herent simplicity. A single redox agent can function as the intermediate oxidizing and reducing agent during the measurement on an enzyme. This use of only one mediator-titrant minimizes the possibility of undesirable interactions between the enzymeb) and mediators and redox titrants which are ordinarily added to the solution during a conventional potentiometric titration. By simply varying the applied potential, repetitive cycling of the enzyme between its reduced and oxidized forms can be carried out with no loss of “titrant” and no increase in solution volume. Redox potentials of individual components in mixtures of redox species can be measured simultaneously when the optical properties are resolvable so that individual [O]/[R] values can be obtained for each applied potential. The small volume of t h e thin layer cell,’ cu. ‘40 fi1 for the cells used here, enables measurements to be made on less than a milliliter of solution with cell designs which eliminate the reservoir cup (35 1. This is an important consideration in view of the very small samples available for some . enzymes.
ACKNOWLEDGMENT The authors gratefully acknowledge C. R. Hartzell, Pennsylvania State University, for purifying a sample of cytochrome c. LITERATURE CITED (1) H. R. Mahier and E. H. Cordes, “Biologkal Chemktry,” Harper and Row, New York, 1971, Chapters 12and 15. (2) N. I . Bishop, Ann. Rev. Biochem., 40, 197 (1971). (3) D. F. Wilson, P. L. Dutton, M. Erecinska. J. G. Lindsay, and N. Sato, Accounts Chem. Res., 5, 234 (1972). (4) R. H. Tiesjema, A. 0. Muijsers, and 8. F. Van Gelder, Biochim. Bbphys. Acta. 305, 19 (1973). (5) J. Vanderkooi and M. Erecinska, Arch. Biochem. Bbphys., 162, 385 11974). (6) D. F. h l s o n , J. G. Lindsay, and E. S. Brocklehurst, Blochim. Biophys. Acta. 256. 277 (1972). (7) W M. Claik, “Oxidat&-Reduction Potentlals of Organic Systems,” Williams and Wilkins, Baitimore, Md., 1960. (8) F. L Rodkey and E. G. Ball, J. Bbl. Chem., 182, 17 (1950). (9) T. 8. Collidge, J. Biol. Chem., 96, 755 (1932). (10) A. H. Caswell and B. C. Pressman, Arch. Biochem. Bbphys., 125, 318 (1968).
(11) L. N. Mackey, T. Kuwana. and C. R. Hartzell, Fed. Eur. Biochem. SOC., Lett., 38, 326 (1973). (12) E. Stotz, A. E. Sidwell, Jr.. and T. R. Hogness, J. Biol. Chem., 124, 11 ( 1938). (13) R. W. Murray, W. R. Heineman, and G. W. O’Dom, Anal. Chem., 39, 1666 (1967). (14) A. Yildiz, P. T. Kissinger. and C. N. Reilley, Anal. Chem., 40, 1018 (1968). (15) T. Kuwana and W. R. Heineman, Bioelectrochem. Bioenergetics, in press. (16) W. R. Heineman, T. Kuwana, and C. R. Hartzell, Biochem. Biophys. Res. Commun., 49, 1 (1972). (17) W. R. Heineman, T. Kuwana, and C. R . Hartzell, Biochem. Biophys. Res. Commun.. 50, 892 (1973). (18) F. M. Hawkridge and T. Kuwana, Anal. Chem., 45, 1021 (1973). (19) S. R. Betso, M. H. Klapper, and L. B. Anderson, J. Amer. Chem. SOC. 94, 8197 (1972). (20) A. T. Hubbard and F. C. Anson in “Electroanalytical Chemistry,” Vol. 4, A. J. Bard, Ed., Marcel Dekker. New York, N.Y.. 1970, Chapter 2. (21) E. Margoliash and 0. F. Waiasek. in ”Methods of Enzymology,” VoI. 10, R. W. Estabrook and M. E. Pullman, Ed., Academic Press, New York, N.Y.. 1967, pp 339-348. (22) R. W. Henderson and W. A. Rawlinson, Biochem. J., 62, 21 (1956). (23) J. Tillmans, P. Hirsch, and E. Reinshagen, 2.Untersuch. Lebensm., 56, 272 (1928). (24) I. M. Kolthoff and W. J. Tomsicek, J. Phys. Chem., 39, 945 (1935). (25) E. Margoksh and A. Schejter in “Advances in Protein Chemistry,” Vol. 21, C. B.Anfinson, Jr., M. L. Anson, J. T. Edsall, and F. M. Richards, Ed., Academic Press, New York, N.Y., 1966, Chapter 2. (26) F. L. Rodkey and E. G. Ball, J. Bid. Chem., 182, 17 (1950). (27) R. Wurrnser and S. FilittiiWurmser, J. Chim. Phys., 35,81 (1938). (28) F. M. Stone and C. B. Coulter. J. Gen. Physiol., 15, 629 (1932). (29) K. G. Paul, Arch. Biochem., 12, 441 (1947). (30) E. G. Ball, Biochem. L,295, 262 (1938). (31) H. E. Davenport and R. Hill. Roc. Roy. Soc.. Ser. B, 139, 329 (1952). (32) D. E. Green, J. Jarnefelt, and H. D. Tisdale, Biochim. Biophys. Acta, 31, 34 (1959). (33) P. L. Dutton, D. F. Wilson, and C. Lee, Biochemistry, 9, 5077 (1970). (34) 6. McDuffie, L. 6. Anderson, and C. N. Reilley, Anal. Chem., 38, 883 (1966). (35) B. J. Norris, University of Cincinnati. Cincinnati, Ohio, unpublished results, 1974.
