Topics in..
. Chemical Insfrumentufion
Edited by S. Z. LEWIN, New York University, N e w York 3, N. Y.
These articles, m s t of which are to be conln'buled by gwat authors, are intended to s m e the readers of this JOURNAL by calling attention to new darelopmenls i n the theory, design, or availability of chemical laboralmy instrumentation, o i by presating useful insights and explanations of topics Wat are of practical importance to those who use, pr teach the use of, modern instrumenlalion and instmmatal techniques.
XXII. Instrumentation for Electrodeposition and Coulometry-Part
One.
Peter F. Lott, Deportment o f Chemistry, University o f Missouri at Konsas City, Kansas City, Missouri.
An electric current may be passed through a solution and used as a n analytical tool without causing any appreciable change in the composition of the solution s in the case for notentiometrv. con-
the solution is used to change the composition of the solution, and this change in solution composition is used as the means of chemical analysis. For example, in the electrodeposition of copper, the electrons could be considered as a gravimet,ric precipitating reagent fur copper. Or, under proper cunditions, the total flow of electrons-that is, the number of equivalents-to cause this change could zlso he measured, and the procedure could be classified as n volumetric process and would be known as acoulametric titration. I t is interesting to note that the basic laws for these procedures were developed by Fsraday in 1834 and that the determination of copper by electrodeposition was developed around 1860. In contrast, not until about 28 years ago were coulometric titration methods firmly estahlivhed in .zndytical chemistry. This came about partieulerly through the work of Lingane and Swift in the United States. Electrodeposition and eoulometry may both be used fur macro and micro qrmntities of material. Both offer the possibility of very high precision in determinations and, compared to cow ventional analysis, they offer the possibility of a, high degree of automationso as not to require the constant attentionof the chemist. Furthermore, their much greater simplicity greatly decreases the danger of mechanical loss and, in both procedures, the change in the solution composition takes place a t the two electrodes. Electrodeposition
Electmdeposition, as well as caulome-
try, may be performed under different conditions. The methods may be classified according to the electrical quantity to which primary control is centered throughout the course of the eleclmlysis.
Figure 1. troly~i.: T Stepdown L, Choke; Voltmeter;
Circuit for "constant current" elec1, Variable voltoge tranrformer; T 2. transformer; X, Full wove rectifier; C, Condensor; A. Ammeter; V, E, Electmlyds cell.
This may be either the initial voltage which is applied across the two electrodes of the eleetmlysis cell (constant voltage electrolysis), or the current which flows through the cell (constant current electrolysis), or the variation of the potential of the electrode a t which the primary electrochemicd reaction occurs (controlled potential analysis). The methods are very closely interrelated. However the diversity of their analytical applications, as well as the complexity of the concordant instrumentation, differs greatly from one to the other. The simplest instrumentation is that for electrodeposition a t constant current. The circuitry far such apparatus is shown in Figure 1 and is probably immediately recognized as equipment which ia used for the electrodeposition of copper in quantitative anslysia courses. A variable voltage transformer (Variac) is used to control the voltage applied to the primary winding of s step-down transformer. I n turn, this changes the voltage developed by the transformer secondary. The seeondsry of the transformer is connected to a. full wave rectifier, with a simple choke and condensor to filter the rectifier output, although not much attention is paid to reducing the voltage ripple. A voltmeter is connected across the terminals of the electrodes and an ammeter is used to indicate the current flow through the electrolysis cell. Initially the current is set to a value well above the deeamposition potential of both copper and some other ion species in the solution which might he hydroniurn ions or p e r h ~ p sa depnlariaer such as nitrate ions. As the electrolysis proceeds, the copper ion concentration is depleted and the cathode assumes a more negative potential. Consequently, if there were only one possible method of conduction through the solut,ion, then according to Ohm's Law, the current should decrease. However, as the potential of the cathode ia well above t,hst of the other ions in the solution, these continue to conduct more and more of the current. Although the current may have dropped con~idembly from its initial value, since the other ions offer an alternative path far the conduction of the current through the solution, the current value, and temk to remain a t a "constantnt" accordingly the process is knnwn as "constant current electrolysis." No attempt is made to control the current,. The "constant current" nomenclature is here really a misnomer, and only serves as a means of describing the type of electrodwosition. Obviouslv, the ~ r i m a r vresctim-the deposition of capper--does not proceed with 100% current efficienry.
(Continued on page A#&?)
