The filter unit is illustrated in Figures 1 and 2. The twopiece paper disk holder (Figure 1) was machined out of 6.35mm thick Lexan. Two screw-cap 250-1111 polycarbonate Erlenmeyer flasks (Nalgene Cat. No. 4108-0250) were each drilled with one 1.32-mm hole (No. 55 drill) at the 130-ml level on a smooth area of the flask away from the graduations and other raised markings. The tops of the flasks required smoothing flat with fine emery paper placed on a flat surface. Two rubber washers, which were fitted between the tops of the flasks and the disk holder, were cut from 0.9-mm thick rubber to 32-mm 0.d. and 29.5-mm i.d. A sample or standard solution of up to 100 ml was measured into one of the flasks, the flask screwed into one side of a No. 40 pinch type ball-and-socket clamp and the second flask screwed into the opposite side of the clamp. The resinloaded disk, presoaked in demineralized water, was placed in the bottom section of the disk holder and the top section fitted. After the two rubber washers were fitted into opposite sides of the disk holder, the holder was inserted between the two Erlenmeyer flasks and the tops of the flasks were seated on the rubber washers in the recesses in the holder. By rotating the flasks, the two air holes in the flasks were located one above the other when the apparatus was standing on the bench. The locking screw of the clamp was tightened to hold the assembly of flasks and disk holder together. To filter the sample, the filter unit was inverted, care being taken to avoid loss of sample through the air hole. To assist filtration, a very slight suction from an aspirator filter pump was applied to the lower air hole, contact being made with the flask by a piece of thick-walled rubber tubing. By slight tilting of the flask and occasional removal of the suction, loss of filtrate through the lower air hole was avoided easily. For field use, suction may be applied from a hand operated vacuum pump (Edmund Scientific, Cat. No. 71301). By inverting the filter unit and repeating the above procedure, the
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A l l d i m e n s i o n s a r e in millimeters.
Figure 2. Assembled filter unit
sample was again filtered through the disk. Repeated filtrations are rapidly performed with this filter unit, seven or eight filtrations having been found sufficient to complete the ion exchange reaction. For good sealing between the suction tubing and the flask, the hole was drilled as described earlier on a smooth area of the flask. Because of the reduced sample handling and greater convenience of operation, together with the ability to be modified for use with other size disks and active filtration areas, this filter unit is considered to be an improvement on the previously described units.
RECEIVED for review October 6, 1972. Accepted November 7, 1972. From a thesis to be submitted by K.A.H.H. to the Graduate Faculty of Arizona State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry.
Signal Generator for Electrochemical Analysis of Mixtures of Electroactive Species G. I. Connor and G. H. Boehme Division of Sciences, Electronics Engineering Group
C. J. Johnson and K. H. Pool Department of Chemistry, Washington State University, Pullman, Wash. 99163 THE EXTENSION OF POLAROGRAPHY to stationary electrode voltammetry (SEV) and cyclic voltammetry has placed a demand of improved versatility on the linear voltage ramp with time. Consequently, several signal generators have been reported recently that offer a multitude of ramp rates and controlled switching potential and hold time (1-5). While these signal generators offer versatility in the analysis of the reduction as well as the oxidation of an electroactive species, they lack the ability to adequately analyze many mixtures of electroactive species. With mixtures, the prob___ (1) J. L. Huntington and D. G. Davis, Chem. Znsrr~m.,2, 83 (1969). (2) R. L. Meyers and I. Shain, ibid.,p 203. (3) J. S. Springer, ANAL.CHEM.,42 (8), 22A (1970). (4) R . H. Bull and G. C. Bull, ibid.,43, 1342 (1971). (5) Chia-Yu Li. Donald Ferrier, and R. R. Schroder, Cliem. Imfrum., 3, 333 (1972).
lems of distortion or concealment of one current peak by the current peak of another electroactive species becomes a serious problem for the electroanalyst. Jones and Perone (6) have discussed this problem in some detail and have developed a computerized fast sweep derivative polarographic method which incorporated timed holds after each reduction of an electroactive species. The timed holds allow sufficient decay between current peaks to analyze mixtures of electroactive species. Using this technique, concentration ratios 1OOO:l of one electroactive species over another could be determined. Farwell and Geer (7) have reported the use of timed holds for the resolution of mixtures of electroactive __ (6) D. 0. Jones and S. P. Perone, ANAL.CHEM., 42, 11 51 (1970). (7) S. 0. Farwell and R. 0. Geer, Abstracts 26th Northwest Regional Meeting of the American Chemical Society, Bozeman, Mont., June 1971, No. 25.
