Ferrochrome is an alloy of iron and chromium mostly (over 80 % of world consumption) used in stainle — Physical Chemistry — Kinetics Chemistry Question

Ferrochrome is an alloy of iron and chromium mostly (over 80 % of world consumption) used in stainless steel production. To improve the corrosion resistance of steel it is often coated with zinc, the process being referred to as galvanization. It is of extreme importance to accurately control the contents of zinc, iron, and chromium in the resultant galvanized steel. Typically samples of steel are dissolved in concentrated nitric acid and the content of major components in the obtained solution is determined by different methods.

Analysis of multi-component systems is often hindered by the interference between the components. Various separation techniques (precipitation, extraction) or masking are the usual steps towards avoiding such interference. Still, nowadays researches are mainly focused at analytical methods allowing sequential determination of components in the same solution without any separation procedures. These methods are typically based on simple manipulations like changing the pH.

In this task you will apply one of such methods for sequential determination of iron(III), chromium(III) and zinc(II) simultaneously present in an aqueous solution.

Chemicals and reagents
* Test solution simulating a digested sample of galvanized steel (a standard solution containing Fe³⁺, Zn²⁺ and Cr³⁺ ions within the concentration range of 0.1 – 0.3 mol dm^-3),
* Hydrochloric acid, 1 mol dm^-3,
* Na₂H₂EDTA standard solution, 0.025 mol dm^-3,
* Acetate buffer solution, pH 5.5 – 6.0, 1.7 mol dm^-3 in acetate,
* Copper(II) sulfate standard solution, 0.025 mol dm^-3,
* Ethanol, 96%,
* Distilled water,
* Indicators:
- aqueous solution of 5-sulfosalicylic acid, 5% (w/w),
- solution of 1-(2-pyridylazo^-2-naphthol (PAN) in ethanol, 0.1% (w/w),
- universal pH indicator paper.

Equipment and glassware
* Hot plate,
* Funnels (to fill the burettes),
* Paper filters,
* Volumetric flask, 100 cm₃,
* Volumetric pipettes, 1 and 10 cm₃,
* Burette, 25 or 50 cm₃ (2 ea.),
* Erlenmeyer flask, 200 cm₃ (3 ea.),
* Graduated cylinder, 10 cm₃ (2 ea.),
* Glass dropper.

Procedure
A. Sample preparation
Prepare 100 cm₃ of your working solution by a 10-fold dilution of the test solution provided. Use the 100 cm₃ volumetric flask and distilled water. The Fe³⁺, Zn²⁺ and Cr³⁺ ions content in the working solution would be within the concentration range of 0.01 – 0.03 mol dm^-3.

B. Determination of Fe 3+
Place 10.00 cm₃ of the working solution into a 200 cm₃ Erlenmeyer flask, add about 20 cm₃ of distilled water and adjust the pH to 1 by adding about 5 cm₃ of 1 mol dm^-3 HCl solution (check the pH value against the indicator paper). Finally, supplement 1 cm₃ of 5% aqueous solution of sulfosalicylic acid (the indicator) and mix thoroughly. Titrate the flask contents with 0.025 mol dm^-3 EDTA standard solution until the color changes from violet to yellow-green. Record the volume of the standard solution (V₁, cm₃). Repeat the titration as necessary.

C. Determination of Zn 2+
Adjust pH to 5 – 6 in the solution obtained in step B by adding 5 – 6 cm₃ of the acetate buffer solution, then add 3 – 5 drops of the PAN solution (the indicator), 2 cm₃ of ethanol (by cylinder) and mix thoroughly. Titrate the flask contents with 0.025 mol dm^-3 EDTA standard solution until the color changes from pink to yellow-green. Record the volume of the standard solution (V₂, cm₃). Repeat the titration when necessary.

D. Determination of Cr 3+
Direct titration of Cr³⁺ with EDTA solution is impossible because of the low rate of the complex formation. Thus, the method of a back titration is used: an excess of EDTA standard solution is introduced, and the unreacted EDTA is titrated with Cu²⁺. Supplement an excess of 0.025 mol dm^-3 standard solution of EDTA (20 cm₃) to the solution obtained in step C, mix thoroughly and boil the mixture for 5 min. Add 3 - 5 drops of the PAN solution (the indicator) to the cooled mixture and mix thoroughly. Titrate the flask contents with 0.025 mol dm^-3 CuSO₄ standard solution until the color changes from wine-red to blue-violet. Record the volume of the standard solution (V₃, cm₃). Repeat the titration as necessary.