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Chapter 17: Problem 22

What are the criteria for choosing an indicator for a particular acid-base titration?

Short Answer

Expert verified
The criteria for choosing an indicator for an acid-base titration are: (1) The pH range over which the indicator changes colour, which should correspond to the equivalence point of the titration; and (2) A distinct colour change over a narrow pH range for better precision. Some common indicators include litmus, phenolphthalein, and methyl orange, and their use would depend on the pH at the equivalence point of the specific titration.

Step by step solution

01

Understanding Acid-Base Titrations

Acid-base titration is a process used to determine the concentration of an unknown acid or base. It is performed by neutralizing the acid or base with a solution of a base or acid of known concentration. An indicator is used to show when the reaction has been completed, by its colour change.
02

Identifying the Role of an Indicator

An indicator is a substance that changes color near the endpoint of a chemical reaction. The choice of indicator is very crucial for accurate results in a titration.
03

Recognizing the Criteria for Choosing an Indicator

The choice of an indicator depends mainly on the pH range over which it changes colour, which should correspond to the equivalence point of the titration, i.e., the point at which the reactants have reacted in stochiometrically equivalent quantities. Moreover, the indicator should have a distinct colour change over a narrow range of pH, for better precision.
04

Identifying Different Indicators and Their pH Ranges

Common indicators and their pH ranges are: Litmus: changes from red in acidic solution to blue in alkaline solution with a neutral point at pH 7. Phenolphthalein: colorless in acidic solution and becomes pink in alkaline solution with change at pH 8.2-10. Methyl Orange: changes from red in acidic solution to yellow in alkaline solution with change at pH 3.1 - 4.4. The selection of these indicators would depend on the pH at the equivalence point of the titration.

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