GEORGE L. GILBERT
tested demonstrations Vanadium Ions as Visible Electron Carriers in a Redox System William D.
are'
Florida State University Tallahassee. FL32306 Wllfredo Resto
University of Florida Gainesville, FL 32611 The principle of oxidation and reduction indisputably ranks among the most important concepts in any general chemistry curriculum, and the teaching of this concept may be aided by the use of demonstrations. These demonstrations, more often than not, involve a change in the oxidation state of a transition metal with a corresponding color change, thereby allowing the students to "see" the oxidation and reduction taking place. Vanadium has four common oxidation states, ranging from 2+ to 5+, shown in the figure and in color on this month's cover. In acidic solutions, vanadium forms stable ions of these oxidation states, each of which exhibits a characteristic color [vanadium(II), viole$ vanadium(III), aqua; vanadium(IV),blue; vanadium(V),yellow], and, as such, has long been a favorite element for this type of demonstration (1,2),as well as being used in reductiometric titration laboratory exercises (33).The demonstration described here uses a column in which these four oxidation states coexist in a steady-state system, creating colored bands at each level of oxidation. In addition to being visually intriguing, it also is attractive from a pedagogical standpoint in that it canlead to discussions involving a wide range of topics including redox equations, electrochemistry, oxidation potentials, reaction intermediates, diffusion, and even biological systems. The redox system described below is roughly analogous to that which is utilized in some biological systems. Asimilar, albeit more complex, system employing electrons, hydrogen ions and a series of "electron carriers" is used to form ATP in photosvnthesis. It is also worth notine - that some organisms do employ vanadium physiologically. The blood of certain species of tunicates (sea squirts) contains vanadium in the 3+ and 4+ states. The reversible oxidatiodreduction ability of this metal had led many researchers to believe that it may be serving a role analogous to that of iron in higher animals. More recent research seems to refute this idea, however (6,7). Although the metal probably participates in some redox mechanism, its exact role remains a mystery. 'Corresponding author. 2~his reaction is actuallv much more cornolex . 110). . . because HCIO, can be fdrlher red~cedr;c12 and C -. The reative amo.nrs of tnese three prodds depends on pH and ternperatde. Tne eqLat on s written n the form aoove to lac1 tare oalanc~ng 692
Journal of Chemical Education
Denison University
Granville, OH 43023
Preparation Stock Solutions, Reagents 1L0.2MV02+in2MHzS04(Add 18.2gV205t0500mLH20.) Slowly add 100 mL concentratedH2S04and dilute to 1L.Add a stir bar and place on a stimngplate for 24 ta 48 h). 100 mL 1M NaC103 (Dissolve 10.6 g NaC103in HZO.Dilute ta 100 mL).
Zinc, mossy Zinc, amalgamated (Prepareby method 2 inVogel(81,substituting mossy zinc for zinc wool). Equimolar Solutions of V2+,V3+,VOzt (prepared as described below.) Fill a 250-mL Erlenmeyer flask nearly to the top (to exclude as much air as possible) with the V02+solution. Add a large stir bar and several pieces of mossy zinc. Cover with a one-hole stopper and place the flask on a stirring plate for several hours, decanting a portion of the solution a t each oxidation level. The yellow solution begins to turn green as the blue V02+is formed and mixes with the yellow VOz'. Decant the V@+ when it is the color of Windex. VG is an aqua blue-green, and Vf is violet. This method creates solutions that will contain some ions of other oxidation states, but because the demonstration is qualitative rather than quantitative, this method is adequate and appears to be the simplest. These ion solutions will remain reasonably stable if they are kept in a stoppered container with little air. The column is prepared from a 75cm piece of Pyrex tubine (approx. 15 mm ID) sealed at one end. Abulb is blown oGth&nd and is flattened so that it may sit on a stirring plate. Alternatively, this can be accomplished by fusing 60 cm tubingonto a 25 mL Erlenmeyer or flat-bottomed flask Procedure
After an introduction to the various vanadium species, the students may be asked to predict the result of mixing solutions of the various ions, and their reactions with zinc and the chlorate ion. Students may use standard potentials (see table) to make these predictions. The color changes that accompany the reactions will indicate the products of the reactions to confirm or refute their predictions. When the reaction products have been determined (assume C103-+ HC102),2the students can write balanced redox equations for each of the reactions, identifying the oxidizing and reducing agents in each. The following reactions give good color changes (although reactions 1and 4 are somewhat slow). Standard Potentials of Pertinent Reactions @25 O C
Half Cells C I W + 3Hi +2e-
tt
HClOz + H z 0
2n2++ 2e-++Zn(s) VOz+ + 2H+ + e- tt vo2++ H 2 0 VO" + 2Hi + e-wv3++Hz0
v3' + e-++ v2+
E "(volts)
1.21
4.76 1.OO
0.34 4.26
indicated by the yellow color of the vanadate ion. Between these two regions, the other oxidation states gradually appear a s vibrantly colored bands, as seen in the figure. These bands are fairly narrow at first and may be difficult to see in large lecture classrooms; however, they can be expanded by moving the stir bar to that region with a handheld marmet and manuallv mixine the solution in this area. The yellow color eventually will disappear completely as the vanadate species is reduced by electron-rich ions migrating up the column. This process can be reversed s the NaCl01 solution to the to^ of bv adding a few d r o ~ of the colm&. This fo-rms a light p e e n layer (a mixture of yellow V02+and blue V02+)that turns yellow after a few -minutes. Tbe system remains stable indefinitely provided there is always a supply of zinc a t the bottom of the column and chlorag ions 2 the top (a small amount of sulfuric acid may be added periodically to maintain acidity).
