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Solvent Extraction and Chemiluminescence Determination of Gold in Silver Alloy with Luminol in Reverse Micelles Imdadullah, Terufumi Fujiwara, and Takahiro Kumamaru' Department of Chemistry, Faculty of Science, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima 724, Japan
A new method, based on the direct comblnatlon of solvent extraction with lwnlnolchemllumlnescence (CL) In a revened mlcellar system of cetyltrlmethylammonlumchloride-water (buffered wlth sodium carbonate)-85 (v/v) chloroformcyclohexane, Is established for the determlnatlon of gold In aqueous solutions. The Interferencebehavlor of 15 metalson the reference gold CL signals was evaluated. By dkperslng an aqueous solution of the lndlvldual Interferent alone and In admixturewith the anaiyte, a comparative lnqulry on their CL behavior was also carriedout Inthe reversedmkellarmedium. Separation of analyte from Interferento by extraction wlth trlrroctylphosphlneoxide In chloroform resulted In reduced or no Interference. The present method was applkd to the determlnatlon of gokt In lndwrtrlalsarnges of silver-ba8ed alloy.
INTRODUCTION Judging by the number of recent publications,l-5 chemiluminescence (CL) has emerged as a successful method of detection in routine chemical analysis. This is the result of new methods and technical innovations that have developed simultaneously. Trace analysis is among the existing applications where CL-baseddetectionsare used, and unparalleled results with confidence for the analytical measurements at ultralow levels are reported. Also, CL detection in liquid chromatography and flow injection analysis (FIA)has received recent attention.2P"l The importance of micelles and microemulsions has been widely demonstrated in view of their analytical applications.s In CL analysis, reverse micelles were used for different including the first ever direct combination of
reversed micellar mediated CL (RMM-CL) detection with solvent extraction.lG19 The CL reaction of tetrachloroauric acid in chloroform with luminol in the presence of reverse micelles has recently been reportad for the quantification of gold in chloroform.l5 Greatly enhanced sensitivity was achieved in reversed micellar medium compared to conventional aqueous solutions.22 To investigate its analytical utilities, this work was extended to the determination of gold in aqueous medium. The CL analysis of gold, similar to iodine,"Q3 in aqueous basic buffer solution of luminol has the disadvantageof irreversible loss due to hydroxide formation, which might be the cause of nonlinearity of the calibration curve obtained in aqueous medium.15 In order to realize the benefits of reverse micelles, gold was quantitatively transferred as the tetrachloroaurate(II1) ion from aqueous acidic medium into chloroform containing tri-n-octylphosphine oxide (TOPO) and was subsequently determined via RMMCL.19 Unlike that in the aqueous medium the dynamic range was linear in reversed micellar medium. Even in the absence of oxidizing reagent like hydrogen peroxide, metal-catalyzed CL oxidation of luminol in aqueous solution has been reported for metal ions like iron(II),24 copper(II),25and gold(III).26 CL reaction in the absence of hydrogen peroxide is simple and relatively more specific. Due to the high sensitivity of the CL reaction toward different metals in aqueous medium, interference from the presence of these species is anticipated. Typically, solvent