edited by ROBERT REEVES Marlborough School 250 S. Rossmore Avenue LOS Angeies. CA 90004
A Laboratory Exercise Introducing Students
to the Pourbaix Diagram for Cobalt Dick Powell Martin County High Schwl, Stuart, FL 33494 Jim Cortez The Bolles School. 7400 San Jose Blvd., Jacksonville, FL 32217 E. K. Mellon Florida State University, Tallahassee, FL 32306 A maior cause of the neglect of simde chemistw in the lahumt~;rY is the ~ i d r s ~ ) r e n d f e e ~ i ~ l g a m teachersihat ing it is difficult to relate iiolated chemical reartiuns tu the theoretical models so prevalent in our beginning chemistry courses. In this paper aseries of reactions of Co2+is founded upon the Pourbaix diagram for the element cobalt, which displays regions of thermodynamic stahility for the various stable aquo-, hydroxo-, and oxo-cobalt species as a function of proton and electron availability. A periodic table based on Pourbaix diagrams was published by Campbell and Whitaker in 1969 ( I ) , but the present authors are not aware of similarly based laboratory exercises published since. Pourbalx Dlagrarns The Pourbaix diagram (2) is a potential-pH plot which displays some of the most thermodynamically stable species for a given element. Regions of the diagram are illustrated for water (or, more precisely, for hydrogen and oxygen) in Figure 1where EH,the redox potential referred to the HTH couple, is plotted on t h e ordinate versus pH (i.e., -log[H30+]) on the abscissa. Pourbaix (3)refers to the neutrality point, or "condition of absolute neutrality", as located at pH = 7.00 and EH = +0.40 V. The two heavy lines, e and f, which meet a t the neutrality point divide the diagram into quadrants
oH
-
2
7
.
C
0
-
2
'
, -
.,. 6
P
C
.
.
?
4
-
6
.--.,:
Figure 1. Various regions of an E pH , diagram superimposedon the Pourbsix diagram for Hzand 0,.Lines b and c indicate the thermodynamicstability field of water, lines a and d the stability field of water expanded to include overPotential, and lines e and f represent electrochemical and acidlbase neutrality. respectively.
I. Electron noor. . ..moton rich 11. Flcrtrm poor, proton poor I 1 1 K l t r t r m rich, proton rich
I\'.
K l r c t r o n rich, proton poor
Along vertical line e water is considered to be neutral (with respect to dissociation into H30+ and OH-) from an acidbase standpoint. Along diagonal line f water is considered neutral (with respect to dissociation into Hp and 02) from a redox standpoint. Above line h the decomposition of water into Ozunder an oxygen pressure of 1atm is thermodynamically spontaneous. Below line c the decomposition of water into H2under a hydrogen pressure of 1atm is thermodynamically spontaneous. In principle, the field of thermodynamic stahility for water should be bounded by lines h and c, with some small adjustment for the partial pressures of Hz and 0 2 . In practice, however, chemistry usually can be conducted outside of the area bounded by lines h and c because the production of hydrogen and oxygen from water is hampered by overpotential. Thus in Figure 1, the stahility field of
water is widened to include the area between lines e and d in accord with the practice suggested by Latimer (4). The Pourbaix diagram for cobalt (Fig. 2) is taken from the "Atlas of Electrochemical Equilibria" (5).Above cobalt metal are displayed the most stable ions, or aauo-, hvdroxv-, or . . oxo-complrxrs, under given cmditions of electron and pn)Inn ava~lability.Coo:?has not been completely characterized and the regioiso labeled may be considered ;chemical terra incognita. CosOa, which is obtained by heating CoC03 at 700 "C for 1 h (61,does not figure in the experiments described here. No kinetic information is conveyed by Pourbaix diagrams. Fortunately, the experiments described herein proceed at rates convenient for beginning observers. The tenuous nature of the water stahility limits used in Figure 2 (the lines are drawn with dashes) is illustrated by the apparent stability of uncomplexed Co3+: actually C03+ reacts with water to liberate 02. Thus the positions of the Volume 64 Number 2
February 1987
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problem rather than "Do this" directions, for example, "What do you know about the behavior of hydrogen ions in solution?" "Does your book have an index with that word in it?"
Things To Watch Foc 1. Limit the volume of the solutions. Small test tube and medicine dropper amounts are appropriate. 2. The laboratory investigation can be run the same period as the demonstration is given or can be run during a separate period the following day. 3. In case students recognize that they can test for the gas being evolved, have waoden splints on hand so they can do so.
STEP
-0.4 .. -...-
..-. .......
-0.8-
-1.2--
....
p. 4rc$..
s?&;
co
-.
Figure 2. The Pourbaixdiagramtor cobalt. The water stabilily field is bonded by lines a and b (dashed llne) from Figure 1. The pmemiais ploned in Steps 1 lhrough 5 (see led) are reasonable appmximatlons. water stability limits in Figure 2 must he taken as only approximate.
