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A simple potentiostat for general laboratory use
Electrochimica Acta, 1961, Vol. 5. pp. 161 to 168.
PergamonPress Ltd. Printedin Northern Ireland
A SIMPLE POTENTlOSTAT FOR GENERAL LABORATORY USE* A. HICKLING University of Liverpool, England Abstract-A very simple electronic potentiostat for general laboratory use is described which will maintain the potential of a working electrode within i-O.01 V of a set value over any desired period, the output current supplied being automatically controlled in the range 0 to 0.3 A. By combining this basic instrument with a saturable reactor currents of any magnitude can be controlled without appreciable waste of electrical power. Applications and further possible developments of the potentiostat are discussed. R&urn&On dCcrit un simple potentiostat Clectronique d’utilisation g&&ale en laboratoire qui permet de maintenir le potentiel d’une Electrode b moins de &O,Ol Vd ‘une valeur donn6e pendant une pCriode de temps indefinie le courant debit.5 &ant contrljle automatiquement dans l’intervalle de 0 g 0,3 A, En combinant cet instrument de base avec un r&cteur saturable des courants de grandeur quelconque peuvent Ctre contrBl& sans perte de puissance apprtciable. On discute les applications et les dCveloppements ult&ieurs possibles de ce potentiostat. Zusammenfassung-Es wird ein sehr einfacher elektronischer Potentiostat beschrieben, der das vorgewlhlte Potential einer Arbeitselektrode innerhalb +O,Ol V fiber einen beliebigen Zeitraum konstant zu halten erlaubt durch automatische Regelung des abgegebenen Stromes im Bereich von 0 bis 0,3 A. Durch Kombination dieses Instrumentes mit einem magnetischen VerstHrker kannen StrGme beliebiger Griisse beinahe leistungslos geregelt werden. Anwendungsbeispiele und weitere EntwicklungsmGglichkeiten des Potentiostaten werden diskutiert. INTRODUCTION
1942 the present author introduced1 the idea of the automatic control of the potential of a working electrode and described an electronic device to achieve this for which Since that time the potentiostatic method has become he coined the name potentiostat. an established electrochemical technique, and a large number of potentiostats, some of very considerable complexity, have been described in the literature.2 All potentiostats operate on the same general principle in that the potential difference between the working electrode and a suitable reference electrode is continually compared with a voltage derived from a potentiometer, and the difference between the two-the error signal-is amplified and used to control the current passing through the electrolytic cell in such a way that the error signal is minimized. The great variety of potentiostats arises from the different methods used for amplifying the error signal, and for the control of the cell current, this latter being variously achieved by electronic, electromechanical or electromagnetic devices. The ideal general purpose laboratory potentiostat should be sensitive to potential shifts of about 0.01 V or less, it should be practically instantaneous in response and of high input impedance, and it should be capable of controlling currents varying from 10W6to at least 10 A; furthermore it should not require any adjustment other than setting the potentiometer voltage, and above all for general utility it should be compact and cheap. IN
* Manuscript received 31 December 1960. 161
162
A. HICKLING
A potentiostat which goes a long way towards meeting these requirements has been in use in the author’s laboratory for the past 6 years. It is the final result of potentiostat development over a long period, and it has proved so trouble-free in operation that it may be of interest to electrochemists generally who require a very simple instrument in order to employ potentiostatic techniques. The basic instrument meets the following specification. It is a rectifier working from the a.c. mains, and its output voltage can be varied from 0 to 300 V and the current from 0 to 0.3 A. This output is controlled entirely by an error signal of about 0.01 V, and the potential of the working electrode can be maintained within &to*01 V of a set value over any desired period. The instrument is all electronic and virtually instantaneous in response, and the current drawn from the control circuit is less than 10e6 A. It is furthermore independent of mains voltage and will work without adjustment over periods of several months. It is completely housed in a cabinet of dimensions 45 x 30 x 25 cm and the cost of the component parts is less than &25. The potentiostat may be used either for anodic or cathodic polarization, or by using it in a bridge circuit current may be passed in either direction through the cell. The instrument will serve also as a current stabilizer and a sensitive and extremely powerful relay for general laboratory purposes; a change of potential in any circuit of 0.01 V will serve to switch on or off about 100 W of d-c. power and this can be very effectively utilized in processes such as potentiometric titration. The potentiostatic control of heavy currents is achieved by using the output of the basic potentiostat as the control current in a saturable reactor. This in turn is used to control the a.c. input to any kind of rectifier which may be desirable. In this way currents of any magnitude can be controlled without appreciable waste of electrical power; for general laboratory work currents of up to 10 A are usually adequate and have been used in this way, but there is no obvious reason why the method could not be applied to much greater currents and even to large scale electrolytic processes. The use of an electromagnetic link such as a saturable reactor necessitates a slight time delay in the response of the cell current to electrode potential change, but for many purposes this is not a serious limitation. BASIC