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solution to the electrodes. Consequently, s,irring is necessary. l-ery often, to further increase the rnte of migration of 0
P
i
,
Figure 3. Eberboch Cmp., Electrolysis opparofur. model i 0 0 0
Figure 2. E. H. Swgenf Co., ElectrolyficAnalyzer
The usual commercial units offer a maximum current setting of about 7.5 amp a t 10 volts DC. The electrolysis would proceed quite slowly (in time) if migration were the only moans of transferring ions from tho hulk phase of the
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the ions, provision is made for heating tho solution. I n convent,ional equipment, as indicated in Figures 2 and 3, the solution is stirred by connecting one of the ole* trodes to a stirring motor. The clcctrolysis is usually carried out a t a current
setting of about 2 to 3 amp. Equipment is also manufactured to provide .z higher currant of approximately 25 nnrp through the solution. This will permit the deposition nf 1 g of copper in about 8 minutes compared to about 45 minutes with the conventional apparatus. To prevent undue heating, the cell is usually water cooled during the elerr trolysis. A novel arrangement is used to stir the solution. A large permanent magnel is fastened below the sample. The magnetic field of the magnet is at a right angle relative to the field produced by the eurrent flow between the trvo cell electrodes. Accordingly, the interaction (Conlinued on page A S @ )
Chemical Instrumentation of the two fields causes the solution to rotate and rapid stirring takes place. These units, ss well as conventional electrodeposition apparatus, may be used to separate ions from s solution by making the cathode connection to a mercury pool. With commercial high-current carrying units, it is possible to separate 0.5 g of iron from a.steel sample in 10 minutes at tho mcrcurv cathode. as mieht be re-
magnet below the mercury pool draws the ferromagnetic materials below the mercury surface and provides a. continuously clean mercury surface as well as preventing the resolution of the deposited metals. A picture of such apparatus is shown in Fiyres 4and 5.
Figure 4. Anolyzer
Eberboch Corp., Ulho-Speed Electro-
Figure 5. Eberbach Corp., Dyno-Coth Mercury Cathode Apparatus
The disadvantage of this conventions1 "constant current" electrolysis apparatus is that it is very difficult to plate out one metal selectively in the prwence of mother unless hydrogen evolution or a
(Continued on page A868)
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Chemical instrumentation similar secondary electrolytio process (e.g., n potential bufier) takes place l o limit the pntential of the electrode below that of the decomposition of the second ion. Thus, copper can he plated out from nickel in an acid solution because hydrogen evolution takes place before the cathode potential becomes negativeenough to permit the deposition of nickel. In a basic solution however, both copper and nickel deposit simultrtneously since now the decomposition potential of nickel is reached before hydrogen evolution takes place. This latter separation and similar separations could still be done if somehow it were possible to limit or control the potential of the cathode. In theory, the initial voltage applied to the two electrodes could be set to a potential necessary only for the deposition of one constituent, and the applied voltage is not changed throughout the electrolysis. This approach is carried out in "constant voltage" electrolysis. Because of the inability to compensate far resistance changes in the solution, na well as the difieulty in assigning values to overvoltages a t the electrodes, "constant voltnge" electrolysis is more of a n aeademic calculntion than a practical means of analysis. The electrolysis in principle could be carried out with the same eqnipment as is used for "constant current" electrolysis.
Controlled Potential Analysis Electrodeposition phenomena tend to follow the Nernst equation; thus in the cane of copper deposition, the potential of the copper electrode formed as won as copper is plated on to the cathode would be given by the expression:
Ec.
=
Ec.'
+ 0.059/2 log C u t +
Accordingly, the potential of the copper electrode would be 0.31 v in a 0.1 hl cupric ion solution, and would decrease to 0.22 v when tire copper concentration has been reduced 1000 fold to 10-4 11. Slmuld it be desired to verify this exponential change of cathode potential as the electrolysis proceeds, we would simply add an independent reference electrode such as the calomel electrode to the circuit and use it to independently measure the potential of the cathode throughout the electrolysis. The addition oi this third electrode is necessary as measurements of the cathode patential are meaningful only with regard to the potential of some reference electrode. I n the described case, as shown in Figure 6, where the primary interest centen on the cathode, the cathode would be designated as the working electrode and the mode, since it serves only to carry the electrolysis current, would be designated as the counter electrode. Obviously, for anodic processes, the reference electrode would be connected to the anode in order to measure anode potentials. In this cam, the cathode would be designated as the counter electrode and the anode, the working eleotrade. (Conlinzwd on page A%%)
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Chemical Instrumentation Conversely, if i t were desired, the electrolysis could be terminated as soon as the cathode reached a set potential, for example 0.22 v as in the electrolysis of copper. The analytical chemiat, however, would be more interested in an instrument in which the voltage across the anode and cathode is varied as needed to automlttieslly maintain this desired potential betw~enthe reference electmde and cathode throughout the electrolysis. Such a procedure would allow the masimum possible current flow between the working and counter electrodes. I t would prohibit the deposition of a second ion if its deposition patential were more negative than the limiting value of the cathode patential. This could be a n ion whioh might have a decomposition patential of 0.20 v in the described case for copper electrolysis. Also, since the only potential measured is the cathode patential, the measurement is independent of secondary changes which occur between the electrolysis electrodes such a,s the overvoltage changes s t the anode, changes in the resistance of the solution as the electrolysis proceeds and line voltage fluctuations. Manual systems to do such controlled electrodepasition were promulgated by Sand (I), and a system of this nature is really shown in Figure 6. The 1 0 VOLTS
Figure 6. Circuit for controlled cathode potential molysir; P 1, Potentiometer for adjusting valtoge applied between onode ond cathode; c, Cothode; a, Anode; r, Reference electrode; P 2, Potentiometer or vocuvrn tube voltmeter for measuring cathode potential; A, Ammeter; V, Vdtmeter.
