Chemical Stripping of Copper with Cerium(lV) John W .
Bixler and Walter F . Staffordl
Department of Chemistry, Lake Forest College, Lake Forest, Ill.
THENONSTOICHIOMETRICCATHODIC DEPOSITIONand subsequent anodic dissolution' of copper in NaC10, medium have markedly different current efficiencies under certain experimental conditions. These investigations were made using a rotated platinum microelectrode (RPE). The quantity of electricity consumed in the dissolution step was significantly less than the amount used for deposition. The overall efficiency of the deposition-dissolution process is defined by Equation 1 : Qu
E ( Z ) = - x 100 QC
where E is the efficiency, Qa is the amount of electricity passed during the anodic stripping process, and Qc is the amount of electricity passed during the plating process. Q, and Q c are defined by Equations 2 and 3:
Qa = iata Qc =
ictc
where t and i are the time and constant current for the respective processes. Similar observations were made by DeGeiso and Rogers (1). This is in marked contrast to the good agreement between values of Qa and Qc observed in anodic stripping experiments with silver (2). Mattsson and Bockris ( 3 ) report an investigation of the kinetics of the deposition and dissolution of copper in sulfate medium. They used galvanostatic techniques and made no measurements which relate the experimental parameters to the quantity of copper plated or stripped. This paper describes the nonfaradic determination of electrodeposited copper by chemical stripping with Ce(1V). This technique has previously been employed for the determination of very dilute oxidant solutions by the chemical stripping of electrodeposited silver (4). Cerium oxidimetry has also been used to determine electrodeposited copper volumetrically (5). The theory of chemical stripping analysis is presented elsewhere (4). If it is assumed that the rate of chemical stripping by Ce(1V) is controlled by mass transfer, the relationship governing chemical stripping is given by Equation 4: AN/At =
k
[Ce(IV)I
(4)
where AN/At is the rate of chemical stripping of copper in moles/sec and [Ce(IV)] is the bulk concentration of Ce(1V). k is a constant defined by Equation 5: 1 Present address, Department of Chemistry, Northwestern University, Evanston, Ill. 60201
( 1 ) R. C. De Geiso and L. B. Rogers, J . Elecfrochem. Soc., 106, 433 (1959). (2) J. W. Bixler and S. Bruckenstein, ANAL.CHEM., 37, 791 (1965). (3) E. Mattsson and J. O'M. Bockris, Trans. Faraday SOC.55, 1586 (1959). (4)S. Errickenstein and I. W . Bixler, ANAL.CHEM., 37, 786 (1965). ( 5 ) L. H. Bradford and P. L. Kirk, IND. ENG. CHEM., ANAL.ED., 13, 64 (1941).
60045
il
=
k[Ce(IV)]
where il is the limiting current of Ce(1V) from voltammetric measurements with the RPE. AN/At is calculated from experimental data using Equation 6:
where QP is the amount of copper plated (expressed in coulombs) and is the chemical stripping time. Substituting values of Q, and Q, into Equation 6 in place of Q p will yield different values of AN/At if E is not 100%. Successive anodic and chemical stripping measurements, in which Q Cis constant, yield the necessary values of Qu and r. From Equations 4 and 5, the shape of a plot of ANJAt cs. [Ce(IV)] should be predictable from limiting current measurements of Ce(1V) solutions. The slopes, therefore, should provide information concerning the relationship between QC, Q,,and Q p . EXPERIMENTAL Apparatus. The RPE consisted of 27-gauge platinum wire sealed into soft glass tubing and was of the form previously described (6). It was mounted in a 600-rpm Sargent Synchronous Rotator. The dual electrolysis cell and the electrode transfer chamber are described elsewhere (2). Currentvoltage and current-time curves were recorded with a Sargent Model XV Polarograph. The controlled current source consisted of a tube-regulated voltage supply in series with a large variable load resistor and a precision resistor for the potentiometric standardization of the current (7). Voltagetime curves were recorded with a Keithley 610-B Electrometer coupled to a variable-speed Bristol High Speed Dynamaster Recorder. Chemicals. Reagent grade chemicals were used without further purification. All solutions were prepared from deionized twice-distilled water, the first distillation being made from alkaline permanganate solution. Approximately 0.1M Ce(1V) and Cu(C104)?stock solutions were standardized volumetrically and electrogravimetrically, respectively. The dilute working solutions were prepared by diluting the stock solutions with supporting electrolyte just prior to use. The working solutions were deaerated with prepurified nitrogen which had passed through an active copper column at 200" C and was water-saturated. Pretreatment of RPE. To obtain reproducible results, the RPE was subjected to a pretreatment prior to each determination. This consisted of anodization and cathodization in 0.1M HC104 followed by equilibration at 0 V us. SCE. This pretreatment is described in detail elsewhere ( 2 ) . Procedure. Plating and anodic stripping were carried out in one of the dual electrolysis cells thermostated at 25' C. Fifty milliliters of the dilute Cu(I1) solution were deaerated in the cell for 15 to 20 minutes. The pretreated RPE was inserted and a constant cathodic current of a value below the limiting current of the Cu(I1) solution was applied for a specified time. The anodic stripping was carried out in the plating cell by applying a constant anodic current to the (6) S. Eruckenstein and T. Nagai, ANAL.CHEM., 33,1201 (1961). (7) W. F. Stafford, Senior Honors Thesis, Lake Forest College, Lake Forest, Ill., 1966. VOL. 40, NO. 2, FEBRUARY 1968
425
I
7t
Table I. Anodic Stripping Results Plating Electrolyte: Cu(I1) in 0.1M NaCIOa [CU(II)I x 104 ic (4) to(set) ia (PA) E(ZJ 1.05 1.05 7.06 7.06 7.06 7.06
