Why methyl orange alkalinity equal to total alkalinity?

Answer 1

Alkalinity is different from basicity, which is directly related to the pH. The higher the pH, the more basic the water.

Like acidity, there are different ways to measure and report alkalinity;

The first is to titrate the water with acid titrant to the phenolphthalein end point. This is called the phenolphthalein alkalinity. Since phenolphthalein changes color at pH~8.3, this corresponds to a pH where all the CO32- present would be protonated.

Second, acid titration to a methyl orange end point, pH~4.3, further converts the bicarbonate to aqueous carbon dioxide. At this end point, some of the weaker conjugate bases are protonated. The methyl orange end point titration indicates total alkalinity.

Answer 2

Methyl orange alkalinity represents the hydroxide and carbonate alkalinity in water. Total alkalinity includes hydroxide, carbonate, and bicarbonate alkalinity. When methyl orange turns red in water, it indicates the presence of carbonate and hydroxide ions, which make up the alkalinity, thus making methyl orange alkalinity equal to total alkalinity.