Experiment 8. Determination of Iron in an Ore by Potentiometric Titration. Iron ores are often completely decomposed in hot concentrated HCl.

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1 Experiment 8 Determination of Iron in an Ore by Potentiometric Titration Introduction The common iron ores are hematite (Fe 2 O 3 ), magnetite (Fe 3 O 4 ), and limonite (3Fe 2 O 3 3H 2 O). Volumetric methods for the analysis of iron samples containing these substances consist of three steps: (1) dissolution of the sample Iron ores are often completely decomposed in hot concentrated HCl. Because iron(iii) tends to form stable chloride complexes, HCl is a much more efficient solvent than either sulfuric or nitric acid. Many iron ores contain silicates that may not be entirely decomposed by treatment with HCl. Incomplete decomposition is indicated by a dark residue that remains after prolonged treatment with the acid. A white residue of hydrated silica, which does not interfere in any way, is indicative of complete decomposition. (2) reduction of the iron to the divalent state Part or all of the iron will exist in the trivalent state in the dissolved sample; pre-reduction must therefore precede titration with the oxidant. Many reductants have been used in determinations of iron, including sulfites, SO 2, H 2 S, SnCl 2, and a variety of amalgamated and free metals. Reduction by zinc in the well-known Jones reductor is avoided because of the inconvenience and expense of maintaining the columns and because of the mercury usually included. The popular method of reduction with excess SnCl 2 and elimination of the excess with HgCl 2 is also 63

2 environmentally objectionable. Zinc is the reductant of choice because of the conceptual and practical simplicity of its use. In this experiment, the reduction of iron(iii) to iron(ii) is carried out in three stages in a small volume of solution strongly acidified with HCl. A high concentration of HCl accentuates the yellow of the iron(iii) to make the progress of reduction easily observed. Since the HCl is exhausted in each stage by reaction with zinc, it is replenished in each following stage. The reintensified provides a visual indicator until reduction is complete. Unreacted zinc is subsequently eliminated by reaction with H 2 SO 4. (3) titration of iron(ii) with a standard oxidant. Potassium permanganate, KMnO 4, is widely used as an oxidizing agent in volumetric analysis. MnO H + + 5e - Mn H 2 O E 0 = 1.51 V In 1 M HClO 4, the formal potential is just 1.70 V. In 1 M HNO 3, it is 1.61 V. The half-reaction shown above for the permanganate ion occurs only in solutions that are 0.1 M or greater in strong acid. In less acidic media, the product(s) may be Mn (III), Mn (IV), or Mn (VI) depending on conditions. The oxidation of iron(ii) with permanganate is based on the reaction: 5 Fe 2+ + MnO H + 5 Fe 3+ + Mn H 2 O Often, this reaction is performed in the presence of moderate concentrations of HCl. But the permanganate ion cannot be used with hydrochloric acid solutions for the slow oxidation of chloride ion, this can be eliminated through removal of the 64

3 hydrochloric acid by evaporation with sulfuric acid or by introduction of Zimmermann-Reinhardt reagent, a mixture of manganese(ii), concentrated sulfuric and phosphoric acids. Because permanganate ion is violet in and manganese(ii) ion is nearly less, no additional indicator is needed for this titration, when one drop excess of potassium permanganate has been added to the sample, the endpoint can be taken as the first pale red/pink. Alternatively, reactions can be monitored with Pt and calomel electrodes. Reagents 1. Potassium permanganate 2. Concentrated hydrochloric acid 3. Concentrated sulfuric acid 4. Zimmermann-Reinhardt reagent Procedure A. Preparation of 0.02 M potassium permanganate solution Measure about 55 ml of standardized potassium permanganate solution. B. Determination of iron in an ore (1) Dissolution of the sample: 1. Dry the ore sample for at least 3 hours at 105 to 110 o C, and cool in a desiccator. Weigh accurately a 0.30 g portion of ore sample into a 250-mL Erlenmeyer flask. 65

