silver, and there is no necessity for an externally stated limiting minimum total silver concentration.
LITERATURE CITED (1) T. M. Hseu and G. A. Rechnitz, Anal. Chem., 40, 1054 (1966). (2) D. C. Muelier, P. W. West, and R. H. Mueller, Anal. Chem., 41, 2038 (1969). (3) R. A. Durst, Ed., "Ion Selective Electrodes", Net. Bur. Stand. (U.S.), Spec. Pub/. 314, Washington, D.C., 1969, pp 402-3. (4) J. Ruzicka and C. G. Lamm, Anal. Chim. Acta, 54, 1 (1972). (5)P. L. Bailey and E. Pungor, Anal. Chim. Acta, 64, 423 (1973). (6) J. W. Ross, Jr., in Ref. 3, p 77. (7) E. A. Butler and E. H. Swift, J. Chem. Educ., 49, 425 (1972). (8) A. K. Covington in Ref. 3, pp 127-36. (9) K. P. Anderson and R. L. Snow, J. Chem. €doc., 44, 756 (1967). (10) T. P. Kohman, J. Chem. Educ., 47, 657 (1970). (1 1) E. Berne and I. Leden, Z. Naturforsch. Ea, 719 (1953). (12) L. Johansson, Coord. Chem. Rev., 3, 293 (1968). (13) K. H. Lieser, Z. Anorg. A/@. Chem., 292, 97 (1957).
(14) V. B. Vouk, J. Kratohvil, and B. Tezak, Arkiv. Kemi, 25, 219 (1953). (15) K. P. Anderson, E. A. Butler, and E. M. Wooliey, J. Phys. Chem., 77, 2564 (1973). (16) K. S.Lyalikov and V. N. Piskunova, Zh. Flz. Khim., 28, 127, 595 (1954). (17) H. Chateau and J. Pouradler, Science lnd. Phot., 23, 225 (1952). (16) G. Bodlaender, "Festschrift fuer R. Dedeklnd", Braunschweig, 1901. (19) G. Bodlaender and R. Fittig, 2.Pbys. Chem., 39, 597 (1902). (20) W. E. Morf, G. Kahr, and W. Simon, Anal. Chem., 46, 1538 (1974). (21) P. S.Smith and E. M. Wooliey, private communication. (22) J. A. Gledhiii and G. M. Malan, Trans. Faraday SOC., 50, 126 (1954); ibid., 49, 166 (1953). (23) A. L. Cummlngs, Dissertatlon, Brigham Young University, Provo, Utah, 1974. (24) B. B. Owen and S.R. Brinkley, Jr., J. Am. Chem. Soc., 60, 2233 (1938). (25) K. Hass and K. Jeliinek, Z.fhys. Chem., A, 162, 153 (1933). (26) A. E. Hill, J. Am. Chem. SOC.,39, 66 (1908). (27) L. G. Sillen, "Soiublllties of Silver Halogenides", Report July 1953 to the
Analytical section, I.U.P.A.C.
RECEIVEDfor review December 2, 1974. Accepted August 8,1975.