RECEIVEDfor review April 19, 1974. Accepted September 3, 1974. The authors gratefully acknowledge the financial support provided by National Science Foundation Grant GP-41981X, Research Corporation Cottrell Grant 6720, and University of Cincinnati Research Council Grant and Summer Faculty Fellowship (W.R.H.).
Digital Function Generator for Electrochemical Applications W. G. Sherwaod, D. F. Untereker,‘ and Stanley Bruckenstein Chemistry Department, State University of New York af Buffalo, Buffak, N. Y. 142 14
A digitally controlled voltage function generator based on a 16-bit digital to analog converter (DAC) Is described. The function generator provides ramp, triangular wave, sawtooth, and a pseudo staircase mode. Independent control of Initial, and upper and lower potential limits is provided by elther manual or computer control, as is the rate of ramping. Voltage resolution is 30 pV when the output range Is 2 volts. In one mode of operation, the digital function generator can emulate the results obtained using an analog integrator type device. In another mode of operatlon, current sampling po(arography and more complicated forms of voltammetry may be carried out. Applications involving the rotating disk electrode and the DME are presented.
* Present address, D e p a r t m e n t of Chemistry, U n i v e r s i t y of North Carolina, Durham, N.C. 84
The need for versatile electrochemical function generators has led to numerous publications describing both analog and digital circuits (1-12). These publications described instruments capable of providing a variety of multicycle potential programs, including triangular waves, square waves, and staircase functions. Also, considerable effort has been spent attempting to obtain a zero drift “hold” function. Most function generators fit into one of three categories. (1) analog integration with analog control of switching (15), (2) analog integration with digital control of switching (6-11)(3) digital to analog conversion with digital control of switching (12-15). The signal generator described here is of the third type and is shown in block form in Figure 1. It has several special features: First, the output voltage resolution, 1 in 216,can emulate the output of an analog inte-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
The design approach described above eliminates the need for binary coded switches; thus reducing cost, simplifying circuit construction, decreasing the size of the control panel, and facilitating both a manual operation mode and a computer or a programmable calculator mode.
Figure 1. Digital function generator
grator when this is desired. Second, a staircase mode output can be emulated. Third, a true zero drift hold function exists. Fourth, a time base with a range of los is provided. Fifth, a simple, rapid and economic method for setting the required initial and limiting voltages is incorporated in the function generator. Finally, the function generator may be used conveniently either manually or under computer control.
PRINCIPLE OF OPERATION The function generator, shown in block form in Figure 1, uses a precision clock to provide the necessary pulse train. The clock pulses are counted by a 16-bit presettable updown counter which drives a 16.bit DAC. The DAC provides an analog output which is further conditioned by an operational amplifier. The digital panel meter displays the output voltage, and the desired initial and limiting values of the ramp are set by observing this display. The 16-bit counter, by appropriate multiplexing, provides digital information to the upper and lower voltage limit data latches. In the manual mode of operation, this feature avoids the expense and inconvenience of manually introducing 16 bits of information by switches. The initial potential can be preset in either of two ways. First, a pulse train may be used to increment the counter until the digital panel meter displays the desired starting voltage. Alternately, a strobe pulse can preset the counters so as to provide a zero voltage output from the DAC, and the desired initial potential is summed in a t the operational amplifier input. The upper (and lower) voltage limit data latches are also preset by appropriate pulse trains to the counter. In the manual mode, the pulse train is gated off when the panel meter displays the desired voltage; while in the programmable calculator or computer controlled mode, the required pulse train is sent to the counter for each voltage limit. We elected to use a single up-down counter, controlled by a pulse train, to set the two data latches. This approach seemed to be the most economical and convenient one to use for manual operation. In practice, it takes less than one minute to set all limits to the least significant digit on the panel meter (1 mV on the 2-volt range). Different input clock frequencies can be used in the count-up and count-down modes, thus providing the capability for generating asymmetric as well as symmetric voltage functions, e.g., symmetrical and asymmetrical triangular and sawtooth profiles.