11.7 11.7 11.6 11.6 11.6 11.6
180 300 180 600 180
33.3" 53.P 33.40 114s 33.3b 11.6b
60 Stripped in plating solution. * Stripped in 1.OM HCI. c Assuming one electron change during stripping.
52 51 64 67 66c 52c
6 -
54-
AN/At x IO"
3-
a
Table 11. Chemical Stripping Results ANlAt X 10'1
9.85 9.85 3.94 3.94 3.94
435 768 438 774 1050
263 484 268 49 1 712
5.98 5.69 2.58 2.43 2.33
3.62 3.58 1.58 1.54 1.58
plated RPE. Voltage-time curves were recorded, which permitted evaluation of the anodic stripping times. Chemical stripping was performed by moving the plated RPE through a nitrogen-filled chamber into the second cell containing 50 ml of deaerated Ce(1V) in 1N HzS04. A potential of +0.15 V us. SCE was applied to the RPE to prevent copper oxidation as the RPE was lowered into the Ce(1V) solution. The rotator was turned on and removal of the applied voltage initiated the chemical stripping. Voltagetime curves were recorded to obtain the chemical stripping times. RESULTS AND DISCUSSION
Some representative data from anodic stripping experiments are presented in Table I. The experimental stripping results were generally reproducible to within f3% deviation. Under our experimental conditions, the overall efficiency varied between 45 and 70% and depended strongly upon Cu(I1) concentration and current. The efficiency was not significantly altered if the plated electrode was stripped in deaerated 0.1MNaC104 or 0.1M HC104containing no Cu(I1). One possible explanation for the low overall efficiency is that a portion of the plated copper is being stripped to Cu(1) rather than Cu(I1). Experiments were performed in which the plated RPE was stripped in 1.OM HC1, which favors the formation of Cu(1). The results of two of these experiments are given in Table I, and suggest that this explanation is incorrect. The limiting current of Ce(1V) concentrations ranging from 3.94 X 1 0 P to 9.85 X lop5 M in 1N were determined voltammetrically. A plot of ill25 us. [Ce(IV)] is shown in Figure 1 . The points appear to be linear and were treated by the method of least squares to obtain Curve A , which has a slope of 4.08 X 10-7 liter-sec-I. From the slope, k was calculated to be 0.0788 amp-litermole-'. Chemical stripping experiments were performed using Ce(1V) solutions in 1N HzS04. The range of Ce(1V) concentrations was the same as used in the voltammetric measurements. The value of Qc was 770 microcoulombs in each case and the average value of Qa was 479 f 10 microcoulombs. 426
ANALYTICAL CHEMISTRY
Figure 1. Comparison of chemical stripping and voltammetry data Curve A .
calculated from limiting current data calculated from Q. Curve B . A calculated from Qo 0
Values of ANlAt were calculated from Q, and Qc for each determination and are plotted cs. [Ce(IV)] in Figure 1. At least two determinations were made at each concentration and the average values were plotted. Both sets of data appear to be linear and were treated by the method of least squares to determine the slopes. The points calculated from Qc yield a slope of 6.17 X liter-sec-1 and a value for k of 0.119 ampliter-mole-', while points calculated from Qahave a slope of 4.04 X liter-sec-l and a value for k of 0.0780 amp-litermole-'. The latter value is in good agreement with that obtained from voltammetry, which strongly suggests that Qa is a rather accurate measure of the amount of plated copper. If chemical stripping is a steady state process controlled by mass transfer, the rate of chemical stripping should be independent of the amount of copper stripped. This was tested at two Ce(1V) concentrations and the results are given in Table 11. This independence is seen only when the rates are calculated from the values of Qa. These results also indicate that electrodeposited copper may be substituted for silver for the chemical stripping determination of Ce(IV), provided the plating conditions are held constant or Q, is used as a measure of the amount of copper plated. An additional experiment was performed to evaluate the efficiency of the anodic dissolution process. A weighed piece of reagent grade copper wire was anodized with a constant current in 0.1MNaC104 solution. The weight loss was 0.0147 of a gram, compared to a calculated value of 0.0146 of a gram assuming the copper was oxidized to Cu(I1). This experiment was reproducible and supports our conclusion that the anodic stripping process is efficient. If the electrode reaction is reversible and the rate is controlled by mass transfer, the electrode potential during plating or stripping is given by Equation 7 at 25' C.
