Complexometric Determination of Water Hardness.
Introduction
A complex ion consists of a metal ion with Lewis bases attached to it. These Lewis bases form strong coordinate covalent bonds with the central complex ion in which both electrons of the bond are donated by the Lewis base. The Lewis bases that form this complex are referred to as ligands and they are typically either neutral molecules or anions. Polydentate ligands have multiple lone pairs of electrons that enable them to bond multiple atoms to the central metal atom. The stability of the resulting complex, or chelate, enables it to be used as a solvent to strip metal ions from various substances. Ethylenediaminetetraacetic acid (EDTA), a common chelating ligand, has four carboxylic acid groups that donate an electron pair to link to the central metal ion.
In this experiment, the affinity of EDTA for metal ions will be applied to tap water that has a particularly high mineral content. Through chelometric (complexometric) titration, EDTA can be used to gauge the concentration of metal wastes found within hard water. Eriochrome Black T will be used to indicate when the EDTA has fully absorbed the metal impurities found in the hard water.
H2In- + M2+ (aq) ↔ MIn- (aq) + 2H+ (aq)
H2In- is the Eriochrome Black T in its normal form where it is blue-colored. Upon reacting with a metal ion (M2+) its color changes to pink. When EDTA is introduced, it attracts the metal ions and thus leaves the Eriochrome Black T indicator solvated (blue-colored). An indicator color change from pink to blue signals the conclusion of the titration.
EDTA (aq) + MIn- (aq) + 2H+ (aq) → H2In- (aq) + MEDTA (aq)
Procedure/Data
0.8652 g of 0.004 M disodium EDTA was dissolved in 500 mL of deionized water within a sealed plastic bottle. Na2EDTA solution was standardized using 10 mL of CaCO3, 30 mL of DI water, five drops of Eriochrome Black T indicator, and 3 mL of NH3/NH4Cl (aq) buffer. Na2EDTA was titrated until the Eriochrome Black T solution
A complex ion consists of a metal ion with Lewis bases attached to it. These Lewis bases form strong coordinate covalent bonds with the central complex ion in which both electrons of the bond are donated by the Lewis base. The Lewis bases that form this complex are referred to as ligands and they are typically either neutral molecules or anions. Polydentate ligands have multiple lone pairs of electrons that enable them to bond multiple atoms to the central metal atom. The stability of the resulting complex, or chelate, enables it to be used as a solvent to strip metal ions from various substances. Ethylenediaminetetraacetic acid (EDTA), a common chelating ligand, has four carboxylic acid groups that donate an electron pair to link to the central metal ion.
In this experiment, the affinity of EDTA for metal ions will be applied to tap water that has a particularly high mineral content. Through chelometric (complexometric) titration, EDTA can be used to gauge the concentration of metal wastes found within hard water. Eriochrome Black T will be used to indicate when the EDTA has fully absorbed the metal impurities found in the hard water.
H2In- + M2+ (aq) ↔ MIn- (aq) + 2H+ (aq)
H2In- is the Eriochrome Black T in its normal form where it is blue-colored. Upon reacting with a metal ion (M2+) its color changes to pink. When EDTA is introduced, it attracts the metal ions and thus leaves the Eriochrome Black T indicator solvated (blue-colored). An indicator color change from pink to blue signals the conclusion of the titration.
EDTA (aq) + MIn- (aq) + 2H+ (aq) → H2In- (aq) + MEDTA (aq)
Procedure/Data
0.8652 g of 0.004 M disodium EDTA was dissolved in 500 mL of deionized water within a sealed plastic bottle. Na2EDTA solution was standardized using 10 mL of CaCO3, 30 mL of DI water, five drops of Eriochrome Black T indicator, and 3 mL of NH3/NH4Cl (aq) buffer. Na2EDTA was titrated until the Eriochrome Black T solution