METAL COMPLEX EQUILIBRIA

Compleximeetry/ Complexometry/ Chelometry

Compleximetric titrations- A titrimetric determination which involves the formation of a soluble but slightly dissociated complex or

complex ion

M+ + :L M – L

Metal ion –

Ligand –

Coordination Number (C.N.) –

Coordinate covalent bond –

Denticity -

Types of Ligands

1. Monodentate or unidentate – 2. Bidentate - 3. Terdentate -4. Quadridentate - 5. Sexidentate – EDTA

EDTA – hexadentate ligand; tetraprotic acid

Complexometric Titration Problems

1. A 50.0mL of a solution which is 0.0100 M in Ca2+ and buffered at pH 10.0 is titrated with 0.0100 M EDTA solution. Calculate the values of pCa2+ at various stages of the titration.Kf = 5.0 x 1010 a. At 0.00 ml EDTA (pCa2+ = 2.00)b. At 10ml EDTA (pCa2+ = 2.18)c. At 25.00ml EDTA (pCa2+ = 6.28)d. At 26.00mL EDTA (pCa2+ = 9.55)

Indicators for EDTA titrations

1. Eriochrome black T2. Calmagite3. Arsenazo I4. Xylenol orange5. Murexide

Problems:

1. A CaCO3 solution that will be used to standardize EDTA was prepared by dissolving 2.8000 g of solid CaCO3 in 100 ml of dilute HCl. A 20.00-mL aliquot was taken for titration with EDTA consuming 29.00 mL of the titrant to reach the endpoint. Express the concentration of EDTA in M and in CaCO3 titer.Ans. 0.1929 M & 19.31 mg CaCO3/ ml EDTA

2. A standard solution of calcium was prepared by the dissolution of 200.0 mg pure CaCO3 in HCl. Then the solution was boiled to remove CO2 and was diluted to 250.0 mL in a volumetric flask. When 25.00 mL of the solution was used to standardize an EDTA solution by titration at pH 10, 22.62 mL of the EDTA solution was required. This EDTA solution was used to determine the hardness of a water sample. A 20.00 ml aliquot (prepared by diluting 25.00 ml sample to 250.00 mL) required 35.40 ml EDTA to reach the EBT endpoint. Determine the hardness of water in ppm CaCO3.Ans. 15650 ppm CaCO3

3. The nickel content of a plating solution is determined by diluting a 1.00 mL sample with water buffered at pH 10

with NH3NH4Cl and treated with 50.00 mL of 0.0496 F EDTA. The excess EDTA was backtitrated with 0.102 F MgCl2 and requires 4.73 mL to reach EBT endpoint. Calculate the nickel content as g/L of NiSO4 . 6H2O (FM = 262.9 g/mol)Ans. 525.15 g/L Nickel

4. A 100-mL sample of H2O containing Ca2+ and Mg2+ was titrated with 15.28 mL of 0.01016 M EDTA in an ammonia buffer at pH 10.00. Another sample of 100mL was titrated with NaOH to form the precipitate Mg(OH)2 and then titrated at pH 13 with 10.43mL of the same EDTA solution. Calculate the concentration of CaCO3 and MgCO3 in the sample in ppm.Ans. 106.06 ppm CaCO3 & 41.54 ppm MgCO3

5. A 100 – ml aliquot of city drinking water was treated with a small amount of an NH3 – NH4Cl buffer to bring the pH to 10.00. After the addition of calmagite indicator, the solution required 21.46 ml of 0.005140 M EDTA for titration. Calculate the water hardness in ppm CaCO3.

