1632
Anal. Chem. 1884, 58. 1632-1640
No measurable (>1ppb) metal carbonyl was found under any ventilation condition using the spray metalizer. All that was measured ww an apparent NO level of approximately 0.5 ppm. Such observations are in accord with what would be expected from thermodynamic considerations since at high temperatures all metal carbonyls are unstable to thermal dissociation. ACKNOWLEDGMENT Special thanks are given to David West for his assistance in designing and construction of the portable detector and to Bobbi Walunas for her assistance in preparing the manuscript. Registry No. Ni(C0I4,13463-39-3; Fe(CO)5,13463-40-6;NO,
10102-43-9.
LITERATURE CITED Wlimot, P. D. Petroleum (London) 1971, 2 4 , 376. “Nickel”; National Academy of Sciences: Washington, DC, 1975. Moffltt. A. E.; Baseit, R.; Grinsfelder, H.; Stedman, D. H. Proposed ASTM E-34 Committee Standard, 1978. Mond, L.; Langer. C.; Quincke, F. J . Chem. SOC. 1890, 5 7 , 749. “Nickel Carbonyl”, (Hyglenic Guide Series) Am. Ind. Hyg. Assoc. J . 1968. 29. 304. Lau, T. J.; Hackett, R. L.; Sunderman, F. W., Jr. Cancer Res. 1972,
32, 2253.
Sunderman, F. W., Jr.; Aiipass, P. R.; Mitchell, J. M.; Baseit, R. C.; Albert. D. M. Sclence 1979. 203. 550. “Documentation of Threshold Limit Values”, American Conference of Governmental Industrial Hygienists, 1977, p 381. Brief, R. S.; Venabie, F. S.; Ajemion, R. S. Am. Ind. Hyg. Assoc. J . 1967, 2 8 , 21. Sunderman, F. W.; Range, C. L.; Sunderman, F. W., Jr.; Donneiiy, A. J.; Lucyszn. G. W. Am. J. Clln. Pafhol. 1961, 3 6 , 477. Stahiy, E. E. U.S. Patent 3246654, 19 Apr, 1966. Sunderman, F. W., Jr.; Roszei, N. 0.; Clark, R. J. Arch. Envlron. Health 1968, 76, 836. Mikak-Devik, M.; Sunderman, F. W., Jr.; Nomoto, S. Clin. Chem. (Winston-Salem, N . C . ) 1977, 2 3 , 948. Chievousky, T. Plyn. 1972, 5 2 , 66. Troy, D. J. Anal. Chem. 1955, 2 7 , 1217. Campana, J. E.; Risby, T. H. Anal. Chem. 1980, 5 2 , 468. ”A Summary of the Properties of Nickel Carbonyl”; INCO Nickel, Ltd.: Toronto, 1975. McDoweil, R. S. Am. Ind. Hyg. Assoc. J . 1971, 3 2 , 621. Mantz, A. W. Appl. Spectrosc. 1978, 30, 539. Morris, E. D.; Niki, H. J . Am. Chem. SOC. 1970, 9 2 , 5641. Stedman, D. H.; Tammaro, D. A.; Branch, D. K.; Pearson, R., Jr. Anal. Chem. 1979, 57, 2340. Stedman. D.H.; Tammaro, D. A. Anal. Lett. 1976, 9 , 81. Stedman, D. H. US. Patent 4205956, 3 June, 1980. Tammaro, D. A. M.S. Thesis, University of Michigan, 1975. Hikade, D. A. Ph.D. Thesis, The University of Michigan, 1981.
RECEIVEDfor review December 29, 1983. Accepted April 9, 1984. This work was supported in part by DOE Contract EE-77-S-02-4499.
Mechanism of Surfactant4nduced Changes in the Visible Spectrometry of Metal-Chrome Azurol S Complexes John H. Callahan and Kelsey D. Cook*
Department of Chemistry, University of Illinois, 1209 West California St., Urbana, Illinois 61801
The mechanisms of surfactant-induced changes in the vlslbie spectrometry of Bez+, Cup+, and Ai3+ complexes of Chrome Aruroi S (CAS) were investigated. Increases in molar absorptivity and red shifts in the wavelength of maximum absorbance were observed for all three metals with a cationic surfactant (hexadecyitrimethyiammoniumbromide, CTAB), and for Bez+ and Cu2+complexes with a nonionic surfactant (Triton X-100, TX-100). Similar effects were noted with an anionic surfactant (sodium dodecyi sulfate, SDS) in solutions of high lonk strength. Strlctiy micellar interactions were involved in the cases of TX-100 and SDS. Both ternary complex formation (Involving surfactant monomers) and micellar interactions were observed with CTAB. Interactions with CTAB ranged from the formation of distinct complexes at k w surfactant concentration to mlceilar interactions above the crltlcal mlcelle concentratlon. Spectra of all “surfactant sensitized” systems were generally simUar, suggesting simHar local environments for the chromophores In ternary compiexes and in mlcelbs.
The addition of surfactants to aqueous solutions of certain metal-dye complexes results in substantial changes in the UV-visible spectrometry of these complexes. Up to 60-fold increases in molar absorptivity (e) (I) and 150-nm red shifts in the wavelength of maximum absorbance )A(, (2) have been reported among the systems summarized in recent reviews (3-5). The mechanism of these surfactant-induced changes 0003-2700/84/0356-1632$01.50/0
(and resulting sensitization of related analytical methods) is not well understood. Improved understanding would greatly facilitate analytical exploitation and should also be useful in explaining surfactant-solubilizate interactions in other applications. This report considers evidence distinguishing between the two major models for sensitization mechanisms: micellar solubilization and formation of ternary complexes involving surfactant monomers. There is evidence in the literature supporting each major model. For example, Sawin et al. (6) found that no spectroscopic changes were induced when the nonionic surfactant OS-20 was added to the A13+ -Chrome Azurol S metal-dye complex until the critical micelle concentration (CMC) was surpassed. Similar observations have been reported for several systems involving nonionic surfactants (7)and at least one system involving a cationic surfactant (8). Additionally, in a recent study in this laboratory involving a cationic surfactant (9), sensitization wag found to be enhanced by excess electrolyte. The degree of sensitization was observed to be dependent on the nature of the anion added, following the known order for counterion binding to a cationic micelle (IO). This is markedly similar to salt effects observed in micellar catalysis (11). Similar effects are not expected in systems where interactions involve exclusively surfactant monomers (instead of micelles) (12). Thus the observed salt effects were interpreted as suggesting that micellar interactions were at least partly responsible for the sensitization process. There is also significant evidence for the alternative model for surfactant sensitization, the formation of ternary complexes (4,13-16). Pertiiient studies almost exclusively involve cat0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1984
ionic surfactants. The hypothetical complexes are characterized by stoichiometries in which typically up t o four surf a a t monomers interact with the metal-dye complex. The induced spectral changes are essentially identical with those attributed elsewhere to micelles. The concentrations at which spectral changes are noted are generally below the nominal CMC of the surfactants involved, although investigators occasionally overlook the lowering of the CMC by cosolutes (17). Much of the conflicting evidence in the literature could be explained by a transition from monomer to micelle interactions, as surfactant concentration is increased. However, most studies cite evidence for one model or the other and do not carefully pursue the alternative. Thus, no such transition has been reported. This study was undertaken to establish, with representative metal-dye systems, whether micellar interactions, ternary complex interactions, or both are responsible for surfactant sensitization. The primary metal-dye system selected was the complex of Be2+with the triphenylmethane dye Chrome Azurol S (CAS). Additionally, studies were carried out on the Cu2+and A13+ complexes of CAS to determine whether the metal involved in the complex influences the sensitization behavior. The dependence of the visible spectrometry of these systems on surfactant concentration will be considered for a nonionic surfactant (Triton X-100, TXloo), an anionic surfactant (sodium dodecyl sulfate, SDS) and a cationic surfactant (hexadecyltrimethylammoniumbromide, CTAB). The results will be used t o provide a model for the interactions involved in the sensitization process. Such a model should remove much of the uncertainty which makes analytical application of the spectral sensitization susceptible to error arising from unrecognized matrix effects (9, 18).
