D. M. Smith Hope College Holland, Michigan 49423
I
A Comprehensive Experiment in ~lec+roand~tica~ Chemistry
A n experiment in electroanalytical chemistry which fulfills some basic criteria for effective instruction involving chemical instrumentation has been desimed and amlied with meat success at Hope College. l ~ e n e r a lguide-lines foilowed in our undergraduate experiences with chemical instrumentation are that experiments should allow the student to discover: (1) the relationship of the phenomenology to the measurement process; (2) the complementarity of instrument components; (3) the limitations as well as applicability of any instrumental method, and (4) the frequent need to augment one type of measurement with another for definitive results. Recent major revisions in the Hope College chemistry curriculum have eliminated many traditional courses such as "Instrumental Methods." Instrumental techniques now are introduced as early as practical, as often throughout the curriculum as useful and always a t a level consistent with the application. Table 1, a summary of the background in electrochemical measurements of our typical undergraduate chemistry majors, places this experiment in perspective. This outline of concepts, brief experimental description, and the instrumentation presently employed shows that the student has exposure to basic electrochemical principles and practice at a generally increasing level of sophistication. It is not suggested that the same pattern of experimental background is necessary for the successful application of the experiment described in this paper, however. While the experiment meets, to some degree, all of the criteria cited above, special emphasis is placed on (1) the principal instrument components as they relate to the phenomenology of the measurement process, and Table 1. Year IV
I1 I
Presented before the Division of Chemical Education at the 161st National Meeting of the American Chemical Society, Los Angeles, California, March, 1971. Hope College is indebted to the National Science Foundation for Instructional Scientific Equipment Program Grant XGY 4969 under which several exeeriments inoluding - this one h w e been developed. 1 WASER,J., "Qusntibtive Chemistry" (Rev. Ed.), W. A. Benjamin, Inc., New York, 1966, p: 279. 2 In s. recent private communicatmn, Dr. Peter E. Sturrock of Georgia Tech has reported excellent qurtntitative results using the generation of bromine from bromide ion to determine As(II1) by a ooulometric titration employing amperometric end point detection. 9recently reported eoulometric analysis of acid primary standards employing amperometric end point detection (YOSHIMOR,, T., AND MhTS,,BLRA, I., ~ ~ 1them. 1 . see. J ~ 43, ~ 2800 (1970)) mould be a novel system to which the experiment described in this paper might be adapted.
+
(Heath EUW401)
Galvanic Cells Temperature dependence of emf Thermodynamic properties (L & N millivolt ~atentiometer) . . pKa and Slmxturi Substituted organic acids (L & N pH meter; glass electrode)
}
" '""';"..",-".
~--LLu1',,1"01
The chemical system to which this experimental approach is applied is a classical and well-defined one from the coulometric analysis standpoint. It is possible to do the coulometric analysis of As(II1) with generated Ia- using simple apparatus to obtain good quantitative results;' the analysis itself is not the primary consideration. The As(II1)-13- system has been used in the development of the experiment simply because it is a well-examined chemical system. In principle the experiment is applicable to a wide variety of reactions employed in c o u l ~ m e t r yor ~ ,conventional ~ titrimetry.
Experiments Amperometry Coulometrie Titration with Amperometric End Point Detection Pb(I1) CmGAs(II1) Is-
,
Polmography
*":A.'-"t-". "U""L"11
Theoretical Considerations
Undergraduate Experiments Involving Electrochemical Measurements
i-E curves Pb(I1) complexes Nitrohenzefie; mechanism (Heath EUW401) I11
(2) the interrelationship of these components. This electroanalytical experiment, as should all, clearly demonstrates the basis of electroanalytical ?nethods in the current-voltage characteristics of the chemical system.
(,
T itration curves, Ka's, formula weights (L & N pH meter; glass electrode)
+
(Fluke 351A Supply) (Heath Umtsi
Chronopotentinndry Theoretical relationships Applications (Fluke 351A Supply) (Moselev 7030 X-Y
Cd,Cdz+,Cd(Hg)
.
D-'-'"--.etrg Titratition cukes; Fe(I1) Formula and Stahhty Constant of AgL,+ (L & N pH meter; Pt, Ag electrodes) "W,'~C.,,,,
The normal sequence of these experiments is upward from left to right in the table. Volume 49, Number 2, February 1972
/
87
~
~
Figure 1.
Theoretical current-voltags chorocteri*icr.
A. i-E curve of solvent, 0.2F NaHCOa; anodic electrode reaction at 4-1.20 V venvr N.H.E. is HrO = '/rOdgl 2H+ 2e-, while B. C. D.
