Indicators

Indicators

Aug 26, 2022, 16:45 IST

Indicators in chemistry

According to Ostwald Theory of acid-base indicator, an acid-base indicator is a weak organic acid or weak organic base which remains partially dissociated/protonated in aqueous solution and of which, ionised and unionised forms possess different colour. If HIn be an indicator (a weak acid) then its dissociation may be represented as: The dissociation constant of an acid indicator is called indicator constant and is denoted by the symbol Kin, Upon decreasing the pH of solution containing the indicator, the dissociation equilibrium shifts towards left and the ratio decreases and at certain ratio, the colour A(colour of the acidic form of the indicator dominates over the colour B (colour of the basic form of the indicator). Contrary to this at certain high pH the ratio becomes high enough to push colour B to dominate over colour A. In between the two extreme ratios the indicator displays intermediate colour. If the minimum pH = x at which colour A predominates over colour B and the maximum pH = y at which colour B predominates over colour A, then x – y is known as pH range of the indicator.

Types of indicators:

1. Acid base indicators: Used in neutralization reactions Change their colour depending on pH of solution. e.g. Phenolphthalein, Methyl red, litmus etc.
2. Redox indicators: Used in oxidation – reduction reactions. Colour changes with the change of emf e.g., KMnO4, N-phenyl anthranilic acid.
3. Adsorption indicators: Change of colour due adsorption of a substance on its surface.
4. Radioactive indicators :  Used in the study of variety of reactions as tracer. Their presence is detected by suitable devices.  e.g., D2, O8, I127etc.

1 g equivalent acid neutralizes 1g equivalent base completely.  But in practice, there is no any visible indication that the reaction is complete. Indicators do the job pretty well.  They show a variation in their colour between certain pH limits, called indicator range or pH range.

Features of a Good Indicator:

• It should change the colour at a pH which falls in proximity to the end point for a given reaction.

• Colour change should be very sharp.

• Colour should be stable and brilliant.

• The two colours of an indicator should be contrasting so that they can easily be distinguished.

Indicators pH range Colour in

Acidic Basic

Methyl orange 3.1 - 4.5 Pink Yellow

Methyl red 4.2 - 6.3 Red Yellow

Litmus 5.4 - 7.5 Red Blue

Phenolphthalein 8.0 - 9.8 Colourless Colourless

Thymolphthalein 9.3 - 10.5 Colourless Blue

Internal indicator: Added into the titration mixture before titration.  e.g., Phenolphthalein, N-Phenyl anthranilic acid, Starch + KI etc.

External indicator: Only a small fraction of the reaction mixture is taken out and checked for end point by adding the indicator e.g.,

Related Topics

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Example of Indicators