Phenolphthalein and methyl orange are acid-base indicators that are chosen based on the neutralization reaction that occurs, however they have some differences, lets check them.
The compounds that change color when exposed to acidic or basic solutions are called indicators. Color indicators are commonly used to measure pH and may be added to the reaction mixture to identify the titration endpoint/equivalence point.
| Table of Contents: 1. Acid-base titration indicators 2. Methyl orange indicator 3. Phenolphthalein indicator 4. Difference between phenolphthalein and methyl orange |
Acid-base titration indicators:
Acid-base titrations employ pH chemical indicators that change color over a range of pH values, rather than a single pH value. Generally, we use phenolphthalein and methyl orange to determine the endpoints of acid-base titrations. The selection of a certain indicator for titration is decided based on a plot known as the pH curve, which is typical of a particular reaction.
Methyl orange indicator:
Methyl orange is a weak acid, a synthetic indicator that provides endpoints for a pH range of 3.1 to 4.4. Its color is yellow in weak acidic or basic conditions (pH>4.4) and turns red in a highly acidic environment (pH<3.1). Its color is orange in the detection range of 3.1 to 4.4 pH.
To indicate the equivalence point, when the pH of the solution falls into the detection range, its color changes from yellow to orange, and when the pH of the solution raises into the detection range its color is red to orange.
Phenolphthalein indicator:
Phenolphthalein is a weak acid, a synthetic indicator that provides endpoints for a pH range of 8.3 to 10. It is colorless in strongly acidic and weak basic conditions (pH<8.3), and it turns pink in the strong basic environment (pH>10). Its color is pale pink in the detection range (pH: 8.8 to 10).
To indicate the equivalence point, when the pH of the solution rises into the detection range the color changes from colorless to pale pink, and when the pH of the solution falls toward the detection range its color changes from pink to pale pink
Difference between phenolphthalein and methyl orange indicator:
- Phenolphthalein is a colorless, weak acid that dissociates in water to form pink anions, while methyl orange is a weak acid that dissociates in water to form orange-neutral molecules.
- Phenolphthalein is suitable for titration of a weak acid against a strong base, while methyl orange is a useful chemical indicator for titration of a weak acid against a strong base.
- The pKa of phenolphthalein is 9.4, while the pKa of methyl orange is 3.5.
- The phenolphthalein indicator has a pH range of 8.2 to 10.0, while the methyl orange indicator has a pH range of 3.1 to 4.4.
- In terms of color change due to pH, phenolphthalein changes from colorless to pink at pH 8.2, while methyl orange changes at pH 4.
- In acid, the methyl orange indicator turns red, while phenolphthalein remains colorless.
- In a basic solution, methyl orange turns yellow, while phenolphthalein turns pink.
References:
- Wikipedia contributors. (2022, January 23). Phenolphthalein. In Wikipedia, The Free Encyclopedia. Available Here
- Wikipedia contributors. (2021, September 27). Methyl orange. In Wikipedia, The Free Encyclopedia. Available Here
- “Acid-Base Indicators.” Chemguide: Helping You to Understand Chemistry - Main Menu, Available Here
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