POTENTIOSTAT-CIRCUIT
AND
CONSTRUCTION
The general circuit of the basic potentiostat is shown in Fig. 1. Direct current is fed from a rectifier El to the output terminals through a high current valve V3 and its magnitude is varied by means of the potential impressed on its grid by the control circuit. This latter is a version of the familiar Schmitt trigger circuit3 in which two halves of a twin triode VI/V2 are coupled through a common cathode resistor RI. For high values of RI the Schmitt trigger circuit functions as a relay in which the current through V2 is switched on and off at two different control voltages,4 but as Rl is decreased these values become closer together until eventually they coincide. In this condition, as discussed by Colebrook, the circuit behaves as a d.c. amplifier of very high sensitivity, the amplification decreasing as Rl is further reduced. By adjusting RI to just below its critical value, stable voltage amplification of about 5000 times can be obtained provided the operating d.c. voltages E2, E3 and E4 are provided from independent small rectifiers. The voltage swing on the anode of V2 is then sufficient when impressed on the grid of V3 through the negative bias E5 to vary the main current from zero up to its maximum value for a change of input control voltage of about
A simple potentiostat for general laboratory use
163
0.01 V. By setting E4 at an appropriate value and using R2 as a fine control, the set input voltage at which the control circuit operates can be accurately adjusted to 0 V. If now the input to the control terminals is provided by a working voltage opposed by a potentiometer in the correct sense, the current will be automatically adjusted to such a value that the two voltages are equal within 0.01 V. For satisfactory continuous operation of the potentiostat, the set input potential at which the control circuit operates must of course remain constant. Although all the d.c. voltages El, E2, E3, E4 and E5 are derived from built-in rectifiers described below and variations in them tend to cancel out to some extent, the circuit as originally
I
Mazda
I
12 El
-+
output -o-
FIG. I. General circuit of basic potentiostat.
used was affected by fluctuations in mains voltage. It was discovered, however, that this difficulty could be very easily surmounted by using a valve as the anode load of Vl instead of a fixed resistance, and this is the function of V4 which is half a twin triode identical with Vl/V2. The resistance of this valve is dependent on its grid bias which is governed by the resistor in its cathode lead. By changing the value of this resistor it was found that the direction of shift of the input voltage with mains voltage variation could be reversed, and an optimum value of the resistor was found at which the critical input voltage was completely independent of mains voltage. With the circuit as shown the critical input voltage is constant within 0.01 V at any a.c. mains voltage between 150 and 250 V without adjustment. No considerable shift in the critical input voltage occurs even in the first 5 min after switching on while thermal equilibrium is being set up, and the potentiostat can be used almost immediately if desired. The circuit components are in general sufficiently indicated in Fig. 1 and need little additional description. The main output valve V3 is a beam tetrode, Ediswan 12E1, which is used triode-connected; it has a very high working anode current of 0.3 A and this can be substantially exceeded for short periods without any apparent ill effects. Any high slope twin triode will serve for the control circuit valve Vl/V2; a Brimar
164
A.
HICKLING
6SL7GT has usually been used, but replacement by a Mullard ECC33 scarcely affected the functioning of the circuit. The milliammeter shown in the anode lead of V2 is a small &5 mA moving coil instrument; it serves a useful purpose in initially setting the potentiostat and in indicating the operation on the control circuit. The circuit diagrams of the rectifier supplies El-E5 are shown in Fig. 2. The main d.c. supply El is a purely conventional half-wave metal rectifier supply with its own independent transformer and condenser-choke smoothing. The subsidiary d.c. supplies are derived from separate secondaries of a common transformer; they use small half-wave metal rectifiers with resistance-capacity smoothing, and the output voltages can be adjusted
FIG. 2. Internal rectifier supplies to potentiostat. (a) heavy duty rectifier El for main output current (b) subsidiary rectifiers E2, 3, 4 and 5.
