A metal`-` insoluble salt electrode consists of

Options

A. A piece of metal placed in a solution containing a sparingly soluble salt.
B. Crystals of an insoluble salt coated with a metal.
C. A piece of metal coated with one of its insoluble salts.
D. A metal fixed with an insoluble salt at high temperature.

Explanation

To solve the question regarding the composition of a metal-insoluble salt electrode, we will analyze each option provided and determine which one correctly describes the electrode.

### Step-by-Step Solution:

1. **Understanding the Definition**: A metal-insoluble salt electrode is typically composed of a metal that is in contact with its own insoluble salt. This means that the metal is not just in a solution but is also coated or associated with its insoluble salt.

2. **Analyzing the Options**:
- **Option 1**: A piece of metal placed in a solution containing sparingly soluble salt.
- This option does not describe a metal-insoluble salt electrode because the metal is not coated with the salt; it is merely placed in a solution.
- **Option 2**: Crystals of an insoluble salt coated with a metal.
- This option suggests that the metal is on the outside, which is not the typical structure of a metal-insoluble salt electrode.
- **Option 3**: A piece of metal coated with one of its insoluble salts.
- This option accurately describes a metal-insoluble salt electrode, where the metal is indeed coated with its own insoluble salt.
- **Option 4**: A metal fixed with an insoluble salt at high temperature.
- This option does not accurately reflect the typical construction of a metal-insoluble salt electrode, as it implies a different method of combination.

3. **Conclusion**: Based on the analysis, the correct answer is **Option 3**: a piece of metal coated with one of its insoluble salts.

### Final Answer:
The metal-insoluble salt electrode consists of a piece of metal coated with one of its insoluble salts.

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