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 or mmol/L Ca2 . 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(In)2+ + EDTA --> Ca(EDTA)^2+ + In

A 50.00-mL sample of groundwater is titrated with 0.0550 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 12.20 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?

M CaCO3 = ?

ppm CaCO3 = ?

Explanation / Answer

Ca+2    and EDTA titration is 1:1

M1V1 = M2V2

M1 x 50 = 0.0550 x 12.20

M1 = 0.01342 M as Ca+2

Because 1 molecule CaCO3 produces 1 molecule Ca2+ , The solution tested is 0.01342 M CaCO3
That is 0.01342 mol CaCO3 in 1.0L of water.

CaCO3 molarity = 0.01342 M

CaCO3 molar mass = 100 g/mol

CaCO3 moles = 0.01342

mass = 100 x 0.01342 = 1.342 g in 1 Litre of water

ppm = mg / L

        = 1342 mg / L

        = 1342 ppm

M CaCO3 = 0.01342

ppm CaCO3 = 1342

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