EXPERIMENTAL SECTION Apparatus. A GCA McPherson Model 707 spectrophotometer or a Hewlett-PackardModel 8450 A spectrophotometerwere used for all absorbancemeasurements. One-centimetermatched quartz cells were used, and all readings were made against a distilled deionized water blank. (None of the salts, surfactants, or buffers used in this study exhibited any absorbance in the 400-700-nm wavelength region employed.) pH measurementswere made with an Orion Model 701 pH meter, using a Model 91-01 pH (glass) electrode. Surface tension measurements were made with a DuNouy platinum ring tensiometer. Calibration of the tensiometer was achieved by measurements on a series of solvents of known surface tension. Values reported represent the average of three determinations, All glassware was cleaned with 10% HN03 and triply rinsed with distilled deionized water. Reagents. Distilled water was used for making all solutions. The Be2+used in this study was obtained from a 1.104 X lo-' M BeClzatomic absorption standard (Aldrich). A 1.104 X M stock solution was prepared by appropriate dilution of the AA standard. The 1.00 X lo-' M Cu2+and A13+standard solutions were prepared by dissolving 0.1 mol of the metal in 18.8 mL and 25.0 mL of concentrated HN03, respectively, and diluting to 1 L. The 1.00 X 10-4M stock solutionswere prepared by appropriate dilution. Chrome Azurol S (CAS) was obtained from Eastman (practical grade) and recrystallized three times by using the method described by Vekhande and Munshi (2) and Martynov et al. (19). A 1.0 X M stock solution was prepared by the addition of enough NaOH (Mallinckrodt) to make the final concentration 3.0 X M in NaOH. Hexadecyltrimethylammoniumbromide (CTAB) was obtained from Sigma Chemical Co. and recrystallized three times from ethanol and diethyl ether. The purity of the CTAB was determined by the method described earlier (9,20) and confirmed by measuring the CMC (8.5 X lo4 M based on surface tension) which was in agreement with literature values (21). Sodium dodecyl sulfate (SDS) was obtained from British Drug House (99+ % purity) and from Sigma Chemical Co. (99% purity). The SDS was purified by extraction three times with diethyl ether and a single recrystallization from ethanol. CMC values of 6.5 X M and 6.3 X M were obtained for the purified BDH and
1633
2.0
WAVELENGTH (nm)
Figure 1. Absorption spectra of CAS, Be-CAS, and Be-CAS-surM CAS; (B) 4.0 X M Be2+, factant complexes: (A) 1.0 X 1.O X lo-' M CAS; (C) 1.00 X M Be2+,1.0 X lo-' M CAS, 1.O M TX-100; (D) 4.0 X lo-' M Be*+,1.0 X M CAS, 1.0 X X M Be2+,1.0 X M CAS, 1.0 X lo-* lo-* M SDS;(E)1.0 X M SDS, 0.5 M NaBr; (F) 1.00 X M Be2+, 1.0 X lO-'M CAS, 1.0 X M CTAB (G) 1.00 X M Be*+, 2.0 X M CAS, 1.0 X
M CTAB.
Sigma SDS, respectively (determined by surface tension measurements), somewhat lower than the literature value of 8.2 X 10-3 M (22). The Triton X-100 (TX-100) used in this study was obtained from Sigma Chemical Co. and used as received. Concentrations were calculated assuming an average molecular weight of 600 g/mol. The CMC obtained from surface tension measurements for this surfactant (1.0 X M-3.0 X lo4 M) corresponds well with the values given in the literature (23,24). The buffer system used in this study was prepared from Aldrich reagent grade hexamethylenetetramine(HMT) and HBr (Baker). The buffer stock solution was 1.00 M HMT and 1.96 X M HBr. It was used in 1:lO dilution in all solutions, giving pH 6.7. All other salts used in this study were reagent grade (Mallinckrodt) and were used as received. Procedure. To avoid the possibility of interferencefrom effects of reaction kinetics in these analyses (25),the order of mixing of all solutions was metal, buffer, CAS, surfactant, and salt (if present), throughout. Preliminary studies determined that the maximum absorbance of all solutions was reached within 30 min after which absorbanceremained constant over a period of at least 12 h. All solutions were allowed to stand at least 45 min prior to analysis. Solutions in which precipitation occurred were stirred just prior to measuring absorbance resulting in a colloidal suspension. As such, resulting absorbance values probably reflect scattering by colloidal particles. pK, values for CAS were obtained by a standard spectrophotometric approach (26) using absorbance data at 496 and 425 nm of the protonated (HzCAS2-)and deprotonated (HCAS3-) forms of the dye, respectively, at the pH employed here). For the free dye (no metal or surfactant), this resulted in an estimated pK, = 5.0 f 0.1, in reasonable agreement with published values (4.7 (27) and 4.9 (28)).
RESULTS AND DISCUSSION Effects of TX-100: A Nonionic Surfactant. At a TX100 concentration of 1.0 X lov3M (above the reported CMC, 1.0 X M-3.0 X M (23,24)) the addition of the surfactant to the Be-CAS system resulted in a significant red shift in Am, as well as an increase in absorptivity (Figure 1C). A corresponding decrease in the absorbance of uncomplexed CAS (425 nm) was also observed. To test whether spectral changes might be attributable primarily to interactions with free dye, the experiment was repeated without metal present. No effect was noted at low surfactant concentrations, but the surfactant did induce appreciable absorbance by CAS at long wavelengths and reduced absorbance at 425 nm as the surfactant concentration reached the CMC region (1.0 X lo4 M) (Figure 2). Thus, the effect of TX-100 on CAS is apparently
1834
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
L .
Table I. Spectroscopy of CAS and Be-CAS Complexes in the Absence and Presence of Surfactantsa
surfactant (M)
complex
Be: CAS
Xmw
nm
f ub
CAS 425 2240OC Be-CAS 538 14350d 1:l Be-CAS TX-100 (1.0 X lom3) 620 100500 + 2400 1:2 -e Be-CAS SDS (1.0 X 5 600 -e Be-CAS SDS (1.0 X 620 148000 f 3100 1:2 Be-CAS CTAB (1.0 X lo-') 608 55000 1700 1:2 Be-CAS CTAB (1.0 X 10-3)g 622 83700 f 1600 1:2 Be-CAS CTAB (2.2 X 10-5)h 604 78000 1:2 Be-CAS CTAB (5.0 X 10-S)h 604 75400 1:2 Be-CAS CTAB (1.0 X 616 79900 1:2 Be-CAS CTAB (1.5 X 10i)j 614 63500 1:2 Be-CAS CTAB (4.0 X 10")' 616 63000 1:2 Be-CAS CTAB (1.0 X 10-3)i 622 114000 1:2 [CAS] = 1.0 X M unless otherwise specified. b e and u represent the least-squares slopes of calibration curves (for Be concentrations from 5.0 X M to 2.0 X IO" M) and their standard deviation, respectively, except where noted. From ref 19. dFrom ref 27. "No apparent shift in A, was observed in this case. e was measured at 620 nm. A stoichiometry could not be obtained. f[NaBr] = 0.5 M. g[NaBr] = 0.1 M. h[Be2+]= 1.1 X M, CAS = 2.20 X 10" M. e values were calculated from single absorbance values. [BeZ+]= 5.50 X 10" M, [CAS] = 1.10 X M, [NaBr] = 0.1 M. e values were calculated from single absorbance values.
