Page 76 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

pH range of Indicators

• The litmus colour change happens over an unusually wide range, but it is
useful for detecting acids and alkalis in the lab because it changes colour

around pH 7.
• Methyl orange or phenolphthalein would be less useful.

• This is more easily seen diagramatically.

• For example, methyl orange would be

yellow in any solution with a pH > 4.4.

• It couldn't distinguish between a weak

acid with a pH of 5 or a strong alkali with

a pH of 14.

• To choose an appropriate indicator, you need
to have a good idea of what pH the titration equivalence point will be.

• You choose an indicator whose pH range is close to the equivalence point
pH.

• This way, the visual endpoint the indicator gives, will be very close to the

actual equivalence point.

• This is because the pH of a titration changes dramatically near the
equivalence point.

• A very small addition of the titrant causes a large change in pH.

• So, if the indicator changes colors near this drastic pH change, the indicator
error will be small.

Types of Acid-Base Indicators

A. Color Indicators: The end point is detected by change in the indicator color.

B. Turbidity Indicators: The end point is detected by appearance of turbidity.

C. Fluorescence Indicators: The end point is detected by emission of light.

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