Ans. 110.4 ppm CaCO3

6. A 25.00 mL aliquot of a solution containing Hg2+ in dilute HNO3 was treated with 10.00 mL of 0.04882 M EDTA and the pH was adjusted to 10 with an ammonia buffer. Two drops of EBT were added and the unreacted EDTA was back-titrated with 0.01137 M Mg2+ solution, requiring 24.66 ml to reach the endpoint. What is the M Hg2+ in the sample?Ans. 8.314 x 10-3 M Hg2+

7. A 25.00 ml sample of unknown containing Fe3+ and Cu2+ required 16.06 mL of 0.05083 M EDTA for complete titration. A 50.00 sample of the unknown was treated with NH4F to protect the Fe3+. The Cu2+ was reduced and masked by the addition of thiourea. Upon addition of 25.00 mL of 0.05083 M EDTA, the Fe3+ was liberated from its fluoride complex and formed an EDTA complex. The excess EDTA required 19.77 mL of 0.01883 M Pb2+ to reach an endpoint using xylenol orange. Find the concentration of Fe3+ and Cu2+ in M.Ans. 0.01797 M Fe3+ and 0.01468 M Cu2+

8. The cadmium and lead ions in a 50.00 mL sample required 40.09 mL of 0.004870 M EDTA for titration. A 75.00 mL portion of the same sample was made basic and treated with excess KCN masking the cadmium as Cd(CN)4

2- .This solution required 31.44 of the EDTA for titration. Calculate the concentration of Cd2+ and Pb2+ in the sample in ppm and M.

9. A solution containing iron (III) and aluminium (III) was analysed by means of consecutive compleximetric titrations. The pH of a 50.00 mL aliquot of the sample solution was buffered at approximately 2, and about 200 mg of salicylic acid was dissolved in it. Then 0.04016 F EDTA titrant was added until the red color of the iron III – salicylate complex just vanished, 29.61 mL being required. Then exactly 50.00 mL of the same EDTA titrant was pipetted into the solution; next, the solution was boiled briefly to ensure the complexation of aluminium and the pH was readjusted to a value of 5. Finally the solution, which contained excess EDTA was back-titrated with 19.03 mL of a 0.03228 F iron (III) solution. Calculate M of Fe3+ and Al3+.Ans. 0.02378 M Fe3+ and 0.02787 M Al3+

10. Calamine is a mixture of Zn and Fe oxides. A 1.022 g of dried calamine was dissolved in acid and diluted to 250.00 mL. KF was added to a 10.00 mL aliquot of the diluted sample solution to mask the iron. After suitable adjustment of pH, Zn2+ consumed 28.71 mL of 0.01294 M EDTA. A second 50.00 mL aliquot was buffered and titrated with 2.40 mL of 0.002727 M ZnY2- solution. Calculate % ZnO and % Fe2O3 in the sample.

Fe3+ + ZnY2- FeY- + Zn2+

Ans. 73.97 % ZnO and 0.5114 % Fe2O3

11. An EDTA solution was prepared by dissolving about 3.0 g Na2H2Y.2H2O in approximately 1L water and standardizing against 50.00 mL aliquots of 0.04517 M Mg2+. An average volume of 32.22 mL was required.a) Calculate M EDTA. b) Calculate volume of EDTA needed to titrate the Ca in a 0.5140 g mineral specimen that

is 81.40 % brushite CaHPO4. 2H2O (172.09 g/mol)

Ans. 0.070096 M EDTA, 34.70 mL EDTA

12. A 50.00 ml sample of water containing both Ca2+ and Mg2+ is titrated with 16.55 mL of 0.0114 M EDTA in an ammonia buffer at pH 10. Another 50.00 mL sample is titrated with NaOH to form the precipitate Mg(OH)2 and then titrated at pH 13 with 9.25 mL. Calculate ppm CaCO3 (100.09) and ppm MgCO3 (84.31)Ans. 211 ppm CaCO3 and 140 ppm MgCO3

13. A 0.2420 g sample containing Ca is dissolved and the metal precipitated as CaC2O4. The precipitate is filtered, washed and redissolved in acid. The pH is adjusted, 25.00 mL of 0.0400 M EDTA added and the excess EDTA was titrated with 33.25 mL 0.01202 M Mg2+ solution. Calculate the % Ca in the sample. Ans. 9.94%