disadvantage of this manual system far controlled potential analysis is that it requires continuous supervision in order to manuslly change the voltage applied across the anode and cathode, in order to keep the cathode at the desired potential. A graph of the current flow through the
(Continued on page A8701
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Chemical instrumentation system as the electrolysis proceeds is shown in Figure 7.
Time, minuter Figure 7. Variation of current with time in controlled potential onolysir.
Potentiostats
To make such controlled potential analysis feasible for routine work requires devices which will do this potential adjustment automat,ieaUy. Such instruments are called potentiostats. Until recently potentiostats were "hmdmade" in the individual laboratories. Currently, they may be purchased eommercially, a t a. cant r e n ~ i n gfrom approximately 5400 to $3,000. I n addition to offering potential control far electrodepositions, potentiostats are also used for polarization studies, electrolytic metal polishing ior the prepamtion of metallurgical samples far microscopic examinntion, and the selective stripping of metals in alloys in order to determine tie lines in phase rule studies of alloys. The operation of these potentiostats may be baaed either on electromechanical or totally electronic principles. Rechnits (2) has listed same guidelines in the selection of ~otentiostats. As criteria in the selection of potentiostats, the following coa-iderations are of importance: ~
~~
~
~.
High current capacity High output volti~ge Fast response time Wide operating range for potential control (5) Highly sensitive and stable potential control (6) Low control current between t.he reference and working electrode (7) Low ripple voltage in the output of the potentiaitat (1) (2) (3) (4)
These criteria apply to the selection of d l potenbiostats, but depending upon the purpose for which the instrument is used, some criteria are of greater importance than others. For normal analytical work, a prime consideration would be the current ca-
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Chemical Instrumentation pacity of the potentiostat. Based upon Fick's law of diffusion and the Nernst equation, to limit the reduction of the ion to diffusion-controlled conditions done would require a current of about 2 amp for a 0.01M solution of an ion of +1 chrtrge. For more concentrated solutions as may often be encountered in practice, a current capacity of around 10 amp would be desirable. Since the solution may have a resistance of about 50 ohms between the electrodes, should diaphragm cells be employed, then based on Ohm's law, the voltage required t o handle a current of only 5 amp would be 250 v. The response time of the potentiostat, may also be an important consideration if the ions to be separated lie closely together. I n the beginning of the electrolysis the potential changes very rapidly, and the potentiostat would have to follow this potential change quite closely to prevent the second ion from depositing simultaneously. Possibly, however, the criterion of responne time may be of greater importance in polarization studies. Generally, if the unit supplies a oontrolled potenbid range from +2.5 to -2.5 v, this would be sufficient for most purposes. From a practical standpoint this is a minor factor ss a "bucking circuit" can he easily constructed by the user to extend the control range. I n certain electrolytic procedures taking place in fused salt and nonaqueous media, control voltages of +5 v may be desirable, and certain commercial patentiostats cover this control range. The last criteria are interrelrtted. The potentiastat should control the potential of the working electrode to withim +10 mv. This, consequently, implies also a ripple current of less than 10 mv. The instrument must be stable within this same voltage range, and this operating stability would be inclusive for such interrelated effects as line voltage fluctuations, electronic drift, and the response time of the potentiostat. All instruments electronically amplify the current flowing between the reference electrode and working electrode. Because of the high input impedance of this amplifier, a negligible current flows between the electrodes; thus the electrode does not become polarized. If the input impedance is sutliciently high, i t is possible to use commercial reference electrodes as are found in p H meters. These electrodes generally have a much higher resistance than individually constructed electrodes which incorporate a law resistance salt bridge.
