EDTA Titration of Ca2+ and Mg2+ in Bottled Water

Extract

[...] The endpoint is indicated by the appearance of a blue tint in the solution. Repeat the titration at least three times more. Prepare a blank using distilled water and perform a blank titration. Record the result and subtract this number from the other volumes obtained in previous titrations. This blank can be used for both parts of the experiment. Calculate the total concentrations of Ca2+ and Mg2+, the individual concentrations of each ion, and the relative standard deviation of the replicate titrations. [...]

[...] Since this is in a 50 mL sample, the molarity of the ions is equal to: 7.36 x 10-5 moles of Ca2+ and Mg2+ = 0.0015 M 0.050 Liters Part II: Titration with EDTA using Hydroxynaphthol Indicator. The concentration of Ca2+ can be calculated from part two of the experiment in which Mg2+ is precipitated using a strong base. The concentration of Ca2+ is related to the number of moles of EDTA needed to reach the endpoint in the titration. [...]

[...] Since this is in a 50 mL sample, the molarity of the ions is equal to: 9.536 x 10-5 moles of Ca2+ = 0.0019 M 0.050 Liters The difference between the number of moles of EDTA needed to complete the first titration and the number of moles needed to complete the second titration can be used to determine the individual concentration of the Mg2+ ion Liters EDTA 0.0231 Liters = 0.0067 Liters EDTA 0.0032 x 0.0067 = 2.144 x 10-5 more moles of EDTA required to reach endpoint in second titration x 10-5 moles of EDTA = Moles of Mg2+ Concentration of Mg2+ = 2.144 x 10-5 moles of Mg2+ = 4.288 x 10-4 M 0.050 Liters The standard deviation for the replicate measurements in part one and is given by: s = [Sum (xi xmean)2/N-1]1/2 Where xi is for each replicate, xmean is and N = 3. [...]