*
WAVELENGTH l n m l
Flgure 2. Absorption spectra of CAS (1.0 X lo4 M) in the presence of various amounts of TX-100. Surfactant concentration: (A) 0 M; (B) 2.0 X IO-' M; (C)1.0 X M; (D) 2.0 X M; (E)4.0 X lom3M; (F) 6.0
x
10-3 M.
micellar in nature. Due to the finite solubilizing capacity of the micelles, spectral changes become more pronounced as the surfactant concentration increases. However, no further changes are observed once there are sufficient micelles present to solubilize all of the dye (above -6.0 X M TX-100). The changes have been attributed to shifts in the degree of protonation (or apparent pKa) of the dye upon solubilization (3,29). Absorption spectra of the solubilized H2CAS2-and HCAS3- can be obtained by adjusting the bulk pH of the micellar solution to more extreme values (pH -4.0 and -9.0, respectively). Assuming that the data at pH 6.7 (Figure 2F) represent a linear combination of these spectra, they can be used to estimate an effective pK, of the solubilized dye. The pKa calculated in this manner (6.4 f 0.1) is roughly 1.4 units higher than that of the aqueous dye. However, it should be noted that while most of the spectral features in Figure 2F are consistent with the linear combination hypothesis, the shoulder at 580 nm is not. Thus, although this is a common approach to interpretation of the effects of solubilization on dye spectra, the precise significance of the calculated pKa is not clear in this case. Nevertheless, the deviations are small and the bulk of the spectral changes probably can be attributed to shifts in protonation equilibria. Despite these spectral effects, the surfactant does not induce significant free dye absorbance at 620 nm, suggesting that most of the absorbance at this wavelength in Figure IC is due to the Be-CAS complex. In fact, the absorbance at 620 nm varied linearly with beryllium concentration over at least the range 5.0 X lo-' M-2.0 X M, confirming attribution to the complex. The slope of this calibration curve was higher than that obtained at 538 nm in the absence of surfactant, suggesting a net sensitization. The metal-dye ratio in the complex was determined by varying the dye concentration while holding the metal and surfactant concentrations constant (mole ratio method). The meta1:dye stoichiometry changed from 1:l in the absence of surfactant (30)to 1:2 in the presence of surfactant. Thus, incorporation of more dye into the complex accounts for part of the decrease in absorbance at 425 nm accompanying addition of TX-100. Table I summarizes the spectroscopic behavior for the B d A S system in the M TX-100. presence of 1.0 X In order to determine whether TX-100 monomers or micelles were responsible for sensitization, absorption spectra for the Be-CAS system were obtained over a wide range of surfactant concentrations. The results are summarized in Figure 3, which shows how the absorbance of the sensitized Be-CAS complex (at 620 nm) varies with TX-100 concentration. Superimposed is a plot of surface tension vs. TX-100 concentration for the same set of solutions. There is little sensitization at surfactant concentrations comparable to the
70
01
12
00
05
IO
15 2
20
25
302 3 5
40
45
' 50
55
60
0
[TX-1001 x I$ M
Flgure 3. Dependence of absorbance (at 620 nm) and surface tension of solutions of the Be-CAS complex on TX-100 concentration: (0) M, [CAS] = 1.10 X A,, , [Be2+]= 5.50 X M; (0)AeZ0, M, [CAS] = 2.20 X M; (A) surface tension [Be"] = 1.10 X
at both Be2+ concentrations. M). Only as concentration of the metal-dye complex the surfactant concentration increases through the broad CMC region is there an increase in absorbance. The CMC in this case is estimated to be approximately 5.0 X M. Repetition of the experiment a t a lower complex concentration lowered all absorbances proportionately and did not otherwise affect the curve shape (Figure 3). These results clearly indicate that micelles are necessary for sensitization by TX-100. However, the low value of the CMC in Figure 3 deserves comment. A surface tension plot obtained in the presence of HMT/HBr buffer alone yielded a curve similar to that of Figure 3, with a CMC of 8.0 X 10" M. The addition of the buffer and the metal-dye complex results in a lowering of the CMC of TX-100. Such effects are common in nonionic surfactant systems (31, 32). In an earlier study (9),it was found that the electrolytemicelle interactions that affect the CMC of cationic surfactants also induce spectral changes in a sensitized dye-metal complex. However, when tests were done for similar effects in conjunction with the present study, sodium salb of NO,, Cl-,
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984 1.2
w
0 0
2
4
6
S
IO
12
14
I
0.4-
I
1635 60
1
\ P 1
18 [SDS] x 104(M)
[SDS] (mM)
Flgure 4. Dependence of absorbance and surface tension (7) of solutions of the Be-CAS complex (pH 6.7) on SDS concentration: M; [CAS] = 8.0 X [Be2+] = 4.0 X (0) A425; (0) AES8;(A) A 620; (V)surface tension (dynlcm).
Flgure 5. Dependence of absorbance (at 020 nm) and surface tension (7) of solutions of the Be-CAS complex on SOS concentration In the [Be2+] = 1.0 X loW5M; presence of NaBr: JNaBr] = 0.5 M; (0) [CAS] = 2.0 X 10- M; (0)[Be2+] = 5.0 X lo-' M, [CAS] = 1.0 X lo-' M; (A)surface tension at both concentrations.
