In the standard notation for a voltaic cell, the double vertical line "[ ]" represents :

Options

A. a phase boundary
B. gas electrode
C. a wire (metal) connection
D. a salt bridge

Explanation

To answer the question regarding the standard notation for a voltaic cell, specifically what the double vertical line "[ ]" represents, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Voltaic Cells**: A voltaic cell consists of two half-cells, one where oxidation occurs (anode) and the other where reduction occurs (cathode).

2. **Components of the Cell**: In the anode half-cell, a metal (like zinc) is oxidized, releasing electrons. In the cathode half-cell, a metal ion (like copper ions) is reduced by gaining electrons.

3. **Role of the Salt Bridge**: The salt bridge is a crucial component that connects the two half-cells. It allows the flow of ions between the two solutions, maintaining electrical neutrality and completing the circuit.

4. **Representation in Notation**: In the standard notation for a voltaic cell, the components are represented in a specific order. The anode is written on the left, followed by a single vertical line (|) to indicate a phase boundary, then the double vertical lines (||) to represent the salt bridge, and finally the cathode on the right.

5. **Conclusion**: Therefore, the double vertical line "[ ]" in the standard notation for a voltaic cell represents the salt bridge, which facilitates the movement of ions and maintains the balance of charge in the cell.

### Final Answer:
In the standard notation for a voltaic cell, the double vertical line "[ ]" represents the salt bridge.

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