extraction is employed as a convenient manner of separating analyte from interferenta.5 However, use of certain organic solvents in such a solvent extraction step prior to CL measurements can present the serious problem of suppressing the CL emission observed from the luminol reaction in aqueousorganic mixtures.7.27 Alternative problems were inherited in using conventional aqueous media for CL after solvent extraction. For example a time-consumingprocedure such as back-extraction into an aqueous phase28 or evaporation of
(1) Fernandez-Gutierrez, A.; Munoz de la Pena, A. In Molecular Luminescence Spectroscopy. Methods and Applications: Part 1; Schulman, S. G., Ed.; Wiley & Sons: New York, 1985; pp 463-546 (see also references cited therein). (2) Townahend, A. Analyst 1990,115,495-500. (3) McGown, L. B.; Warner, I. M. Anal. Chem. 1990,62,255R-267R. (16) Fujiwara, T.; Tanimoto, N.; Huang, J.-J.; Kumamaru, T. Anal. (4) Warner, I. M.; McGown, L. B. Anal. Chem. 1992,64,343R3-352R. Chem. 1989,61, 2800-2803. (17) Fujiwara,T.; Kumamaru, T.ProceedingsoftheInternationalTrace (5) Fujiwara,T.; Kumamaru, T. Spectrochim.ActaReu. 1990,13,399406. Analysis Symposium '90 held at Sendai and Kiryu, Japan, July, 1990; pp (6) Bahowick,T. J.;Murugaiah,V.;Sulya,A.W.;Taylor,D.B.;Synovec, 187-190. R. E.; Berman, R. J.; Renn, C. N.; Johnson, E. L. Anal. Chem. 1992,64, (18) Fujiwara, T.; Tanimoto, N.; Nakahara, K.; Kumamaru, T. Chem. 255R-270R. Lett. 1991, 1137-1140. (7) Niemann, T. A. In Chemiluminescence and Photochemical Re(19) Imdadullah; Fujiwara, T.; Kumamaru, T. Proceedings of the actionDetectioninChromatography; Birks, J. W., Ed.; VCH: New York, International Congress on Analytical Sciences held at Makuhari-Messe, Japan, August, 1991; Anal. Sci. 1991, 7 (Suppl.), 1399-1402. 1989; pp 99-123. (8) Georges, J. Spectrochim. Acta Reu. 1990, 13, 27-45. (20) Hayashi, J.; Yamada, M.; Hobo, T. Anal. Chim. Acta 1991,247, (9) Inpexa International B. V. Neth. Appl. NL. 82 01,713 16 Nov 1983, 27-35. (21) Hayashi, J.; Yamada, M.; Hobo, T. Anal. Chim.Acta 1992,259, 8 pp; Chem. Abstr. 1984, 100, 111995f. 67-72. (10) Cohen, M. L.;Arthen, F. J.; Tseng,S. S. Eur. Pat. Appl. EP 96,749 (22) Lukovskaya, N. M.; Terletakaya, A. V.; Bogoslovskaya, T. A. Zh. 28 Dec 1983, 21 pp; Chem. Abstr. 1984,100, 182970e. (11) Yeda Research and Development Co., Ltd. Isr. IL 59,263 30 Nov Anal. Khim. 1974,29,2268-2270; Chem. Abstr. 1976,82, 132556~. 1982, 10 pp; Chem. Abstr. 1984, 100, 15173r. (23) Seitz, W. R.; Hercules, D. M. J. Am. Chem. SOC.1974,96,4094(12) Belyaeva, E. I.; Brovko, L. Yu.; Ugarova, N. N.; Klyachko, N. L.; 4098. Levashov, A. V.; Martinek, K.; Berezin, I. V. Dokl. Akad. Nauk SSSR (24) Seitz, W. R.; Hercules, D. M. Anal. Chem. 1972,44, 2143-2149. 1983,273,494-497; Chem. Abstr. 1984, 100, 134856a. (25) Klopf, L. L.; Nieman, T. A. Anal. Chem. 1983,55,1080. (26) Lukovskaya, N. M.; Bogoslovskaya, T. A. Ukr. Khim. Zh.(Ukr. (13) Hoshino, H.; Hinze, W. L. Anal. Chem. 1987,59, 496-504. (14) Igarashi, S.; Hinze, W. L. Anal. Chem. 1988, 60, 446-450. Ed.) 1975, 41, 268-273; Chem. Abstr. l975,83,107713a. (27) Delumyea, R.; Hartkopf, A. V. Anal. Chem. 1976,48,1402-1405. (15) Imdadullah; Fujiwara, T.; Kumamaru, T. Anal. Chem. 1991,63, 2348-2352. (28) Montano, L. A,; Ingle, J. D., Jr. Anal. Chem. 1979,51,926-930. 0003-2700/93/0385-0421$04.00/0
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Table I. Composition Weight Percentage of the Industrial Samples of Silver Alloy* sample Au Ag Pt Pd otherb 79.9 0.1 0.5 7.5 A 11.9 80.4 0.2 0.6 8.1 B 10.7 C