EXPERIMENTAL Instrumentation and Chemicals. The electrochemical equipment and techniques used are described in references (16) and (17).Previously studied and well understood electrochemical systems were used to test the function generator (18J9).Reagent grade chemicals and triply distilled water were used. Potentials are reported US. the SCE. The function generator uses Signetics 7400 series logic, and the DAC converter is a Date1 Model DAC-l69B, modi fied as described in the next section. Two clocks have been used. The first is a 2-MHz, ’r% compatible, crystal oscillator. A series of decade and bincLL >, dividers provide selectable pulse repetition rates from 2 MHz to 0.06 Hz from the 2 MHz oscillator. This clock pro vides ppm timing level accuracy. When it was necessarq t i generate a particular pulse repetition rate not available from the crystal clock, a Teledyne Model 4705/01 V t o F converter (*44 ppm stability) was used in place of the crystal oscillator. Continuous control of the pulse repetition rate is obtained by adjusting the input voltage to the V to F converter. The dividers mentioned above can also be used with the V to F converter. “Computer” control and data acquisition were provided by a Wang Model 720C programmable calculator. The T2L logic and operational amplifier circuits providing the 1.6quired interfacing will be described elsewhere (20 ). CIRCUIT DESIGN A schematic of the function generator, including the control circuitry, is jhown in Figure 2. The analog voltage output is provided by a 16-bit DAC hard wired to provide a current output of -1 mA, 0, and +1 mA for inputs of 0, 2’ ), and 216, respectively. The output of the DAC is fed into an Analog Devices 2345 chopper stabilized amplifier used as a current to voltage converter. The DAC operates a t a 5-MH7 update rate and has a 750-nsec settling time (0.005% F.S.) The maximum voltage step rate,,,S ,, from the signal generator is given by
where Ai is the current change corresponding to a 1-Sit A), St is the count increment at the DAC inputs (3 X seconds) time required for the DAC settling (7.5 X and Rf is the range scaling feedback resistor in the 234J operational amplifier. The voltage resolution, r, is 1 part in 216 of the desired maximum potential range, AV. A IO3!! feedback resistor ( R f )yields A V = 20 V, S,,, = 400 Vlsecond and r = 300 pV, while if Rf = lO3Q, AV = 2.0 V, S,,, = 40 V/second, and r = 30 pV. These calculated step rates are quite conservative if the voltage accuracy desired does not require waiting for settling to 0.005% of full scale. NOR 1 gates pulses of frequency 1 and NOR 2 gates pulses of frequency 2, and the desired frequencies are introduced through front panel pinjacks. Frequency 1 and frequency 2 are picked off the digital divider chain. The state of flip-flop 1 (FF1) determines whether NOR 1 or NOR 2 pass pulses, as well as the scan (count) direction. The pulse frequency through NOR 1 determines the positive (anodic) scan rate and the frequency through NOR 2 determines the negative (cathodic) scan rate, thus providing for asymmetrical waveforms. If the same frequency i q
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
e
85
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Figure 2.