Alternating Current Polarographic Evidence for the Reversible Reduction of Trivalent Gallium from Acidified 1.O-Molar Sodium Thiocyanate at 60 OC Edward D. Moorhead and Gustaf A. Forsberg Department of Chemical Engineering, University of Kentucky, Lexington, Ky. 40506
Clinical research ( 1 - 5 ) has established that neoplastic sarcomas and certain categories of carcinoma (lung, breast, colon, etc.) (6) exhibit a pronounced tendency to sequester serum gallium citrate and other gallium salts. The metal's therapeutic effectiveness in regressing or arresting malignant growth-either as absorbed nonradiogallium ( 6 ) or as (principally) the 67 isotope used for in situ radiotherapy (2)-has prompted exploration of new benchtop analytical procedures ( 7 ) for the measurement of trace Ga which might speed up tissue assay efforts and aid in the eventual identification of the responsible biochemical mechanism. Were it not for the classically severe kinetic complications which are associated with the aqueous gallium electrode reaction (GER) (8-12), the electroanalysis of Ga(II1) a t trace and ultratrace concentration levels could be readily accomplished using a variety of acidified supporting electrolyte media. As described (10-14) in several earlier studies, the chemical composition of the electrolyte can play a catalytic role in moderating the actual charge transfer rate. For example, it has been found (IO) that a kinetically fast (Le., Nernstian) gallium electrode process is attainable a t the dropping mercury electrode (DME) if the supporting electrolyte is adjusted to satisfy two criteria simultaneously: 1) the presence of an inert salt at a very high ionic strength ( J ) ;and 2) the incorporation of >0.1 M SCN(12) or NB- ( 1 3 ) (halides are also marginally effective a t extremely high J values of 1 1 3 M ( I d ) ] .A supporting electrolyte comprised of acidified NaSCN/G.OM NaC104 was successfully employed in three recent investigations in which trace levels of gallium, including gallium in ashed tissue, were measured by ac phase-selective anodic stripping (PSAS) procedures (15-177, and Demerie et al. employed essentially the same media as the basis for their pulse polarographic analysis of gallium in samples of gallium arsenide semiconductors (18). However, binary electrolytes of the above types-which to our knowledge are the only ones known so far to produce room temperature
GER reversibility-can pose a variety of practical analytical difficulties ( 1 9 ) , due to the densely concentrated salts required, problems that could be substantially alleviated by discovery of milder electrolyte conditions. We wish to report in this note evidence that the degree of kinetic reversibility of the gallium electrode process exhibits a pronounced reciprocal dependence on ionic strength and temperature. Results obtained from single sweep alternating current phase-selective polarograms (PSP) show that a simple 30-OC increment in reaction temperature removes the historical necessity for densely concentrated inert salts and enables one to obtain a Nernstian voltammetric current readout from acidified NaSCN alone a t the 1.0-molar level (J = l),and an analytically useful peak in 0.1M NaSCN.
EXPERIMENTAL Apparatus and Reagents. The apparatus and reagents used to obtain the phase-selective polarographic results reported here have recently been described in detail elsewhere (16) and will not be reiterated. The mercury capillary electrode used was supplied by Princeton Applied Research. The capillary was driven by a Princeton Applied Research Model 174A drop knocker which was adjusted for a 2.0-sec drop time. Reaction temperatures between 20 and 70 O C were obtained using a Forma-Temp Model 2095 thermostated water bath which supplied a glass-jacketed Metrohm EA-876-20 electrolysis cell. Supply and return lines to the cell jacket were enclosed in foam insulation to lessen heat loss, but the .cell structure itself was not SO protected. Nonetheless, cell temperature was controllable to f l . O "C at 70 "C. Elevated solution temperatures combined with argon deaeration of the cell resulted in some evaporative loss of the test solution over prolonged periods of time, and therefore it is recommended that repeated runs on the same solution under such conditions should probably be avoided.
RESULTS AND DISCUSSION The rather extreme electrolyte concentrations required to effect reversible pseudohalide catalyzed voltammetric behavior present a t least two obstacles that are significant
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2313
9
Table I. Peak Current (rrns) vs. Gallium Concentration Ga(II1) molant) ( x lo5)
2 .oo 4 .OO 8 .OO
I I
1
0:5
0:6
x
I
8
0:9 -
0.5
I
0.6
07
0.8
- E vs 4 q / A q C I
3.9
I
ips,
w?
i i D Ga(II1)I x 10.'