E.
+
+
++
e- = '/nHdgl OH-. cathodic reaction a t -0.50 is HnO i-E curve of 0.1 F KI in 0.2 F NaHCOl; anodic electrode reoction a t +O.5 V versus N.H.E. is 31- = Is2e-. i-E curve of system B in which Is- has been generated; cathodiselectrodo reoction ir lr2e- = 31-. AE = EI,. - iR; potential difference between ampemmetric electrode poi.. Elodmde potentid of mulometric m o d e during generation of Is-.
+
+
The electrode reactions associated with the coulometric generation are a t the anode, and 2H20
+ 2e-
=
Hn(g)
+ 20H-
(2)
a t the cathode. The reaction in solution is 11-
+ HsAsOa+ HsO = HIAsO~-+ 31- + 3H+
(3)
makimg the overall cell reaction BO
+ H8As08= Hdg) + HAsOI- + H+
(4)
The quantity of As(II1) in each sample is calculated using the relationship
where G = mass of substance (As) reacted; (Mln) = equivalent weight; F = the Faraday constant (96, 487 coulombs); Q = quantity of electricity (coulombs) all in consistent units. Since the coulometric titration is a constant current process, eqn. (5) becomes
Figure 1 shows the theoretical current-voltage (i-E) characteristics of the system if a Pt working electrode is employed. The solvent, 0.2F aqueous NaHC03, has anodic and cathodic limits a t f1.20 V and -0.50 V versus the normal hydrogen electrode (N.H.E.), respectively. These limits, shown in Figure lA, are imposed by the oxidation and reduction of HzO a t the Pt electrode under these solution conditions (pH 8). Figure 1B represents the i-E relationship which should be obtained if the same solvent contains a significant concentration (O.lN) of iodide ion. The anodic limit now appears a t about +O.M V versus N.H.E., however, because of the oxidation of I- to Is-. Finally, the addition of Is- to the NaHCOa-I- system should produce the i-E relationship shown in Figure 1C. The 1,-/I- couple is quite reversible a t the Pt electrode under these solution conditions, producing the com88
/
Journal of Chemical Education
posite wave which crosses i = 0 at about +0.5 V versus N.H.E. As stated earlier, the relationships of the i-E curves of the system determine the efficacy of the method, a constant current coulometric process (titration) with dual polarized electrode amperometric end point. F i s t , the potential of the generating electrode (anode) must remain between +0.50 and +1.20 V versus N.H.E. in order that not only a significant current will flow but also 100% current efficiency is maintained. Second, the composite I--Ia- curve shape determines the potential difference (AE = E.,,, - iR) which must be applied to the amperometric electrode pair. AE must be chosen to yield measurable current when excess Is- is generated within the solution by the anode of the coulometric circuit. Since AE is fixed, the potentials of the amperometric electrodes will individually float such that +i = -i; that is, the currents through the amperometric anode and cathode will be equal in magnitude. Apparatus
All of our voltammetry experiments, normally done in the senior year as indicated in Table 1, are coordinated about a core of instrumentation in a way which not only maximizes their pedagogical value but also minimizes cost. In addition to the Health EUW401 Polarography System, this instrument core consists of a Fluke Model 351A Precision DC Constant Current Supply and a Moseley 7030AM X-Y Recorder. Figure 2 is a diagram of the arrangement of components for this experiment. It is this apparatus with which all i-E curves are obtained, the generation effected and the analysis made. Figure 3 is a photograph of the assembled apparatus, and Figure 4 provides a close-up of the electrode arrangement. The Fluke constant current supply (wired with a socket for an electric timer) directly connected to standard Sargent platinum electrodes is the generation system. The cathode of the generating system is contained in a fritted sealing tube filled with 0.2F NaHC03 to prevent H2from entering the titration cell and As(V) from reaching the cathode. The cell consists of a 250-1111 high form (Berzelius) beaker which is clamped in position over a magnetic stirrer and fitted with a stopper containing all electrodes. The Heath EUW-19A Polarograph control unit and associated 3-electrode
Figure 2. Diagram of apparotu% A. Fluke Model 351A Precision DC Constant Current Supply. 8. Cell ( 2 5 0 4 Berzelius beokerl: (I)shielded Pt cathode of covlometrie circuit, (21 PI anode of coulometric circuit, (3) Agl-Ag reference electrode, (4.51 PI electrodes doubling as the amperometric pair ond workina-ouiiliarv o d r for voltammetrv. " C. Heath external control relay. D. Heath EUW-401 Polorography System (control unit): ( 6 ) Reference (71 Counter and 18) Test positions. E. Heath EUW 20-A-26 Recorder. F. SPDT knife switch. G. Timer rocket
..
