In acid-base titration, the sudden change in color of acid-base indicator is used to indicate the end point of titration. Acid-base indicator usually is a weak organic acid or an organic weak base. In alkaline solution, the indicator lost protons that it changes from the acid to basic (expressed in In-), and give protons in acidic solution changing from the basic to acid (expressed in HIn), accompanied by changes of structure and color. Since the equilibrium constants KHlπ are different from variety of indicators, the color range is different. Some discoloration happens in an acidic solution, some happens in the vicinity of neutral, some discoloration occurs in an alkaline solution. Thus, you can select the appropriate acid-base indicator based on acid-base titration equivalence point of pH value and the jump range. For example, strong base titrate weak acid, etc. the solution appear weak alkaline at the titration endpoint, pH leap is also included in the alkaline range near the titration endpoint, and the choice of thymol blue or phenolphthalein indicator is correct, while the methyl orange which change color in an acidic solution is not proper. On the contract, strong acid weak base titration, the solution appear weak acidic at the titration endpoint, pH leap is also included in the acidic range near the titration endpoint. The choice of methyl red, methyl orange and bromine cresol green indicator are very appropriate.