0.07
3.50 5.25 4.25 10.0 4.20 20.0 4.15 40.0 4.10 80.0 4.01 100.0 4.02 Average 4.19 Supporting electrolyte of 1,OM NaSCN adjusted to pH 2.0 (HC101) and 60 "C. Excitation signal = 3.0 mV (rms) at 50 Hz; 2.0-second drop time; 1.0 mV sec-l scan rate. 0.21 0.34 0.42 0.83 1.64 3.21 4.02
I
and b equal to 4.49 and 0.28, respectively; that is, i, for the SCN--catalyzed In(II1) process increased less rapidly with increase in centigrade temperature.] Except for the 2O-OC datum, the Figure 2 points were found to fit the empirical equation i, = a exp [b(l/T)], where b and the preexponential factor a are -4896 and 1.15 X lo7, respectively, as determined by least squares [r = -0.99981 ( T = O K ) . If the temperature-induced increase reflects a rate-controlling, preceding chemical transformation, then multiplication of b by the gas constant yields -9729 cal mol-' for an apparent Arrhenius activation energy. Plausible precursor reactions include prior combination of Ga:&, with electrodeadsorbed SCN- to produce a rapidly reduced intermediate as in the case of In(II1) (21), or a prestep (22,23) involving desolvation to a more labile aquo Ga3+ moiety. Since a 1 3 0 OC rise in reaction temperature renders Ga(II1) almost as susceptible as In(II1) to SCN- electrocatalysis (21, 24), experimental comparison of these two redox couples a t elevated temperatures might aid in resolving the GER's mechanistic perference for pseudohalide [attempts at 60 "C to obtain some evidence of a halide-catalyzed peak by replacing NaSCN with 1.OM NaX (X = C1-, Br-, I-) were entire20 3C 40 50 60 70 ly fruitless]. Temperature 1°C) The high temperature reaction's response to increases in Flgure 2. Variation of PSP (rms) peak current with temperature for ionic strength was ascertained using the Figure 2 condi1.0 X 10-3M Ga(C104)3 in 1.OM NaSCN at pH 1.5. Experimental tions. For this series of solutions, electrolyte J values were conditions same as Figure 1, except 5.0 mV (rms) excitation signal incremented with NaC104. The i, vs. J profile deviated most conspicuously from the 6.OM NaC104 room-temperature response (12, 25) in that a) the plot depicted a linear for Ga(II1) trace analysis: 1)the requirement for NaC104 or increase in i, in the region 1 < J < 3, and b) i, was indesome other salt a t concentrations L6.OM can serve as a p e n d e n t of ionic strength in the region 3 < J < 6. source of contaminant metal ions which seriously interfere A plot of i, vs. NaSCN (not shown) at constant unit ionic with peak readout; and 2) high J value supporting electrostrength (NaC104) and pH 2 yielded an increase in current lytes place a severe constraint on the amount and nature of which limited a t ca. 0.8M NaSCN. The shape of this plot accompanying organic molecules making it impossible, or mimicked the earlier dependence found in 6.OM NaC104 at least very difficult, to utilize voltammetric peak behavior for both dc (12,18) and ac (12) polarograms. to study, e.g., the combination of Ga(II1) with various From room-temperature measurements, Moorhead and amino acids. Frame (12) found that ac peak magnitudes (in 6.OM Curve A of Figure 1depicts the current-voltage envelope NaC104) remained unchanged between pH 0 and 2, but (DME drop oscillations) for the ac phase-selective polarodropped to zero in the range 2 to 3. In the present study, gram obtained at 60 OC for 1.0 m M Ga(II1) in 1.OM NaSCN measurements a t 70 OC yielded substantially the same bebase electrolyte (pH 1.5 adjusted with HC104). Figure 1, havior, viz., i,'s were invariant up to ca. pH 2.5, then gradCurve B, represents an identical run using 0.1M NaSCN. ually decreased to zero at pH 4.5. Above pH 4, visible opalAnalysis of Curve A showed that it conformed to the equation E d c = E;$; ( 2 R T / n F ) In [(1p/1)1/2 - ((1, - 1)/1)'12], escence evidenced the onset of hydrous oxide formation. DC polarographic (12) and