Question

Calculate the concentration of an EDTA solution of which 24.22 mL were needed to titrate the Ca2+ ions present in a 50.00 mL solution containing 246.7 mg CaCO3. Express your answer in terms of (a) molar concentration of EDTA and (b) Ca titer (mg Ca per mL of EDTA)

Answer 1

In the estimation of calcium ions by EDTA , Ca²⁺ ion solution is pepared by dissolving CaCO₃ in HCl.

CaCO₃ + 2 HCl → CaCl₂ + CO₂ + H₂O

Note that EDTA is a hexadentate ligand , it forms an octahedral complex with calcium ions. The reaction of EDTA ( Ethylene diammine tetra acetate ) with Ca²⁺ takes place as follows :

(EDTA)^4- + Ca²⁺   →Ca²⁺( EDTA) ^4-  

Ca-EDTA complex is very strong complex.

Now in the given problem :

Ca²⁺ solution is prepared by dissolving 246.7 mg   ( ie 0.2467 gm) CaCO3 in 50 ml ( ie 50x 10^-3 litre).

Hence number of moles of CaCO₃ =   weight / molar mass = 0. 2467 / 100 = 2.467 x 10^-3 moles

Molarity of Ca²⁺ ions = number of moles / volume of solution in litre

                                   = 2.467 x 10^-3 / ( 50x10^-3)      = 2.467 / 50 = 0.04934 M

Volume of EDTA = 24.22 ml

                              EDTA        -----------------------------          Ca²⁺ ions

                               M₁V₁                        =                                         M₂V₂

                               M₁ x 24.22   =   0.04934 x 50

                                  M₁ = 0.04934 x 50 / 24.22 = 0.1018 M

Molar Concentration of EDTA = 0.1018 M

(b) Ca titer (mg Ca per ml of EDTA)

In the lab total 24.22 ml EDTA was used to titrate 246.7mg Ca

Hence 1 ml EDTA was used to titrate = 246.7/24.22 = 10.185 mg Ca

Hence Ca titer is = 10.185 mg Ca per ml of EDTA