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Experiment 3
Separation of the Oxidation States of Vanadium
Prelab Assignment Calculate how to dilute 6 M HCl with distilled water to make 65 mL of 3 M HCl and 50 mL each of 1.0 M and 0.4 M HCl. Introduction The transition element vanadium has four common oxidation states: V 5+ , V 4+ , V 3+ , and V2+. The syntheses and separation of these are investigated using ion exchange chromatography in this experiment. Starting with V 5+ in the form of the ionic compound ammonium metavanadate (NH 4 VO 3 ), a series of reductions is carried out to generate the lower oxidation states of vanadium. The first reduction is carried out in an acidic solution (HCl) to yield vanadium(IV) in the form of the vanadyl ion, VO 2+ , according to the reaction scheme below. VO 3
- (aq) + 4 H + + e - VO 2+ (aq) + 2 H 2 O (1) A second reduction with zinc amalgam yields [V(H 2 O) 6 ]3+^ and [V(H 2 O) 6 ]2+. VO2+^ (aq) + 2H+^ + e-^ V3+^ (aq) + H 2 O (2) V 3+ (aq) + e
V 2+ (aq) (3) Each oxidation state of vanadium exists as an aquo complex, where H 2 O or OH¯ ligands fill the coordination shell of vanadium in a tetrahedral or octahedral complex. Under our experimental conditions, the species in solution are as follows: oxidation state short-hand notation complex V5+^ VO 3 ¯ [V(OH)O 3 ]^2 -^ (contains three V=O bonds) V 4+ VO 2+ [V(H 2 O) 5 O] 2+ (contains one V=O bond) V3+^ V3+^ [V(H 2 O) 6 ]3+ V2+^ V2+^ [V(H 2 O) 6 ]2+
These oxidation states are readily distinguishable by their different colors, as the VO2+^ ion gives blue aqueous solutions, while the V(H 2 O) 6 3+ and V(H 2 O) 6 2+ ions give green and violet colored solutions, respectively. Ion exchange: Ion exchange chromatography uses a charged resin that binds oppositely charged ions. In the present lab, you will use a cation-exchange resin that is composed of an inert polymer functionalized with sulfonate (-SO 3 ¯ ) functional groups. By equilibrating the resin with HCl, you ensure that no cations other than H
are initially present on the column, bound to the SO 3
- groups. The cations we hope to separate, such as VO 2+ (which is really [V(H 2 O) 5 O] 2+ ) compete with H+^ for the negatively charged sulfonate, and the most highly charged positive ions take the longest time to pass through the column. The concentration of H
will affect the rate of travel through the column, as high [H
] will tend to displace other cations from the sulfonate binding sites. Please remember that the charge of the molecular species (the coordination complex) is NOT the same as the oxidation state of vanadium.
the resin into the beaker. Do not throw the resin away, but dispose of it in the ceramic bowl in the waste hood. After allowing the slurry to settle, add about 15 mL of 3 M HCl to the column, elute the acid, and then rinse the column with two volumes of distilled water (~20 mL each). If the eluate is from the column is not clear, wash the resin with more distilled water. The addition of HCl assures that H+^ is the only cation bound to the column, and the washing with water removes any excess, unbound acid. The column will contract when you add acid, but will re-expand upon washing with water. Suggest in your notebook why this occurs. During the settling, acid elution, washing, etc., obtain ~25-30 test tubes and a rack for collecting fractions. In separate flasks, make up 50 mL each of 3M, 1M and 0.4 M HCl by diluting a stock 6M HCl solution (label them to avoid confusion). It is OK to use the approximate (~10% accuracy) gradations of your glassware for measuring your water and HCl, as the acid concentrations do not need to be known to a high degree of precision. Separation of VO 3 ¯ and VO 2+ In the hood , add 2.0 mL of concentrated HCl (not 6M HCl) to 200 mg (1.71 mmol) of NH 4 VO 3 in a 150-mm test tube. Swirl to dissolve. The solution should not be slightly blue, and must be re-made if it is. Heat the mixture to boiling over a Bunsen microburner for about four minutes. Keep the test tube pointed away from yourself and persons near you! To prevent bumping, do not heat the test tube at the bottom, but rather heat along the edges while gently moving the tube. After four minutes, stop heating the solution and dilute it with 10 mL of distilled water, mixing it well by swirling. The solution should change from its original color to a bright green color. VO 3 ¯ is yellow, and VO 2+ is electric-blue – the solution is green, as it is a mixture of these two vanadium oxidation states. Pour the bright green solution onto the cation exchange column. A clear separation on the column into a yellow VO 3 ¯ band and a blue VO2+^ band need not be seen. It is the different