pulse polarographic (18) curwhich is a major criterion for ac reversibility (20). rents, e.g., from 6.OM NaC104 0.1M NaSCN, are known T o gauge the dependence of the Curve A peak on temto vary linearly with bulk Ga(II1) concentration, and it was perature, replicate PSP's were run on solutions comprised therefore of considerable interest to test this relationship of l.OmM Ga(II1) in (pH 1.5) 1.OM NaSCN using a 5.0-mV under the much milder electrolyte conditions of the (rms) 50-Hz excitation signal. The results obtained for 20 present study. The P S P results obtained a t 60 "C using a through 70 "C are shown as Figure 2. [Substitution of base electrolyte of 1.OM NaSCN adjusted to pH 1.5 1.OmM In(II1) for Ga(II1) in otherwise identical experi(HC104) are shown in Table I. A plot of the Table I data ments yielded only a 30% increase in i, from 30 to 60 OC followed by a least-squares analysis ( r = 0.99993) gave an and gave an excellent fit ( r = 0.9993) to i, = ~ ( 5 "with )~ a
Figure 1. AC phase-selective polarograms (envelope of drop oscillatlons) of 1.O X 10-3M c%(CIO~)~in 1 .OM NaSCN (Curve A ) and 0.1 M NaSCN (Curve B), both at pH 1.5 (HC104) and 60 'C. For Curves A and B: f = 50 Hz; 2.50-mV (rms) excitation signal: 2-second drop time: 1.0 mV sec-' scan rate
+
+
2314
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13. NOVEMBER 1975
(9) E. D. Moorhead and N. H. Furman, Anal. Chem., 32, 1507 (1960). intercept and slope of 0.0238 pA and 3.96 r A d-', re(10) E. D. Moorhead, J. Am. Chem. Soc., 87, 2503 (1965). (11) E. D. Moorhead and G. M. Frame, J. Phys. Chem., 72, 3684 (1968). spectively. Adaptation of these greatly simplified electro(12) E. D. Moorhead and G. M. Frame, Anal. Chem., 40, 280 (1968). lyte conditions to gallium anodic stripping voltammetry (13) E. D. Moorhead and G. M. Frame, J. Electroanal. Chem., 18, 197 (16) should enable ppb and sub-ppb Ga measurements to (1968). (14) C. Stoll, G. M. Frame, and E. D. Moorhead, Anal. Lett., 1, 861 (1968). be carried out with a useful reduction of interference prob(15) E. D. Moorhead and P. H. Davis, Anal. Lett., 7, 781 (1974). lems and incompatability with dissolved organics. A study (16) E. D. Moorhead and P. H Davis, Anal. Chem., 47, 622 (1975). of gallium stripping voltammetry utilizing dilute thiocy(17) P. H. Davis and E. D. Moorhead, Anal. Lett., 8, 387 (1975). (16) W. Demerie. E. Timmerman. and F. Verbeek, Anal. Lett., 4, 247 (1971). anate media is presently in progress and those results will (19) E. D. Moorhead and P. H. Davis, Anal. Chem., 46, 1879 (1974). appear at a later date. (20) D. E. Smith, "Alternating Current Polarography", in "Electroanalytical
LITERATURE CITED H. Dudley, Nucl. Scl. Abstr., 3, 284 (1949). C. L. Edwards and R. L. Hayes, J. Nucl. Med., 10, 103 (1969). Y. Ito, S.Okuyama, K. Sato, K. Takahashi. T. Sato, and I. Kanno, Radi-
ology, 100, 357 (1971).
R. L. Hayes, B. Nelson, D. C. Swartzendruber, J. E Carlton. and B. L. Byrd, Science, 167, 269 (1970). M. Hart, C. F. Smith, S.T. Yancy, and R. H. Adamson, J. Nat. Cancer Inst., 47, 1121 (1971). M. M. Hart and R. H. Adamson, Proc. Nat. Acad. SCl. USA, 68, 1623 (1971). (7) R. Zweidinger, L. Barnett, and C. Pitt, Anal. Chem., 45, 1564 (1973). (8) W. M. MacNevin and E. D. Moorhead. J. Am. Chem. Soc., 81, 6382 (1959).
Chemistry-A Series of Advances", A. J. Bard, Ed., MarceCDekker, N.Y., 1966. (21) L. Pospiscil and R. delevie, J. Electroanal. Chem., 25, 245 (1970). (22) Ya. I. Tur'yan and L. M. Makarova, Nectrokhlmiya, 9, 1334 (1973). (23) Ya I. Tur'yan and L. M. Makarova, Nectrokhlmiya. 10, 1359 (1974). (24) E. D. Moorhead, Anal. Chem., 38, 1796 (1966). (25) G. M. Frame, Ph.D. dissertatlon, Rutgers University, New Brunswick, N.J.. 1968.
RECEIVEDfor review May 15, 1975. Accepted July 30, 1975. We gratefully acknowledge support of this work by the National Institutes of Health (Grant No. GM 204702).