8.6
81.8
0.4
0.7
8.4
Composition establishedwith inductivelycoupled plasma atomic emission spectrometry. Mainly Ca, Cu, and Pb. the solventz9 is required before CL detection. The direct coupling of RMM-CL with prior separation has the advantages of eliminating some of the aforementioned problems associated with solvent extraction, thereby allowing development of an on-line CL method based on the combination of a continuous-flow extraction system using a microporousTeflon membrane." In this work, RMM-CL determination of gold in the presence of different potential interferents is addressed. The effects from the presence of various metal ions at different levels on the CL reaction with luminol in reverse micelles were evaluated. In the absence of hydrogen peroxide, the effect of individual metal ion, excluding gold(III), on the luminol CL reaction in reverse micelles had not been reported previously. In this paper, CL from luminol in reverse micelles is also recorded for certain metal ions in relation to their interfering behavior. After manipulation of the experimental conditions, gold in the presence of different interfering agents was extracted from aqueous solution into chloroform using TOPO as extracting reagent and selectively determined via RMM-CL. The method was successfully applied to the direct determination of gold in artificial and industrial samples of silverbased alloy containing some of the aforementioned metals. The method developed has the advantages of simplicity, sensitivity, and convenience since the lengthy processes associated with solvent extraction are avoided. Additionally, minimum sample manipulation is involved before analysis.
EXPERIMENTAL SECTION Reagents. Chloroauric acid, 1000gg mL-' standard solutions of gold and other metals, hydrochloric acid (electronic grade), nitric acid (electronicgrade),and sodium carbonate were obtained from Kanto Chemical Co., Inc. (Tokyo, Japan). Luminol was purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI). Cetyltrimethylammoniumchloride (CTAC)and tri-n-octylphosphine oxide (TOPO) were obtained from Tokyo Kasei Kogyo Co., Ltd. (Tokyo, Japan). Cyclohexane and chloroform (containing 0.5-0).9%(v/v)ethanol as stabilizer)were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). All chemicals were used without further purification. Deionized water, freshly collectedfrom an Advantec Toyo (Tokyo, Japan) Model GSU-901 water purification apparatus, was utilized in the preparation of all aqueous solutions. All working solutions were made before use. Silver-based alloy samples of preestablishedcomposition (Table I) were procured from SumitomoMetal Mining Co., Ltd. (Niihama, Japan). Apparatus. The FIA system used was composed of a Hitachi (Tokyo,Japan) Model K-1000 flow injection analyzer, equipped with a 16-port rotary, programmed automatic injection valve, a Tosoh (Tokyo,Japan) Model CCPM computer-controlled pump unit, and a Niti-on (Funahashi, Japan) ModelLF-800photometer with a coiled flow cell (70pL) which were used like before15JS for routine CL analysis. The CL signals were recorded with an ordinary strip chart recorder. A Nippon Jarrell-Ash (Tokyo, Japan) Model AA-8200two-channelspectrophotometer was used for graphite furnace atomic absorption (AA) measurements. All hollow cathode lamps used were manufactured by Hamamatsu (29)Boyle, E.A.; Handy, B.; van Green, A. Anal. Chem. 1987,59, 1499-1503.