concentrations of acid that separate the compounds and remove them from the column, not the resolving power of the column itself. After applying the vanadium solution to the column, begin to elute and collect 3 mL fractions. Save the very first fractions that come off the column, since a yellow VO 3 ¯ fraction elutes quite quickly even at the low acid concentration of the column. Do not collect aliquots greater than 3 mL; to estimate how much solution to elute as one fraction, put 3 mL of distilled water in a test tube, and collect fractions that are approximately that volume. Do not count drops to estimate 3 mL. The yellow VO 3 ¯ fractions are quite dilute and may appear clear. Hold them against a white background to more effectively gauge their color. Probably not many aliquots of yellow will be eluted. If the yellow fractions start to become tinged with blue and appear faintly green, collect a few fractions to count as "yellow" VO 3 ¯, but stop collecting fractions as soon as the green color becomes more apparent. Only collect the first few fractions that appear faintly green to count as "yellow" VO 3 ¯. Now allow the level of liquid to fall to close to the top of the resin, and then add 0.4 M HCl as necessary to keep the burette full during elution. This will more efficiently remove yellow VO 3 ¯, so collect more yellow fractions. When the fractions become more than faintly green (as discussed above), add a 1.0 M HCl solution to the top of the column. This greater concentration of H
will remove the blue VO2+. The first few fractions that are eluted may be blue-green, and should be set aside (do not throw them away). Continue collecting 3 mL blue fractions. The total number of fractions you collect may consist of ~9 mL of yellow and ~24 mL of blue solution, with fractions of yellow- green and blue-green in between. However, you may find that your total number of fractions deviates greatly from these. This is not a problem. When you have finished eluting, record the number and color of the fractions you collected. Make sure the top of the column is still wet, close the burette, and seal the top with a stopper. You will use the column again in separating the V+3^ and V+2. Combine your bluest VO 2+ fractions into one container and the darkest yellow VO 3 ¯ fractions into another container, but avoid combining any fractions that are definitely yellow-
some possible reasons for it. Also, if after the purple V+2^ has been removed, a pale blue fraction elutes, count this as the green V 3+ fraction (not as the purple V 2+ ). Record the number and color of fractions collected, and combine some individual fractions in the same manner as in the first separation. Keep approximately 2 mL of the best V+ and V
solutions for titrations, as you did above. After carrying out your titrations, regenerate the column to its original H+^ concentration by washing 3.0 M HCl through it until the eluate is clear, and then passing 20-30 mL of water over the column. Remove the resin from the burette by pouring it out into a large beaker, and decant as much of the water/acid solution as possible off of the resin into the acid waste container. Dispose of the resin in the ceramic bowl in the waste hood. If you have inadvertently poured zinc amalgam onto your column, dispose of the resin in the mercury waste. Characterization of the Different Vanadium Oxidation States. KMnO 4 (potassium permanganate) is a strong oxidizing agent, which will oxidize vanadium to its maximum oxidation state. Titrate a fraction from each vanadium complex with KMnO 4 to determine the concentration of vanadium. Place precisely 1.0 mL of each of the four eluted solutions in four different test tubes, and add 0.0010 M KMnO 4 dropwise (via a Pasteur pipette) to each tube while shaking or swirling the tube continuously. Record the exact number of drops needed to give the first persistent trace of the pale pink KMnO 4. (In some cases, tens of drops of KMnO 4 may be needed, while in others, many fewer. Do this addition carefully in order to catch the true endpoint of the titration, and thus obtain accurate oxidation state data.) Hold the test tube against a white background during the titration to see the first indication of pale pink. At the true endpoint of the titration, the solution should remain pale pink for 30 - 60 seconds even though you are shaking the tube. The pink color will fade after this time because of further reactions of the permanganate. So, only add enough KMnO 4 to maintain the pink color for 30 - 60 seconds with shaking of the tube.
If the solution begins to turn pale orange during the titration, this usually means that just a few more drops of KMnO 4 are necessary. The endpoint of the titration is reached when the solution remains very pale orange or pink-orange for 30 - 60 seconds while shaking. In your lab report Data Analysis section you will calculate the concentrations of the different vanadium complexes. Table 1. Reduction Potentials relevant to this lab. Reaction E^0 (V)
MnO 4
+ 8H + + 5e - Mn 2+ + 4 H 2 O 1.
Cl 2 + 2e
2Cl - 1.
- VO 3 -^ + 4H+^ + e-^ VO2+^ + 2H 2 O 1.
- VO2+^ + 2H+^ + e-^ V3+(aq) + H 2 O 0.
- V 3+ (aq) + e - V 2+ (aq) - 0.
- Zn 2+ + 2e - Zn(Hg) - 0. Data Analysis
- (7 points) For each reaction that you carried out in this experiment, write a balanced equation indicating the oxidation state of the vanadium species and its color. Since these reactions are reductions, you will need to use half-reactions to write the balanced equations. Refer to the table of half-reactions given above.
- (9 points) Write balanced equations showing the various reactions with the oxidant KMnO 4 , which re-oxidizes vanadium to V+5. Show the oxidations from one oxidation state to the next as separate redox reactions, until you reach V
- Use the table of half-reactions given above.
- (9 points) Calculate the concentration of the V+4^ , V+3, and V+2^ complexes, using the approximation that each drop of 0.0010 M KMnO 4 is 0.025 mL in volume.