4 2. Add 8 ml of concentrated HCl. Heat nearly to boiling in hood until the ore is dissolved, about 5~15 minutes. Some white residue of silica may remain and can be ignored. If evaporation reduces the volume significantly, add additional HCl; that is, iron oxide is reprecipitated. Boil the solution gently for a few minutes to expel as much HCl (g) as possible (Note 1). (2) Reduction of the iron to the divalent state: 1. Cool briefly, and add slowly ~1.5 g of zinc metal in hood. Heat or cool the flask as necessary while swirling it gently, to maintain a vigorous but not violent reaction, until the reduction is complete (Note 2). 2. When the solution appears to be completely reduced, add another 0.75 g of zinc and heat nearly to boiling in hood for ~ 5 minutes. 3. Add 15 ml of distilled water and 3 ml of concentrated H 2 SO 4. Mix well. Add 12 ml of Zimmermann-Reinhardt reagent and stir the solution until the remaining zinc is dissolved. 4. Transfer into a 100-mL volumetric flask. and wash the residue in the beaker with distilled H 2 O. Dilute the solution to the mark, and mix thoroughly. (3) Titration of iron(ii) with a standard oxidant: 1. For the rough titration, pipet 20.0 ml aliquots of the sample solution into a 250-mL Erlenmeyer flask. Titrate immediately with standard ~0.02 M KMnO 4 solution until the pink persists for 15 s. 2. Then comes the careful titration, pipet 40.0 ml of the sample solution into a 100-mL beaker containing a magnetic stirring bar. Position the Pt combination electrode in the solution so that the stirring bar will not strike the electrode. Set 66

5 the ph meter to measure potential rather than ph. 3. Titrate immediately with standard ~0.02 M KMnO 4 solution. The equivalence volume will be about two times greater than it was in Step 1 (2V 1 ). Firstly add 2.00 ml of MnO - 4 solution from the buret, after mix well, record the volume, potential and. When you are within 1 ml of the indicator end point (2V 1-1), add titrant in 0.1 ml increments. Continue with 0.1 ml increments until you are 1.0 ml past the indicator end point (2V 1 +1). The equivalence point has the most rapid change in potential. Add three more 2.0 ml aliquots of titrant and record the potential after each. 4. Make the following plots of the data: (1) potential versus titrant volume (2) ΔE/ΔV vs. average titrant volume. From the equivalence volume (V e ) of the titration curve, determine the wt% of iron in the ore sample. Notes 1. The HCl concentration cannot drop much below 6 M, but that does reduce somewhat the amount of zinc required for reduction. 2. The blue-green hue of iron(ii) will be apparent in a solution that is completely reduced; a solution that appears green with a trace of yellow will require an additional portion of zinc. References l. Skoog, West, Holler and Crouch, Fundamentals of Analytical Chemistry, 8 th ed., 67

6 Chap. 20, pp D.C. Harris, Quantitative Chemical Analysis, 7 th ed., Chap Skoog, West, Holler, Crouch, and Chen, Introduction to Analytical Chemistry, 2011, Chap. 11, 12, 13A, & 13B 68

7 Data A. Preparation of 0.02 M potassium permanganate solution The Molarity of KMnO 4 : M B. Determination of iron in an ore (1) Dissolution of the sample: Unknown No. Weight of sample (g) g (3) Titration of iron(ii) with a standard oxidant: 1. Rough titration Initial buret reading Final buret reading Volume of KMnO 4 (V 1 ) ml ml ml 69

8 3. Careful titration Estimated volume of end point for 40 ml unknown sol n (2V 1 ) ml volume of indicator end point (V i of step 3) ml 70

9 Data analysis 1. Construct a graph of potential(y-axis) versus titrant volume(x-axis). Make on your graph where the indicator change was observed. 71

10 2. Compute the first derivative (the slope, ΔE/ΔV) for each data point within ± 0.5 ml of the indicator end point volume (V i of step 3). Construct graph of ΔE/ΔV v.s V avg. E/ V (mv/ml) V avg (ml) Graph of E/ V versus V avg : 72

11 3. From the equivalence volume (V e ) of the titration curve, determine the wt% of iron in the ore sample. wt% of Fe in the ore sample % Accurate value of %Fe % Relative error % Discussion 73

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