"Determination of nickel ions Ni²⁺ by using EDTA with the help of back titration"

 

Experiment # 1: Determination of nickel ions Ni²⁺ by using EDTA with the help of back titration

Objective:

The objective of this experiment is to determine the concentration of Ni²⁺ ions in a given solution through a back titration technique using ethylenediaminetetraacetic acid (EDTA) as a complexometric titrant while employing magnesium sulfate as a masking agent.

Introduction:

Back titration is a chemical analysis technique used to determine the concentration of a substance by reacting it with an excess of a known reagent, then titrating the unreacted excess with another reagent of known concentration.

Nickel ions (Ni²⁺) can be quantitatively determined by complexometric titration with EDTA. In this back titration, excess EDTA is added to react with the metal ions, and the unreacted EDTA is then titrated with a standardized solution of a metal ion (usually Zn²⁺) to determine the amount of excess EDTA. The addition of magnesium sulfate helps prevent interference from other metal ions.

Materials Required:

• Nickel sample solution with an unknown concentration
• EDTA solution of known concentration
• Buffer solution (pH ~ 10)
• Standardized Zn²⁺ solution (0.01M ZnSO₄)
• Eriochrome Black T indicator solution

Equipment Required:

• Burette
• Conical flask/Erlenmeyer flask
• Pipette
• pH meter
• Glass stirring rod
• Analytical balance
• Distilled water

Chemical Equation:

Procedure:

A. Preparation of 0.01 M of EDTA solution:

Weigh out 0.29224 grams of EDTA… Transfer the weighed EDTA into a clean and dry container (e.g., a beaker). Add a small amount of distilled or deionized water. Once the EDTA is fully dissolved, transfer the solution to a 100 mL volumetric flask (or any container that can hold 100 mL) and add more water to reach the 100 mL mark. Be sure to mix the solution thoroughly.

B. Preparation of 0.01 M of ZnSO₄

Weigh out 0.16138 grams of ZnSO₄… Transfer the weighed ZnSO₄ into a clean and dry container (e.g., a beaker). Add a small amount of distilled or deionized water. Once the ZnSO₄ is fully dissolved, transfer the solution to a 100 mL volumetric flask (or any container that can hold 100 mL) and add more water to reach the 100 mL mark. Be sure to mix the solution thoroughly.

C. Preparation of Buffer solution

Weigh out 32 grams of NH₄Cl accurately using a balance. In a suitable container (such as a glass beaker or flask), add 200 mL of distilled water. Slowly add the 32 grams of NH₄Cl to the water and stir until it is completely dissolved. This will create a solution of NH₄⁺ ions. Next, carefully add the required amount of concentrated ammonia (NH₃) to the solution. Given that you need about 1/10th the amount of NH₃ compared to NH₄Cl, you may start by adding approximately 3.2 mL of concentrated ammonia solution. However, pH measurements are crucial in this step, as the concentration of the concentrated ammonia may vary. After adding the initial amount of NH₃, stir the solution thoroughly. Check the pH of the solution using a pH meter or pH indicator paper. If it is not at the desired pH of 10, you may need to add more concentrated ammonia in small increments while monitoring the pH until you reach the target pH. Once you achieve a pH of 10, the buffer solution is ready. You can use it for your intended application.

D. Preparation of 0.01 M of NiCl₂

Part I - Standardization of EDTA Solution:

1. Weigh an accurately measured amount of Zn²⁺ sample (ZnSO₄) (approximately 0.2-0.3 g) using an analytical balance and transfer it to a clean Erlenmeyer flask. Or you can also use alternative like few milliliters of magnesium sulfate solution to the flask. The magnesium sulfate will help mask other metal ions, preventing their interference.
2. Add a few milliliters of distilled water to dissolve the Zn²⁺ sample.
3. Add a few drops of buffer solution to maintain the pH around 10.
4. Add a few drops of Eriochrome Black T indicator solution to the flask. The solution will turn from red to blue.
5. Titrate the Zn²⁺ solution with the standardized EDTA solution until the blue color changes to a wine-red or violet color, indicating the endpoint of the titration. Record the volume of EDTA solution used.
6. Repeat the titration two more times for accuracy. Calculate the average volume of EDTA solution used.

Part II – Back Titration of Nickle sample with excess EDTA

Determination of Ni²⁺ by Back Titration:

1. Pipette a 25 mL aliquot of the unknown Ni²⁺ solution (NiCl­₂) into a clean Erlenmeyer flask.
2. Add a few milliliters of buffer solution to the flask to maintain the pH at approximately 10.
3. Add a few drops of Eriochrome Black T indicator solution to the flask. The solution will turn from red to blue.
4. Titrate the Ni²⁺ solution with the standardized EDTA solution until the blue color changes to wine-red or violet. Record the volume of EDTA solution used.
5. Repeat the titration two more times for accuracy. Calculate the average volume of EDTA solution used.

Calculations:

A. To prepare a 0.01 M solution in 100 mL

Moles of EDTA = 0.01 moles/L × 0.1 L = 0.001 moles

Calculate the mass of EDTA required:

Use the formula:

Mass (g) = Moles (mol) × Molecular Weight (g/mol)

Mass (g) = 0.001 moles × 292.24 g/mol = 0.29224 g

 

B. To prepare a 0.01 M solution in 100 mL:

Moles of ZnSO4 = 0.01 moles/L × 0.1 L = 0.001 moles

Calculate the mass of zinc sulfate required:

Use the formula:

Mass (g) = Moles (mol) × Molecular Weight (g/mol)

Mass (g) = 0.001 moles × 161.38 g/mol = 0.16138 g

1. Calculate the moles of Zn²⁺ used in the standardization titration.
2. Calculate the moles of EDTA used in the standardization titration.
3. Calculate the moles of excess EDTA used in the Ni²⁺ titration (average volume of EDTA used in Part II - average volume of EDTA used in Part I).
4. Calculate the moles of Ni²⁺ in the sample solution using the moles of excess EDTA and the stoichiometry of the reaction between Ni²⁺ and EDTA.
5. Calculate the concentration of Ni²⁺ in the original sample solution.

Safety Precautions:

• Handle chemicals and glassware with care.
• Use appropriate personal protective equipment (lab coat, gloves, safety goggles).
• Work in a well-ventilated laboratory.
• Dispose of chemical waste according to laboratory guidelines.