in use these subsidiary d.c. by means of potentiometers as shown. For convenience supplies are built into small individual aluminium boxes made to plug into 5-pin valve bases; the contacts on these serve for the a.c. input and d.c. output, and the boxes are earthed through the remaining pin. All the circuits are set up on a common steel chassis and contained in a steel cabinet. On the panel are mounted positive and negative output terminals, with an output switch and 1A fuse. There are also the grid and cathode control terminals, the milliammeter and the control spindles of the rheostats Rl, R2 and R3; these rheostats are of the pre-set type with slots for screwdriver adjustment. No particular care is necessary in wiring the potentiostat, but it is desirable to keep all leads in the control circuit as short and direct as possible and to avoid introducing stray capacities; good insulation between the separate d.c. supplies in the control circuit is also essential. It is noteworthy that the potentiostat was originally used in a very primitive laboratory The only time when trouble was encountered hook-up and gave complete satisfaction. with it was after it had been professionally wired by an electronics engineer. This was
A simple potentiostat
for general laboratory
use
165
traced to the long runs of connecting wires side by side in neat bundles which apparently led to some interference in the control circuit, and disappeared when direct point-to-point wiring was restored. ADJUSTMENT
AND
EXTERNAL
CONNECTIONS
The potentiostat is used in conjunction with an external potentiometer-voltmeter, and a multirange milliammeter is employed for current measurement. A useful set-up
output
6
Control +i
A
i
P
P I
Control
resistance
+ 0
Output
-
4 A
(b)
Cdl
+
I
output
-
Control
+i
-L
_
j
s
c-I T
Control +i
j
1,
FIG. 3. External connections of potentiostat (a) for current stabilization (b) for control of anode potential (c) for control of cathode potential
for initial adjustment and trial is the current stabilizer circuit shown in Fig. 3(a). S is a standard Weston cell and with the 2-way switch to the right the control circuit is first adjusted so that V2 switches on and off when the potentiometer voltage is changed from 1.01 to I.03 V. This is done by starting with a high value of Rl, so that V2 switches on and off at two different potentiometer voltages, and decreasing it to just below the critical value at which the two voltages coincide. E4 and R2 are then adjusted until the current through V2 is a maximum at 0.01 V below the standard cell voltage and reduced to zero at 0.01 V above the standard cell voltage. R3 is adjusted
166
A. HICKLING
until this balance point is independent of a.c. mains voltage. The potentiostat is now set and should require no further adjustment. The stability of the critical control voltage can be checked at any time by balancing the potentiometer-voltmeter against the standard cell as indicated above. As a rough guide to adjustment, the following representative values for a potentiostat in use are quoted, but it should be emphasised that exact values must be found by trial. Rl 56Q, R2 750 Q, R3 820 Q. El 300 V, E2 260 V, E3 275 V (V2 off), 240 V (V2 on), E4 150 V, E5 280 V. In first adjusting the potentiostat it is useful to check with a microammeter for grid current in VI and V3; no detectable deflection should be observed. Now with the 2-way switch to the left and the potentiostat output switched on the current flowing should always be such that the voltage drop across the control resistance box is equal to any excess voltage set up on the potentiometer. This can be tested very readily for a range of values of control resistance and potentiometer voltage, and it has been found to hold with control resistances varying from 5 to 10,000 Q and currents of lo4 to 0.2 A. At any particular value the current is quite independent of the series resistance box in circuit (provided the maximum output voltage of the potentiostat is adequate); for example, with a 200 Q control resistance and I.00 V excess voltage on the potentiometer the current which passes is 5.00 x 10e3 A and no perceptible change in this can be detected when the series resistance is abruptly changed from 0 to 10,000 R. Practically any degree of current stabilization can be achieved if the potentiometer voltage is made large enough and if it can be maintained steady. For potentiostatic electrolysis the circuits normally used are shown in Fig. 3(b) and 3(c) for anodic and cathodic polarisation respectively. With a saturated calomel reference electrode, for a potentiometer voltage P, the corresponding working anode potential CTis given by rr=Pt- 0.25- 1.02V and the corresponding
cathode
potential
by
n = -P + 0.25 + 1.02V. Thus anode potentials can be obtained at any value more positive than -0.77 V on the hydrogen scale, and cathode potentials at any value more negative than + 1.27 V on the hydrogen scale, and this covers any range likely to be required in practice (these starting potentials can, of course, be further displaced by using two standard cells in series). With the circuits indicated the potentiostat will pass current only in one direction through the electrolytic cell, and this current will be automatically increased or decreased in the range 0 to 0.3 A for any potential arbitrarily chosen for the working electrode. This way of working is what the present author has generally found to be the most convenient. If, however, it is desired that current shall pass in either direction through the electrolytic cell, this can be readily achieved by passing a separate direct current through the cell in the opposite sense to that provided by the potentiostat. With this kind of bridge circuit a net current will flow in either direction as necessary according to the chosen potential of the working electrode. A Solatron Vari-Pack (Model SRS 153) has been found to be a convenient d.c. source to use in conjunction with the potentiostat in this way. Its output current is controlled by a built-in pentode
A simple potentiostat
for general laboratory
use
167
circuit and it can be set to its maximum value of 0.1 A; there is then available potentiostatically controlled current of -0.1 to $-O-2 A through the cell.
a
CONTROLOFHEAVYCURRENTS
The maximum current of the basic potentiostat can of course several output valves in parallel, but for really heavy currents it is current from the potentiostat as a signal current in another power which has many advantages in simplicity and convenience is the
be raised by using simplest to use the amplifier, and one saturable reactor.6
current
Control from potentiostot
d.C. output
Rectifier -
FIG.