Photonics (Toyooka, Japan). Background and backgroundcorrected AA signals were recorded with an ordinary strip chart recorder. Samples were injected with a Gilson Medical Electronics (Villiers-le-Bel,France) Pipetman P-20digitalmicropipet. A Taiyo (Koshigaya, Japan) Model SR-IIw time-controlled mechanical shaker was used for the shaking required in solvent extraction. Procedure. Luminol reagent was prepared daily by mixing a certain volume of the stock luminol solution (1.0 mM) in 0.2 M sodium carbonate (pH 11.5) with a 6 5 (v/v) chloroformcyclohexane mixture containing 10 mM CTAC.15 The reversed micellar solution had a molar concentration ratio ([HzO]/ [CTAC]) of 22.2 and was 4.0 gM with respect to luminol concentration. Workinggold and other metal-containingsolutions were prepared by serial dilution with 2 M HC1. A 50 ng mL-l solution of gold was used both for optimization and interference studies. Chloroform containing 20 mM TOPO was used as a solvent for extraction. With the exception of the aqueous to organic volume ratio, the rest of the experimental conditions in the present work were the same as reported earlier.l9 Aqueous gold solution (10 mL), containing the desired amount of the interfering species,was placed in a separating funnel, and 5 mL of chloroform containing TOPO was added to it. The mixture was shaken for 10 min and was allowed to stand awhile. The lower organic layer was collected in a clean test tube and was subsequently CL analyzed. Parallel AA measurements were carried out for the same samples using a 20 MLsample size, according to the conventional operating Both the analytical signals for CL and AA were taken as the difference in the observed peak heights for the analyte and blank. Industrial samples of silver-based alloy containing gold, originally collected as byproduct from the copper electrodeposition process, were digested according to the reported proced~re.~ Eachsample l (2.51-2.96 mg) was placed in a 10-mLbeaker and was digested with aqua regia, added in 2-mL portions, until the sample was completely dissolved. The digest was heated until it became tacky; then 2 mL of concentrated HC1was added, and an excess of nitric acid was expelled with heating. After cooling, each sample was made to 100mL with 2 M HC1. A blank solution was also prepared under the same conditions. All working solutions were prepared by serial dilution of the stocks with 2 M HC1. In addition, an artificial sample containing 100 ng mL-l Au(III), 625 ng mL-' Ag(I), 7.5 ng mL-' Pd(II), 2.5 ng mL-' Pt(II), 0.25 ng mL-l Cu(II), 0.25 ng mL-l Pb(II), and 0.25 ng mL-l Ca(I1) was prepared in 2 M HCl and was diluted accordingly before use.
RESULTS AND DISCUSSION Metal-CatalyzedChemiluminescence. In aqueous medium, certain metal ions are capable of catalyzing the CL reaction of luminol even in the absence of hydrogen peroxide. Relatively small CL signal for M (0.6 ng mL-1) copper(I1) and fairly high signals for 10-6 M iron(I1) and cobalt(I1) have been rep0rted.2~ The CL intensities with iron(I1) and cobalt(11) were 390 and 7 times, respectively, greater than those with copper(I1). In our previous work, the gold(II1)-catalyzed CL of luminol in reverse micelles in the absence of hydrogen peroxide has been estab1i~hed.I~In a comparative study, aqueous solutions of metal ions, related to inferference phenomenon, were dispersed in reversed micellar medium and their CL signals were recorded using the experimental conditions optimized for gold. The results are summarized in Table 11. Besides gold(III), at 100 ng mL-' of metal ion, only cobalt(I1) and copper(I1) produced fairly large CL signals. However, the CL intensities withgold(II1)are 2.5 times greater than that of cobalt(I1) and 4.5 times greater than that of copper(I1). At 500 ng mL-l of metal ion, the magnitude of the signals for iron(II1) and chromium(II1) was observed to be close to that of copper(II),but the signalintensity of cobalt(11) was 7.6 times greater than that of copper(I1). (30)Ham, S.; Matsuo, H.; Kumamaru, T. Bunseki Kagaku 1986,35, 503-507. (31)Xu,S.;Sun,L.; Fang, Z. Anal. Chim. Acta 1991,245,7-11.