Extraction and Concentration of Copper by Anodic Stripping from a Mercury Thin Film Electrode Lawrence L. Edwards' and Beniamino Oregioni international Laboratory of Marine Radioactivity, Oceanographic Museum, Principality of Monaco
The flameless atomic absorption (AA) technique has a limit of detection of about 3 pg Cu in aqueous solution (1). This corresponds to a concentration of about 0.12 pg 1.-' (1.9 X 10-9M), near the level of Cu in unpolluted oceanic waters (2). However, to determine such a concentration accurately by AA spectrometry, the Cu must be extracted to concentrate it and to remove the interfering salts ( 3 ) .Recently, Fairless and Bard ( 4 ) have extracted Cu from seawater by reduction into a hanging Hg drop electrode. The Hg drop was subsequently transferred to and vaporized within a carbon filament atomizer ( 5 ) of an AA spectrometer. The particularly attractive aspect of this procedure was that a minimum of foreign substances came into contact with the sample. We have determined Cu in seawater by a variation of this method. An electrode consisting of a thin film of mercury deposited on a wax-impregnated graphite rod (6, 7) was used instead of a hanging Hg drop electrode. Instead of analyzing the Hg solution for dissolved Cu, the Cu was completely stripped out of the Hg into a small volume of distilled water. This water was then analyzed for Cu by flameless AA spectrometry.
EXPERIMENTAL Apparatus. AA. A Perkin-Elmer model 403 AA spectrometer equipped with a HGA 72 flameless AA attachment was used. The following program was used in each analysis: 100 "C for 30 sec; 450 "C for 30 sec; 2400 OC for 5 sec. The absorption a t 324.7 nm was monitored. Anodic Stripping. The reduction was carried out in 250-ml or 500-ml polyethylene wash bottles. The spout was used for the introduction of prepurified nitrogen. Stirring was accomplished with a Teflon-coated magnetic stirring bar. The electrode system conPresent address, Department of Chemistry, California State University, Northridge, Northridge, Calif. 91324.
sisted of an 8-mm diameter graphite rod and a 1-mm diameter platinum wire rigidly positioned relative to one another with the platinum wire extending below the graphite. They were supported by a plastic cap which fitted securely into the mouth of the wash bottle. The graphite rod had about 5 cm2 of surface area on which the Hg could deposit. The procedure was carried out in a Laminaire Corporation model V624R laminar flow hood. Reagents. The seawater samples were collected from various stations along the northwestern Mediterranean Sea (2) and filtered through 0.45-pm Millipore filter paper. An electrolyte solution of salinity 35%0was prepared from 3.8 g of Na2S04 and 30.2 g of NaCl diluted to 1 liter with distilled water. The Hg stock solution was prepared by dissolving 4 g of Hg (Kock Light Laboratories Ltd) in 2 ml of concentrated " 0 3 (Merck suprapur) and diluting to a final volume of 1000 ml. It contained 60 Fg Cu 1.-' (1.0 X 10-6M) as impurity. The graphite rod was cut from a spectroscopic grade carbon electrode (Perkin-Elmer Corporation). The standard solutions of Cu, Cd, and Zn used in spiking the samples and the Cu standard solution used in calibrating the AA spectrometer were all prepared every other day by dilution of stock solutions of concentration 1000 mg 1. -1. Procedure. Two-hundred-fifty or 500 ml of sample were placed in a cleaned wash bottle. Nitrogen flushing was started and continued throughout the electrolyses. Then 500 fil (10-5 mol of Hg) of the stock Hg solution were added to the sample. The electrode system was positioned and about 2.6 volts impressed across the two electrodes with the graphite being negative. (In a separate experiment with a third, reference electrode, the graphite was determined to be a t about -1.6 volts relative to a Ag/AgCl electrode under these conditions.) The Hg was deposited for a few hours. The electrode system was then slowly extracted from the wash bottle with the voltage unchanged and placed in a test tube containing a few milliliters of distilled water. During extraction, the graphite electrode cleared the solution surface before the platinum electrode. The two electrodes were then shorted together for about 20 minutes to strip out metals deposited, thus cleaning the Hg film. The sample was then changed and the electrode system placed back into the wash bottle. A new electrolysis was carried out for anywhere from 6 to 16 hours, long enough to reduce over 97% of the available Cu. Then the electrode system was again placed into
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2315
