Electronic Controlled-Potential Coulometric Titrator M. T. KELLEY, H. C. JONES, and D. J. FISHER Analyfical Chernisfry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b A simple electronic instrument for coulometric redox titrations a t controlled potential is described. The potential of only the electrode a t which the desired reaction occurs is controlled by a stabilized difference amplifier combined with a transistor current amplifier. The electrolysis current is integrated by a stabilized amplifier. The integral is read out a s a voltage. Either manual or automatic titrations may b e made. The instrument is alternating current lineoperated. It is very stable and gives titration results of good precision which are obtained by calculation from Faraday‘s law.
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electronic instruments for coulometric redox titrations a t controlled potential have been described by Booman ( 1 , 2 ) and illerritt, Martin, and Bedi (5). Electrolysis at controlled potential provides electrochemical selectivity. The principles and applications of controlled potential coulometric titrations have been reviewed by these authors and by Lingane (4). The instruments described by Booman and by Merritt et al. electronically integrate the electrolysis current by different methods and results of good precision are obtained by both. The electronic controlled-potential coulometric titrator uses the methods of control of electrode potential and of electronic integration described by Booman. It has the advantage of simplicity, which is reflected in lower costs of fabrication and of maintenance. Only the potential of the electrode a t which the desired reaction occurs is controlled. The second potential control amplifier system that is required in Booman’s instrument has been eliminated by using a difference amplifier for potential control. The use of a transistor current amplifier stage greatly simplifies the attainment of high initial electrolysis current necessary for short titration times, because it is used to control the current delivered from a simple and compact low voltage supply. The vacuum tube regulating circuits used by Booman must operate a t high voltage and waste power. The cell current integrator differs from that designed by Booman only in that one GAP/R Model USA-3-M3 printed circuit universal stabilized operational amplifier LEGANT
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replaces the GAP/R Models K2-X and K2-P operational rbmplifiers used by Booman (6). All mercury batteries have been eliminated and this instrument is alternating current line-operated. It may also be used for either oxidation or reduction coulometric titrations a t controlled potential and titrates with good precision. Either manual or automatic titrationf, can be made with this instrument. Titrations are made in an inert atmosphere provided by a nitrogen blanket. Titration assemblies have been designed and ’abricated for bench top analyses and for entirely remotely manipulated analyses in the high radiation level analytical facility, which has been described (3). For controlled anode-potential oxidation titrations, such as those of I- :r iron(II), platinum electrodes are used, and the cathode is isolated by a salt bridge and frit barrier. For controlled cathode-potential reduction titrations, such as those of uranium(VI) or copper(II), the anode is a platinum wire that is iirolated by a sulfuric acid salt bridge and a frit barrier and the cathode is a mercury pool. For reduction titrations, a stirrer driven by a constant speed motor is positioned vertically so that it, thoroughly agitates the mercury-solutim interface to obtain a high initial c ~ r r e n t . A standard saturated calomel electrode or silversilver chloride reference electrode, positioned as close t c the controlled electrode as possible, is used in both cases. The titration is Oerminated when the current drops to :Lpredetermined fraction of the initial wrrent, usually at 50 paThe principles of operation of the electronic contrclled-potential coulometric titrator for reduction titrations are shown in Figure 1; Figure 2 shows the operation for oxidation titrations. The constituents :ire cell, control potential source, contrcll amplifier, cell power supply, operatiolial amplifiers power supply, current integrator, and integral readout device. ‘L‘o obtain the correct circuit connectioiie that are shown in Figures 1 and 2 for oxidation and reduction operalion, sections of the “reduce-oxidize” switch perform the following functiciis. The switch sections are labeled in Figure 3. Sections 1 and 2 reverse the polarity of connections to the cell current meter.