Figure 3.
View
d assembled appamtur.
Recorded currant-voltage svrvsl of sy9tem. A. Solvent, 0.2F N~HCOZ;8. 0.1 F KI in 0.2F NaHCOs; C. 3 X lo-' N Is- (gem erotedl in 0.1 F KI, 0.2F NaHCOa. Reference electrode is Ad-Ag.
Figure 5.
activated prior to current generation; we often do not employ it, using the manual recorder control instead. While the specific equipment employed in this experiment has given excellent results and is convenient to use, certainly any similar combinations of other polarizing units, constant current sources and recorders may be equally as effective. Rather than the Fluke supply used here, for example, a variety of less expensive commercial or homemade units may be suitable. The Experiment
Figure 4.
The
arrangement of electrodes.
circuit is the heart of the detection system. The 3electrode system employs two more standard Sargent P t electrodes as indicator and auxiliary electrodes, and a Sargent Ag electrode as the reference electrode, for the i-E work. The clean Ag electrode is conveniently anodized right in the I--containing solution to produce a AgI-Ag reference electrode. The Pt electrodes double, through switching, as the amperometric pair for end point detection. The AgI-Ag reference electrode is available for checking the potential of either generating electrode or amperometric electrode, using a VTVM, a t any time during the course of a coulometric titration. The SPDT switch is convenient to transfer from i-E studies to the application of AE across the amperometric pair. The REFERENCE and COUNTER (auxiliary) positions of the polarograph control unit are thus connected so that the INITIAL POTENTIAL setting may be used to apply the desired AE to the amperometric pair. The Heath EUW 20A-26 Recorder is used to obtain the i-E curves of the chemical system and the i-time plot during generation. The external control relay is merely a convenience in starting the recorder as the amperametric circuit is
Figure 5 is a recording of the i-E curves of the chemical system as obtained by the student and is to be compared with the theoretical ones of Figure 1. Prior to ohtaining these i-E curves the student is riven the instrument comDonents and reauired to properly interconnert them from hie krwwledge of clrrtn~rnalgrical principles, the inatrwtor rherkirag the n?.cmbly print. I r e irs u.e. This has proved 14 br en rvtremel~in-rrut:rive lr~giuniw to the experiment as the student must soceessfdy cope not only with electronic principles but also fundamental concepts of electrorhemicsl cills. By obtaining tho i-E curve of the solvent system (0.2F NaHCOa) havine anodic and cathodic limits a t about +1.0 V and - 0 . 3 v versus AgI-Ag, respectively, the st,udent becomes aware that a total working potential range of about 1.3 T' is available (Fig. 5A). The i-E curve of the solvent to which has been added an aliquot of a 1.OF K I solution (used as a blank for chemical andysis pnrposes), with the anodic wave of I- a t about +0.5 V versus AgI-Ag, is Figure 5B and is the next one obtained. The student discovers from i-E curve 5B that to both maintain significant current and 100yo current efficiency, the potential of the generating anode must remain between +0.5 and 1.0 V versus AgI-Ag. The next step is the generation of sufficient Is- within the solution to yield an observable reduction wave of 13-when again obtaining the i-E curve of the system. Ten minutes of generation a t 5 milliamperes (0.031 milliequivslents) is sufficient to produce enough 4- to yield the composite curve 5C using the same recorder sensitivity. Figure 5C demonstrates the essential reversibility of the Ia-/I- couple under these conditions, and rtllows the select,ion of an appropriate AE 14 be applied to the srnperometric pair so that a measurable current will flow as excess la-is produced by the generating electrode (Refer to Fig. I). After the i-E curves are recorded, the cell is cleaned and refilled with distilled water and an aliquot of the blank solution ( I F NaHC08, 1F KI, trace As(II1)). Upon establishing a stable zero current line, Is- is generated a t constant current while a AE of 100 millivolts is applied across the amperometric pair. During the generation the potential of the anode versus the Agl-Ag reference electrode is checked using the Aeath VTVM. At a generating current of .5 me, under theae solution conditions, for example, the mode sits a t +0.87 V versus AgI-Ag, thus allowing the student to confirm from his own i-E curves that the generaVolume 49, Number 2, Febrwry 1972
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89
Figure 6. Recorded ompsromotric end point* Shows reproducibilily of blonk 10.2F N ~ H C O I , 0.1 F KI, trace Asllll)) determindiom
Figure 7. Recorded ompemnetric end points. A. Determination of blonk (Fig. 6 ) ofAr two daysi 8. Cell mlution containing fresh starch indicator; C. Cell solution containing aged rtorch indicetor.