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Table 11. Relative CL Intensities for Individual Metal Ion with Luminol in Reverse Micelles relative CL inbnsitvb ~
~~
[metall ng mL-1
Au(II1)
100 500
100 506
Cr(II1) 9
110
Fe(II1) COW) Cu(I1) 3 41 22 118 882 116
Ag(1)
0 0
*
a Dispersed in reverse micelles. All CL intensities are relative to the CL signal (~100) for 100 ng mL-l Au(II1).
Solvent Extraction. As the luminol CL reaction is sensitive toward different species, manipulations would be required to eliminate potential interferenta either before or during the solvent extraction process or to eliminate their effects in the detection process. Our initial investigation of the AuC14--catalyzed CL oxidation of luminol in reverse micelles and the extraction of the catalyst involved from 2 M HC1 suggested the possibilities of selective gold CL determination. The solvent extraction of the AuCh- ion with TOPO is believed to proceed through an ion pair f0rmation3~as follows:
H++ AuCl;
+ xH,O + nTOPO(org) = H+.nTOPO.wHzO..*AuC1;(~ - w)H20(org)
Since solution chemistry (e.g. degree of hydration of the ions, polarizability,difference in hydrogen bond capability, degree of electrostaticinteraction in ion-pairing) of one species differs from another species in aqueous medium,33 a difference in the extraction behavior of TOPO and partial or complete elimination of certain interferents is expected. There are several possibilities that gold could be selectively transferred into chloroform under appropriate experimental conditions. The reported decreasing order of extractability of metals from 2 M HC1 solution by TOP033 is Au(I1) >> Fe(EI1) > Ag(1) > Pt(I1) > Pd(I1) > Pb(I1) > Cu(I1) > Ni(I1) > Co(I1) > Mn(I1) > Cr(II1). The significance of reverse micelles in CL analysis is considered to be due to its unique structure (shape/size)34 and composition (ionic strength, local pH, etc.). An increase in HC1 concentration beyond the optimized level for the extraction of gold resulted in a decrease in the CL signals but no effect on AA signals.lg This could be attributed to an increase in the accumulationof free HC1in chloroform during solvent extraction which brings a change in the local pH inside the reversed micellar aqueous pool the very moment the sample and reagent are mixed for the desired CL reaction. In a separate study, a 50% decrease in the CL signals was observed for an aqueous solution containing equivalent amounts of gold and HC1but was 0.1M with respect to HN03. Higher extractability for HN03 compared with HC1 up to 2 M acid in the aqueous phase by TOPO has been reported.36 It is difficult to explain this phenomenon explicitly, but in view of the respective aqueous anion hydration for C1- and NO3-, relatively more free HN03 would be extracted into chloroform causing an alteration of the luminol buffer in the reverse micelles. Similarly, an increase in the aqueous to organic volume ratio, V(aq)/ V(org),beyond the optimum level led to an increased accumulation of free or excess acid in chloroform and a negative effect on the resulting CL signals.lg (32)Bucher, J. J.; Zirin, M.; Laugen, R. C.; Diamond, R. M. J.Inorg. Nucl. Chem. 1971,33,3869-3883. (33)Marcus, Y.; Kertes, A. S. Ion Exchange and Solvent Extraction of Metal Complexes; Wiley & Sons: New York, 1969;pp 694-736. (34)See, e.g.: Pileni, M. P. In Structure and Reactiuity in Reuerse Micelles;Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989 (see also references cited therein). (35)Bucher, J. J.; Conocchioli, T. J.; Held, E. R.; Labinger, J. A.; Sudbury, B. A.; Diamond, R. M. J.Inorg. Nucl. Chem. 1975,37,221-227.
