Master alloys are formed by mixing a base metal such as Al, Ni or Cu with a high percentage of one o — Physical Chemistry — Kinetics Chemistry Question
Determination of iron and copper by iodometric titration
Master alloys are formed by mixing a base metal such as Al, Ni or Cu with a high percentage of one or two other metals. Master alloys are widely used in industry as semi-finished products. In metallurgical plants, master alloys are added to other molten metal mixtures for some purposes such as alteration of the composition to achieve certain chemical, electrical or mechanical properties in the final product.
In this experiment, a sample solution of master alloy containing iron and copper ions will be analyzed by a two stage titrimetric method. First, the amount of Fe(III) ions will be determined by precipitation with pyrophosphate in acidic solution and then, the amount of Cu(II) will be calculated from the total amount of ions present in solution determined by an indirect iodometric titration with standard sodium thiosulfate solution.
Chemicals and reagents
* Test solution (c = 0.10 mol dm-3) simulating a digested sample of alloy containing both Fe3+ and Cu2+ ions in 4 - 6 g dm-3)
* Sodium thiosulfate standard solution, Na2S2O3, 0.050 mol dm-3
* Sodium pyrophosphate solution, Na4P2O7, 5.0 % (w/v)
* Hydrochloric acid solution, HCl(aq), 4.5 mol dm-3
* Potassium iodide solution, KI, 10 % (w/v)
* Starch solution, 5.0 % (w/v).
Apparatus and glassware
* Burette, 50 cm3
* Graduated cylinder, 50 cm3
* Erlenmeyer flask, 250 cm3 (2)
* Pipettes, 5 cm3 and 10 cm3
* Watch glasses (2)
PROCEDURES:
A. Determination of copper(II) ion
1. Transfer 10.0 cm3 of the test solution into a 250 cm3 Erlenmeyer flask, add 50 cm3 water and mix thoroughly.
2. To the same flask add 20 cm3 of 5.0% (w/v) pyrophosphate, 5.0 cm3 of 4.5 mol dm-3 HCl and 40 cm3 of 10% (w/v) KI. When pyrophosphate is added a precipitate may form.
3. Close the flask with a watch glass and leave in dark for 3 – 5 min for the formation of white precipitate.
4. Titrate the content of the flask with standard 0.020 mol dm-3 Na2S2O3 until a pale yellow color is obtained.
5. At this point, add 5 cm3 of starch indicator (5% w/v) and titrate until the color of solution changes from dark blue to milky white.
6. Record the volume of sodium thiosulfate solution added.
B. Determination of total amount of copper(II) and iron(III) ions
1. Transfer 10.0 cm3 of the test solution into 250 cm3 Erlenmeyer flask, add 50 cm3 water and mix thoroughly.
2. To the same flask add 2 cm3 of 4.5 mol dm-3 HCl and 40 cm3 of 10% (w/v) KI into the solution and mix thoroughly.
3. Close the flask with a watch glass and leave in dark for 3-5 min. A small amount of white precipitate may be observed.
4. Titrate the solution with a standard 0.050 mol dm-3 Na2S2O3 until a pale yellow color is obtained.
5. At this point, add 5.0 cm3 of 5.0 % (w/v) starch indicator and titrate until the color of the solution changes from dark blue to milky white.
6. Record the volume of sodium thiosulfate solution added.
Write the equations for the titration processes.
Model Answer
2 Fe3+(aq) + 2 I– (aq) ⇌ 2 Fe2+(aq) + I2(aq)
I2(aq) + I– (aq) ⇌ I3– (aq)
I3– (aq) + 2 S2O3 2–(aq) ⇌ 3 I– (aq) + S4O6 2–(aq)
2 Cu2+(aq) + 4 I– (aq) ⇌ 2 CuI(s) + I2(aq)
2 S2O3 2–(aq) + I2(aq) ⇌ S4O6 2–(aq) + 2 I– (aq)
2 Cu2+(aq) + 2 S2O3 2–(aq) + 2 I–(aq) ⇌ 2 CuI(s) + S4O6 2–(aq)
Explain why the solution is acidified.
Model Answer
Reaction proceeds quantitatively in neutral or slightly acidic solutions. In strongly alkaline or acidic solutions the oxidation of the thiosulfate does not proceed by a single reaction. In the former, the thiosulfate ion is oxidized to sulfate as well as to the tetrathionate. In the latter, the thiosulfuric acid formed undergoes an internal oxidation-reduction reaction to sulfurous acid and sulfur. Both of these reactions lead to errors since the stoichiometry of the reactions differs from above reactions. In many cases the liberated iodine is titrated in the mildly acidic solution employed for the reaction of a strong oxidizing agent and iodide ion. In these cases the titration of the liberated iodine must be completed quickly in order to eliminate undue exposure to the atmosphere since an acid medium constitutes an optimum condition for atmospheric oxidation of the excess iodide ion.
Explain why starch is added close to the end of titration.
Model Answer
Without starch usage, the end point of titration may not be observed. Because color of solution is very pale yellowish color in the absence of starch. Therefore end point is harder to observe when the concentration of iodine.
Calculate the amounts number of moles of Cu2+ and Fe3+ ions present in the test solution.
Model Answer
Common stoichiometric calculations should be used for obtaining the required results. The calculations are based on chemical equations in 35.1..
Calculate the mass ratio of Cu2+ and Fe3+ ions.
Model Answer
Common stoichiometric calculations should be used for obtaining the required results. The calculations are based on chemical equations in 35.1..