Br-, and SO:- and chloride salts of Li+, Na+, K', and "4' were all observed to have no spectroscopiceffects when added (0.1 M) to the TX-100 sensitized system. This may be attributable to the weaker micelle-electrolyte interactions characteristic of nonionic surfactants (33) or may suggest that the sensitized complex is further from the micelle surface than it is with ionic surfactants. Effect of SDS: An Anionic Surfactant, CAS and its complexes are generally negatively charged and are not expected to interact with anionic surfactants or their micelles. As expected, SDS had no effect on the spectroscopy of uncomplexed CAS at any SDS concentration. However, Figure 1D shows that small increases in the absorbance at longer wavelengths were observed when SDS was added to the BeCAS system. These were accompanied by a decrease in the absorbance at 538 nm. Figure 4 shows the dependence of surface tension and complex absorbance on SDS concentration. The surface tension plot indicates that the CMC is approximately 5.0 X M. There is negligible change in absorbance prior to the apparent CMC indicating that interactions are dependent on the presence of micelles. Above the CMC there is a decrease in absorbance at 538 nm which is accompanied by an increase and then a decrease in absorbance at 620 nm. There is a slight increase in the free dye absorbance (425 nm) as the absorbance at 620 nm decreases. This suggests that micelles f i s t sensitize the binary complex and then promote its dissociation. The sensitization is not great, either because of low concentrations of the sensitized absorber (due to an unfavorable equilibrium) or a low molar absorptivity. The dissociation may result from competition for the metal between the anionic micellar surface and the dye. Similar effects have been reported in a t least two cases (34,35). We have found that the effect is particularly significant for CAS complexes of lanthanide ions. In such cases, there is no sensitization of the binary complex but in fact complete dissociation of the complex at the CMC of the surfactant (35). Significant changes in the spectroscopy of the micellar Be-CAS-SDS system accompanied the addition of salts (e.g., 0.5 F NaBr, Figure 1E). The absorptivity at 620 nm increased substantially, surpassing that of the unsensitized complex (at 538 nm). The m e a d y e complex stoichiometrychanged from 1:l to 1:2 as determined by a mole ratio study (at 620 nm). Curvature in the mole ratio plot suggested that the formation constant for the sensitized complex is small. Additionally, when the m e a d y e ratio exceeded the 1:2 stoichiometry, the absorbance of the complex began to decrease slightly, falling from 1.2 to 1.0 absorbance units as the metakdye ratio was
Table 11. Effect of Electrolytes on the Spectroscopy of the Be-CAS-SDS System" added electrolyte 0.5 M NaBr 0.5 M NaCl 0.5 M NaN08
0.5 M LiCl 0.5 M NH&l n[Be2t] = 5.0 X lo-' to 2.0 [SDS] = 1.0 X lo-' M.
u620
(620
1.48 x 1.45 X 1.43 x 1.38 x 1.35 X X
105
10' 105 105
10'
3.1 x 1.5 x 1.1 x 1.7 x
103 103 103 103 1.4 x 103
M; [CAS] = 1.0
X
M;
increased from 1:2 to 1:4 ([Be2'] = 1.0 X M). A similar decoloration is not observed in the absence of surfactant and suggests that interaction between excess dye and the surfactant micelle may cause dissociation of the sensitized complex, expulsion of the complex from the micelle, or formation of a spectroscopicallydistinct dye-rich complex. This results in substantial curvature in the working curves obtained at 620 nm in SDS micellar systems with high ionic strength. When working curves are run in the presence of a constant CAS concentration, a larger excess of CAS a t low metal concentrations promotes more extensive decoloration resulting in lower than expected absorbances at low metal Concentration. The absorbance and surface tension of solutions of the complex with 0.5 F NaBr were measured at several SDS concentrationsas a test for micellar effeds (Figure 5). Surface tension measurements indicate that the CMC is lowered to approximately (4.0-5.0) X lo4 M by added salt (Figure 5 and similar data obtained with only buffer and 0.5 M NaBr). Only as the surfactant concentration increases through and beyond the CMC does the absorbance at 620 nm increase, leveling out near 1.0 X M. This strongly suggests that the sensitization process is dependent on the presence of micelles. Furthermore, the fact that the onset of sensitization occurs at a surfactant concentrationindependent of the concentration of the metal-dye complex (rather than at a specific metaldye-surfactant stoichiometry) also supports the hypothesis of micellar interactions. Studies were again conducted to determine whether variation of the composition of the added salt affected the sensitization process. Working curves were obtained for the Be-CAS-SDS system in the presence of 0.5 M NaC1, NaN03, NaBr, LiC1, and NH4C1. E values and their standard deviations were obtained from the least-squares slopes of the working curves (Table 11) (assuming a nonzero intercept, using all points despite the curvature noted above). The nature of the electrolyte anions would not be expected to affect the prop-
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
erties of negative SDS micelles and in fact there were no significant differences (at the 95% level) among the slopes obtained for the various sodium salts. By contrast, values obtained with chloride salts decreased significantly in the order NaCl > LiCl > NH4Cl. In an earlier study, the degree of micellar sensitization in the B d A S - C T A B system was noted to follow the known order of counterion binding to the CTAB micelle (9). However the trend observed here with SDS does not follow the relative strength of counterion binding to the SDS micelle (NH4+> Na+ > Li+ (36)). Further studies will be required to account for the observed order of sensitization. The dramatic effect of salt on SDS sensitization is evident in comparing curves D and E of Figure 1,which differ greatly despite the fact that micellar interactions are active in both cases. Absorbance at 620 nm begins to increase only when the NaBr concentration exceeds roughly 0.05 M, leveling out a t salt concentrations above 0.4 M. It has been established by several investigators that significant changes in the size, aggregation number, and solubilization properties of the SDS micelle occur upon the addition of 0.1 M-0.6 M salt (37-40). These changes apparently result from a greater effective screening of the charge on the micellar surface. The possible reduction of surface charge would lessen the expected Coulombic repulsions between the micellar surface and the anionic metal-CAS complex, allowing more extensive interactions between the complex and the micelle. The micellar environment produced under these conditions is apparently very similar to that produced by TX-100, as similar changes in stoichiometry and spectroscopy accompany sensitization. The spectroscopic behavior of the Be-CAS complex in the presence of SDS is summarized in Table I. The relative lack of sensitization in the absence of salt is in agreement with the Hartley sign rule of 1934 (41). The addition of salt produces an exception to that rule. Previous reports of sensitization by anionic surfactants are relatively rare. In fact, Chernova (34) indicated that the spectroscopy of metal triphenylmethane dye complexes was not affected by anionic surfactants, although it is not likely that the effect of added electrolyte was considered. More recently, systems sensitized by anionic surfactants have been reported, primarily involving derivatives of the dye 2-(2-pyridylazo)-5-(diethylamino)phenol (PADAP) (42-44). Unlike CAS, this azo dye can form cationic complexes, thus its behavior with SDS may mirror that observed with CAS and cationic surfadants (for example, ternary complex formation has been reported in at least one case (44)). The results reported here are not in agreement with those from a previous study in this laboratory (45),wherein substantial sensitization of Be-CAS by SDS occurred even in the absence of salt. Our inability to reproduce those surprising results has prompted extraordinary care in testing reagent purity and controlling solution conditions. The results of these experiments have been obtained with surfactants from two different suppliers and are reproducible. Furthermore, these results seem more reasonable in view of the chemical properties of the components involved. It appears that an unidentified reagent impurity may have accounted for the earlier sensitization (which was observed by two different investigators in this laboratory using SDS obtained from two different suppliers). This suggests that sensitization in this system may be very sensitive to the balance between COUlombic and hydrophobic interactions. Further studies will be necessary to determine the specific changes leading to enhanced SDS micelle-complex interactions upon the addition of salt. Effect of CTAB: A Cationic Surfactant. Figure 1F shows the spectrum of the Be-CAS system in the presence of 1.0 X M CTAB. There is sensitization qualitatively similar to that observed with TX-100 and SDS/NaBr. Again
1:
616
w
'i
I.21
c
lwo1 604
jm
0.21 Y
0
I
5
I
I
I
I
a5
I
I
l
1
1
10
1
1
l
l
1.5
l
l
l
l
l
l
2.0
l
l
l
I
I
2.5
[CAS] x IO'IM)
Flgure 6. Dependence of absorbance (at 615 nm) and ,A, of the Be-CAS complex on CAS concentration in the presence of CTAB: [Be2+] = 1.10 X IO-, M, [CTAB] = 1.0 X M; (0)A,,,; (A)Am.