Circuit diagram of digital function generator
S-1 = SPDT, center off, S-2 = SPDT, PB = SPST, momentary on. Computer input jacks I = pulse input, II = hold-scan, 111 = anodic scan, IV = cathodic scan, V = anodic limit set, VI = cathodic limit set, VI1 = initial potential set. 16-Bit anodic (and cathodic) limit latches: 4. 4-Bit data latches (7475); 8 required. 16-Bit presettable up-down counters: 4 cascaded 4-bit counters (74193); 4 required. 16-Bit anodic (and cathodic) limit comparators: 4 cascaded 4-bit digital magnitude comparators (7485): 8 required. NOR gates: Quad two-input NOR gate (7402); 1 required. NAND gates: Quad two-input NAND gate (7400): 2 required. AND gates: Quad two-input AND gate (7408); 1 required. Inverters: Hex inverter (7404); 1 required. J-K Flip-flop (7476); 1 required
introduced to both pulse inputs, identical anodic and cathodic sweep rates are obtained. Alternately, computer point I a t NOR 5 may be pulsed from the Wang 720C to provide precise potential steps of variable magnitude and duration. The count direction is determined by the output states of NAND 6 and 7 . One output remains high while the other alternates between logic 0 and 1 a t either frequency 1 or frequency 2 (or a common frequency) depending upon the scan direction. The three position toggle switch, S-1, a t input B of AND 4 controls the “hold” function for the ramp. In one position, input B is high (swrep); in the second position, B is low (hold); and in the third position, input B is controlled from the Wang 720C. The voltages (count value) a t which the function generator reverses its scan direction (count direction) are set using a bistable data latch connected to the counter outputs as described above. When the counter outputs are equal to the desired lower potential limit. push button 1 (PB-1) is pressed (in manual mode) or input A of NAND 10 is pulsed from the computer, thereby latching the desired lower limit a t the latch outputs. These outputs are connected to one set of inputs of a digital magnitude comparator, while the counter outputs are wired to the comparison inputs. The upper potential limit is set in an analogous manner using another data latch and magnitude comparator. The 16-bit magnitude comparators used for limit detectors produce a logic zero a t one of three outputs (A < B, A = B, A > B) when the 2 16-bit digital numbers under cornparison have the appropriate relationship. This circuit uses only one of these outputs for each of the limit detectors, and the selected output is a logic one when the DAC output is between the anodic and cathodic potential limits. The outputs of the two limit detectors are connected to AND 8, the output of which is wired to AND 9. PB-3 is connected to the other input and, when depressed, produces a change of scan direction by toggling FF-1. The state of FF-1 may also be changed by strobing either the Clear or Set input from the computer. Light emitting diodes on the front panel are used to indicate the direction of the potential sweep. 86
Switch S-2 (one pole, two position) changes the output of NAND 12 to either the Clear or the Preset pin of the 16-bit counter. In one position, the Preset input can be strobed from PB-4 (NAND 12). If S-2 is in the other position a pulse a t either input of NAND 12 clears the counter (DAC output to -1 mA). The 2345 operational amplifier voltage output is continually monitored on a 3H digit panel meter which can be ranged to read voltages from f 2 mV t o f 2 0 V.
RESULTS AND DISCUSSION Electrical Characteristics. The 16-bit DAC is used in the current output mode and has an output range of fl mA, corresponding to a current resolution of 30 nA. The linearity temperature coefficient is fO.O005%/”C, the long term stability is f0.06%/year, and the absolute linearity and accuracy are 0.005% of full scale. We have used a 234 J chopper stabilized amplifier as a current to voltage converter at the output of the DAC, although an internal amplifier is available for this purpose. The 2345 has a slew rate of 3OV/psec, and the stability specifications exceed those of the DAC. The output noise measured a t the 2345 is indistinguishable from that present in the power supply. Comparison of Digital and Analog Ramp. The digital signal generator output was used to drive a three-electrode potentiostat to determine the magnitude of the electrochemical effects occurring as a result of the discrete staircase nature of the “triangular” voltage function. Figure 3, curve A, shows a cyclic voltammogram of 0.2M H2S04obtained a t a stationary gold disk electrode using the digital function generator to scan at 75 mV/sec. Similar electrochemical data were obtained using an analog integrator type triangular wave generator ( Z Z ), and results obtained with the analog function generator were indistinguishable from those shown in Curve A. As a further test, the surface roughness of the gold disk electrode was determined using the method devised by Brummer and Makrides ( 2 1 ) and also investigated by Cadle and Bruckenstein (22). The gold disk electrode was potentiostated a t 1.20 V for 5 minutes, after which the potential was swept in the cathodic direction, and the charge
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