deaeration would improve both accuracy and precision. tion of I$- is proceeding with 100% 'ourrent ifficiency. The generation is allowed to continue beyond the end point as it is indicated by the recorder. The blank titrations are accomplished with a. reproducibility of within 1% using typical extrapolation procedure. Figure 6 is a recording of a typical, triplicate blank determination, current in microamperes being recorded s~ a function of time in seconds. The analysis is completed by adding an aliquot of the sample solution to the 1F NaHCOslF KI-trace As(II1) blsnk in distilled water and generating beyond the end point as indicated by the recording in Figure 6. The calculation of the mass of AszOais based upon eqn. (6), taking the form
I t is instructive to have the student complete the same analysis using the traditional starch indicator end point. Since the blue starch-iodine complex cannot he detected until a few seconds later than the eleetrometrio end point indication, it is necessary to determine the blsnk for this method also. Only the generation circuit need be employed in this determination. At one paint in the development of the experiment we began adding strtrch indicator to the system while simultmeously employing amperometric detection. The amperometric end point is 1-3% higher in the presence of starch and the shape of the Gt enrve may he quite different. Figure 7C shows an unusual i-t curve which results when aged starch is used. The lower slope of the i-t curve beyond the end point in the presence of starch may he a result not only of the equilibrium Complex
Is-
+ Stsrch
but also the decreased diffusion coefficient of Ia-. The shrtpe of curve 7C appmently results from the presence of an oxidation product of starch. Finally, Figure 7A demonstrates the effect of air oxidation of As(II1) and I- in solution under these conditions. The end point time of 74.5 see compares with the mesn of 79.9 see obtained two days earlier (Fig. 6). It is for this reason that the hlank generation time must be determined a t the time of analysis.
Analytical Results
Table 2 lists analytical results obtained by Hope seniors who did this experiment during the 197@71 academic year. Three analyses involving large determinate errors in sample preparation are not included; this experiment is manipulation-limited as far as accuracy is concerned. The reported analyses have an average relative error of 1.4%. The precision of the method is good as demonstrated by the ranges, w, and standard deviations, s, estimated from the range, these two parameters averaging 0.14 X lo-' g and 0.08 X lo-' g, respectively. Analyses of the same samples where the starch indicator end point was employed also yielded an average relative error of 1.4% but the precision was somewhat higher, w and s averaging 0.03 X lo-' g and 0.02 X lo-' g, respectively. Certainly a careful scrutiny of quantitative technique and experimental variables such as AE and solution
90 ' / Journol of Chemical Education
Summary
The exueriment described in this uauer has been spectacul~rlyeffective in bringing the student to a discovery of the basis of electroanalytical methods in the current-voltage characteristics of the chemical system. Assemblv of the instrument comuonents utilizine the basic theory of the experiment and his concepts of electronics and electrochemical cells represents an important learning experience for the student. By obtai&ng his own i-E curves of the system the student discovers
-
1. The total working potential range available for the experiment is determined by the anodic and cathodic limits established by the particular electmdes snd solvent 2. The potential range in which the generating anode must remain to insure both significant current and 100$7' current efficiency 3. The technique of following the electrode potential versus a reference electrode 4. The reversibility of a couple (4-/I-) 5. The basis upon which the A E to he applied across the amperometric electrode pair is determined
Not only does the experiment effectively relate theoretical principles to instrumental arrangement, but an accurate and precise analysis is obtained which is a satisfying completion of the experiment. Most important, perhaps, is the fact that the student must understand and be able to relate the phenomenology to instrument design in order to carry out the experiment. Acknowledgment
The author wishes to thank A. Bentz for aid with the electronics, Dr. D. H. Klein for helpful discussions, and many Hope College students for their patience and cooperation. Table 2. 1970-71 Student Results Am~erometricEnd Point Detectiond Student'
Resultb
w
s
Actmlb
Data. obtained by J. Braininard, J. Koert, R. Peree, V. Roelofs, M. Van Dort, and G. Van Kempen. Mean value of 3 4 results re1 Mass of AsnOzin g X 10'. ported. Standard deviation as estimated from the range (s = Kw). . (BLAEDEL, W. J., AND MELOCHE, V. W., "Elementary Quant~tative Anslysls" (2nd Ed.) Harper and Row, New York, 1963, p. """ ~ 0JL.j
d Analyses of these samples employing the oonventional starch indicator averaged the same relative error (1.4'%) with a lowers (0.02 versus 0.08).
