Hardness in groundwater is due to the presence of metal ions, primarily Mg2 and

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?

Explanation / Answer

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

Answer:

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

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