4. Potentiostatic
control of heavy currents using a saturable reactor.
Penther and Pompeo’ first used a saturable reactor in a potentiostat circuit, but in conjunction with a mechanical motor control. Lingane pointed outs that it should be possible to retain the inherent advantages of saturable reactor control while dispensing with mechanical components, and this has been achieved in the present work. The basic circuit is shown in Fig. 4. The saturable reactor is put in series with the a.c. input to a suitable d.c. rectifier source, and the output current from the basic potentiostat is passed into the control coil of the reactor. With no signal current passing the impedance of the reactor is very high, and the output from the rectifier is minimal. As the signal current increases so the reactor impedance falls and the rectifier output rises. Thus such a current will pass as is appropriate to the selected potential difference between the working and reference electrodes. The saturable reactor used in the present work is one of Type 820 supplied by Electra Methods Ltd. Its output is 200 W controlled by a signal current of 0.04 A This resistance has been increased to flowing in a control coil of 2200 Q resistance. 5000 !J by means of a series resistor and the output from the basic potentiostat is applied directly to it. The saturable reactor is placed in series with the a.c. input to two Labgear Eliminac units, Type B2027. These are useful laboratory rectifier supplies providing well-smoothed d.c., and each has a maximum voltage of about 20 V and maximum current of 5 A. By using them in series or parallel according to requirements potentiostatically controlled outputs of up to 40 V or 10 A are available. Two minor Since a saturable reactor, like other practical difficulties have had to be overcome. electromagnetic devices, has an appreciable response time of a fraction of a second, whereas the basic potentiostat is practically instantaneous, there is the possibility of
168
A. HICKLING
‘hunting’ if the signal current is varying rapidly. This can be completely avoided by introducing a definite delay in the operation of the basic potentiostat, and this is easily achieved by placing a large capacity condenser between the anode and negative lead of V2. A 200 ,uF, 350 V working electrolytic condenser is used in the present instrument, arranged so that it can be switched in or out as desired. The overall response time of the apparatus is still short compared to that of most mechanical potentiostats. A further feature of the saturable reactor is that even with zero signal current it will normally pass a small standing current, so that the d.c. output from the rectifiers does not fall completely to zero even when the current from the basic potentiostat does. Where this minimal residual current is undesirable it can readily be balanced out by applying a cancelling d.c. output across the cell, and in the present set-up a third Labgear Eliminac unit is used for this purpose. POSSIBLE
DEVELOPMENTS
Attention has been concentrated in the present work on devising a very simple potentiostat for general laboratory use, but the instrument has obvious possibilities for further development in particular applications. Much interest has been taken recently in devising very high-speed potentiostats. s The basic instrument described here may have possibilities in this connection since the time of operation of the Schmitt circuit is of the order of 10e5 sec. Furthermore if a single stage of pre-amplification were used, the sensitivity of the potentiostat could probably be increased to &to.001 V. If it is desired to increase the output current while retaining the rapid action of the basic potentiostat, several output valves in parallel, or the Ediswan beam tetrode 13E. 1, which will pass up to 1 A anode current, might be employed. There seems to be no effective limit to the currents which can be dealt with when a saturable reactor is employed in conjunction with the basic potentiostat. Laboratory pattern saturable reactors with outputs of 5000 W for O-3 A signal current are available, and even larger types could be operated using the rectified output from a small reactor to supply a substantial current. Thus large scale electrolytic processes could be potentiostatically controlled if desired. Possible applications suggest themselves in electrolytic preparations, in electrodeposition and anodic polishing of metals, and in processes such as cathodic protection. Furthermore the same apparatus could be used not only for controlling electrolysing currents, but equally for controlling any processvariable such as temperature, agitation etc., where an indicator electrode responsive to such changes in the system is available. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
A. HICKLING,Trans. Faraday Sec. 38,27 (1942). J. J. LINGANE,Electroanalytical Chemistry (2nd Ed.) p. 308. Interscience, New York (1958). 0. H. SCHMITT,J. Sci. Znsrrum. 15, 24 (1938). R. H. MUELLER and J. J. LINGANE,An+. Chem. 20, 795 (1948). F. M. COLEBROOK, Wireless Engr. 15, 138 (1938). M. H. ARONSON,Instruments 25, 608 (1952). C. J. PENTHER and D. J. POMPEO,An+. C/zem. 21, 178 (1949). J. J. LINGANE,Electroanalytical Chemistry (2nd Ed.), p. 327. Interscience, New York (1958). H. GER~SCHER and K. E. STAUBACH,Z. Elektrochem. 61, 789 (1957); A. BEWICK,A. BEWICK, M. FLEISCHMANN and M. LILER,Elecfrochim. Acta 1, 83 (1959); F. G. WILL, Z. Elektrochem. 63, 484, 689 (1959); R. SCHINDLER, H. WILL and L. HOLLECK,ibid. 63, 596 (1959).
