The most commonly used indicator electrode is the glass electrode. It is a type of electrode that is used to measure the pH of a solution. The glass electrode consists of a glass bulb that is filled with a solution of hydrochloric acid. The bulb is connected to a reference electrode, such as a calomel electrode. When the glass electrode is placed in a solution, the hydrogen ions in the solution diffuse into the glass bulb. This causes a voltage to be generated between the glass electrode and the reference electrode. The voltage is proportional to the pH of the solution.
The other options are not as commonly used as the glass electrode. The calomel electrode is a type of reference electrode that is used in potentiometric measurements. It consists of a mercury electrode that is immersed in a solution of mercury chloride. The silver electrode is a type of reference electrode that is used in potentiometric measurements. It consists of a silver electrode that is immersed in a solution of silver nitrate. The silver-silver chloride electrode is a type of reference electrode that is used in potentiometric measurements. It consists of a silver electrode that is coated with silver chloride.
The glass electrode is the most commonly used indicator electrode because it is relatively inexpensive, easy to use, and has a wide range of applications. It is also very stable and can be used in a variety of solutions.