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 2 , F E B R U A R Y 1973
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- TO relay A
Figure 1. Signal generator-block schematic
-TO relay
diagram and
B
-15,
Figure 2. Program control logic-schematic diagram
I
From Clock Output (Ramp/Hold Control Logic, Fig. 3 )
species. They have developed a digital waveform generator that used diode transistor logic (DTL) and a digital clock for the accurate timing of ramp and hold functions for the analysis of organic-electrochemical problems. This article describes a solid state signal generator which provides the versatility of previous signal generators coupled with electronically timed holds which augment the electroanalysis of mixtures of electroactive species. Transistor transistor logic (TTL), including one-shot capacitive decay timing, is used which allows accurately controlled switching and hold times even at relatively rapid voltage ramps. Cathodic and anodic voltage ramps can be preset independently over a contiouously variable range of 10-1000 mV/sec or at single values of either 10 or 100 V/sec. Four variable holds can be preset at various voltage levels on the cathodic as well as the anodic ramp. Single cathodic, single anodic, single cathodic and anodic ramp or repetitive cathodic and anodic ramps are provided. The versatility of this generator can be a real aid in research on electrochemical systems or in the analysis of mixtures of known electroactive species. The wide range of ramp rates and hold times adds a new tool for the analysis of EC and ECE reactions. Qualitative analysis of the effect of inducing ramp, hold, ramp, hold, etc. potential control can be used to indicate the presence of a chemical reaction occurring at the electrode in addition to the known electrochemical one (8). This particular generator adds the (8) C. J. Johnson and K. H. Pool, unpublished data, 1972. 438
additional capability for ramp, hold, ramp, hold, etc. potential control on cathodic and anodic sweeps of the same run. The ability of the generator to aid in the analysis of mixtures of electroactive species is illustrated using SEV of mixtures of Cu(II), Pb(II), Tl(I), and Cd(I1). EXPERIMENTAL
Apparatus. The electrochemical cell consisted of a 100ml borosilicate glass electrolysis beaker fitted with a rubber stopper with holes to accommodate the electrodes. The working electrode was a hanging mercury drop electrode (HMDE) similar to that suggested by Underkofler and Shain (9). A single mercury drop was taken from a conventional DME and transferred to the HMDE using a Teflon (Du Pont) scoop. The electrode area was calculated to be 0.033 sq. cm. A platinum wire auxiliary electrode was used and a fiber junction saturated calomel electrode was used as a reference electrode. The signal generator described below was coupled to a solid state three-electrode polarograph designed by Bezman and McKinney (10) in place of their function generator. A Hewlett-Packard 5265A Digital Voltmeter and a Honeywell Electronic 19 recorder were used to record the currentpotential curves, A voltage ramp of 40 mV/sec was used for analysis of the solutions. Figure 1 is a block diagram and schematic of the signal generator. Closure of the reed relay contacts A or B allows (9) W. L. Underkofler and I. Shain, ANAL.CHEM., 33, 1966 (1961). (10) R. Bezrnan and P. S . McKinney, ibid., 41,1561 (1969).
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 2, F E B R U A R Y 1973
Inverted output
(Fig,!)
+5v b Cock
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To Rogram
Control Logic (Fig.2) +5v
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Figure 3. Ramp/hold control logicschematic diagram
1
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output (Fig.1)
To relay B (Fig.1)
Iselector switch]
the voltage set at A or B to be integrated to produce a respective negative or positive voltage ramp. The voltage at A and at B are set independently; thus, this generator can produce a positive voltage ramp of different magnitude than the negative ramp. The opening of both relay A and B produces a “hold” state. The drift of the FET operational amplifier (Int., Figure 1) and its integrating capacitor are negligible for holds of a few minutes or less. The output of the integrator, either direct or inverted (Inv. operation amplifier, Figure l), is the programmed output voltage of the signal generator. Closure of reed relay C (Figure 1) discharges the integrator and reverts the output of the signal generator to zero. The Program Control Logic (shown in detail in Figure 2 ) controls the closure of reed relay C depending on signals sent from the RamplHold Control Logic (shown in detail in Figure 3), which controls the closure of reed relays A and B. The program selector switch (Figure 2 ) can be set to provide a single A ramp, which can be inverted to provide the equivalent of a single B ramp, one A and B ramp, or continuous A and B ramps. These cycles are controlled by the edge-triggered D flip-flop (Figure 2), which determine the open or closed status of the integrator clamp relay C (Figure 1). Further details of construction and/or operation will be supplied upon request. The signal generator output or the inverted output ties into a three-electrode polarographic circuit at the summing point of the potentiostat operational amplifier, which drives the auxiliary electrode. Reagents. Stock solutions of 1M potassium nitrate, 0.01M cupric nitrate, lead nitrate, thallous nitrate, and cadmium nitrate were prepared by dissolving weighed quantities of reagent grade chemicals and then diluting to volume with deionized and distilled water, which was made 0.012M in nitric acid. Solutions for analysis were made up by taking 10 ml of the potassium nitrate solution and varying amounts of the other four metal ion solutions and diluting to 100 ml with deionized and distilled water. Prepurified nitrogen (Airco Industrial Gases) was used for deaeration. RESULTS AND DISCUSSION
Analysis of mixtures of Cu(II), Pb(II), Tl(I), and Cd(I1) in 0.1M KNO,I using SEV at a HMDE illustrates some of the capabilities of the signal generator when coupled to a three-
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Figure 4. Normal and interrupted stationary electrode voltammetry of 5 X lO+M Cu(II), Pb(II), TI(I), andCd(I1) at a HMDB in 0.1M KNOI Stationary electrode voltammogram Voltage ramp (40 mV/sec) C. Interrupted stationary electrode voltammogram D. Interrupted voltage ramp
