Question

Hardness in groundwater is due to the presence of metal ions, primarily Mg2 and Ca2 . Hardness is generally reported as ppm CaCO3. To measure water hardness, a sample of groundwater is titrated with EDTA, a chelating agent, in the presence of the indicator eriochrome black T, symbolized here as In. Eriochrome black T, a weaker chelating agent than EDTA, is red in the presence of Ca2 and turns blue when Ca2 is removed.

Red blue

Ca (ln)2+ +EDTA------->Ca EDTA 2+ + In

A 50.00-mL sample of groundwater is titrated with 0.0350 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 11.70 mL of EDTA is required to titrate the 50.00-mL sample, what is the hardness of the groundwater in molarity and in parts per million of CaCO3 by mass?

Answer 1

Molarity of EDTA solution (M₁) = 0.0350 M

Volume of EDTA required for the titration (V₁) = 11.70 ml

Volume of water sample (V₂)= 50 ml

From the relation M₁V₁ = M₂V₂ we get,

0.0350 x 11.70 = M₂ X 50

or, M₂ = (0.0350 x 11.70) / 50 = 0.00819 M

Mass of CaCO₃ = 0.00819 mol x 100.09 g mol^-1 = 0.8197371 g = 819.7371 mg

Volume of water = 50 mL = 50/1000 L = 0.05 L

Hence hardness of water in mg/L is 819.7379 mg/ 0.05 = 16394.742 mg/L

819.7379 mg/ 0.05 = 16394.742 mg/L = 16395 ppm

hardness of water in molarity = 0.00819 Molar

hardness of water in mg/L = 16394.742 mg/L

hardness of water in ppm = 16394.742 ppm