pH indicators are usually weak acids or weak bases that change their color depending on their dissociation (protonation) state. Sometimes both forms are colored, sometimes only one. In most cases you may assume that to completely change color of bicolored indicator pH must change by 2 units. However, human eye is more sensitive to some colors than to others, thus some color changes can be perceived over wider pH range.

pH indicators can be used to check pH of the solution, although they are rarely added directly. Only in acid-base titrations indicator should be added to the solution. To check pH it is much more convenient to use pH strips. It is worth of noting here that pH strips are nothing else but pieces of paper impregnated with indicator or a mixture of indicators.