a decrease in dye absorbance a t 425 nm is also noted. Interactions of CAS with CTAB (and other cationic surfactants) have been noted elsewhere and have been reported to reduce apparent pK:s and therefore change the spectroscopy of the dye at pH values near the dye pK:s (3, 4 ) . (By contrast, TX-100 micelles were observed to increase the pK, of CAS, as noted above, indicating a fundamental difference in the interactions involved.) Salt effeds similar to those which have been observed elsewhere (46-48) have been noted, suggesting an electrostatic interaction between the anionic dye and the cationic micellar surface. Although such interactions undoubtedly take place in this system at pH 6.7, only slight decreases in extinction coefficients and no change in A, result. The absence of spectral effects probably results from the predominance of the monoprotonated form of the dye at this pH, and the inability of the surfactant to promote loss of the final, least acidic proton. Thus, the decrease in free dye absorbance accompanying addition of CTAB evident in Figure 1F cannot be attributed to changes in free dye spectroscopy. In fact, mole ratio studies (Figure 6) show that the Be:CAS ratio is increased to 1:2 upon sensitization by the cationic surfactant, as has been observed elsewhere for the Be-CAS system (34,49,50).It should be noted that, unlike the corresponding data for TX-100 but similar to that for SDS, Figure 6 shows that the addition of CAS in excess of a 1:2 metakdye ratio results in a decrease in absorbance. The reasons for this behavior will be further considered below. In order to determine whether or not the presence of micelles is required for sensitization, absorption spectra were obtained over a range of surfactant concentrations. The results are summarized in Figure 7, along with data for the surface tension of these solutions. It is obvious upon inspection that the behavior of the CTAB system differs in several respects from that of TX-100 or SDS. Although surfactant concentrations are far below the CMC (CMC of 4 x M in the presence of buffer, determined by surface tension measurements), sensitization begins immediately with the first added surfactant, suggesting interactions with monomer. Absorbance increases until the surfactant concentration is just double that of the metal. This 1:2 metaksurfactant ratio for optimum absorbance persists as the metal concentration is varied (Figure 7) indicating that a stoichiometric ternary complex is being formed. Surface tension data (Figure 7) support this hypothesis. No decrease in surface tension is noted until the 1:2 ratio is reached, indicating that surfactant monomers are not initially free to migrate to the surface as they would do if not constrained by interactions with other solutes. A mole ratio plot obtained at fixed metal and surfactant concentration (Figure 8) shows that the metal:CASsurfactant stoichiometry
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
I-
0
20
IO
O
50
40
30
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1637
1
[CTAB]=I,O x I6,M-,
I
I
I
I
450
500
550
600
I
650
WAVELENGTH (nm)
[CTAB] x IO4 M
Figure 7. Dependence of absorbance and surface tension (7)of solutions of the Be-CAS complex on CTAB concentration: (V)A 805 M, [CAS] = 4.40 X M. For all others, [Be2+] = 2.20 X [Be2+] = 1.10 X M, [CAS] = 2.20 X M (0)AMs; (A)A,& (0)surface tension.
Figure 8. Absor tion spectra of the Be-CAS complex in the presence M; solid of CTAB: [Be"] = 1.10 X loT5M, [CAS] = 2.20 X line (-), [CTAB] = 1.0 X lo-, M; broken line (---), [CTAB] = 3.0 M:
o' 0.7
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
5.0
9.0
0.0
[CAS] x IO' M
Flgure 8. Dependence of absorbance (at 605 nm) of the Be-CAS complex on CAS concentration in the presence of CTAB: [Be2+] = M; (0)[CTAB] = 2.50 X M; (0) [CTAB] = 5.00 1.10 X
x
10-5 M.
for this initial ternary complex is 1:22. An identical 1:22 ratio has been reported for the Be-CAS-zephiramine (benzyldimethyltetradecylammonium chloride) system (47). Additional studies have cited the formation of submicellar ternary complexes of Be-CAS with cetylpyridinium chloride (CPC) (34) and CTAB (50),although specific surfactanbto-complexratios were not reported. Figure 7 shows a region where absorbance measurements are uncertain (denoted by a dashed line). This corresponds to surfactant concentrations where precipitate was detected in solution. Further addition of CTAB resulted in dissolution of the precipitate and resensitization of the analysis. These processes occur at particular Be-CTAB ratios, suggesting stepwise formation of a second (insoluble) and a third ternary complex. The sharp onset of precipitation when the 1:2 ratio is just exceeded suggests that the equilibrium constant for formation of the insoluble complex is much less than that of its 1:2:2 precursor. (By analogy, consider the sharp break that occurs in a titration curve for a multiprotic acid when subsequent K i s differ greatly.) Thus while mole ratio plots (Figure 8) confirm a 1:2 metakdye ratio for the sensitized complex both above and below the precipitation region, Figure 7 allows only a rough estimation of the stoichiometricamounts of surfactant in the second and third ternary complexes (1:2:3 and 1:2:4, respectively). It should also be noted that the behavior observed in the mole ratio plots of Figure 8 differs from that observed in Figure 6, in which the absorbance of
[CTAB] x IO' M
Flgure 10. Dependence of absorbance (at 620 nm) of the Be-CAS complex on CTAB concentration: [Be"] = 5.5 X lo-' M, [CAS] = 1.1 X M; (A)no added salt; (0)0.1 M NaBr added. the metal-dye complex decreases with excess CAS. These differences are probably attributable to the presence of micelles in the system of Figure 6 (see below). Results described thus far clearly establish that distinct ternary complexes are formed in the presence of CTAB up to a metdsurfactant ratio of 1:4. In considering Figure 7 , it is apparent that additional spectroscopic changes take place a t still higher surfactant concentrations. Specifically at a surfactant concentrationof about 3.0 X lo4 M, the absorbance of the sensitized complex at 605 nm begins to decrease. Examination of the absorption spectra over this concentration region (Figure 9) reveals that an additional 14-nm red shift in A- for the sensitized complex has taken place. In contrast to the other behavior with CTAB, the surfactant concentration range over which this change takes place was found to be independent of initial binary complex concentration. Furthermore, this red shift roughly coincides with the CMC for CTAB in the presence of buffer alone (-4 X M), indicating that interactions between the complex and surfactant micelles are involved. This hypothesis cannot be confirmed by the surface tension data presented in Figure 7 , because it is complicated by the precipitation and resensitization processes which obscure the CMC. However, evidence of micellar behavior can be inferred from spectral dependence on the presence of excess electrolyte (see discussion above). Our earlier study (9) established that the addition of excess electrolyte to this system results in increases in sensitivity which are dependent on the anion of the added salt, as expected for micellar interactions. Figure 10 shows the de-
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6
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
Table 111. Spectroscopy of Cu-CAS and AI-CAS Complex in the Absence and Presence of Surfactantsn L s ,
complex
CU-CAS Cu-CAS CU-CAS CU-CAS CU-CAS Cu-CAS Cu-CAS CU-CAS
surfactant (M)
nm
586 594 SDS (1.0 X IO-') 596 SDS (1.0 X lo-')' 620 CTAB (2.0 X 588 CTAB (1.0 X 10-4)d 603 CTAB (1.0 X 10-3)d 620 CTAB (1.0x 10-3)d9e 620 545 TX-100 (1.0 X 545 SDS (1.0 X lo-') 545 SDS (1.0 X lo-')' 610 CTAB (2.0 X 10")' 654 CTAB (1.0 X lo-*)' 650 CTAB (1.0 X 10-3)f 650 CTAB (1.0 X 10-3)e*f 650
TX-100 (1.0
X
eXmaxf
U*
20800 f 1000 26000 f 300 17 100 rt 1900 36200 f 2600 114000
117500 48000
iiiooo
metal: CAS 2:l 1:2 1:2 1:2 1:2 1:2 1:l 1:l
