5- Lab5- Permanganate And Dichromate Titration 306q5s

Chem 223:

Experiment 5

Redox Oxidation with Potassium Permanganate and Dichromate- Determination of Iron in an Ore. References: Harris: Chap. 16. Harvey. 9D Kealy: pp. 90-94 This experiment is partly based on the experiment 7- Chem 1502-Villanova University and Expt. 31- Chem 10 – UWI- Mona University.

A. Redox Titration with Potassium Permanganate. I. Introduction: In this experiment, the percentage of iron in an unknown iron oxide sample is determined by titration with a standard solution of potassium permanganate. The overall reaction is: MnO4+ 8 H+ + 5 Fe2+ Mn2+ + 5 Fe3+ + 4 H2O To ensure that all of the iron in the unknown is in the Fe2+ state prior to titration, the unknown is treated with tin(II) chloride, which reduces any small amount of Fe3+ that might be present: Sn2+ + 2 Fe3+ Sn4+ + 2 Fe2+ The excess Sn2+ is then removed by reaction with mercury(II) chloride, which produces insoluble mercury(I) chloride: Sn2+ + 2 HgCl2 Sn4+ + Hg2Cl2(s) + 2 Cl The potassium permanganate is previously standardized against sodium oxalate in acid solution: 2 MnO4 + 5 H2C2O4 + 6 H+ 2 Mn2+ + 10 CO2 + 8 H2O Potassium permanganate acts as its own indicator both in the standardization and in the iron determination. II. Reagents Provided: * Prepared 0.02 M KMnO4 solution as follow: Dissolve about 3.2 g of KMnO4 in 1 L of deionized water using a large beaker. Cover that beaker with a watch-glass and heat to boiling (using a hot plate in the hood first) to destroy reducible substances (traces of organics remaining in deionized water). Keep the solution at a gentle boil for about 1 hr. Let the solution stand overnight. Remove MnO2 by filtering through a filter crucible. Transfer the solution to a clean amber glass-stoppered bottle; store in the dark when not in use. If the solution is kept in the dark and is not acidified, its concentration will remain stable for several weeks. Important: clean the filter and beakers using a solution of sodium bisulfite (use about 1g in 400 mL of water. Store this solution in an amber bottle).

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Prepare 1 L of 1 M H2SO4 by slowly adding 60 mL of concentrated acid to about 700 mL of water then diluting to 1 L.

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Dry about 1.5 g of primary-standard sodium oxylate, Na2C2O4, at about 110 °C for at least 1 hr . Cool in the desiccator.

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Dry the unknown iron ore at 1100C for at least 3 hr . Keep it in the desiccator.

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A fresh solution of the SnCl2 reducing agent needs to be prepared each lab session. Dissolve 6 g of SnCl2 H2O in 10 mL of concentrated HCl, using a 100 mL volumetric flask. After the SnCl2 dissolves, dilute to volume. Keep this solution tightly sealed to prevent air oxidation. Mercury(II) chloride, 5% (w/v).: Dissolve 50 g of HgCl2 in 1 L of distilled water. Zimmerman-Reinhard reagent or "preventive" solution: Dissolve 300 g of MnSO4. 4 H2O in 1 L of water. Cautiously add 400 mL of concentrated H2SO4, 400 mL of 85% H3PO4, and dilute to 3 L.