PergamonPress Ltd. Printedin Northern Ireland
A SIMPLE POTENTlOSTAT FOR GENERAL LABORATORY USE* A. HICKLING University of Liverpool, England Abstract-A very simple electronic potentiostat for general laboratory use is described which will maintain the potential of a working electrode within i-O.01 V of a set value over any desired period, the output current supplied being automatically controlled in the range 0 to 0.3 A. By combining this basic instrument with a saturable reactor currents of any magnitude can be controlled without appreciable waste of electrical power. Applications and further possible developments of the potentiostat are discussed. R&urn&On dCcrit un simple potentiostat Clectronique d’utilisation g&&ale en laboratoire qui permet de maintenir le potentiel d’une Electrode b moins de &O,Ol Vd ‘une valeur donn6e pendant une pCriode de temps indefinie le courant debit.5 &ant contrljle automatiquement dans l’intervalle de 0 g 0,3 A, En combinant cet instrument de base avec un r&cteur saturable des courants de grandeur quelconque peuvent Ctre contrBl& sans perte de puissance apprtciable. On discute les applications et les dCveloppements ult&ieurs possibles de ce potentiostat. Zusammenfassung-Es wird ein sehr einfacher elektronischer Potentiostat beschrieben, der das vorgewlhlte Potential einer Arbeitselektrode innerhalb +O,Ol V fiber einen beliebigen Zeitraum konstant zu halten erlaubt durch automatische Regelung des abgegebenen Stromes im Bereich von 0 bis 0,3 A. Durch Kombination dieses Instrumentes mit einem magnetischen VerstHrker kannen StrGme beliebiger Griisse beinahe leistungslos geregelt werden. Anwendungsbeispiele und weitere EntwicklungsmGglichkeiten des Potentiostaten werden diskutiert. INTRODUCTION
1942 the present author introduced1 the idea of the automatic control of the potential of a working electrode and described an electronic device to achieve this for which Since that time the potentiostatic method has become he coined the name potentiostat. an established electrochemical technique, and a large number of potentiostats, some of very considerable complexity, have been described in the literature.2 All potentiostats operate on the same general principle in that the potential difference between the working electrode and a suitable reference electrode is continually compared with a voltage derived from a potentiometer, and the difference between the two-the error signal-is amplified and used to control the current passing through the electrolytic cell in such a way that the error signal is minimized. The great variety of potentiostats arises from the different methods used for amplifying the error signal, and for the control of the cell current, this latter being variously achieved by electronic, electromechanical or electromagnetic devices. The ideal general purpose laboratory potentiostat should be sensitive to potential shifts of about 0.01 V or less, it should be practically instantaneous in response and of high input impedance, and it should be capable of controlling currents varying from 10W6to at least 10 A; furthermore it should not require any adjustment other than setting the potentiometer voltage, and above all for general utility it should be compact and cheap. IN
* Manuscript received 31 December 1960. 161
162
A. HICKLING
A potentiostat which goes a long way towards meeting these requirements has been in use in the author’s laboratory for the past 6 years. It is the final result of potentiostat development over a long period, and it has proved so trouble-free in operation that it may be of interest to electrochemists generally who require a very simple instrument in order to employ potentiostatic techniques. The basic instrument meets the following specification. It is a rectifier working from the a.c. mains, and its output voltage can be varied from 0 to 300 V and the current from 0 to 0.3 A. This output is controlled entirely by an error signal of about 0.01 V, and the potential of the working electrode can be maintained within &to*01 V of a set value over any desired period. The instrument is all electronic and virtually instantaneous in response, and the current drawn from the control circuit is less than 10e6 A. It is furthermore independent of mains voltage and will work without adjustment over periods of several months. It is completely housed in a cabinet of dimensions 45 x 30 x 25 cm and the cost of the component parts is less than &25. The potentiostat may be used either for anodic or cathodic polarization, or by using it in a bridge circuit current may be passed in either direction through the cell. The instrument will serve also as a current stabilizer and a sensitive and extremely powerful relay for general laboratory purposes; a change of potential in any circuit of 0.01 V will serve to switch on or off about 100 W of d-c. power and this can be very effectively utilized in processes such as potentiometric titration. The potentiostatic control of heavy currents is achieved by using the output of the basic potentiostat as the control current in a saturable reactor. This in turn is used to control the a.c. input to any kind of rectifier which may be desirable. In this way currents of any magnitude can be controlled without appreciable waste of electrical power; for general laboratory work currents of up to 10 A are usually adequate and have been used in this way, but there is no obvious reason why the method could not be applied to much greater currents and even to large scale electrolytic processes. The use of an electromagnetic link such as a saturable reactor necessitates a slight time delay in the response of the cell current to electrode potential change, but for many purposes this is not a serious limitation. BASIC
