The compound which changes color when exposed to the acidic or basic solution is called an indicator. Color indicators are widely used to detect pH and can be added to reaction mixtures to determine titration endpoints or equivalence points.
In chemistry, several types of indicators are used in various forms of titration, including acid-base, precipitation, redox, and complexometric. Each one has its pKa and pH range and generates a distinct hue at various pH levels. Ostwald's idea and the quinonoid theory are the two theories that explain the acid-base indicator.
What is methyl red?
Methyl red (2-(N, N-dimethyl-4-aminophenyl) azobenzenecarboxylic acid), the indicator dye also known as C.I. Acid Red 2, becomes red in acidic solutions. It is an azo compound that differs from methyl orange in that the sulphonic acid function has been substituted with a carboxylic acid function.
Methyl red (C15H15N3O2) is a weak acid that dissociates in water to form red neutral molecules. It's an azo dye that's used as a pH indicator and comes in the form of a dark red crystalline powder. It turns red below pH 4.4, yellow above pH 6.2, and orange in the middle, with a pKa of 5.1.
Prperties of methyl red:
| Formula of Methyl red | pKa of Methyl red | pH range of methyl red | Color in acidic medium | Color in basic medium |
|---|---|---|---|---|
| C15H15N3O2 | 5.1 | 4.4 to 6.2 | Red | Yellow |
How does methyl-red work as an indicator?
The acid-base indicators are weak acids or bases that come in both ionized and non-ionized forms and have a variety of colors. The degree of dissociation is controlled by the pKa value of the analyte. The two forms are equivalent when the pH equals the pKa. One variant predominates above or below this. We select compounds with the suitable pKa and consequently pH for the color change, that provide good, distinctive color contrast.
The pH-dependent color change of methyl red is caused by a structural change. The unprotonated form on the left is a simple diazo dye, due to the extended conjugation, it appears yellow. When the dimethylamino group is protonated at pH's less than 4.0 to 6.0 the predominant structural form is the quinone diimine form on the right.
What is the color of methyl red in the acidic medium?
The color of methyl red in an acidic medium is red.
What is the color of methyl red in the basic medium?
Methyl red has a yellow color in an alkaline medium and neutral solution.
What titration is methyl red used for?
Methyl red is suitable for the titration of a strong acid with a strong base; Due to its low pH range, it is good for acid titration.
Can methyl red be used for NaOH titration with HCl?
Yes, we can use methyl red, phenolphthalein, or methyl orange as indicators in the titration of a strong base with strong acid.
Why was methyl red used as an indicator rather than phenolphthalein?
If the endpoint of the compounds that have in between pH 4.0 to 6.2, methyl red indicator used in that titration which turns red to yellow, Whereas phenolphthalein is used for pH 8.2 to 10.0, which turns from colorless to pink.
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