Which of the following devices is commonly used as a reference electrode?

The calomel electrode is a well-established reference electrode, essential in electrochemical measurements. It provides a stable and known electrode potential due to the presence of a saturated potassium chloride solution, which maintains a constant ionic concentration. This stability is crucial when determining the potential of other electrodes in electrochemical cells, as it allows for accurate comparisons and measurements.

Reference electrodes like the calomel electrode are designed to have a constant voltage output regardless of the conditions in the solution. This stability ensures that variations in the system being studied do not affect the reference voltage, which is pivotal for obtaining reliable and reproducible results in electrochemical experiments.

The other types of electrodes mentioned do not fulfill the role of a reference electrode. An ion-selective electrode responds to specific ions in solution but doesn't provide a consistent reference voltage across different samples. The working electrode is the site where the reaction of interest occurs, and its potential can vary during the process. Similarly, the counter electrode serves to balance the charge flow within the electrochemical cell without providing a stable reference potential. Hence, these devices cannot be relied upon for the same purpose as a reference electrode like the calomel electrode.