POTENTIOSTAT-CIRCUIT
AND
CONSTRUCTION
The general circuit of the basic potentiostat is shown in Fig. 1. Direct current is fed from a rectifier El to the output terminals through a high current valve V3 and its magnitude is varied by means of the potential impressed on its grid by the control circuit. This latter is a version of the familiar Schmitt trigger circuit3 in which two halves of a twin triode VI/V2 are coupled through a common cathode resistor RI. For high values of RI the Schmitt trigger circuit functions as a relay in which the current through V2 is switched on and off at two different control voltages,4 but as Rl is decreased these values become closer together until eventually they coincide. In this condition, as discussed by Colebrook, the circuit behaves as a d.c. amplifier of very high sensitivity, the amplification decreasing as Rl is further reduced. By adjusting RI to just below its critical value, stable voltage amplification of about 5000 times can be obtained provided the operating d.c. voltages E2, E3 and E4 are provided from independent small rectifiers. The voltage swing on the anode of V2 is then sufficient when impressed on the grid of V3 through the negative bias E5 to vary the main current from zero up to its maximum value for a change of input control voltage of about
A simple potentiostat for general laboratory use
163
0.01 V. By setting E4 at an appropriate value and using R2 as a fine control, the set input voltage at which the control circuit operates can be accurately adjusted to 0 V. If now the input to the control terminals is provided by a working voltage opposed by a potentiometer in the correct sense, the current will be automatically adjusted to such a value that the two voltages are equal within 0.01 V. For satisfactory continuous operation of the potentiostat, the set input potential at which the control circuit operates must of course remain constant. Although all the d.c. voltages El, E2, E3, E4 and E5 are derived from built-in rectifiers described below and variations in them tend to cancel out to some extent, the circuit as originally
I
Mazda
I
12 El
-+
output -o-
FIG. I. General circuit of basic potentiostat.
used was affected by fluctuations in mains voltage. It was discovered, however, that this difficulty could be very easily surmounted by using a valve as the anode load of Vl instead of a fixed resistance, and this is the function of V4 which is half a twin triode identical with Vl/V2. The resistance of this valve is dependent on its grid bias which is governed by the resistor in its cathode lead. By changing the value of this resistor it was found that the direction of shift of the input voltage with mains voltage variation could be reversed, and an optimum value of the resistor was found at which the critical input voltage was completely independent of mains voltage. With the circuit as shown the critical input voltage is constant within 0.01 V at any a.c. mains voltage between 150 and 250 V without adjustment. No considerable shift in the critical input voltage occurs even in the first 5 min after switching on while thermal equilibrium is being set up, and the potentiostat can be used almost immediately if desired. The circuit components are in general sufficiently indicated in Fig. 1 and need little additional description. The main output valve V3 is a beam tetrode, Ediswan 12E1, which is used triode-connected; it has a very high working anode current of 0.3 A and this can be substantially exceeded for short periods without any apparent ill effects. Any high slope twin triode will serve for the control circuit valve Vl/V2; a Brimar
164
A.
HICKLING
6SL7GT has usually been used, but replacement by a Mullard ECC33 scarcely affected the functioning of the circuit. The milliammeter shown in the anode lead of V2 is a small &5 mA moving coil instrument; it serves a useful purpose in initially setting the potentiostat and in indicating the operation on the control circuit. The circuit diagrams of the rectifier supplies El-E5 are shown in Fig. 2. The main d.c. supply El is a purely conventional half-wave metal rectifier supply with its own independent transformer and condenser-choke smoothing. The subsidiary d.c. supplies are derived from separate secondaries of a common transformer; they use small half-wave metal rectifiers with resistance-capacity smoothing, and the output voltages can be adjusted
FIG. 2. Internal rectifier supplies to potentiostat. (a) heavy duty rectifier El for main output current (b) subsidiary rectifiers E2, 3, 4 and 5.
