why phenolphthalein is added first then methyl orange in alkali mixtur titration
Phenolphthalein and methyl orange are both commonly used as indicators in acid-base titrations to determine the endpoint of the reaction. The choice of which indicator to use and the order in which they are added depend on the nature of the reaction and the pH range in which the indicators change color.
In the case of alkali mixture titration, where a solution containing a mixture of strong and weak bases is being titrated with an acid, phenolphthalein is added first, followed by methyl orange.
Phenolphthalein is a pH indicator that changes color in the pH range of approximately 8.2 to 10. It is colorless in acidic solutions and turns pink or magenta in alkaline solutions. Since phenolphthalein changes color in the alkaline range, it is suitable for titrating weak bases and the endpoint is indicated when the pink color persists.
Methyl orange, on the other hand, is a different type of indicator that changes color in a lower pH range, specifically around pH 3.1 to 4.4. It is red in acidic solutions and changes to yellow in alkaline solutions. Methyl orange is more suitable for titrating strong bases. When the solution reaches the endpoint, it changes from red to yellow.
As a result, phenolphthalein is added first in alkali mixture titration to indicate the endpoint for the weak base in the mixture. Once the phenolphthalein has turned pink, indicating that the weak base has been completely neutralized, methyl orange is added to determine the endpoint for the strong base, as its color change occurs in a lower pH range.
Ultimately, the selection and order of indicators in titration depend on the specific reaction being studied and the pH range at which the indicators change color.