The standard electrode potential of a half cell is measured experimentally using which electrical instrument in combination with a reference electrode?

Introduction / Context:
This electrochemistry question focuses on how standard electrode potentials are measured. Standard electrode potential is a voltage difference between a given half cell and the standard hydrogen electrode under defined conditions. To determine this potential, an instrument that measures electrical potential difference is required. Identifying the correct instrument reinforces understanding of how electrochemical data is obtained in practice.

Given Data / Assumptions:
- The options include pyrometer, electrometer, thermometer, and voltmeter.
- The question asks specifically about measuring standard electrode potential of a half cell.
- We assume standard laboratory practice where a half cell is connected to a reference electrode in a galvanic cell and the potential difference is measured.

Concept / Approach:
Standard electrode potential is expressed in volts and represents the tendency of a half cell to gain or lose electrons relative to the standard hydrogen electrode. To measure such a potential difference, an instrument that reads voltage is needed. A voltmeter is designed to measure electrical potential differences in volts. A pyrometer measures high temperatures, a thermometer measures temperature, and an electrometer is a very sensitive apparatus for measuring small electric charges, but standard laboratory determination of electrode potentials uses a voltmeter. Therefore, the correct instrument is a voltmeter.

Step-by-Step Solution:
Step 1: Recognise that standard electrode potential is defined as an electric potential difference between a half cell and a reference standard hydrogen electrode.
Step 2: Electric potential difference is measured in units of volts.
Step 3: An instrument that directly measures potential difference in volts is called a voltmeter.
Step 4: A pyrometer is used to measure very high temperatures, for example in furnaces, and does not measure electric potentials.
Step 5: A thermometer measures temperature in degrees and also does not measure potential difference.
Step 6: An electrometer is a specialised instrument for detecting very small electric charges or potentials, but in standard electrochemistry experiments a voltmeter is commonly specified for measuring electrode potentials.
Step 7: Therefore, when the question refers to the instrument for measuring standard electrode potential, the appropriate answer is voltmeter.

Verification / Alternative check:
Typical electrochemistry laboratory manuals describe setting up a galvanic cell by connecting the half cell under study to a standard hydrogen electrode. The output terminals of this cell are then connected to a high impedance voltmeter, which reads the potential difference. The value obtained under standard conditions is reported as the standard electrode potential. These descriptions consistently mention a voltmeter, confirming that it is the correct instrument here.

Why Other Options Are Wrong:
- Pyrometer: This instrument measures high temperatures, not electrical quantities, so it is unrelated to electrode potential measurements.

Why Other Options Are Wrong (continued):
- Thermometer: It measures temperature and cannot measure voltage, so it is not used for standard electrode potentials.
- Electrometer: While it can detect small electric charges or potentials, the standard basic instrument referred to in most educational contexts for electrode potential measurement is a voltmeter, not an electrometer.

Common Pitfalls:
Some learners may be distracted by the similarity between the words electrometer and electrode and choose electrometer without considering the practical context. Others may not distinguish instruments by what physical quantity they measure. A useful approach is to always link an instrument name to the quantity it measures: voltmeter for voltage, ammeter for current, thermometer for temperature, and so on. This habit makes such questions straightforward.

Final Answer:
Voltmeter is the electrical instrument used to measure standard electrode potentials of half cells.