Complexometric Determination of Water Hardness.

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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 turned light blue.

Trial | Amount of Na2EDTA Titrated (mL) |
1 | 25.5 |
2 | 25.5 |
3 | 23.1 |

Unknown Water Sample B was selected and then titrated with the standardized Na2EDTA. 25 mL of the Unknown Sample B was combined with 20 mL DI water, 3 mL NH3/NH4Cl (aq) buffer, and five drops of Eriochrome Black T indicator. The Na2EDTA was again titrated until this solution turned light blue.

Trial | Amount of Na2EDTA Titrated (mL) |
1 | 13 |
2 | 13.4 |
3 | 13.6 |

Data Analysis

Standardization of the disodium EDTA solution:

Trial 1:
[Na2EDTA]= 10.00 mL CaCO3 solution x 1.000 g CaCO3 x 1 mol CaCO3 x 1 mol Na2 EDTA = 0.003910 mol 25.5 mL Na2EDTA solution 1 L CaCO3 solution 100.1 g CaCO3 1 mol CaCO3

Trial 2:
[Na2EDTA]= 10.00 mL CaCO3 solution x 1.000 g CaCO3 x 1 mol CaCO3 x 1 mol Na2 EDTA = 0.003910 mol 25.5 mL Na2EDTA solution 1 L CaCO3 solution 100.1 g CaCO3 1 mol CaCO3

Trial 3:
[Na2EDTA]= 10.00 mL CaCO3 solution x 1.000 g CaCO3 x 1 mol CaCO3 x 1 mol Na2 EDTA = 0.004325 mol 23.1 mL Na2EDTA solution 1 L CaCO3 solution 100.1 g CaCO3 1 mol CaCO3

Mean molarity:

[Na2EDTA]mean = 0.003910 + 0.003910 + 0.004325 = 0.004048 mol 3

Absolute deviation for each trial:

Trial 1:
0.004048 – 0.003910= 0.000138

Trial 2:
0.004048 – 0.003910= 0.000138

Trial 3:
0.004048 – 0.004325= -0.000277

Mean of absolute deviation:
0.000138 + 0.000138 + (-0.000277) = -3.33 x 10-7
3

Relative Precision:

-3.33 x 10-7 x 1000 ppt = -0.000082 parts per thousand
0.004048

Calculating hardness of unknown water sample

Trial 1:
13 mL Na2EDTA solution x 0.004048 mol Na2EDTA x 1 mol CaCO3 x 100.1 g CaCO3 = 210.706 ppm 0.02500 L CaCO3 solution 1 L Na2EDTA solution 1 mol Na2EDTA 1 mol CaCO3

Trial 2:
13.4 mL Na2EDTA solution x 0.004048 mol Na2EDTA x 1 mol CaCO3 x 100.1 g CaCO3 = 217.19 ppm 0.02500 L CaCO3...
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