Electromechanical Devices The Analytical I n s t ~ m e n t s ,Inc., PCtentiostat would be classified as an electromechanical type potentiostat. By means of a servo mechanism, it automates the operations which were done manually by the Sand system of controlled cathode potential analysis. The operation of this potentiostat for an electrolytic reduction {where the potential of the working (Continued on page A878)
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+
electrode is with respect to the saturated cdomel electrode) in illustrated in Figure 8. The motar-driven auto-transformer (Variac) T 801 supplies a n AC voltage to the rectifier and filter through a stepdawn transformer TZOZ. TheDC outout of the
the working electrode is represented as a mercury pool and illustrates the applicability of controlled cathode potential analysis to separations with a mercury cathode. The potential difference between the working and saturated calomel electrodes is connected in series opposition with the known DC output (which r e p r e sents the preselected control potential of a ten-turn helical potentiometer, R ZOS), and also in series with the servo amplifier. The amplifier senses any difference between the control potential and the potential of the cell arising between the working electrode and the saturated calomel electrode. This potential difference (error signal) is amplified and is used to cause the control motor to turn the auto-transformer in such a direction that the error signal is reduced to zero. For quantitative work, the electrode F a y be weighed if the metal is deposited on the working electrode. Or, if the electrolysis reaction proceeds with 100% current efficiency, the current flowing during the electrolysis may be integrated.
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Figure 8. Analytical inshumentr, inc. a1 Potentiostat with Current lntegatar bl Block diagram of potentiortat
A current integrator is supplied for the Analytical Instruments, Inc., potentiostat and used to obtain an accurate measure of the amount of the substance electrolyzed. The operating mode of the current integrator, which is plugged directly into the potentiostat, is illustrated sche(Continued a page AZ78)
Chemical Instrumentation matieally in Figure 9. The current to be integrated, that is the current flowing between the cathode and anode of the electrolysis cell, is passed through a precision resistor. Since the resistance of this resistor does not change and is accurately known, the voltage drop is proportional to the current. This voltage drop ia opposed by the voltage produced by a DC generator (a, permanent magnet DC motor, operated as a. generator). If there is any difference between the voltage drop across the resistor and the generator output, the amplifier converts this L C voltage difference to AC, and amplifies it to operate a reversible two phase AC motor which drives the DC generator. This results in the generator being driven a t a speed sufficientto maintain its output very nearly hut not quite equal to the apposing voltage drop developed in the standard resistor, R. As the output voltage of the generator is proportional to the speed of the rotation of its shaft, the counting rate uf a revolution counter mechanically connected to the generator shaft by appropriate gearing is directly proportional to the generator output voltsge, and thus to the instantaneous current in the cell circuit. Thus, by proper choice of the standard resiston and gearing, the revolution counter sums the total current whieh haa passed through the cell.
Figure 9. Block diagram of Anolyticd Inrtrurnenh, Inc. current integrator
A similar mechanical type potentiastat has been recently introduced by Fisher Scientific Co. The unit is very suitable for electrodepositions or electroseparations; a current integrator is not incorporated into the instrument by the manufacturer. [Current integration may also be done by connecting conventional current integrators such as a, ball disc integrator, a printing integrator, or chemical coulometer into the circuit. Only the commercially offered units are described. Information on adapting equipment as well as a description of Inhoratory constructed potentiostats, and coulometers may be found in J. J. Lingane, "Eleetraanalylical Chemistry," John Wiley, New York, 1958.1 The unit incurparates two control channels and permits two determinations to be made simultaneously for either the same or two direrent metals. As shown in Figure 10, the instrument cunsists of two units maunt,ed permanently together. The
(Contintad on page A2821
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Chemical Instrumentation lower unit contains the power unit, combination heaters, beaker supports, beaker cover plates, stirring spindles, meters, fuses and master power switch. The upper unit is a self-contained electronically controlled and regulated amplifier control, designed to oontrol either mode or cathode potentials. Figure 11 indicates the circuit diagram for the analy~erunit (lower unit) and shows the
Figure 11.
:isher Scientific Co., Model 40 po-
Figure tentiortot
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Schematic diagram of ondyrer unit, Fisher Scierttiflc Co., Model 40 potentiortd
two servo-motors, MTR-1 and MTI1-8, which regulate two separate DC rect,ificr circuit?; .Motors MTR-3 and MTR-!+ are used to stir the solution. The unit incorporates a servo amplifier system which keeps the voltage of the working electrode from exceeding a preset value or from drifting more than a few millivolts from the preset value. As a gmup, eloctromechanical units have the ability to handle large currents.
Compared to tot,nlly electronic units, they could be considered to suffer from a relatively slow response time. Because of their larger current capacity the electrolysis can he done more rapidly which is an advantage far routine type determinations, as might he found in control work. P a d Two, the conclusion of "Instrun~entation for Eleclrodeposilion and Coulometry" will appear in the M a y issue.
