The key difference between methyl orange and phenolphthalein is that the color of methyl orange changes from red to yellow when changing from acidic to basic medium, whereas the color of phenolphthalein changes from colorless to pink when changing from acidic to basic medium.
An indicator is a component used in titrimetric analysis to find an endpoint where the reaction ends. We can determine the used analyte amount to find many different chemical parameters accordingly.
CONTENTS
1. Overview and Key Difference
2. What is Methyl Orange
3. What is Phenolphthalein
4. Methyl Orange vs Phenolphthalein in Tabular Form
5. Summary – Methyl Orange vs Phenolphthalein
What is Methyl Orange?
Methyl orange is a pH indicator that shows red color and yellow color at different pH values. It is used frequently in titration techniques because of its distinct and clear color variation. It shows red color in acidic medium and yellow color in basic medium. The color changes at the pKa, so it is usually used in titration for acids. Although it does not have a full spectrum of color change, it has a sharp end point. When a solution becomes less acidic, methyl orange changes the color from red to orange. Finally, it turns yellow, giving the endpoint of the titration.
The chemical formula of methyl orange is C14H14N3NaO3S. It has a molar mass of 327.33 g/mol. Its appearance can be described as orange or yellow solid. The density of methyl orange is 1.258 g/cm3. Its melting point is >300 degrees Celsius, and it decomposes at higher temperatures. It is poorly water-soluble. In diethyl ether, methyl orange is insoluble. The pKa of this indicator is 3.47 in water at normal room temperature (25 degrees Celsius).
Moreover, we can derive another indicator using methyl orange, and it is known as xylene cyanol. It changes from grey-violet to green as the solution becomes more basic. It is known as a modified indicator. However, methyl orange has mutagenic properties. Therefore, it is considered a toxic substance that should be handled with care.
What is Phenolphthalein?
Phenolphthalein is a pH indicator that is useful as an acid-base titration indicator. It is a common indicator often used in laboratory titration processes. The chemical formula of phenolphthalein is C20H14O4. This name is abbreviated as “Hin” or as “phph.” The acidic color of phenolphthalein is colorless, while the basic color of the indicator is pink. Therefore, when going from acidic to basic medium, the color changes from colorless to pink. The pH range for this color change is around 8.3 – 10.0 pH.
Moreover, a phenolphthalein indicator is slightly water-soluble, and it often dissolves in alcohols. This is why we can easily use them in titrations. Phenolphthalein is a weak acid that can release protons into the solution. The acidic form of phenolphthalein is nonionic and colorless. The deprotonated form of phenolphthalein is pink colored and is an ionic form. If we add a base to the reaction mixture consisting of a phenolphthalein indicator, the equilibrium between the ionic and nonionic forms tends to shift towards the deprotonated state because the protons are removed from the solution.
When considering the synthesis of phenolphthalein indicator, we can produce it from the condensation of phthalic anhydride in the presence of two equivalents of phenol under acidic conditions. Moreover, this reaction can be catalyzed using a mixture of zinc chloride and thionyl chloride.
What is the Difference Between Methyl Orange and Phenolphthalein?
The key difference between methyl orange and phenolphthalein is that the color of methyl orange changes from red to yellow when changing from acidic to basic medium, whereas the color of phenolphthalein changes from colorless to pink when changing from acidic to basic medium. In methyl orange, the pH range for this color change is around 3.1 – 4.4, whereas, in phenolphthalein, the pH range for this color change is around 8.3 – 10.0 pH.
The below infographic presents the differences between methyl orange and phenolphthalein in tabular form for side-by-side comparison.
Summary – Methyl Orange vs Phenolphthalein
Methyl orange and phenolphthalein are pH indicators that are useful as titration indicators. The key difference between methyl orange and phenolphthalein is that the color of methyl orange changes from red to yellow when changing from acidic to basic medium, whereas the color of phenolphthalein changes from colorless to pink when changing from acidic to basic medium.
Reference:
1. “Acid Base Titration – Titration Curves, Equivalence Point & Indicators of Acid Base Titration.” BYJU’S, 18 May 2022.
Image Courtesy:
1. “Methyl-orange-sample” By Ben Mills – Own work (Public Domain) via Commons Wikimedia
2. “Ca(OH)2 and Phenolphthalein ^^” By Friedemann W.-W. (CC BY-NC-SA 2.0) via Flickr