Sections 3 and 4 reverse the output polarity of the potential control amplifier. Section 5 reverses the sign of the I,, compensating current. Sections 6 and 7 reverse the polarity of the cell power supply and switch the transistor connections. Sections 8 and 9 reverse the leads to the readout device. The latter switching operation is done for convenience because the sign of the current integrator output voltage is negative for reduction and positive for oxidation. Relays 1 and 2 are actuated only when the toggle switch is in the “titrate” position and not when it is thrown to “off” or t o “reset.” The contacts of relay 1 connect the control amplifier either to the titration cell or to a dummy cell consisting of a 120and a &ohm resistor in series. The A contacts of relay 2 connect the cell power supply t o the working electrode of the cell only when the toggle switch is in the titrate position. The B contacts of relay 2 are used to switch the connections to the base of the transistor for titration or for nontitration operation. The command signal to the control amplifier is the algebraic sum of the control potential from the control potential source and the potential of the controlled electrode with respect to the solution, as seen through the reference electrode. The control potential is a selected fraction of the constant potential across a silicon voltage reference (Zener) diode. A simple half-wave silicon diode power supply supplies the voltage to the voltage reference diode. The potential drop across the reference diode is very constant a t about 5 volts and has a very small temperature coefficient (7). The control amplifier consists of a difference amplifier that controls a current amplifier. The difference amplifier, by negative feedback, maintains its two inputs a t equal potential. The chopper-stabilized difference amplifier is a modified GAPPJR Model USA3-M3 printed circuit universal stabilized amplifier (6). It is modified (Note 4, Table I) so that the chopper references against input R (pin 9 of connector A ) rather than against ground. It then becomes a chopper-stabilized difference amplifier, and its output signal is directly proportional to the difference between its two input signals. This means that it is not necessary t o maintain one of the cell electrodes a t ground potential, as must be done in Booman’s
instrument, by a second potential control amplifier system. The identities of other pins of connector A are as follows: pin 1 is the regular input terminal, pin 6 is the output terminal, and pins 2, 4, and 10 are grounded terminals. The two input channels of the difference amplifier do not have similar frequency responses. The regular input channel has wide frequency response but the reference input channel, because of a long time constant output filter, and because i t is a carrier-type chopper stabilizing amplifier, has limited frequency response. The reference channel will not respond immediately to a step-type input change. It is possible, by operating the chopper amplifier portion of USA-3 No. A from an added independent power supply, to improve the frequency response of the reference channel. However, higher
frequency response in the reference channel is not needed in this instrument and this modification has not been made. The output signal from the difference amplifier is fed into the base of the transistor current amplifier. The transistor acts essentially as an appropriately variable resistance and so controls the current delivered to the cell from the cell power supply. The transistor is a germanium PNP highvoltage power transistor, Motorola Type 2N375 (Motorola Inc.), or equivalent (Note 8). It is only about 1 inch in size, yet can control as much as 3 amperes and costs less than $6.00. The transistor amplifier system is very stable because of the feedback loop that is completed through the cell. The effect of the intrinsic current, L,of the transistor which flows in the collector circuit with a zero base current signal,
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Figure 1 Electronic controlled-potential coulometric titrator block diagram, switched for reduction
is eliminated by a larger compensating current of opposite sign, obtained from the GAP/R Model R-100A power supply. Thcugh is temperature dependent, its value is of no importance as long as i t ' s about the same or less than this conipensating current. The absolute difference between I,, and the compensating current is cancelled by current delivered by the control amplifier, so that i t is not seen by the current integiator or the cell. The control amplifier also causes current that is required by the cell during the electrolysis to be delivered and continuously prevents the potential of the controlled electrode 21s. the reference electrode from exceeding the control potential. The cell poner supply is a full-wave silicon diode voltage doubler circuit. It could delivei. up to 500 ma. but the 160-ohm current limiting resistor is added because the cells being used cannot safely carry currents substantially greater than 300 ma. The maximum collwtor to base voltage rating of the transistor is stated to be 80 volts and the maximum collector to emitter voltage l,o be 60 volts. Reliable operation has k'een obtained with the 55-volt cell pclwer supply. With a 55-volt power supply, the transistor will not be ruptured by momentary voltage transients due l o switching or t o line voltage surges. The cell power supply may be designed to deliver higher currents or voltage;$if two transistors are cascaded to conLrol currents a t higher voltages, if cells that can handle higher currents are ufjed. Because of the limited heat dissipation capacity of the cells, the maximum current delivered by the particular ciicuits shown is limited by the 160-ohm resistor and by the resistance of thi: cell to values well below the maximum rating of the transistor. As a precaution, the transistor is mounted on IL heat sink. At the beginning of a titration, the current may be limited by thefe resistances SO that it will remain for a vhile a t a value equal 160). Although to 55 volts/(Rcell during this period the controlled electrode potential will be less than the control potential, the only effect is to lengthen the titra1,ion time. The circuit of the current integrator is a conventional analog computer circuit described b3. Philbrick (6). Its output voltage is directly proportional to the time integrd of the cell current. With the input re,Gstors that are used, 10/400,000 of the cell current is passed through the integrator. A Stabelex D capacitor is used that has a dielectric material of high quality. A limit light is included, which is illuminated if the limiting potential of the amplifier is reached. The smdl zero offset of the integrating amplifier must be biased out so that the ou1,put voltage remains
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Figure 2. Electronic controlled-potential coulometric titrator block diagram, switched for oxidation
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Figure 3.