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III. Experimental procedure 1. Standardization of potassium permanganate solution: The standardization and the determination of the unknown should each be based on at least three titrations. After your first determination, it is preferable to use the procedure described in Note. Procedure: Weigh about 0.2 g Na2C2O4 (to the nearest 0.1 mg) and put a sample into a 400 mL beaker. Dissolve each sample in about 250 mL of 1 M H2SO4. Heat each solution to 80 - 90 °C, and titrate with KMnO4 while stirring with a thermometer. The pink color imparted by one addition should be permitted to disappear before further titrant is added. The temperature should not drop below 60 °C. The end point is marked by the appearance of a faint pink color that persists at least 30 s. Determine a blank by titrating an equal volume of the 1 M H2SO4. Correct the titration data for the blank, and calculate the concentration of the permanganate solution. Notes: 1. Add the permanganate directly into the oxylate solution (not down the walls of the beaker). Promptly wash down any KMnO4 that spatters on the walls of the beaker into the bulk of the liquid using a wash bottle. 2. Finely divided MnO2 will form if the KMnO4 is added too rapidly and will cause the solution to acquire a faint brown discoloring. This is not a serious problem if sufficient oxylate remains to reduce the MnO2 to Mn2+; simply discontinue the titration until the brown color disappears. 3. The surface of the permanganate solution rather that the bottom of the meniscus can be used to measure titrant volumes. While you may use either the meniscus or the top of the solution, you must be consistent, always making the same choice. 4. Partial decomposition of the permanganate to MnO2 may occur if it remains in the buret for a long time. Clean the buret with a dilute sodium bisulfite solution. 2. The determination of iron in an ore: Use a first sample size of about 0.5 g if your unknown is < 30% iron and 0.4 g if > 30% iron.

Sample preparation. Weigh a sample into a 100 mL glass beaker. Add 10 mL of concentrated HCl. Cover the flask with a small watch glass. Heat the solution in the hood at just below boiling until the sample is dissolved, about 5-10 min. A white residue of hydrated silica may remain. . All sample solution should be transferred to the 100 mL volumetric flask (solution A). This solution is also used for the redox titration with K2Cr2O7. Reduction with tin(II) chloride. Take 10.00 mL of solution A to the 250 ml conical flask (individually through this step to avoid air-oxidation of iron(II)). Adjust the sample size to about 15 mL by dilution. Heat the solution nearly boiling. Add sufficient KMnO4 solution to impart a faint yellow color to the solution (thus ensuring that there is not a large excess of tin(II)). Now add the SnCl2 drop by drop until the color changes from yellow to colorless or a very light green; then add two more drops. Cool to room temperature, and rapidly add 10 mL of 5% HgCl2 solution. A small amount of silky white Hg2Cl2 should precipitate. An absence of precipitate indicates that insufficient SnCl2 was used and the reduction of iron(III) was incomplete. A gray residue indicates the presence of elemental mercury resulting from the use of too much SnCl2. If no precipitate forms or if the precipitate is gray or black, the trial must be discarded. Titration. Wait 2 to 3 minutes after adding the HgCl2. Then add 10 mL of ZimmermanReinhard reagent (Caution! This is a caustic mixture containing concentrated sulfuric acid and phosphoric acid.) and 100 mL of water. Titrate immediately with standard KMnO4 to the first faint pink that persists for 15 to 20s. Do not add the KMnO4 rapidly at any time. Correct the titrant volume for the blank if it has been determined. B. Determination of iron using potassium dichromate: Redox indicators I. Introduction As an oxidant, dichromate has some advantages over permanganate, but, as it is less powerful, its use is much more limited. Furthermore, Cr(VI) is thought to be a carcinogen, so any use of dichromate is discouraged so that the Cr(VI) burden in the environment is minimized. It is obtainable in a state of high purity and can be used as a primary standard. Solutions of dichromate in water are stable indefinitely. The half reaction for the dichromate system is: Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O

E° = 1.33 V

The most important application of dichromate is in its reaction with iron(II) in which it is often preferred to permanganate. The relevant half reaction is : Fe2+ → Fe3+ + e-

E° = -0.77 V

and the total reaction is: Cr2O72- + 6 Fe2+ + 14H+ → 2Cr3+ + 6 Fe3+ + 7H2O

E° = 0.56 V

Unlike permanganate, dichromate titrations require an indicator. There are three indicators that may be used for the titration of Fe2+ with K2Cr2O7. These are diphenylamine, diphenylbenzidine and diphenylamine sulfonate. The colour change for all three indicators is green to violet and the standard electrode potentials are all ca 0.78 V. According to Kolthoff and Sandell, this should lie between the electrode potentials of the two reduction reactions.

