Question

Why two indicators are used in determination of alkalinity?

Answer 1

Water Alkalinity is mainly due to carbonate and bicarbonate salts in a water sample, the carbonate ion has two end points, one from carbonate to bicarbonate and another from bicarbonate to carbon dioxide.

Phenolphthalein end point pink to clear

Methyl orange end point yellow to red.

Answer 2

The two indicators are used to determine the alkalinity of a solution as one indicator is used for carbonate and the other is used for bicarbonates.

Detailed answer

The first indicator is phenolphthalein is used for carbonates to bicarbonates

• methyl orange is used for indicating bicarbonates to carbon dioxides.
• Usually, the carbonate endpoints have two or more endpoints.
• When there is a presence of two carbonates we have to use two indicators to determine the alkalinity.
• $CO23+H^++HCO3$ ---- phenolphthalein
• $HCO3+H^{+} CO2+H2O$--- Methyl orange