in use these subsidiary d.c. by means of potentiometers as shown. For convenience supplies are built into small individual aluminium boxes made to plug into 5-pin valve bases; the contacts on these serve for the a.c. input and d.c. output, and the boxes are earthed through the remaining pin. All the circuits are set up on a common steel chassis and contained in a steel cabinet. On the panel are mounted positive and negative output terminals, with an output switch and 1A fuse. There are also the grid and cathode control terminals, the milliammeter and the control spindles of the rheostats Rl, R2 and R3; these rheostats are of the pre-set type with slots for screwdriver adjustment. No particular care is necessary in wiring the potentiostat, but it is desirable to keep all leads in the control circuit as short and direct as possible and to avoid introducing stray capacities; good insulation between the separate d.c. supplies in the control circuit is also essential. It is noteworthy that the potentiostat was originally used in a very primitive laboratory The only time when trouble was encountered hook-up and gave complete satisfaction. with it was after it had been professionally wired by an electronics engineer. This was
A simple potentiostat
for general laboratory
use
165
traced to the long runs of connecting wires side by side in neat bundles which apparently led to some interference in the control circuit, and disappeared when direct point-to-point wiring was restored. ADJUSTMENT
AND
EXTERNAL
CONNECTIONS
The potentiostat is used in conjunction with an external potentiometer-voltmeter, and a multirange milliammeter is employed for current measurement. A useful set-up
output
6
Control +i
A
i
P
P I
Control
resistance
+ 0
Output
-
4 A
(b)
Cdl
+
I
output
-
Control
+i
-L
_
j
s
c-I T
Control +i
j
1,
FIG. 3. External connections of potentiostat (a) for current stabilization (b) for control of anode potential (c) for control of cathode potential
for initial adjustment and trial is the current stabilizer circuit shown in Fig. 3(a). S is a standard Weston cell and with the 2-way switch to the right the control circuit is first adjusted so that V2 switches on and off when the potentiometer voltage is changed from 1.01 to I.03 V. This is done by starting with a high value of Rl, so that V2 switches on and off at two different potentiometer voltages, and decreasing it to just below the critical value at which the two voltages coincide. E4 and R2 are then adjusted until the current through V2 is a maximum at 0.01 V below the standard cell voltage and reduced to zero at 0.01 V above the standard cell voltage. R3 is adjusted
166
A. HICKLING
until this balance point is independent of a.c. mains voltage. The potentiostat is now set and should require no further adjustment. The stability of the critical control voltage can be checked at any time by balancing the potentiometer-voltmeter against the standard cell as indicated above. As a rough guide to adjustment, the following representative values for a potentiostat in use are quoted, but it should be emphasised that exact values must be found by trial. Rl 56Q, R2 750 Q, R3 820 Q. El 300 V, E2 260 V, E3 275 V (V2 off), 240 V (V2 on), E4 150 V, E5 280 V. In first adjusting the potentiostat it is useful to check with a microammeter for grid current in VI and V3; no detectable deflection should be observed. Now with the 2-way switch to the left and the potentiostat output switched on the current flowing should always be such that the voltage drop across the control resistance box is equal to any excess voltage set up on the potentiometer. This can be tested very readily for a range of values of control resistance and potentiometer voltage, and it has been found to hold with control resistances varying from 5 to 10,000 Q and currents of lo4 to 0.2 A. At any particular value the current is quite independent of the series resistance box in circuit (provided the maximum output voltage of the potentiostat is adequate); for example, with a 200 Q control resistance and I.00 V excess voltage on the potentiometer the current which passes is 5.00 x 10e3 A and no perceptible change in this can be detected when the series resistance is abruptly changed from 0 to 10,000 R. Practically any degree of current stabilization can be achieved if the potentiometer voltage is made large enough and if it can be maintained steady. For potentiostatic electrolysis the circuits normally used are shown in Fig. 3(b) and 3(c) for anodic and cathodic polarisation respectively. With a saturated calomel reference electrode, for a potentiometer voltage P, the corresponding working anode potential CTis given by rr=Pt- 0.25- 1.02V and the corresponding
cathode
potential
by
n = -P + 0.25 + 1.02V. Thus anode potentials can be obtained at any value more positive than -0.77 V on the hydrogen scale, and cathode potentials at any value more negative than + 1.27 V on the hydrogen scale, and this covers any range likely to be required in practice (these starting potentials can, of course, be further displaced by using two standard cells in series). With the circuits indicated the potentiostat will pass current only in one direction through the electrolytic cell, and this current will be automatically increased or decreased in the range 0 to 0.3 A for any potential arbitrarily chosen for the working electrode. This way of working is what the present author has generally found to be the most convenient. If, however, it is desired that current shall pass in either direction through the electrolytic cell, this can be readily achieved by passing a separate direct current through the cell in the opposite sense to that provided by the potentiostat. With this kind of bridge circuit a net current will flow in either direction as necessary according to the chosen potential of the working electrode. A Solatron Vari-Pack (Model SRS 153) has been found to be a convenient d.c. source to use in conjunction with the potentiostat in this way. Its output current is controlled by a built-in pentode
A simple potentiostat
for general laboratory
use
167
circuit and it can be set to its maximum value of 0.1 A; there is then available potentiostatically controlled current of -0.1 to $-O-2 A through the cell.
a
CONTROLOFHEAVYCURRENTS
The maximum current of the basic potentiostat can of course several output valves in parallel, but for really heavy currents it is current from the potentiostat as a signal current in another power which has many advantages in simplicity and convenience is the
be raised by using simplest to use the amplifier, and one saturable reactor.6
current
Control from potentiostot
d.C. output
Rectifier -
FIG.
4. Potentiostatic
control of heavy currents using a saturable reactor.