AI-CAS 15400 f 1500 A1-CAS 15400 rt 400 AI-CAS 5800 f 300 AI-CAS 6500 & 300 AI-CAS 60000 1:3 AI-CAS 115000 1:3 A1-CAS 32000 1:3 AI-CAS 77000 1:3 [CAS] = 1.0 X M, unless otherwise specified. b e and u represent the least-squares slopes of calibration curves (for Cu and A1 concentrations from 5.0 x lo4 to 2.0 X low6M) and their standard deviation except where noted. ' [NaBr] = 0.5 M. [Cu2'] = 1.0 x M, [CAS] = 2.0 x M. B values were calculated from single absorbance values. e[NaBr] = 0.1 M. f[A13+] = 1.0 X M, [CAS] = 3.00 X M. B values were calculated from single absorbance values. pendence of this effect on the concentration of CTAB. As expected, added salt has little effect on the spectroscopy of the ternary complexes described above. Significant salt-induced increases in sensitivity occur only in the postprecipiM in buffer tation region near the CMC (CMC F= 5.0 X and 0.1 M NaBr, determined by surface tension), as would be expected if micellar interactions are involved. The small accompanying red shift may indicate a reduction in the polarity of the absorber's solvent environment (51) as would accompany solubilization in or near the micellar core. Further study is required to ascertain whether an identifiable ternary complex persists in the micellar environment or if specific monomers are no longer associated with particular sites in the dye-metal complex. Additional evidence of micellar interactions in the BeCAS-CTAB system is apparent when the mole ratio plot at micellar CTAB concentrations (Figure 6) is compared with those at lower surfactant concentrations (Figure8). Decreases in absorbance at 620 nm are observed at CAS concentrations above a 1:2 meta1:CAS ratio only in the micellar system. It appears that the sensitized complex is displaced from the micelle by excess dye, suggesting a stronger micellar affinity for the (more highly charged) dye. A, undergoes a blue shift with increasing CAS concentration (Figure 1, curves F and G , and Figure 6) suggesting release of either the ternary complex or the unsensitized binary complex (both of which absorb at shorter wavelengths than the micellar complex). Due to the overlappingspectra of the species involved it is difficult to determine the exact nature of this displacement process. However, the fact that the effect is observed in Figure 6 but not Figure 8 clearly indicates its micellar nature. Effects of Surfactants on Other Metal-CAS Systems. The effects of surfactants on the spectroscopyof the Cu-CAS and AI-CAS systems were also tested. Cu2+ forms a 2:l metakdye complex (52)and Ai3+forms a 1:l complex (53)in the absence or surfactants. Pertinent spectroscopic data are summarized in Table 111. A. Effect of TX-100. The addition of 1.0 X lo4 M TX-100 to the Cu-CAS and Al-CAS systems resulted in spectroscopic changes only in the former case. A red shift in ,A, and a 2-fold increase in sensitivity were observed (Table 111). Mole
ratio studies indicated that a 1:2 Cu:dye complex was formed in the presence of the Surfactant. Spectroscopic changes showed a dependence on surfactant concentration similar to that observed for Be. These results confirm that micellar interactions are again responsible for sensitization. As with the Be system, no salt effects were noted. B. Effect of SDS. In the absence of added excess electrolyte, it was found that SDS had little effect on the spectroscopy of the Cu-CAS complex (Table 111). As the surfactant concentration approached the CMC, ,A, shifted from 586 nm to 596 nm, and the absorptivity of the complex increased slightly, indicating a slight sensitization. Further increases in surfactant concentration to 1.0 X M resulted in decreases in the absorptivity of the complex, however. Accompanying increases in free dye absorbance indicated that the complex was dissociated as the SDS concentration surpassed the CMC, an effect similar to that observed with Be. As with the Be-CAS system, the addition of 0.5 M NaBr to the Cu-CAS complex in the presence of 1.0 X M SDS resulted in an increase in the absorptivity and a red shift in A,, to 620 nm (Table 111). Variation of the surfactant concentration indicated that sensitization coincided with micellization. There was no decrease in absorbance at higher surfactant concentrations, indicating that the addition of salt reduces the ability of the SDS micelle to dissociate the complex. The addition of 1.0 X M SDS to the AI-CAS complex resulted in a significant decrease in the absorptivity of the On the basis of increases in complex with no change in A,,=. the free dye absorbance when SDS is added to the AI-CAS complex, it appears that the complex is dissociated by the SDS micelle. The addition of 0.5 M NaBr resulted in a shift in ,,A to 610 nm but only produced a slight increase in the absorptivity of the complex. Again, based on the free dye absorbance relative to that observed in the absence of surfactant or salt, it appears that significant dissociation of the binary complex still occurs in the presence of salt. That complex which is not dissociated undergoes spectroscopic changes, however. C. Effect of CTAB. The behavior of both A1 and Cu complexes was similar to that of the Be-CAS system in the presence of CTAB. Each complex exhibited an initial sensitization at low surfactant concentration, followed by precipitation and redissolution. By methods described above, it was established that 1:2:2 and 1:3:2 metal:CAS:surfactant complexes were initialy formed for Cu2+and A13+,respectively. The stoichiometry of the Al complex agrees with that reported by Tikhonov et al. (54) but these authors reported a 1:3:3 Cu:CAS:CTAB ternary complex in an earlier study (55). Tikhonov specifies neither reagent purity nor purification procedures in his studies, thus the apparent agreement with his data in one case and lack of agreement in another may result from use of different sets of reagents. Precipitation occurred at a 1:3 metahurfactant ratio and resensitization occurred at approximately a 1:4 ratio for both Al and Cu. As with the Be system, this behavior was indicative of the formation of additional ternary complexes. The 1:2:4 stoichiometry of the redissolved Cu complex agrees with that reported by Nishida (56)for a Cu-CAS-Zephiramine complex. Other authors hypothesizing ternary A1 or Cu-CAS-cationic surfactant complexes have not indicated a surfactant:complex stoichiometry (34,57, 58). A red shift of 12 nm was observed in the Cu-CAS complex as the surfactant concentration was increased over the range of 1.0 x M to 5.0 x 10-4 M. No such shift was observed for A13+. For both metals, the absorbance of the sensitized complex began to decrease at surfactant concentrations above 1.0 X M (Figure 11). This decrease was accompanied
ANALYTICAL CHEMISTRY, VOL. 56,
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[CTAB] x IO' M
Figure 11. Dependence of absorbance of CU-CAS and AI-CAS complexes on CTAB concentration: (0)As,,, [Cu"] = 1.0 X M, [CAS] = 2.0 = IO-' M; (0)A,,, [AI3+] = 1.0 X M, [CAS] = 3.0 x 10-5 M.
by a corresponding increase in the absorbance of uncomplexed CAS at 425 nm. In contrast with the beryllium system, these complexes evidently began to dissociate as the surfactant concentration increased. The dissociation of both complexes is probably a result of competition between complex and micelles for the available CAS, analogous to the competition between dye and SDS micelles for metal (described above). As in anionic surfactant systems, the dissociations observed here were much less extensive than those reported elsewhere for lanthanide metals (35). Furthermore, the micellar dissociation mechanism operative here contrasts with the evident involvement of surfactant monomers in the case of lanthanide complexes. The variation in dissociation mechanism with the metal employed suggests that the relative stability of the sensitized complex may be important in the dissociation process. Further studies will be aimed at determining Kf values for the sensitized systems in order to better characterize the dissociation process.