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Discussion In the column, reactions take place by diffusion (or perhaps "assisted diffusion"). This is evidenced by the green color that forms between the yellow vanadium(V) band and the blue vanadium(N)band. In this case, the diffusion results in a mixture of two species. Further down the column, however, this diffusion gives rise to the reactions listed above. As VOzi ions diffuse downward, Vf ions diffuse upward. Where these two ions meet, VOZ+is formed. All four of the reactions listed above can be seen occurring by diffusion in the wlumn. A ouestion mav arise as to the nature of the net reaction e reaction can be found by adding the ofthTs system. ~ i l net individual reactions ieqs 2 and 3 are doubled, which gives:
This photograph shows solutions of vanadium in oxidation states from +2 to +5 (leftto right vanadium(lV),vanadium(\/),vanadium(lll), vanadiumlllll in the flasks and containers on the left. The bubbles in the vanad/u+l) solution are caused by zinc (the reducing agent) reacting with the acidic medium. The column at the right contains all four of these species together in a steady-state system. The lowest oxidation state (c2)is on the bonom, and the highest state (+5)ison the top. The vanadium ionsact as electron carriers,transporting electrons through the column. The metal pieces atthe bottom of the column are zinc, the reducing agent. This illustration is shown in color on the cover of this issue. (Photo by Stephen Leukanech.)
Zn + 2V'+ + 2nZ++ 2vZ+
v2++ VO"
+2Ht+ 2v3++ HzO
v3++ VOZ1' 2v02+ 2v02++ C10,- + HzO + 2VOd + HCIO, + H'
(1) (2)
(3) (4)
Now the chemical playing field is altered by filling the Pyrex wlumn (leave about 10 cm a t the top to accommodate additional reagents) with the VOz+solution. A few small pieces of amalgamated zinc and a small (318 in. x 118 in.) stir bar are added to the column, and it is placed on a stirring plate supported by a ring stand and a clamp. Ask students to predict how the system will develop, then turn on the stirring plate and observe the column for several days. Alternatively,the column can be prepared a t least 48 h earlier to allow for immediate observation. The vanadium ions in the column are reduced slowly by the zinc, and after about a day, the purple color of the V% ion can be seen a t the bottom of the column. The limited mixing afforded by the small stir bar allows the vanadium a t the top of the column to remain in the +5 state as is
This shows that although the vanadium ions are in some way taking part in the chemical process (as evidenced by the color changes), they are not found in the net reaction. They serve only as intermediates, being continually consumed and produced, as they transport electrons through the column from the zinc to the chlorate ions. Although the movement of each individual ion is random, the reversible redox reactions of this system create a scenario in which the net movement of eleckrons is effectively methodic, with the vanadium ions serviw a s a "chemical conveyor belt". This set of reactions is approximate model-for some types of biological electron transport. Disposal Vanadium is a toxic metal and must be disposed of by a licensed company. The waste volume can be reduced by oxidizing or reducing the solution to the +4 state and precipitating with hydroxide at a pH range of 7-8 (9).
Acknowledgment This project was completed with the support of the National Science Foundation (Grant Number MDR-87Literature Cited 1. Ppacocke. T A. H . J. Chom. Educ. 1959.36.A-415. 2. Summerlin, L.R.; Ealy, J. L JI Chamlml hrnonsfmtions:A Soumbmh for %rh=WACS: Washington. DC, 1985: pp 10M08. 3. n e t % %H.R. J. Chrm.Educ. 1863,40,34444.5 4. Dauis,J.M.J. Chom. Educ. 1968,45,473. 5 . Hentr, E C.Jr: long,G. G. J Cham. Educ. 1918,55,5546.
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6. Sen0zan.N.M. J. Ckem.Edve 1914,52,50345. 7. Michibata, H.; Sakurai H.JnVonodium in Bido@col SysBms:Physiology ondBio chamislry:Chasteen, D.N., Ed.; Kluver Academic Pub.: Booton, 1990:ChapterIX 6. V0gel.A. I.Vog& lktboob o f h c t i m l OlgonicChemistw, 5thfh.;Wiley:New York, 1989; p 467.
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9. National Academy Resr.PrudududfPmefi-sfor Disposml o/Chemicolsfrom Loborofor&%National Academy Re-: Washingfon, DC, 1983. 10. Cottan,F.A.;Wikinson,G.AduonmdInoqanicChemlstry,5thed.: W L I ~ ~ : N ~ W Y O ~ ~ , 1989; p 567.
