Introduction Complexometric titration is one of the most common methods of volumetric analysis in ch — Organic Chemistry Chemistry Question

Introduction
Complexometric titration is one of the most common methods of volumetric analysis in chemical laboratories. In this method, the most widely used complexing agent is disodium salt of ethylenediaminetetraacetic acid (EDTA). EDTA is a polyprotic acid (H₄Y). The solution Na₂H₂Y is used as a standard solution for titration of metal ions (Mn⁺):
Mn⁺ + H₂Y 2- + 2 H₂O → MYn^-4 + 2 H₃O⁺

The formation constant (Kf) of MYn^-4 depends on the nature of metal ions. For example:
[VISUAL]

Metal ions can be titrated separately due to the formation of MYn^-4 complex with conditional formation constant at a suitable pH condition. In practical, the metallochromic indicators are usually used for the detection of the end point. These indicators are the color complexing agents for the metal ions. The color of the indicator differs from that of the metal-ion-complex and metal indicator complex (Mind) is much less stable than metal^-EDTA-complex. Therefore, the titration of a metal ion solution is carried out with a given indicator at suitable pH of the solution:
MInd + H₂Y 2- → MYn^-4 + Ind

This task is to help participants to use techniques of the volumetric analysis for the determination of metal cations.

Chemicals and reagents
* Standard solutions of 0.050 mol dm^-3 EDTA (prepared from Na₂H₂Y. 2 H₂O)
* Zn(NO₃)₂, solution (c = 0.050 mol dm^-3 )
* Buffer solutions of 1 mol dm^-3 CH₃COOH and 1 mol dm^-3 NaCH₃COO (pH = 4.7)
* Solution of 2 mol dm^-3 NH₃ and 2 mol dm^-3 NH₄Cl (pH = 9.2)
* Solution of HCl (c = 1 mol dm^-3)
* Solution of NaOH (c = 1 mol dm^-3)
* Distilled water
* Indicators: xylenol orange (1% in KCl)
* Eriochrome black T (ET – 00) (1% in KCl); The congo red (pH indicator); universal pH indicator paper; 5% solution of sulfosalicylic acid

Apparatus and glassware
* Burette, 50 cm₃
* Volumetric pipette, 25 cm₃
* Erlenmeyer flask, 250 cm₃
* Graduated cylinder, 10 cm₃
* Glass beaker, 250 cm₃
* Funnels and filter papers (the red tape)
* Droppers
* Hot plate.

Experimental procedure
Step 1. Sample preparation
Prepare a sample solution containing three ions of Fe³⁺, Al³⁺, and Mg²⁺ within concentration range of 0.01 mol dm^-3 to 0.1 mol dm^-3.

Step 2. Titration of iron, aluminum, and magnesium ions
1. Titration of Fe³⁺
Add 5.00 cm₃ of the sample solution into an Elenmeyer flask. Adjust the pH of the solution to 2 using NaOH or HCl solution with the Congo red paper. Heat the solution to 80 – 90 oC on a hot plate and then add 0.5 cm₃ of sulfosalicylic acid as indicator. The solution is titrated with EDTA solution from a colorless to yellow end point. Record the volume of the standard solution (V₁ cm₃).

2. Titration of the total amount of Fe3+and Al³⁺
Add 25.00 cm₃ of the sample solution into an Elenmeyer flask. Add 50 cm₃ of EDTA standard solution into the flask. Heat the solution to 80 – 90 oC and add 10 – 15 cm₃ of the buffer solution CH₃COOH/ NaCH₃COO and a small amount of xylenol orange. Titrate the remaining amount of EDTA with the Zn²⁺ standard solution until the solution changes from red to yellow at the end-point. Record the volume of the standard solution (V₂, cm₃).

3. Titration of Mg²⁺
Measure 25.00 cm₃ of the sample solution to a glass beaker. Add slowly the NH₃ + NH₄Cl buffer solution while stirring the mixture. After the precipitation reaction is complete, add 5 cm₃ of buffer solution and swirl the mixture. Filter the solution to an Elenmeyer flask through filter paper in a funnel. Wash the precipitate (3 – 4 times) with warm water. Finally, titrate Mg²⁺ with standard EDTA solution using ET – 00 until the solution changes from pale red to blue. Record the volume of the standard solution (V₃, cm₃).