Electronic controlled-potential coulomelric titrator
constant after the input current becomes zero, Terminal No. 9 of connector B is the zero offset trim terminal of this ampliier. A suitable potential that is derived from the adjustable voltage and -300-volt divider across the regulated power supply is used for this purpose (Note 9). It is essential to carry out the following construction practices to obtain satisfactory performance of the integrating amplifier. Ground loops in the input circuitry must be avoided-for example, the 10-ohm resistor must be returned to ground by a direct connection to pin E of the power supply connector and the leads a t the connectors of the USA-3 amplifiers that are wired to chassis ground are removed. To prevent drift due to leakage currents, all electrical connections to terminal 1 of connector B must be made with well insulated wiring. To avoid integrating switching
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transients, th:se electrical connections must also be riade with shielded wire. A fraction of the output voltage that is directly pi oportional to the time integral of tl-c cell current is selected by a voltage divider and presented to the readout device. On X1 1 volt of readout is produced by about 40 coulombs thi ough the cell. Substitution of a Helipot with a calibrated dial for the dividw would make it possible to have the readout directly in micrograms titratrd. The integral may be read in either the titrate or off toggle switch positions but is erased to zero magnitude nlien this switch is thrown to the reset position. A readout device of low precision could consist of a current reading meter inserted in series with this divider. For titrations that are inherently of low electrochemical precision, a d a t i v e l y inexpensive readout meter :lees not compromise the
precision. For results of high precision, a correspondingly good readout device must be used. Examples of the latter include a Rubicon Catalog No. 2730, Pointerlite potentiometer (Rubicon Co.) and a Non-Linear Systems Model 481 digital voltmeter (Non-Linear Systems, Inc.). For research or method development purposes, a recorder would provide information about events occurring during the titration. If it is necessary to ground a terminal of the readout device, sections 8 and 9 of the “reduce”‘‘oxidize” switch must be eliminated. Because of the “end point” switch, there is an option of either manual or automatic titrations. If this switch is in the “manual” position, the titration proceeds until the operator throws the toggle switch from titrate to off, I n the “automatic” position, the titration current and integration process are automatically terminated when the cell
current falls to the value preset on the low-limit contacts of the meter relay. Manual titrations are preferred for method development or special analyses. Automatic titrations are satisfactory for replicate determinations by established procedures. The cell current meter indicates the electrolysis current delivered to the working electrode, which is numerically equal to the current consumed a t the controlled electrode. Of the five current ranges of the meter, four are linear and one is nonlinear or logarithmic. X silicon diode prevents meter overload and burnout on all ranges. The diode has no effect on the highest linear current range. On the three lowest linear current ranges, the diode has no effect below each respective full scale current value. At and above these current values, the potential drop across the shunted meter and a series resistor is sufficient t o maintain forward conduction through the diode and the diode bypasses the major portion of the cell current around the meter. Cell currents many times greater than these full scale current values cannot damage the meter, though it be switched to a low range. An adjustable series resistor is provided for the nonlinear logarithmic range, so that the point a t which the diode starts to conduct may be set to an intermediate scale value. For example, if this value be 400 pa., the meter will read linearly and correctly from 0 to 400 pa., but will respond logarithmically to currents above 400 pa. The purpose of the logarithmic current range is to enable the observation of the current without switching ranges during the titration and without eliminating a sensitive meter response near the end point. While i t would be possible to calibrate the titrator relatively by standard solutions, it is usually calibrated absolutely in coulombs per volt of readout. The absolute calibration procedure is as follows: Throw the “control potential supply” switch to the “out” position. Insert a 1.3-volt mercury battery between the reference and the working electrode leads. For reduction calibration, connect the negative battery terminal to the reference lead. For oxidation calibration, reverse the polarity of the battery. Insert a precisely calibrated resistor of approximately 100 ohms between the working and the controlled electrode leads. With the readout device, measure the potential developed across this resistor. Calculate the value of the current. Observe the readout voltage that results from the integration of this current over a measured time interval. The circuit diagram of this controlled-potential coulometric titrator is given in Figure 3. Construction notes (Table I) have been prepared t o assist in the fabrication of this instrument.
Table 1. Construction Notes 1. A GAPjR universal stabilized amplifier model USA-3 (in modular enclosure USA-3-
M-3) plugs into each Amphenol 26-4200-16s conriector. The amplifier that plugs into connector A is modified as described in Note 4.. Cut chassis ground from 10 of both connectors. 2. GAP/R components are manufactured by George A. Philbrick Researches, Inc., 285 Columbus Ave., Boston 16, Mass. 3. Relays are manufactured by Allied Control Co., In(:.,2 East End Ave., New York 21, N. Y .
