How do phenolphthalein, bromothymol and methyl orange behave in acid base titration
Phenolphthalein, bromothymol blue, and methyl orange are commonly used as acid-base indicators in titrations.
1. Phenolphthalein: Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions. In an acid-base titration, it is typically used in the endpoint detection of strong acid-strong base titrations when the pH is around 8-10. It remains colorless in acidic solutions and changes to pink when excess base is added and the solution becomes slightly basic.
2. Bromothymol blue: Bromothymol blue is yellow in acidic solutions, greenish-blue in neutral solutions, and blue in basic solutions. It is used as an indicator in weak acid-strong base and weak acid-weak base titrations. In the endpoint detection, bromothymol blue changes its color from yellow to greenish-blue or blue when the pH becomes slightly basic.
3. Methyl orange: Methyl orange is red in acidic solutions and yellow in basic solutions. It is commonly used in strong acid-strong base titrations as an indicator. Methyl orange undergoes a color change from red to yellow at a pH of approximately 3.7-4.4, indicating the endpoint of the titration.
In acid-base titrations, these indicators are added to the analyte solution being titrated and the color change is observed to determine the endpoint of the titration. The color change occurs due to the change in the concentration of hydrogen ions (H+) or hydroxide ions (OH-) in the solution as the titrant (acid or base) is added. The specific pH range at which the color change occurs depends on the indicator being used.