0 1 10 IO* lo3 Interferant :gold weight r a t i o
Figure 1. Effects of interferents on the solvent extractbn/CL determlnatlon of gold(111) wlth lumlnol In reverse micelles. Increaslng amounts of interferents(0,Fe(II1); A,AO(1); 0, R(I1); 0, Cu(I1); A, Co(I1); m, Cr(II1); V,Ca(II), MNII), NYII), W I I ) , and Pb(I1)) were added to a 50 ng mL-' solution of gold. Dashed lines include 10% error band.
Although there was a drastic change in the CL signals, the AA studies showed the absence of any effect on the extraction of gold with a change in V(aq)/V(org). In spite of the fact that an optimized V(aq)/V(org) ratio of 10 was already established,lg a reduced ratio of 2 was used instead in order to minimize waste (consumptionof less reagents and samples) and simultaneouslyto improve the CL signal reproducibility. Interference Effects. To evaluate interference with respect to concentration, various amounts of each interfering ion were separately added to the 50 ng mL-l standard of gold in aqueous2 M HCland after solvent extraction the CL signals were compared to reference gold signals. Keeping in mind the composition of the prospective alloy samples (Table I), different metal ions were tested and their tolerable limita established. In CL analysis no generalization concerning the behavior of metal ions with respect to their relative position in the periodic table or oxidation state per se has been reported. Similarly, in the present work, involving the use of reverse micelles as a medium for CL measurements, categorization is made merely on the basis of the interference effect of the individualspecieson the reference gold CL signal. The results fall in the following groups: (a) no effect, (b) enhancement effect, and (c) inhibition effect. (a) No Effect. Species like lithium(I), sodium(I), potassium(I), calcium(II), barium(II), manganese(II), nickel(II), palladium(II), and lead(I1)were found to be benign to the CL determination of gold. When present at an interferentlgold weight ratio of 1O00, no effect of the individual species was observed either on the extraction of gold or ita CL signal, as shown in Figure 1. All results were within 10% error band. (b) Enhancement Effect. Figure 1 shows a mild enhancement in the CL signals for chromium(III), cobalt(II), and copper(I1) when either one was present above a weight ratio of 50. However, AA measurements showed no effect on the extraction of gold either above or below this ratio. A blank CL signal (Le. in the absence of gold) was observed for copper(I1) only when its concentration was 500 times more than the correspondinganal* concentration in the aqueous solution. This indicated that after slight modification in the existing experimental conditions, copper(I1) could be extracted and CL detected. Obviously,a fairly good relationship exists between the earlier mentioned order of extractability and the resulting order of interference levels for the metals, Fe(II1) > Ag(1) > Cu(I1) > Co(I1) > Cr(II1) > Pt(II), as given in Figure 1. Iron(II1) had a very strong enhancement effect on the gold CL signals when present at an iron/gold weight ratio of 5
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15, 1993
Table 111. Relative CL Intensities for the Mixture of Metal Ion and Gold(II1) with Luminol in Reverse Micelles relative CL intensity*
Table IV. Determination of Gold in Artificial and Industrial Samples of Silver Alloya sample
[metall ,n ng mL-'
Cr(II1)
Fe(II1)
COW)
Cu(I1)
Ag(1)
100 500
153 129
104 790
188 430
97 143
102 104
A B C
Dispersed in reverse micelles. All CL intensities are relative to the CL signal (=loo) for 100 ng mL-l Au(II1) alone.
