An acid-base indicator is a weak acid or base that will be protonated or deprotonated at a certain pH. It can be used for both titrations and general pH control. The behavior of acid-base indicators is explained by two theories.
Indicators are compounds that change color when exposed to acid/base. Color indicators are commonly used to measure pH and can be used to identify the titration endpoint/equivalence point by adding them to the reaction mixture.
Several types of indicators are employed in different types of titrations, including acid-base, complexometric, redox, and precipitation. Each has its own pKa and pH range and produces a different color at different pH levels.
In chemistry, there are three types of indicators: natural indicators, artificial indicators, and olfactory indicators, while artificial and natural indicators are the two most commonly employed forms.
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What is an acid-base indicator?
A weak acid or weak base that changes color as the concentration of hydrogen (H+) or hydroxide (OH-) ions in an aqueous solution changes is known as an acid-base indicator. In titration, acid-base indicators are most commonly used to determine the endpoint of an acid-base reaction.
The acid-base indicators can be classified into three groups, phthaleins, sulphophthaleins, azo indicators, triphenylmethane indicators, etc.
Examples of the acid-base indicator with their color change:
- Phenolphthalein (Colorless to pink)
- Methyl orange (Red to yellow)
- Bromphenol blue (Yellow to blue)
- Methyl red (Red to yellow)
- Phenol red (Yellow to red)
- Thymol blue (Red to yellow)
- Bromcresol green (Yellow to blue)
- Thymolphthalein (Colorless to blue)
The acid-base indication is a type of indicator in which the analyte displays a visual signal confirming the presence of an acid or a basic. Below we briefly explain the principle of how acid-base indicators work.
Theory of acid-base indicator:
Ostwald's theory and the quinonoid theory are the two theories that explain the acid-base indicator.
- According to Ostwald's theory, the color change is due to the ionization of the acid-base indicator. The color of the unionized form differs from that of the ionized form.
- According to the quinonoid theory, the ionization of the indicator is greatly affected in acids and bases as it is either a weak acid or a weak base. If the indicator is a weak acid, its ionization in acids is relatively low due to the presence of common H+ ions, whereas it ionizes well in bases. Due to the common OH- ions, if the indicator is a weak base, its ionization is very high in acids and low in bases.
How an acid-base indicator works:
The acid and its conjugate base are distinct colors if the indicator is a weak acid. The base and its conjugate acid show distinct colors if the indicator is a weak base.
According to the chemical equation, equilibrium is reached in a solution of a weak acid indicator with the molecular formula HIn:
HIn(aq)
+ H2O(l) ↔ In-(aq) + H3O+(aq)
HIn(aq) is an acid that’s color is different from the base In-(aq). The concentration of the hydronium ion H3O+ is higher when the pH is lower, and the equilibrium is toward the left, resulting in the color A. Since the concentration of H3O+ is lower at higher pH, the equilibrium tends to the right side of the equation, resulting in the color B.
Phenolphthalein, a weak acid that dissociates in water to generate a magenta or red-purple anion, is an example of a weak acid indicator. Because equilibrium is to the left in an acidic solution, the solution appears colorless, however as pH rises, equilibrium shifts to the right, revealing the magenta color.
Below equation can be used to find the equilibrium constant of a reaction:
KIn = [H3O+][In-] / [HIn]
In which the KIn is the indicator dissociation constant. At the moment where the acid and anion base concentrations are equal, the color changes:
[HIn] = [In-]
This is the point at which half of the indicator is in the form of acid and the other half is its conjugate base.
Frequently Asked Question (FAQ):
What is the use of acid-base indicators?
Acid and base indicators are used to detect the pH of a sample and to determine the endpoint of the reaction. In chemistry, a substance to be studied (Titrand) is placed in a conical flask, 2-4 drops of a suitable indicator is added, then a drop by drop addition of titrant of known strength from a burette until the chemical reaction is complete.
Acid and base indicators are used to detect the pH of a sample and to determine the endpoint of the reaction. In chemistry, a substance to be studied (Titrand) is placed in a conical flask, 2-4 drops of a suitable indicator is added, then a drop by drop addition of titrant of known strength from a burette until the chemical reaction is complete.
How acid-base indicator is selected?
The indicator for an acid-base titration is chosen based on its pH range falling within the reaction's pH change.
The indicator for an acid-base titration is chosen based on its pH range falling within the reaction's pH change.
Why indicator changes color?
The color change is caused by an electron isomerism resonance. Because different indicators have different ionization constants, they change color at different pH intervals.
The color change is caused by an electron isomerism resonance. Because different indicators have different ionization constants, they change color at different pH intervals.
References:
- Anne Marie, “Definition and Examples of Acid-Base Indicator” ThoughtCo, 18 Jan. 2011 Available Here
- ‘PH Indicator’. Wikipedia, 22 Feb. 2022. Wikipedia, Available Here:
- Acid-Base Indicator - an Overview | ScienceDirect Topics. Available Here:
- Reactions of Acids and Bases in Analytical Chemistry. Hulanicki, A. and Masson, M.R. New York: Halsted Press, 1987.
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