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msec, the disk current and potential were sampled. This process was repeated until the disk electrode potential sweep covered the range in Curve A of Figure 3. The 175msec delay allows the current from double layer charging to approach zero, while the current associated with surface processes also decreases appreciably before sampling. The apparent height of the gold oxide reduction peak (in Curve B, Figure 3) is reduced to 60% of that normally observed for the continuous 75 mV/sec potential sweep in Curve A. The decrease of the background current by this mode of operation, without resorting to extremely slow scan rates, decreases the lower level of detection for most electroactive species which can be determined using solid electrodes. Complex Voltage Function. An additional decrease in currents related to surface processes on a gold electrode was obtained using a more complex voltage program than that described above. The potential of a gold disk electrode was stepped from 0.0 to 1.2 V and a cathodic scan direction was selected. A 20-KHZ pulse train was introduced into the counter for 8.8 msec (53 mV) and, after a 33-msec delay,
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passed during the reduction of the oxidized gold surface was measured. The gold/oxygen ratio is unity a t 1.20 V and the surface charge density due to the oxidation of gold is 400 FC/cm2. Averaging of 10 repetitive cathodic potential scans obtained using the digital function generator yielded results that differed by no more than 2% from those determined using the analog sweep generator in an identical averaging experiment. A precision of 2% is the typical reproducibility attained in these experiments, and thus we conclude there is no observable difference between cyclic voltammetric experiments done using the digital and analog function generators. It should be noted that currents a t solid electrodes due to surface processes are particularly sensitive to the imposed voltage program, and the above results represent a rigorous test of the ability of the digital ramp to emulate an analog ramp in electrochemical applications. Staircase Mode. Software control of the digital function generator was used to generate a cyclic staircase voltage sweep on a gold disk electrode in 0.2M H2SO4. The disk electrode potential was stepped from 0.0 to 1.2 V, and an initial cathodic scan direction was selected. A pseudo-staircase function was produced by gating a 20-KHZ pulse train into the 16-bit counter for 2.2 msec and, after a delay of 175
the scan direction was reversed and the 20-KHZ pulse rate was introduced to the counter for 7.0 msec (42 mV). After an additional delay (98 msec), the disk current was sampled. This procedure produces a net 11-mV potential step in an elapsed time of 147 msec, or an equivalent scan rate of 7 5 mV/sec. This procedure was repeated until a complete voltage cycle was obtained, and the resultant i-E curve is shown in Figure 3, Curve C. The peak current for the reduction of gold oxide has been reduced to 40% of the value observed for a conventional linear sweep rate of 75 mVlsec. Tast Polarography. Calculator control was used to obtain Tast current voltage curves a t a dropping mercury electrode (DME) in the presence of 1.0 X 10-6MCd2+ and 1.0 X 10-5M Cd2+ in 0.5M KCl (Figure 4, Curves A and B, respectively). The voltage-time program used to obtain the polarograms in Figure 4 was the following. The potential of the DME was maintained at 0 V for three complete drops (drop time is 5 seconds a t ECM). When the third drop fell, the DME potential was stepped to the value present a t the output of the function generator, and the current was sampled 3.00 seconds after the birth of the fourth drop. Twelve data points were acquired at 20-millisecond intervals and the mean value was recorded. At the end of the fourth
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
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(4) R. P. Buck and R. W. Eidridge, Anal. Chem., 35 1829 (1963). (5) G. A. Philbrick Researches, Inc. Boston, Mass.. "Applications Manual for Phiibrick Octal-Plug-in Computing Amplifiers," 1956. (6) J. S. Huntington and D. J. Davis, Chem. Insfrum., 2, 83 (1969). (7) R. L. Meyers and I. Shain, Chem. Instrum., 2, 203 (1969). (8) R. H. Bull and G. C. Bull, Anal. Chem. 43, 1342 (1971). (9) Chia-Yu Li, Conaid Ferrier, and R. R . Schroder. Chem. Instrum., 3, 333 (1972). (10) R. Bezman and P. S.McKinney. Anal. Chem., 41, 1560 (1969). ( 11) D. F. Untereker, T. M. Riedhammer, W. G. Sherwood, and S. Bruckenstein, submitted for publication. (12) D. 0. Jonesand S . P. Perone, Anal. Chem., 42, 1151 (1970). (13) S.0. Farwell and R. 0. Geer, Abstracts 26th Northwest Regional Meeting of the American Chemical Society, Bozeman, Mont., June 1971, No.
drop, the DME potential was set back to zero, the digital function generator advanced 5 mV, and the above process repeated 4 drops later. Figure 4 gives the average of the twelve sampled currents as a function of potential. The Tast polarograms are quite satisfactory and compare favorably with those obtained using dedicated instruments (23).
