tested demonstrations GEORGE L. GILBERT

Denison University Granville, OH 43023

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 demon- strations, more often than not, involve a change in the oxi- dation 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 solu- tions, vanadium forms stable ions of these oxidation states, each of which exhibits a characteristic color [vana- dium(II), viole$ vanadium(III), aqua; vanadium(IV), blue; vanadium(V), yellow], and, as such, has long been a favor- ite 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 discus- sions 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. Asimi- lar, albeit more complex, system employing electrons, hy- drogen 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 oxida- tiodreduction ability of this metal had led many re- searchers 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 met- al probably participates in some redox mechanism, its ex- act role remains a mystery.

'Corresponding author. 2~his reaction is actuallv much more cornolex 110). because HCIO, . . .

can be fdrlher red~ced r;c12 and C -. The reative amo.nrs of tnese three prodds depends on pH and ternperatde. Tne eqLat on s writ- ten n the form aoove to lac1 tare oalanc~ng

Preparation

Stock Solutions, Reagents 1 L0.2MV02+in2MHzS04(Add 18.2gV205t0500mLH20.) Slowly add 100 mL concentrated H2S04 and 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 NaC103 in HZO. Dilute ta 100 mL). Zinc, mossy Zinc, amalgamated (Prepare by method 2 inVogel(81, substi- tuting mossy zinc for zinc wool). Equimolar Solutions of V2+, V3+, VOzt (prepared as de- scribed below.)

Fill a 250-mL Erlenmeyer flask nearly to the top (to ex- clude 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 cre- ates solutions that will contain some ions of other oxida- tion states, but because the demonstration is qualitative rather than quantitative, this method is adequate and ap- pears 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 tub- ine (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 poten- tials (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 predic- tions. When the reaction products have been determined (assume C103-+ HC102),2 the students can write balanced redox equations for each of the reactions, identifying the oxidizing and reducing agents in each. The following reac- tions give good color changes (although reactions 1 and 4 are somewhat slow).

Standard Potentials of Pertinent Reactions @25 O C

Half Cells E "(volts)

C I W + 3Hi +2e- tt HClOz + H z 0 1.21

2n2+ + 2e-++ Zn(s) 4 .76

VOz+ + 2H+ + e- tt vo2+ + H 2 0 1 .OO

VO" + 2Hi + e-wv3++ Hz0 0.34 v3' + e-++ v2+ 4.26

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This photograph shows solutions of vanadium in oxidation states from +2 to +5 (left to 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) re- acting 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 elec- trons through the column. The metal pieces atthe bottom of the col- umn 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+ (1)

v2+ + VO" +2Ht+ 2v3+ + HzO (2)

v3+ + VOZ1' 2v02+ (3)

2v02+ + C10,- + HzO + 2VOd + HCIO, + H' (4)

Now the chemical playing field is altered by filling the Pyrex wlumn (leave about 10 cm at the top to accommo- date 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 at 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 at the bottom of the column. The limited mixing afforded by the small stir bar allows the vanadium at the top of the column to remain in the +5 state as is

indicated by the yellow color of the vanadate ion. Between these two regions, the other oxidation states gradually ap- pear as 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 ex- panded by moving the stir bar to that region with a hand- held marmet and manuallv mixine the solution in this - - area. The yellow color eventually will disappear com- pletely as the vanadate species is reduced by electron-rich ions migrating up the column. This process can be reversed bv adding a few d r o ~ s of the NaCl01 solution to the to^ of the colm&. This fo-rms a light peen 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 at the bottom of the column and chlorag ions 2 the top (a small amount of sulfuric acid may be added periodically to maintain acidity).

Discussion In the column, reactions take place by diffusion (or per-

haps "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 col- umn, however, this diffusion gives rise to the reactions listed above. As VOzi ions diffuse downward, Vf ions dif- fuse 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 ofthTs system. ~ i l e net reaction can be found by adding the individual reactions ieqs 2 and 3 are doubled, which gives:

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 con- sumed 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 as 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 pre- cipitating with hydroxide at a pH range of 7-8 (9).

Acknowledgment This project was completed with the support of the Na-

tional Science Foundation (Grant Number MDR-87-

Literature 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. net%% 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. 9. National Academy Resr.Prudududf Pmefi-sfor Disposml o/Chemicolsfrom Loboro- 7. Michibata, H.; Sakurai H. JnVonodium in Bido@col SysBms:Physiology ondBio for&% National Academy Re-: Washingfon, DC, 1983.

chamislry:Chasteen, D.N., Ed.; Kluver Academic Pub.: Booton, 1990:ChapterIX 10. Cottan,F.A.;Wikinson, G.AduonmdInoqanicChemlstry, 5thed.: W L I ~ ~ : N ~ W Y O ~ ~ , 6. V0gel.A. I.Vog& lktboob o f h c t i m l OlgonicChemistw, 5thfh.; Wiley:New York, 1989; p 567.

1989; p 467.

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