(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 Chemistry-A Series of Advances", A. J. Bard, Ed., MarceCDekker, LITERATURE CITED N.Y., 1966.
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).
(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
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a small volume of distilled water and the Cu completely stripped out. Finally, using an Eppendorf pipet, 25 ~1 of this solution were placed into the graphite furnace of the flameless AA attachment. This last step was repeated several times.
RESULTS In the first set of experiments, samples of the electrolytic solution were first purified by the above method and then spiked to various values [in the range of 0.5 to 1.0 pg 1.-' (0.8 to 1.6 X 10-9M)] of Cu, Cd, and Zn. We analyzed for Cu and found the correct values to within f5%. The next set of experiments was identical to the first, except that seawater was used. The third set of experiments consisted of measuring the concentrations of Cu in various samples of seawater. The results agreed, within the combined experimental errors of f1596, with those obtained by Fukai and Huynh-Ngoc using anodic stripping voltammetry on the same samples of seawater (2). The final set of experiments was a series of measurements of the concentration of Cu in one particular sample of seawater. We obtained the value 0.57 f 0.04 pg Cu 1.-l (9.0 f 0.6 X 10-9M) making a total of 8 independent measurements. After each measurement, the concentration of Cu in the then metal-free sample was determined again. These results were always below 0.03 pg CU1.-l (0.5 X 10-9M). DISCUSSION The result of the anodic reduction and stripping is the transfer of essentially all the available Cu from the seawater to the distilled water. The concentration factor is thus the ratio of the two volumes, which was typically 100. The reduction and stripping of 100% of the metal is important as it results in a considerable simplification in the method. If some constant fraction of the metal is to be reduced,
then the reduction must be timed and the conditions maintained constant from run to run. This imposes stringent conditions on the cell geometry, mechanism and rate of stirring, geometry of electrode, and even the nitrogen bubbling (7). These conditions are normally so variable that it is often customary to calibrate each sample by the method of standard additions (7). In our method, such variations merely change the half-time for reduction and/or stripping. Since we wait many half-lives in both steps, such changes are not significant. In fact, we do not use the method of standard additions a t all. We merely prepare a standard solution of concentration in the range of the expected result, i.e., 100 times more concentrated than the seawater sample. A single standard is sufficient since the flameless AA technique is linear in this concentration region. ACKNOWLEDGMENT We would like to thank L. Huynh-Ngoc for his advice and assistance throughout the project. One of us (LLE) would like to express his appreciation to the International Laboratory of Marine Radioactivity in Monaco for the opportunity to work there for an extended period.
(1) (2)
LITERATURE CITED J. D. Winefordner and R. C. Elser, Anal. Chem., 43 (4). 24A (1971). R. Fukai and L. Huynh-Ngoc, XXiV Congres Assoc. Wen. Monaco, 6-14 Dec. 1974.
(3) D. A. Segar and J. G. Gonzelez. Anal. Chim. Acta. 58, 7 (1972). (4) C. Fairless and A. J. Bard, Anal. Chem., 45, 2289 (1973). (5) C. Fairless and A. J. Bard, Anal. Lett.. 5, 433 (1972). (6) W. R. Matson, D. K. Roe, and D. E. Carritt. Anal. Chem.. 37, 1594 (1965). (7) R. G. Clem. G. Litton, and L. D. Ornebs. Anal. Chem., 45, 1306 (1973).
RECEIVED for review May 5, 1975. Accepted August 4, 1975.
Direct Potentiometric Measurement of Several Thiols Paul K. C. Tseng and W. F. Gutknecht Department of Chemistry, Duke University, Durham, N.C. 27706
The sulfide ion-selective electrode has previously been used for the analysis of various thiol compounds. In 1971, Gruen and Harrap used the sulfide ion-selective electrode and silver nitrate as a titrant to determine potentiometrically the concentrations of cysteine and glutathione (1). Pungor et al. made similar titration measurements with thioacetamide (2) and phenylthiourea and N,N-diphenylthiourea ( 3 ) . In these analyses, the ion-selective electrode was used to monitor the change in [Ag+] during the course of the titration. Some success has been attained in the direct potentiometric analysis of several simple thiols. In 1972, Guilbault and von Storp used the sulfide ion-selective electrode to monitor the production of various thiocholine salts ( 4 ) . Peter and Rosset used the sulfide ion-selective electrode for the direct measurement of several thiols in a benzeneethanol solvent mixture ( 5 ) . Their results did not show good agreement with the Nernst relationship. In this paper, work is reported wherein several thiols in aqueous NaOH solution have been measured by the direct potentiometric method using a Ag2S membrane electrode. The electrode responses to these various thiols are Nern) to a detection limit as stian (i.e., Emeasd = f(1n U A ~ + ) down predicted by a model developed by Morf et al. (6). Subse2316
quently, the slopes of the response curves (Ememd vs. log cthiol) have been used to determine the stoichiometry of the electrode response reactions and the apparent formation constants for the silver-thiol complexes formed as a part of these reactions. The potential measured with an electrode utilizing a silver salt membrane can be described by the equation
EA^+ = E ' A ~ ++ RT -In U A ~ + F
where a A g + is the activity of the silver ion at the sample solution-electrode membrane interface. In the absence of complexing agents, this activity is described as the sum of: 1) the silver ion activity in the sample solution; 2) the silver ion activity due to the dissolution of the electrode membrane; and 3) the silver ion activity due to interstitial or defect silver ions. For a membrane composed of compressed, polycrystalline Ag& the silver ion activity due to membrane dissolution is much less than the defect silver ion activity, which is about 10-5.5M (6). Now, in the presence of thiols (silver ion complexing agents), the reaction between the silver ion and the thiol is
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
Ag+
+ pRS-Y
= Ag(RS)p'-PY