Penther and Pompeo’ first used a saturable reactor in a potentiostat circuit, but in conjunction with a mechanical motor control. Lingane pointed outs that it should be possible to retain the inherent advantages of saturable reactor control while dispensing with mechanical components, and this has been achieved in the present work. The basic circuit is shown in Fig. 4. The saturable reactor is put in series with the a.c. input to a suitable d.c. rectifier source, and the output current from the basic potentiostat is passed into the control coil of the reactor. With no signal current passing the impedance of the reactor is very high, and the output from the rectifier is minimal. As the signal current increases so the reactor impedance falls and the rectifier output rises. Thus such a current will pass as is appropriate to the selected potential difference between the working and reference electrodes. The saturable reactor used in the present work is one of Type 820 supplied by Electra Methods Ltd. Its output is 200 W controlled by a signal current of 0.04 A This resistance has been increased to flowing in a control coil of 2200 Q resistance. 5000 !J by means of a series resistor and the output from the basic potentiostat is applied directly to it. The saturable reactor is placed in series with the a.c. input to two Labgear Eliminac units, Type B2027. These are useful laboratory rectifier supplies providing well-smoothed d.c., and each has a maximum voltage of about 20 V and maximum current of 5 A. By using them in series or parallel according to requirements potentiostatically controlled outputs of up to 40 V or 10 A are available. Two minor Since a saturable reactor, like other practical difficulties have had to be overcome. electromagnetic devices, has an appreciable response time of a fraction of a second, whereas the basic potentiostat is practically instantaneous, there is the possibility of
168
A. HICKLING
‘hunting’ if the signal current is varying rapidly. This can be completely avoided by introducing a definite delay in the operation of the basic potentiostat, and this is easily achieved by placing a large capacity condenser between the anode and negative lead of V2. A 200 ,uF, 350 V working electrolytic condenser is used in the present instrument, arranged so that it can be switched in or out as desired. The overall response time of the apparatus is still short compared to that of most mechanical potentiostats. A further feature of the saturable reactor is that even with zero signal current it will normally pass a small standing current, so that the d.c. output from the rectifiers does not fall completely to zero even when the current from the basic potentiostat does. Where this minimal residual current is undesirable it can readily be balanced out by applying a cancelling d.c. output across the cell, and in the present set-up a third Labgear Eliminac unit is used for this purpose. POSSIBLE
DEVELOPMENTS
Attention has been concentrated in the present work on devising a very simple potentiostat for general laboratory use, but the instrument has obvious possibilities for further development in particular applications. Much interest has been taken recently in devising very high-speed potentiostats. s The basic instrument described here may have possibilities in this connection since the time of operation of the Schmitt circuit is of the order of 10e5 sec. Furthermore if a single stage of pre-amplification were used, the sensitivity of the potentiostat could probably be increased to &to.001 V. If it is desired to increase the output current while retaining the rapid action of the basic potentiostat, several output valves in parallel, or the Ediswan beam tetrode 13E. 1, which will pass up to 1 A anode current, might be employed. There seems to be no effective limit to the currents which can be dealt with when a saturable reactor is employed in conjunction with the basic potentiostat. Laboratory pattern saturable reactors with outputs of 5000 W for O-3 A signal current are available, and even larger types could be operated using the rectified output from a small reactor to supply a substantial current. Thus large scale electrolytic processes could be potentiostatically controlled if desired. Possible applications suggest themselves in electrolytic preparations, in electrodeposition and anodic polishing of metals, and in processes such as cathodic protection. Furthermore the same apparatus could be used not only for controlling electrolysing currents, but equally for controlling any processvariable such as temperature, agitation etc., where an indicator electrode responsive to such changes in the system is available. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
A. HICKLING,Trans. Faraday Sec. 38,27 (1942). J. J. LINGANE,Electroanalytical Chemistry (2nd Ed.) p. 308. Interscience, New York (1958). 0. H. SCHMITT,J. Sci. Znsrrum. 15, 24 (1938). R. H. MUELLER and J. J. LINGANE,An+. Chem. 20, 795 (1948). F. M. COLEBROOK, Wireless Engr. 15, 138 (1938). M. H. ARONSON,Instruments 25, 608 (1952). C. J. PENTHER and D. J. POMPEO,An+. C/zem. 21, 178 (1949). J. J. LINGANE,Electroanalytical Chemistry (2nd Ed.), p. 327. Interscience, New York (1958). H. GER~SCHER and K. E. STAUBACH,Z. Elektrochem. 61, 789 (1957); A. BEWICK,A. BEWICK, M. FLEISCHMANN and M. LILER,Elecfrochim. Acta 1, 83 (1959); F. G. WILL, Z. Elektrochem. 63, 484, 689 (1959); R. SCHINDLER, H. WILL and L. HOLLECK,ibid. 63, 596 (1959).