This not being the case, phosphoric acid is added to reduce the electrode potential for the Fe3+ → Fe2+ reaction by stabilizing the ferric ion. 3+

As the titration proceeds, the sample solution will turn green due to the presence of Cr . The endpoint is reached when the very fine yellow color (at the beginning of titration curve) of the 6+

Cr titrant appears (at the end of titration curve). II. Reagents: Provided: - Potassium dichromate can be used as a primary standard if it is dried in an oven at o

150- 200 C for two hours to remove any bound water. - Zinc metal; conc. H2SO4; conc. H3PO4 To be prepared: Prepare a standard dichromate solution by dissolving an accurately weighed sample of about 0.4 g in water and make up to 250 mL in a volumetric flask. Calculate the concentration of dichromate solution in mol.L-1. (FW of K2Cr2O7 = 294.19). III. Experimental procedure The reduction step: Add about 1 g of zinc metal in 10.00 mL of solution A, replacing the watch glass immediately. You should carry out titrations with 3 portions of solution A. Do not add all of the zinc at once since this may reduce the Fe3+ to metallic iron. Heat the flask as necessary while swirling it gently, to maintain a vigorous but not violent reaction until all of the zinc is consumed. A few flecks of non-reactive black residue may be ignored. Add a second 1 g of zinc in 3 portions and heat the solution as necessary. Repeat the addition of zinc in smaller portions until reduction is complete. Usually this means that the solution is clear with no trace of yellow. This reaction is, in essence, a titration of the Fe3+ with Zn(s) . Adding too much extra zinc metal will greatly increase the time required to dissolve all of the excess zinc. (In concentrated Fe2+ solutions, a very light green hue of iron(II) may be apparent in a solution that is completely reduced; a solution that appears green with a trace of yellow will require a small additional portion of zinc. In addition, a solution that is too low in volume may appear to show a yellow-green color — add some distilled water and see if the color of the solution turns clear, a sign that all of the iron(III) has been reduced.) When the solution appears to be completely reduced, add another 0.1 g of zinc and heat nearly to boiling for about 5 min. (Approximately 3 g of zinc will suffice for reduction of 0.4 g samples dissolved as described above. The final portion of zinc should not be added until one is ready to complete that particular titration, since iron(II) undergoes air oxidation. If any zinc remains undissolved, add 5 mL of concentrated H2SO4 in 20 mL of water, and swirl the flask until the remaining zinc is dissolved. Very fine bubbles of H2 will emanate from any undissolved zinc as opposed to the larger bubbles of a boiling solution. Set your reaction mixture off the hotplate and allow the boiling to stop to check for any "fizzing" unreacted zinc. Titration with K2Cr2O7 After waiting only about 2 to 3 minutes, add to the pre-reduced solution, about 5ml of concentrated sulfuric acid and 7ml of syrupy phosphoric acid (measured in a graduated cylinder). Dilute with distilled water to bring the volume to about 125ml. Again cool the

solution to room temperature by running the outside of the flask under cold water. Add 8 drops of diphenylamine sulfonate indicator and slowly titrate with your standard K2Cr2O7 solution from a blue-green, through a greyish tinge to the first permanent violet, which is the end point. The titration should be conducted dropwise when the grey tinge is noted because the oxidation of the indicator is somewhat slow at this point. Do a trial run first to see the end point and determine the sample weight. Then do three good trials. The average standard deviation should not exceed 0.02 mL. Blank There is a significant indicator blank for this titration. To run a blank, all the reagents except for the iron unknown should be added to a 250ml Erlenmeyer flask and the volume should then be brought up to the approximate volume at the end point using distilled water. the actual end point change is slow. The blank should be no more than 1 to 4 drops (about 0.20ml) and its value needs to be subtracted from the volume of K2Cr2O7 required to titrate each iron sample. Do the blank twice. NOTE: Solutions containing Cr or Hg should not be poured down the sink, since these metals are highly toxic. Dispose of these solutions in the waste containers that are provided. FOR YOUR REPORT 1. How else could you determine the end points in dichromate and permanganate titrations? 2. A solid is known to contain iron(III). Outline how you could determine the iron content using a redox titration. 3. In other research, the scientists use the above methods to determine iron in natural magma using concentrated sulphuric and hydrofluoric acids for sample digestion. Explain the role of HF in this experiment. What kind of beaker should be used? 4. Report the % Fe found in the unknown for each determination, the mean value, standard deviation, and relative standard deviation