Choosing the right acid-base indicator is essentially important for a successful titration experiment. In this article, we have explained what acid-base indicators are, their definition, function and the chemistry behind their color change in an acid-base neutralization reaction. So, without any further delay, let’s start reading!
What is an acid-base indicator-Definition
An acid-base indicator is defined as a chemical substance which marks the endpoint of an acid-base titration experiment by giving a color change. Chemically, the acid-base indicator is either a weak organic acid or a weak organic base. As the concentration of hydrogen (H+) ions and hydroxide (OH–) ions change, the pH of the reaction mixture changes, and the indicator gets ionized or deionized. The indicator thus displays a different color in its dissociated and undissociated forms.
Examples of acid-base indicators
The most widely used acid-base indicators are:
- Litmus
- Methyl red
- Methyl orange
- Phenolphthalein
- Bromothymol blue
- Phenol red
A universal indicator is a mixture of different acid-base indicators that give a gradual color change at a wide pH range.
How an acid-base indicator works
An acid-base indicator which is a weak organic acid, is represented by HIn. HIn represents the weak acid present in an acidic titration mixture (pH < 7) in its undissociated form. The dissociation of a weak Bronsted-Lowry acid by the loss of a proton (H+) yields a strong conjugate base represented by In–. The indicator possesses a different color in its undissociated (HIn) and dissociated (In–) forms.
Example:
The titration of an unknown solution of a base (such as NaOH) with an acidic solution of known concentration (such as HCl).
- An aqueous solution of NaOH is taken in the titration flask.
- As a few drops of an acid-base indicator such as methyl orange are added, it dissociates into In– ions which gives a yellow color in the titration mixture.
- As HCl solution is added from the burette into the titration flask, the H+ ions from HCl start neutralizing OH– ions present in the mixture.
- At a point where [H+] = [OH–], the pH of the solution rises rapidly from an acidic pH (< 7) to neutral (pH = 7). This is known as the equivalence point.
- As [H+] > [OH–] in the titration flask, In– ions readily combine with H+ ions to form InH.
- With one additional drop of acid from the burette, the equilibrium reaction maintained in the titration flask immediately shifts backwards according to Le Chattlier’s principles.
- At the point where [In–] = [HIn], i.e., half the indicator is in its dissociated form while the other half reverts to the undissociated form, the indicator changes color from yellow to red. This is known as the endpoint, marking the completion of the acid-base titration. The endpoint occurs at a pH approximate to the pH of the titration mixture at the equivalence point.
The indicator dissociation constant KIn can be determined from the equation given below.
pKIn = Endpoint of the indicator
How to choose the best acid-base indicator
For different types of acid-base titrations, the choice of indicator differs. It depends on the pH range in which the indicator changes color. Indicators do not give a sharp color change at a particular pH; instead, they change color at a narrow pH range. The equivalence point for a specific titration should fall within the pH range of the indicator used. In this way, the indicator endpoint coincides with the equivalence point of the acid-base titration.
An ideal indicator must also:
- Give a rapid color change at the endpoint.
- Give an easily detectable color change.
- Be non-reactive with the components of the titration mixture.
Below is a list you can use while choosing the best acid-base indicators for your titration experiment.
List of acid-base indicators
Organic structures of acid-base indicators
We told you that the acid-base indicator gives a different color in its HIn and In– forms. But, if you are wondering what structural changes leads to this color change, then here are the organic structures of some commonly known acid-base indicators.
Other than synthetically prepared indicators, natural indicators can also be prepared by extracting colored pigments from natural compounds such as red cabbage, Rose, and Hibiscus flowers.
Revise your concepts through this video tutorial on acid-base indicators.
Do you know!
The vibrant colors of organic molecules are due to the conjugation present in their structures. The organic molecules absorb visible radiation of a specific color and emit radiations of a complementary color and wavelength, as we learnt in UV-Vis spectroscopy.
References
1. Barbosa, J. 2005. ‘INDICATORS | Acid–Base.’ in Paul Worsfold, Alan Townshend and Colin Poole (eds.), Encyclopedia of Analytical Science (Second Edition) (Elsevier: Oxford).
2. Christian, Gary D., Purnendu K. Dasgupta, and Kevin A.Schug. 2013. Analytical Chemistry.
3. Pietrzyk, Donald J., and Clyde W. Frank. 1979. ‘Chapter Eight – Neutralization in Analytical Chemistry.’ in Donald J. Pietrzyk and Clyde W. Frank (eds.), Analytical Chemistry (Academic Press).