CONCLUSIONS The results of this study clearly demonstrate that changes in metal-dye complex spectroscopy can be induced by micellar solubilization and the formation of stoichiometric ternary complexes involving surfactant monomers. The micellar mechanism is important in the case of all three types of surfactants (cationic, anionic, and nonionic) although the addition of high concentrations of salt is necessary in order to observe sensitization with SDS. The degree of sensitization varies appreciably with the surfactant and metal employed (to the extreme of no sensitization of the A1 complex by TX100). Similar variability has been reported elsewhere, e.g., by Sawin et al. (6, 7). Despite these variations in degree, the overall spectral similarity of solubilized complexes suggests localization in similar environments, regardless of the aurfactant involved. For the systems investigated in this study, the role of ternary complexes is limited to cationic surfactants. Rather than a single, simple stoichiometry as has been suggested for these (34,49, 50, 54-56) and other systems ( 4 , 5 ) ,stepwise incorporation of multiple surfactant molecules can result in formation of several stable complexes (we detect at least three for the systems studied here), some of which may be insoluble (accounting for occasional references to turbidity in other literature reports (4,57,59,60)).However, to our knowledge multiple ternary complexes have been specifically discussed elsewhere only by Xi-Wen and Poe (61,62),who did not report precipitation. The dual (ternary and micellar) interactions responsible for sensitization in the system studied here have not been reported in other dye-metal systems but do resemble those
NO. 9,
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observed in binary surfactant-dye systems by Reeves (17) and Tomlinson and Davies (12). These authors note that charged organic dyes undergo a range of interactions with oppositely charged surfactants, including the formation of stoichiometric complexes at submicellar concentrations, the formation of mixed micellar or premicellar aggregates at higher concentrations, and the eventual micellar solubilization of surfactant-dye complexes. The initial complex formation is apparently driven by the strong tendency for the dye molecules to form ion pairs with surfactant molecules, probably due to water structure enforced ion pairing (63,64). The dyes also interact with the micellized surfactant (46, 47). Further understanding and predictive capabilities will be important for realization of the analytical applications that have been widely proposed for these systems. Equilibria among ternary complexes complicate their spectroscopy and render them impractical for use in real analytical systems. (Note, for example, the variations in c for the Be-CAS-CTAB system, summarized in Table I.) On the other hand, the chemistry of these complexes may well serve to illustrate the mechanism of surfactant interactions and thus provide an avenue to improved understanding of the reason for observed spectral changes in all of these Systems. In this regard, it is interesting to note that the change in metddye ratio induced by added surfactant is invariant for each complex studied here, regardless of the surfactant used or its concentration, increasing the likelihood of similarity between micellar and ternary complex environments. Further study is clearly necessary to establish the structure of the ternary complexes. Additionally, further study will be necessary to determine what changes in micellar properties promote sensitization of the Be-CAS complex by SDS when excess electrolyte is present. This information should facilitate prediction of micellar interactions, upon which practical applications (with due attention to micellar salt effects) may ultimately be based. Registry No. CTAB, 57-09-0; TX-100,9002-93-1; SDS, 15121-3.
LITERATURE CITED Evtlmova, B.; Nonova, D. Anal. Chlm. Acta 1973, 6 7 , 107-112. Vekhande, C.; Munshi, K. N. Microchim. J. 1978, 2 3 , 28-41. Hinze, W. L. I n "Solutlon Chemistry of Surfactants"; Mlttal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, pp 79-127. Tikhonov. V. N. Zh. Anal. Kh/m. 1977. 32. 1435-1447 Marczenko, 2. Chem. Anal. (Warsawj1979, 551-587. Sawln, S. B.; Chernova, R. K.; Kudryatseva, L. M. Zh. Anal. Khim. 1978. 33. 2127-2133. Sawln, S. 6.; Chernova, R. K.; Kudryatseva, L. M. Zh. Anal. Khim. 1979, 3 4 , 66-76. Marczenko. 2.; Kalowska, H. Anal. Chim. Acta 1981, 723, 279-287. Callahan, J. H.; Cook, K. D. Anal. Chem. 1982, 5 4 , 59-62. Bunton, C. A.; Romsted. L. S.;Sepulveda, L. J. Phys. Chem. 1980, 8 4 , 2611-2618. AI-Lohedan, H.;Bunton, C. A.; Romsted, L. S.J. Phys. Chem. 1981, 8 5 , 2123-2129. Tomlinson, E.; Davles, S.S.; Mukhayer, 0.I. I n "Solution Chemistry of Surfactants"; Mlttal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, pp 3-43. Chester, J. E.; Dagnall, R. M.; West, T. S . Talanta 1970, 77, 13-19, Dagnall, R. M.; West, T. S.; Young, P. Analyst (London) 1987, 9 2 , 27-30. Koroleva, G. N.; Poluetkov, N. S.;Kirllov, A. J. Zavod. Lab. 1978, 42, 139-140. Pohetkov, N. S.;Ovchgr, L. A.; Lauer, R. S. Zh. Anal. Khim. 1973, 2 8 , 1958-1961. Reeves, R. L. J . Am. Chem. SOC. 1975, 97, 6019-6024. Strelow, F. W. E.; Welnert, C. H. S. Anal. Chem. 1975, 47, 2232-2293. Martynov, A. p.; Novak, V. P.; Reznlck, B. E. Zh. Anal. Khim. 1977, 3 2 , 519-525. Chaimovlch, H.; Boniiha, J. B. S.;Pollti, M. J.; Quina, T. H. J . phvs. Chem. 1979, 8 3 , 1851-1654. Czernlawski, M. Rocz. Chem. 1988, 40, 1935-1945. Elworthy, P. H.; Mysels, K. J. J. Colbid Sci. 1988, 27, 331-347. Crook, E. H.; Fordyce, D. B.; Trebbl, G. F. J . Phys. Chem. 1983, 6 7 , 1987-1994. Crook, E. H.;Trebbl, G. F.; Fordyce, D. B. J. Phys. Chern. 1984, 6 8 , 3592-3599. Kudryatseva, L. M.; Chernova, R. K. Zavod. Lab. 1978, 44, 522-523. Cheng, K. L. I n "Spectrochemlcal Methods of Analysis"; Winefordner, J. D., Ed.; Wiley: New York, 1971; pp 349-350.
