Question

100ml of water sample on titration with N/50 H2SO4 required 12.4ml of the acid

for neutralization to phenolphthalein end point and 15.2ml of the acid to methyl

end endpoint. Determine the type and extent of alkalinity present in the water

sample.

Answer 1

$Alkalinity=\frac{A*N*50000}{ml\>of\>sample}$

Phenolphthalein Alkalinity$=\frac{12.4×0.02×50000}{109}=124ppm$

Methylorange alkalinity

$=\frac{25.6×0.02×50000}{100}=156ppm$

The alkalinity is due to OH^- and CO₃^2- ions

Half of CO₃^2- ions $=156-124=28ppm$

OH^- Alkalinity $=124-28=96ppm$

$CO_3^{2-}$ Alkalinity $=28×2$

$=56ppm$