemical half-cells):

• Glass Electrode (Measuring Half-Cell): This electrode has a special hydrogen-sensitive glass membrane. Its potential changes as a function of the hydrogen-ion concentration ($H^+$) in the test solution.

• Reference Electrode (Reference Half-Cell): This electrode provides a constant, stable, and known potential that is independent of the pH of the test solution (commonly a Calomel or Silver/Silver Chloride electrode).

• In modern systems, these two half-cells are often physically housed inside a single glass tube (known as a "combination pH electrode"), but they remain electrochemically distinct as two separate cells.

Step 3: Final Answer:
A standard pH meter has two cells (two electrochemical half-cells: the measuring electrode and the reference electrode).