(7) where Ec,' is the formal potential for the reduction of Cu(I1) to Cu (us. SCE), [CU~+] is the bulk concentration of Cu(I1) and kcu is the proportionality constant between limiting cathodic current and bulk copper concentration. The applied current, i, is negative under anodic conditions. The activity of copper
metal was tactically assurried to be unity when the copper coverage was greater than about 10 monolayers. By use of 9.91 X 10-3M Cu(I1) in 0.1M NaC104, potential measurements were made with anodic and cathodic currents ranging from 35.0 to 1010 PA. The anodic and cathodic segments of plots of E US. log ([Cu2+] - i/kc,) were not continuous, as would be predicted from Equation 7, and neither segment was linear throughout the entire current range. Apparent linearity was observed for the points obtained with current values between 228 and 406 PA, yielding slopes of 0.53 and 0.34 for anodic and cathodic measurements, respectively. These values are in marked contrast to the value of 0.0296 predicted by Equation 7 for a reversible reaction. An additional experiment was performed in which stirred 1.OOM Cu(I1) perchlorate solution was electrolyzed at anodic and cathodic current densities low enough to ensure negligible concentration polarization. Pure copper wire electrodes with uniform areas were used. Plots of E us. log i yielded anodic and cathodic segments which became linear at an overvoltage of about 60 mV. This suggests that the rate-determining step in the reduction involves the transfer of the second electron, for which the following mechanism can be postulated: Cu(I1) Cu(1)
+ e+ e-
4
Cu(1) (fast)
4
Cu
(slow)
This mechanism would result in an inefficient plating process, because a portion of the Cu(1) would escape from the electrode surface by convective mass transfer. Additional studies are required to elucidate the kinetic parameters of the reaction. Mattsson and Bockris (3) conclude that the surface diffusion of adions is the rate-controlling step in thedeposition of copper from HzS04 medium at low current densities. Either this
explanation or the slow reduction of Cu(1) to Cuo could account for the low plating efficiency under convective conditions, In either case, the absence of stirring should enhance the plating efficiency significantly. This was tested by repeating several anodic stripping experiments under quiescent conditions, taking care not to exceed the chronopotentiometric transition time for Cu(I1). In each case, the value of overall efficiency exceeded 8 6 z . When an experiment under quiescent conditions (ec= 1400 microcoulombs) was repeated without pretreatment or oxidation of the platinum surface, the efficiency increased to 9 3 x , but did not increase further when the experiment was again repeated without pretreatment. This effect was reproducible and represents an increase of 100 microcoulombs in the amount of copper stripped. From physical measurement of the electrode area, we estimate that about 90 microcoulombs of copper are required to form a uniform monolayer on the RPE, assuming the electrode surface is smooth. This suggests the formation of a copper-platinum alloy. However, this effect appears to account for only a small fraction of each of the low efficiencies shown in Table I. Secondary waves which could be attributed to alloy oxidation were not observed during anodic or chemical stripping of the plated RPE. Thus, if an alloy is formed, it is apparently oxidized at the potential attained during the anodic step of the pretreatment. Further studies are in progress in an attempt to relate the observed efficiencies to experimental parameters and postulated mechanisms. RECEIVED for review July 31, 1967. Accepted October 30, 1967. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Gas Chromatographic Determination of 2,3=Butanediol in 1,2=Propanedio1 Using Tetrahydrox yethylethylenediamine as Stationary Phase B. A. Swinehart’ Wyandorte Chemicals Corp., Analytical Research Dept., Wyandotte, Mich. 48193
THE SEPARATION OF POLAR COMPOUNDS by gas chromatography has proved to be a generally troublesome problem. A typical example was presented by the need to determine trace quantities of 2,3-butanediol in 1,Zpropanediol. Phifer and Plummer ( I ) attacked this problem by choosing a partially deactivated solid support and by using water as the liquid phase and water vapor as the carrier gas. An alternative approach, as suggested by Smith and Johnson (2), is to choose Present address, Corning Glass Works, Sullivan Park, Corning N.Y. 14830
(1) L. H. Phifer and H. K. Plurnrner, Jr., ANAL.CHEM. 38, 1652 (1966). (2) E. D. Smith and J. L. Johnson, Zbid., 35, 1204 (1963).
a solid support which shows selective interaction with one of the components in a mixture. The systematic application of this technique to a variety of solid supports and liquid phases indicated that untreated Chromosorb W, regular, coated with tetrahydroxyethylethylenediamine (THEED) was a suitable combination for achieving the desired separation. Nadeau and Oaks (3) used a THEED column to separate ethylene and propylene glycols but they did not apply the procedure to other diols nor did they obtain sufficient sensitivity to determine parts-per-million quantities. This paper describes a method which has been applied generally to the separation of various diols and specifically to the determination of partsper-million quantities of 2,3-butanediol in 1,2-propanediol. (3) H. G. Nadeau and D. M. Oaks, Zbid., 32, 1760 (1960). VOL 40, NO. 2, FEBRUARY 1968
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