Step 1: Demonstration T h e following demonstration incorporating vivid color changes is appropriate here to engage students' visualization. Place80mLof3.0M NaOH ina400-ml. beakrrand brginstirring withamagneticrtirrer,ur haveastudent dothestirring. Add 6.0gof V20. weighed out prior torlass. Slowly add it tu the ROmLof %OH. The suspension changes from yellow to green yellow hefore clearing to a greenish straw color. When the solution reaches the straw color add 200 mL of 2.5 M H2SOI very slowly. The color changes will be yellow, orange, red, returnine to vellow. (Caution: Do not substitute h~droehloric acid f o r h f & i c acid in the procedure just above.) In strongly basic solution vanadium(V) exists as the colorless orthovanadate ion.. V O. 3 (note the analoev with orthodrops, the v & a d i u m ( ~ ) is phosphate, P043-). AS the sequentially converted (7)into condensation produds such as divanadate, trivanadate, tetravanadate, and decavanadate and various orotonated derivatives. T h e color of the solution deepens &ward red as the number of condensed vanadium atoms gets larger. In highly acidic media the yellow d i o x o v a n a d i u m ( V ) i o n , V02+ ( p r o b a b l y c i s [V02(H20)4]+ (a),is formed. Typically, students will want to know how you make those colors. Ideally, this gives you the opening needed to lead them to the laboratory to investigate for themselves: li you want to inwsrignre some different color changes, here is something you ran try. Let's shift elements from vanadium to cobalt.
Step 2: Student lnvestlgatlon Afford the student an opportunity t o investigate without the bounds of someone else's questions and methods. Laboratory technique and method should be developed to such an extent that the student is able to operate safely in the laboratory. If students require more structure it can bein the form of Gestions that wiil suggest a possible approach to a 166
Journal of Chemical Education
Step 3: Reconclllatlon: Potential-pH Diagrams After completion of the investigation, many students will need a quick and simple review of concepts to help them shape what they have seen and done. One method of doing thi'is to re-run the lab in a systematic sequence of steps, building a portion of the potential-pH diagram on the board with each step. The series of directions below illustrates a technique to accomplish this important step linking observation to chemical equation. Use the same concentrations the students used in the lab. The only difference should be the volume used. For the first-year students the Pourbaix diagram may he interpreted in terms of the quadrant model (electron poor, electron rich, etc.) alone, while for second-year and AP students quantification in terms of the Nernst equation ( 2 , 5 ) may beintroduced. The "steps" identified in the directions refer to the numbered steps in Figure 2. 1. Prepare the following solutions: 0.5 M NnOH, 6 M HCI, 0.1 M CoCI, ,118gCoCI2.6H2Oplus 1 mL6M HCIdilutedtuO.51.,31. H A 2. To 4 mL of 0.1 M CoClzadd in one portion with stirring 4 mL of 0.5 M NaOH. The a-Co(0H)z (blue, crystalline) which forms immediately isomerizes within 1min into tan-pink ~ - C O ( O H(9). ) ~(See Step 1 in Fig. 2.) 3. Allow the solution from Direction 2 to stand for at lest 3 h. The @-CO(OH)~ has now been air oxidized to dark brown CO(OH)Z (better: CoO(OH).aq (10). (Step 2) This step is accomplished much more rapidly by the addition of 3%HzOz. 4. Add 4 mL of 6 M HC1 to the solution from Direction 2 and heat to 90 OC. Most often, pink, hydrated Co' is formed. (Step 3) If the chloride ion concentration is high enough, blue COCI~~forms,then reverts to pink Co2+upon cooling (ice bath). The following set of reactions is best done as a demonstration because of the caustic nature of concentrated KOH. Caution: Use eye protection! 5 Prepwe the fdhming adutions: I4 M KOH (15.7 g KOH dtswived in 20 m L d HIO with heat, stirring);Re< H& 6 Heat 20mLuf 14 M KOH to80H1"Candadd 4 mLaft1.l MCoCI,.
Cobalt hlue HCoOl- (better: Co(0H)F (11)) is formed. (see
....
Sten 4.) .
7 . Cod the solurion from 1)ircction5 to below 6 0 T and add 2 ml. of Re, H?O>slowly to minimm effervescence.Co(OH),is formed
tStep51 along with02from the rohnlt-mmlyzed dsrornposition of the hydrogen peroxide. 8. Add 6 M HC1 to the cooled solution from Direction 6 until
acidic, and heat. Aqueous cahalt(I1) forms. (SF Step 3.) 9. Use a safety shield and eye protection! Finally, use of CoC126H20 as the source of aqueous cobalt(I1) allows some chemistry not displayed on the Pourbaix diagram to be sampled: 10. To 4 mL of 0.1 M CoC12 add 0.5 M NaOH dropwise without stirring. S-CO(OH)Z forms first and is rapidly converted into a green, basic cobalt chloride (best represented as 4Ca(OH)y CoCl(OH).4H?O ( 1 2 ) ) . On a month-lone standine in contact wirh nrltxrotls~obaltrili.the firat-formeipreen,basic chloride inrerrcmvrrtc into n pink form. Co2CI!OH,,.Ruth badic chlurder mn?.he oxidized toCoAS
. 9. 10. 11. 12.
Hemy, H.T r w t i s r on lnorgonk Chemistry: Elsevier: Amsterdam, 1956, Val. 2, p 293.
Ref7. p 1297. Ref8. p768. Ref%Vol. 2, p 298.
Literature Cited 1. Camnhell. J. A : Whitaker, R. A. J Chem.Educ. 1969,46,90. 2. Barnum. D. W.J. Chem. Educ. 1982.59.809. S. P , ~ U ~ M. ~ ~~~~t~~~~ ~ X , on ~ ~ ~ ~corrosion; t ~ereen, ~ J. A.~ s..~hm n s ~.plenum: : ~ i N ~ ~W m k , ,9 7 2 , ~115.
~
'
copies of the Laboratory Exercises in Chemistry may be ~ Single i obtained from E. K. Mellon at the above address.
Volume 64 Number 2
February 1987
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