A. B.
electrode polarographic circuit. Figure 4A shows the typical stationary electrode voltammogram of a solution that is 5 x 10-5M in copper, lead, thallium, and cadmium nitrate. The uninterrupted voltage ramp applied to the HMDE is shown in Figure 4B. While the peak currents for the copper and lead peaks can be evaluated, the peak current for the thallium or cadmium peak is much less certain. Figure 4C
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 2, F E B R U A R Y 1973
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c (IO~XM) Figure 6. Lead reduction peak current as a function of Pb(I1) concentration 0.1M KNO, with 7 X 10-4M CuOI) added 0 0.1M NKOa [no Cu(II) added]
t Figure 5. Normal and interrupted stationary electrode voltammetry of 7 X 10-4M Cu(II), and 2.5 X 10-5M Pb(II), TI(I), and Cd(I1) at HMDE in O.lMKNOB A. Stationary electrode voltammogram B. Voltage ramp (40 mV/sec) C. Interrupted stationary electrode voltammogram
D. Interrupted voltage ramp shows a stationary electrode voltammogram of the same solution when analyzed with the interrupted voltage ramp illustrated in Figure 4 0 . The eight-second hold 250 mV after the lead peak potential offers sufficient decay of the current to resolve the smaller thallium peak current which follows. Extrapolation of the decaying current from the lead reduction offers accurate evaluation of the thallium peak current. Similarly, an eight-second hold after the thallium peak gives good resolution of the cadmium peak current. While equimolar mixtures are difficult to resolve using uninterrupted SEV, the real problem manifests itself when one attempts the analysis of a mixture of a large excess of a less cathodically reduced substance followed by relatively low concentrations of more cathodically reduced electroactive species. Figure 5A shows the stationary electrode voltammogram of such a mixture using the uninterrupted voltage ramp shown in Figure 5B. The reduction peak current of the 7 X 10-4M Cu(I1) makes it extremely difficult to assess with any quantitative reliability the peak currents of the 2.5 x 10-5M Pb(II), Tl(I), or Cd(1I) in 0.1M KNO,. However, using a 40-second hold after the copper peak and eightsecond holds after the lead and thallium peaks (Figure 5D), one can obtain the large copper peak current and then in-
440
crease the sensitivity, as the copper reduction current decreases, and analyze quantitatively the reduction peak current for the lead, thallium, and cadmium (Figure 5C). In this manner, relatively small concentrations of the more cathodically reduced ions, Pb(II), Tl(I), and Cd(II), can be determined even in an excess of copper. Using the interrupted voltage ramp provided by the signal generator does not appear to alter the magnitude of the current peaks when a mixture of electroactive species is present. The lead reduction peak current is shown as a function of the lead concentration in Figure 6. The solid points were measured on a solution containing 7 X 10-4M Cu(I1) with additions of Pb(I1) from 2 X 10-5M to 1.3 X 10-4M. The hollow points were measured on a blank solution (no added copper) with additions of 1 x 10-5M Pb(I1). All solutions were analyzed using the voltage ramp shown in Figure 5D. Least squares analysis of the data points shows that the two sets of data (with and without added copper) do have slightly different slopes, but these slopes are not statistically different at the 95 confidence level. Thus, even with electrochemical analyses which approach one-minute duration, diffusion control is still reasonable and calibration charts can be made for the individual ions of interest and applied to the analysis of a mixture of ions with good reliability. As shown, this signal generator does have definite capabilities in the analyses of mixtures of electroactive species. The generator is relatively inexpensive to construct and can be tied into many of the three-electrode polarographic circuits rlready available. In addition, the sequential ramp and hold potential control on cathodic and anodic ramps offers much versatility for many electrochemical research applications.
RECEIVED for review September 15, 1972. Accepted November 21, 1972. Part of the equipment used in this study was funded by the National Science Foundation under Grant No. G48530.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
