Question

A water sample is alkaline to both phenolphthalein as well as methyl
orange. 200 mL of water sample on titration with N/25 HCl required 5 mL
of the acid to phenolphthalein end point. When a few drops of methyl
orange are added to the same solution and titration was further continued,
the yellow colour of the solution just turned pink after the addition of another 12 ml of acid solution. elucidate on the type and extent of alkalinity present in water sample.​

Answer 1

Answer:

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. ... The methyl orange end point titration indicates total alkalinity.