Complexometric Determination of Water Hardness
Complexometric Determination of Water Hardness
By Nick Williams
CHM 152
Lab dates: Aug 28th 2013
Dr. Weide
Abstract:
When a polyatomic ligand with multiple lone pairs of electrons available for bonding to a central metal ion forms a complex with a metal ion, a process known as chelation takes place. Metal ion impurities can be found by using disodium salt of EDTA to determine the concentration of M2+ by complexometric or chelometric titration. Erichrome Black T makes it easy to see when the EDTA has completely chelated the metal impurities. When the metal cation is still present in the water, the solution will be a complex with a pink tint. Once enough EDTA has been added to the M2+ solution the metal ions complex to the EDTA leaving the indicator solvated causing a color change from pink to blue. Once the color has changed to blue, the titration has reached an end point and the M2+ metal ion impurities in hard water can be calculated.
Procedure:
Prepare 500 mL of 0.004 M disodium EDTA solution by dissolving 0.7-0.8g of Na2EDTA in 500 mL deionized water. Standardize the solution by using a 10 mL transfer pipet to add 10 mL of standardized calcium ion stock solution to a 250 mL Erlenmeyer flask and mixing it with 30 mL of deionized water. Add 3 mL of ammonia/ammonium chloride buffer (pH 10) to ensure that the calcium ions remain in the solution for accurate results. Add four drops of Eriochrome Black T indicator solution so that a color change can be visible during titration. Once the solution is prepared, titrate it with the disodium EDTA solution. Add the solution quickly at first and once the color changes from pink to violet slow down the titration until the color changes from violet to blue. Then record the results.
Part 2 Unknown
After receiving an unknown prepared water sample record the unknown in your lab notebook (unknown #76). After the unknown has been recorded, transfer 25 mL of the sample into a 250 mL Erlenmeyer flask
By Nick Williams
CHM 152
Lab dates: Aug 28th 2013
Dr. Weide
Abstract:
When a polyatomic ligand with multiple lone pairs of electrons available for bonding to a central metal ion forms a complex with a metal ion, a process known as chelation takes place. Metal ion impurities can be found by using disodium salt of EDTA to determine the concentration of M2+ by complexometric or chelometric titration. Erichrome Black T makes it easy to see when the EDTA has completely chelated the metal impurities. When the metal cation is still present in the water, the solution will be a complex with a pink tint. Once enough EDTA has been added to the M2+ solution the metal ions complex to the EDTA leaving the indicator solvated causing a color change from pink to blue. Once the color has changed to blue, the titration has reached an end point and the M2+ metal ion impurities in hard water can be calculated.
Procedure:
Prepare 500 mL of 0.004 M disodium EDTA solution by dissolving 0.7-0.8g of Na2EDTA in 500 mL deionized water. Standardize the solution by using a 10 mL transfer pipet to add 10 mL of standardized calcium ion stock solution to a 250 mL Erlenmeyer flask and mixing it with 30 mL of deionized water. Add 3 mL of ammonia/ammonium chloride buffer (pH 10) to ensure that the calcium ions remain in the solution for accurate results. Add four drops of Eriochrome Black T indicator solution so that a color change can be visible during titration. Once the solution is prepared, titrate it with the disodium EDTA solution. Add the solution quickly at first and once the color changes from pink to violet slow down the titration until the color changes from violet to blue. Then record the results.
Part 2 Unknown
After receiving an unknown prepared water sample record the unknown in your lab notebook (unknown #76). After the unknown has been recorded, transfer 25 mL of the sample into a 250 mL Erlenmeyer flask