4. 7VirTng changes in USA-3 (control potential) amplifier ( A )are as follows: connect point a on printed circuit to terminal No. 14 on blue ribbon male connector (264100-16P). connect point c t o terminal No. 13; connect point b t o terminal No. 12; connect pdint d t o terminal No. 11; move pin No. 7 of Airpax A-175 (chopper) from ground t o offset trim terminal No. 1 on printed circuit; replace 12AX7 tube (Vl) in stabilizer section of amplifier with 5751 tube. Leads from terminals 11, 12, 13, 14 to the “reduce”-“oxide” switch should be as short as possible. 5. The silicon voltage reference (Zener) diode shoiild have an approximately zero temperature coefficient (Transitron SV-5 or equivalent). Transitron SV-5 manufactured by Transitron Electronic Corp., Wakefield, Mass. 6. Enough ventilation must be provided so that the ,emperatwe within the cabinet is less than 149”F. Considerable heat is liberated from the GAP/R R-100A and must be removed. 7. For manual readout, the Rubicon Pointerlite Cata og No. 2730 potentiometer which has a range of 1.61 volts is suitable. For automatic,readout, digital voltmeters could be used. 8. The Motorola 2N375 transistor should be provided with a heat sink according t o the manufacturer’s instructions. Delco engineeringdata indicates that their type 2N553 transistor should also be suitable for use in this circuit. 9. The integrator bias should be adjusted for essentially zero drift of the integral readout voltage at the order of magnitude of the voltage reached at the end of the titration. 10. Stabelex-D capacitors are manufactured by Industrial Condenser Corp., 3243 North California Ave., Chicago 18, Ill. 11. Sarkes Tarzian ill 500 (also designated 40 M and 1P11084)rectifiers are manufactured by Sarkes Tarzian, Inc., Rectifier Division, 415 N. College Ave., Bloomington, Ind. 12. All items in quotation marks to be engraved on panel with the exception of the “Integrator Bias” adjustment, “Cont. Pot. Supply”, “Cell” connector, “Readout” connector, and “Power Supply” connector, located on the chassis rear apron. 13. Make the cable length equal to 12 feet. 14. Make the cable length equal to 8 feet. 15. Make the cable length equal to 6 feet. Replace 1,he line cord to the R-100A by a 3-wire cord. 16. Use Teflon insulated wire throughout. 17. A11 electrical connections to terminal 1 of amplificr B must be made with shielded wire and must be made with the highest possible quality of insulation.
Chassis layout drawings and a full size circuit diagram are available from the authors. The circuits of the GAP/R components are available from the manufacturer (6). The drift is so small when the integrator bias adjustment has been made (Note 9) that it can be ignored if the electrolysis time is less than 45 minutes. For precise work, the instrument should be warmed up for 1 hour. The following data illustrates its performance in reduction titrations. Ten 1-ml. aliquots of standard copper sulfate solution each containing 2.263 mg. of copper were titrated with the potential limited to -0.1 volt us. silver-silver chloride. The supporting electrolyte was 5 ml. of 1 N sulfuric acid. The titration times were about 12 minutes. The relative standard deviation of these titrations is 0.04%. The amount of copper calculated from an absolute calibration agrees with the amount present to 0.2%. As an example of the performance of the titrator for an oxidation reaction, the following data was obtained in the titration of iodide. The control potential was 0.6 volt us. a silver-silver chloride reference electrode. The supporting electrolyte was 10 ml. of N sulfuric acid. The standard samples each con-
tained 5.60 mg. of potassium iodide as I-. For ten titrations, each requiring about 12 minutes, the relative standard deviatio 1 is 0.24%. ACKNOWLEDGMENT
Quoted data defining the performance of this ir strument are the work of 117. D. Shults :[I and B. B. Hobbs, of this laboratory. The principle of shunt diode n-eter protection was suggested by F. M . Glass, also of this laboratory. LITERATURE CITED
(I) Boornan, G. L., ANAL. CIIEY. 119.57) \----,-
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(2) Booman, G. L., Holbrook, W. B., Rein, ,J. E., Ibid., 29, 219 (1957). (3) Frederick, E. J.,Nucleonics 12, No. 11, 36-7 (19545. ’ t41 Lineane. J. J.. ANAL.CHEM.30. 1716 . 11958s. ‘ (5) M y mitt, L. L., Jr., Martin, E. L., Jr., Bedi, 3.D., Ibid., 30,487 (1958). (6) Geoige A. Philbrick Researches, Inc., Boston 16, Mass., “GAP/R Electronic Analol: Computers” (catalog data sheets). (7) Trarlsitron Electronic Corp., Wakefield, Mass., L‘TransitronSilicon Regulators” (catalog data sheets). RECEIVED for review September 10, 1058. Accepted January 9, 1959. Division of Analytical Chemistry, ACS, Chicago, Ill., Septemker 1958. .
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