CONCLUSIONS In one mode of operation, the digitally controlled function generator yields cyclic voltammograms at solid electrodes indistinguishable from those obtained using a conventional analog generator. Replicate electrochemical experiments give an absolute voltage reproducibility limited by the voltage recording technique, rather than the function generator accuracy, which is several orders of magnitude better than that typically obtained with analog integration techniques. This feature is especially useful when signal averaged voltammograms are required. Complicated reproducible voltage programs can be readily produced under computer control by the digital function generator. In the manual mode, the function generator is easy to use and also is highly reproducible. LITERATURE CITED (1) Dennis C. Johnson, Department of Chemistry, Iowa State University, Ames, Iowa, personal communicatlon, 1968. (2) W. L. Unerkofler and I. Shain, Anal. Chem., 35, 1778 (1963). (3) G. Lauer, H. Schlein, and R. Osteryoung, Anal. Chem., 35, 1780 (1963).
95
6'1.Connor, G.
(18) (19) (20) (21) (22) (23)
H. Boehme, C. J. Johnson, and K. H. Poole, Anal. Chem., 45, 437 (1973). J. S. Springer, Anal. Chem., 42 (8) 22A (1970). D. Napp, D. Johnson and S. Bruckenstein, Anal. Chem., 39,481 (1967). D. Untereker, Ph.D. Dissertation, State University of New York at Buffalo, 1973. M. Z. Hassan, Anal. Chem., 43, 7 (1971). W. G. Sherwood and S. Bruckenstein, unpublished work, 1973. D. F. Untereker. T. M. Riedhammer, W. G. Sherwood. and S. Bruckenstein, unpublished work. S. Brummer and A. C. Makrides, J. Nectrochem. SOC., 111, 1122 (1964). S. Cadle, Ph.D. Dissertation, State University of New York at Buffalo, Buffalo, N.Y., 1972. P. 0. Kane, J. Polarogr. Soc., 8, 10 (1962).
RECEIVEDfor review April 22, 1974. Accepted September 5, 1974. This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research USAF, under Grant No. AFOSR-74-2572.
Simultaneous Determination of Cyanide and Sulfide with Rapid Direct Current Polarography D. R. Canterford' Deparfment of Physical Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia
Rapid (short controlled drop time) dc polarography provides a simple method for simultaneous cyanidehulfide analysis. The rapid method is superior to conventional dc polarography since it extends the sulfide concentration range over which these two species can be simultaneously determined. The optimum supporting electrolyte pH is In the range 9 to 10. At higher pH values, the anodic hydroxide wave interferes with the cyanide wave. Detection limits are 5 X 10-sM (cyanide) and 4 X 10-6M (sulfide). With the shortest drop time used (0.16 sec), up to 5 X 1OV2Mcyanide and 5 X 10-3Msulfide could be tolerated. With shorter controlled drop times, higher concentrations of sulfide could be tolerated. Partial or complete coverage of the electrode by a film of HgS has no effect on the cyanide limiting current. Iodide and thiosulfate interfere with the determination of cyanide.
Because of their extreme toxicity, both cyanide and sulfide are of importance in water quality control programmes. With most analytical techniques, cyanide and sulfide interfere with each other, which obviously precludes IPresent address, Research Laboratory, Kodak (Australasia) Pty Ltd, P.O. Box 90, Coburg, Victoria 3058, Australia. 88
their simultaneous determination. For example, the ionselective electrode ( 1 , 2 ) responds to both species, as do many amperometric (3, 4 ) , spectrophotometric (5, 6), and spectrofluorimetric (7, 8) methods. Distillation is usually an integral part of the procedure for the determination of both species (9). Although distillation removes many interfering materials it does not separate cyanide and sulfide from each other. For cyanide analysis, the standard method of removing sulfide is by precipitation as a heavy metal sulfide (e.g., PbS) (9).However, an appreciable amount of cyanide can be occluded by the precipitate, resulting in low cyanide values ( 1 0 ) . This procedure should therefore be avoided if possible. Cyanide and sulfide both give anodic polarographic waves corresponding to mercury compound formation. The rapid polarographic technique, which is based on the mechanical dislodgment of mercury drops from a capillary at short time intervals, possesses a number of advantages over conventional direct current (dc) polarography for such processes (11). A t normal drop times, sulfide produces up to four dc waves spread over a very large potential range. Under rapid conditions, however, fewer sulfide waves are recorded, which reduces the possibility of interference from other anions which also give anodic waves. The determination of sulfide in the presence of cyanide was used to illustrate this advantage of the rapid technique (12).
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