amt with CL,b wt %
amt with AA, w t %
Artificialc 12.9,13.6
13.6, 14.0
Industriald 11.6,11.8 10.3,11.4 8.4, 8.9
11.4,11.5 10.3,lO.g 9.4,9.6
Results in duplicate. The relative standard deviation (n = 5) of the analytical signals was in the range of 3%. The composition is given in the text: Au, 13.6 w t %. Amounts of the alloy samples, A, B, and C, in 10-mL working solutions used for analysis are 8.87, 8.72, and 7.53 gg, respectively. (I
(Figure 1). However, neither a blank CL signal for iron(II1) nor an effect on the extraction of gold was observed. In spite of such a significant enhancement of CL emission in the presence of iron(II1) a t this ratio, AA measurements on the same samples indicated the absence of any detectable amount of iron extracted independently or coextracted with gold as expected above. To explain this unique involvement of iron(III)in the postextractionluminol-AuCL- CL reaction, further investigations are required. One can speculate that in the extraction process, a very minute quantity of iron(II1) is most probably trapped in the H+-TOPO-H~O-AUCL- complex, which ultimately causes an effective catalytic oxidation of luminol. Iron(II1) can presumably be masked prior to extraction by making the aqueous solution of gold containing iron(II1) 0.1 M with respect to fluoride. If so masked, no enhancement in the gold CL signals was observed. In addition to measuring CL intensity due to individual metal ion only, a similar comparative study for the metals in this group under the same experimental conditionswas carried out by dispersing an aqueous mixture of gold(II1) and the desired metal ion in the reversed micellar medium. The results showed the influence of these metals on the CL of gold with luminol in the reversed micellar system as given in Table 111. When present at a metal/gold weight ratio of 1, only chromium(II1) and cobalt(I1) interfered while the rest of the metals showed interference at a weight ratio of 5. In comparison to Figure 1,this obviously indicated the separation of the corresponding metals from the analyte, with solvent extraction resulting in reduced or no interference at all. In this regard a unique behavior was observed for iron(II1).When present alone, the relative CL signals of iron(II1) were almost identicalto those of chromium(1II)(Table 11). However, when a mixture of gold(II1) and one of the two metals was used to catalyze the CL reaction in reverse micelles, a significant enhancement in the CL signals was observed only for the iron-gold mixture when present a t a metal/gold weight ratio of 5 (Table 111). This might be due to the gold-iron interaction which strongly influenced the metal-catalyzed oxidation of luminol, as assumed for a similar involvement of iron(111) in the postextraction luminol-AuC14- CL reaction mentioned earlier. (c) Inhibition Effect. Both silver(1) and platinum(I1) inhibited the gold CL signals (Figure l),but the difference in their respective tolerable amounts is great. For silver(I), no impact on the CL signals was observed when present up to a silver/gold weight ratio of 10. Additionally, when dispersed in reverse micelles, neither CL activity nor an
inhibition effect was observedfor silver(1)when present alone (Table 11) or in an admixture with gold (Table 111). For platinum(II), inhibition in the CL signals occurred when ita weight ratio to gold exceeded 500. Gold Determination. The application of the solvent extraction-CL detection method developed was applied to the direct determination of gold in both artificial and industrial samples of silver-based alloys containing other metals (Table I). Without pretreatment of the sample solutions gold was directly determined in the presence of metals like platinum, palladium, copper, calcium, and lead using a calibration graph. The results are given in Table IV. The data are in good agreement with the results obtained by means of standard additions. To evaluate the method critically, results obtained with CL were checked through parallel AA measurements performed for the same samples (Table IV). Both the CL and AA data are in good agreement with the results for gold with inductively coupled plasma atomic emissionspectrometry established compositionof the respective samples (Table I). This indicates that the luminol CL method of gold determination in the presence of aforementioned potential interferents is valid, simple, rapid, and cheap. A substantial advantage of the proposed method is that neigher preconcentration nor screening are required prior to analysis. However, before this method can be applied to samples of biological and environmental nature, analytical efforts are certainly needed to identify potential interferents. This and on-line solvent extraction RMM-CL determination of gold will be included in our further work.
ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research, No. 04453038, from the Ministry of Education, Science, and Culture, Japan, and one of the authors, Imdadullah, also thanks this Ministry for awarding a Scholarship. We thank the Analytical Center of Sumitomo Metal Mining Co., Ltd., Niihama, Japan, for providing us silver-based alloy samples. RECEIVEDfor review September 10, 1992. Accepted November 16, 1992.
