Which one of the following is commonly used as an indicator to measure the exact pH?

Acids are compounds that, when dissolved in water, form free hydrogen ions (H+ ions). When bases are dissolved in water, they form hydroxyl ions (OH-). The pH scale is used to determine the acidity or basicity of an acid or base, going from 0 to 14. pH indicators are made up of several chemical compounds known as universal indicators, which are used to measure the exact pH.