24,
(16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
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(27) Malat, M. Anal. Chim. Acta 1961, 25,289-291. (28) Langmyhr, F. J.; Klausen, K. S. Anal. Chim. Acta 1963, 29, 149-167. (29) Sawin, S.B.; Marov. I. N.; Chernova, R. K.; Kudryatseva. L. M.; Shtykov, S. N.; Sokolov, A. B. Zh. Anal. Khim. 1981, 36,850-859. (30) Sommer, L.; Kuban, V. Anal. Chim. Acta 1969, 4 4 , 333-344. (31) Mukerjee, P. J. Phys. Chem. 1970, 7 4 , 3824-3826. (32) Schick, M. J.; Gilbert, A. H. J. Colloid Scl. 1965, 20,464-472. (33) Rosen, M. J. “Surfactants and Interfacial Phenomena”; Wiley: New York, 1978; p 87. (34) Chernova, R. K. Zh. Anal. Khlm. 1977, 32, 1477-1486. (35) Klopf, G. J.; Cook, K. 0.Anal. Chlm. Acta, in press. (36) Ray, A.; Nemethy. G. J. Am. Chem. SOC. 1971, 93, 6787-6793. (37) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976, 80, 1075-1085. (38) Hayashi, S.;Ikeda, S. J . Phys. Chem. 1980, 8 4 , 744-751. (39) Llanos, P.; Zana, R. J. Phys. Chem. 1980, 8 4 . 3339-3346. (40) Anacker, E. W. I n “Solution Chemistry of Surfactants”; Mlttal, K. L., Ed.; Plenum Press: New York; 1979; pp 247-265. (41) Hartley, G. S. Trans Faraday SOC. 1934, 30,444. (42) Hung, S.-C.; Qu, C.-L.; Wu, S. S. Talanta 1982, 29, 85-88. (43) Hung, S.-C.; Qu, C.-L.; Wu, S.S. Talanta 1982, 29, 629-631. (44) Zhang, C.-P., Qi, D. Y.; Zhou, T.-2. Talanta 1982, 29, 1119-1121. (45) Baxter-Hammond, J.; Cook, K. D. I n “Solution Behavior of Surfactants”; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2, pp 1283-1305. (46) Rosendorfova, J.; Cermakova, J. Talanta 1980, 2 7 , 705-708. (47) Skarydova, V.: Cermakova, J. Collect. Czece. Chem. Commun, 1982, 4 7 , 776-783. (48) Funsaki, N. J. J. Phys. Chem. 1979, 83, 1998-2003. (49) Nishida, H. Bunseki Kagaku 1971, 20, 1080-1084. (50) Marczenko, Z.;Kalowska, H. Chem. Anal. (Warsaw) 1977, 22, 935-941.
(51) Cheng, K. L. I n “Spectrochemical Methods of Analysis”; Winefordner, J. D.. Ed.; Wiley: New York, 1971; p 363. (52) Semb, A.; Langmyhr, F. J. Anal. Chlm. Acta 1986, 35. 286-292. (53) Srivastava, S.C.; Sinha, S. N.; Dey, A. K. J. Prakt. Chem., Ser/es 4 , 1963, 20,70-80. (54) Tikhonov, V. N.: Ekaterinina, L. A. Zh. Anal. Khlm. 1975, 30, 1507-1512. (55) Tikhonov, V. N.; Tikhonova, E. S. Zh. Anal. Khim. 1978, 37, 465-469. (56) Nishida, H.; Nishida, T. Bunseki Kagaku 1972, 21, 997-1004. (57) Tikhonov, V N.; Danilova, S. G. Zh. Anal. Khlm. 1980, 35, 1264-1 272. (58) Tikhonov, V N.; Aleksandrova, N. P. Zh. Anal. Khlm. 1981, 36, 242-247. (59) Horiuchi, Y.; Nishida, H. Bunsekl Kagaku 1969, 18, 180-184. (60) Horluchi, Y.; Nishida, H. Bunsekl Kagaku 1969, 78, 694-698. (61) Xi-Wen, H.; Poe, D. D. Talanta 1981, 28,419-424. (62) Xi-Wen, H.; Poe, D. D. Anal. Chim. Acta 1981, 131, 195-203. (63) Diamond, R . M. J . Phys. Chem. 1963, 6 7 , 2513-2517. (64) Sawin. S. B. Cherova, R. K.; Belousova, V. V.; Sukhova, L. K.; Shtykov, S N. Zh. Anal. Khim. 1978, 33, 1473-1480.
RECEIVED for review November 14, 1983. Accepted April 9, 1984. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Support from the University of Illinois Campus Research Board and Biomedical Research Committee (funded by N.I.H.) is also gratefully acknowledged.
Colorimetry with a Total-Reflection Long Capillary Cell Keiichiro Fuwa, Wei Lei, and Kitao Fujiwara* Department of Chemistry, IJniversity of Tokyo, Tokyo 113, Japan
By appllcatlon of a solvent wlth a refractlve Index higher than that of the cell materlal to a long capillary cell (LCC), high transmlsslon of source light was attalned (“total reflection” LCC). Slnce the transmlsslon efflclency of LCC Is Independent of the cell shape, free-shape LCCs (0.7-50 m, 0.25-2 mm id.) were constructed for colorlmetry determlnatlon In the visible region. The sample lntroductlon system to the LCC wlth aperture to source llght was also glven. Absorbance of solutlon was enhanced about two to elght times higher than the apparent cell length. Phosphomolybdenum heteropoly blue In carbon dlsulflde (absorbance, 5 X lo-’) was enhanced about 3 X l o 4 tlmes wlth 50-m LCC. Transmlsslon efflclency of the LCC was observed for varlous solvents. The present LCC method was applled to the colorlmetries of phosphorus, Iodide, copper, and mercury, and the sensitivities were improved about 300-3000 tlmes hlgher than the ordlnary colorlmetry.
A long capillary tube (1 m length) made of Pyrex glass (1 to 2 mm i.d.) was found to be usable as a cell for colorimetry (1). With this tube, sensitivity is improved by 100-300 times with a sample volume less than a few milliliters. For the purpose of enhancing the absorbance higher than that obtained with a 1 m long capillary cell (LCC), elongation of cell length from a few to tens of meters is necessary. However, such long LCC is difficult to handle in an ordinary laboratory. For practical use of LCC with lengths over 1 m, the transformation of cell shape is favorable. When the aqueous solution is applied to the LCC, the transmission efficiency is much dependent on the linearity of the cell shape: It was 0003-2700/84/0356-1640$0 1.50/0
found that only 0.001% of the incident light can reach to the exit of the single-looped cell (2 mm id., 1m). Moreover, even when the LCC was covered with light-reflective aluminum tape, attenuation of the incident light is serious. In fact, the double-looped cell of the same dimension could not transmit any source light including lasers (I). The wave-guide phenomena is well-known and the technique of light transmission through the fiber of which diameter is less than 100 pm has been extensively developed: The transmission efficiency is independent of the shape of the transmission media. In this case successive total reflections occur through the light transmission process. To attain this process, Snell’s law has to be satisfied, where the angle of the incident light, 0, exceeds the critical value expressed by sin 6 = n l / n z
(1)
where nl and n2 are refractive indexes of the first and the second light passing media. The incident source light transmits in the analytical solution via total reflection a t the inside wall of capillary, when the solution is used of which refractive index is greater than that of Pyrex glass or cell material. Recently, the hollow fiber filled with liquid is becoming the important subject in Raman spectroscopy; Le., Raman scattering including inverse Raman and coherent anti-Stokes Raman have been observed for the solvents of high refractive index such as benzene and tetrachloroethylene in the hollow fiber (2-Tj). In the present paper, a wave-guide “total reflection” cell was applied for the absorption colorimetry. Namely, the performance and efficiency of long capillary “total reflection” cell were investigated in terms of the transmission spectra, G 1984 American Chemical Society
