Anal. Chem. 1992, 64, 1490-1495
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Kinetic Determination of Antimony( III)Based on Its Accelerating Effect on the Reduction of 12-Phosphomolybdate by Ascorbic Acid in a Micellar Medium Dolores Sicilia, Soledad Rubio, and Dolores PBrez-Bendito' Department of Analytical Chemistry, Faculty of Sciences, University of Cbrdoba, Cbrdoba, Spain
The nonlonlc surfactant Trlton X-100 was used as a mlcellar medlum to develop the reductlon of 12-phosphomolybdate to a blue heteropdycompoundby ascorbk acld. Antimony( I I I), whlch accelerates this reactlon In aqueous medla by the formatlon of a mixed heteropoly compound wRh molybdenum and phosphorus, le concentrated on the mlcellar surface, thereby augmentinglts effectlve concentratlon In the reaction medlum. The mlcellar catalysls of Trlton X-100 permlts the klnetlc-photometrlc determlnatlon of antimony( I I I ) over the llnear range 0.1-1.8 mg/L wRh a detectlon llmlt of 0.07 mg/L, Le. 7 tlmes lower than that of the correspondlng method developed In an aqueous medlum. The relatlve preclslon for 0.8 mg/L antlmony was 1.8%. Slgnlflcantly Improved 88IectIvRy was also obtained. Thus, Ions such as arsenate and slllcate are tolerated at concentratlons 10- and 1000-fold, respectlvely, of that of antlmony. Some observatlons on the mechanlsm via whlch TrRon X-100 acts on the reaction are made.
INTRODUCTION The fact that surfactant micelles can accelerate reactions has been increasingly frequently exploited in the last few years to improve the features of both ~atalyticl-~ and noncatalytic kinetic495 methods. Several quantitative kinetic treatments have been developed to assess the intrinsic reactivity in aqueous micelles,6-11 and the results obtained in this respect suggest that the major source of the rate enhancements in most of the reactions is the increased reactant concentration in the micellar pseudophase. This "local concentration" of reactants permits their determination to be more sensitive. In addition, micelles can greatly improve the selectivity of analytical kinetic method^.^ A crucial factor in analysing kinetic data from micellecatalyzed reactions is the distribution of reactants between micelles, particularly when reactant concentrations are similar to or higher than the micellar concentrations. Such a
* Corresponding author.
(1)Rubio, S.; PBrez-Bendito, D. Anal. Chim. Acta 1989,224,185-198. (2)Lunar, M. L.; Rubio, S.; PBrez-Bendito, D. Anal. Chim. Acta 1990, 237, 207-214. (3)Sicilia, D.;Rubio, S.; PBrez-Bendito, D. Talanta 1991,38,11471153. (4)Athanasiou-Malaki, E.; Koupparis, M. A. Anal. Chim. Acta 1989, 219, 295-307. (5)Archontaki, H.A.; Koupparis, M. A.; Efstathiou, C. E. Analyst 1989,114,591-596. (6)Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (7)Bunton, C. A. Pure Appl. Chem. 1977,49,969-979. (8) Berezin, I. V.; Martinek, K.; Yatsimirskii, A. K. Russ. Chem. Rev. 1973,42, 787-802. (9) Cordes, E. H. Reaction Kinetics in Micelles; Plenum Press: New York, 1973. (10)Quina, F.H.; Chaimovich, H. J . Phys. Chem. 1979,83,1844-1850. (11)Chaimovich, H.;Bonilha, J. B. S.; Politi, M. J.; Quina, F. H. J . Phys. Chem. 1979,83,1851-1854. 0003-2700/92/0364-1490$03.00/0
distribution will be determined by the association constants of the reactants with the micelles and the relative concentrations of the reactants.'* In catalytic reactions where the determination of the catalyst is of interest, direct association between the catalyst and the micelles is often hindered, in our experience,owing to exclusion phenomena from substrates present at concentrations that are frequently higher than that of the catalyst by a few orders of magnitude. Thus, in studying various oxidation reactions of organic compounds by bromate, catalyzed by vanadium(V), we found that the catalyst was displaced from the micellar surface as the bromate concentration in the reaction medium increased (unpublished results). Similar effects were observed in other reactions. Therefore, in the absence of a "local concentration" of the catalyst on the micelle and of changes in its intrinsic reactivity, one must seek new alternatives for improving the sensitivity of catalytic kinetic methods in micellar media. Thus, an increase in sensitivity by a factor of ca. 11 in the kinetic determination of the catalyst vanadium(V) was obtained by using the bromate oxidative coupling reaction of p-phenetidine with catechol in the presence of the cationic surfactant cetylpyridinium chloride (CPC).2 The reaction takes place in two steps that involve the surfactant and the catalyst, respectively, and "micellar catalysis" increases the effect of the chemical catalysis. This paper reports a study of the influence of micelles on a type of reaction that allows for "local concentration" of the accelerator or catalyst on the micellar surface, which results in increased sensitivity. The accelerating effect arises from the formation of a complex between the accelerator or catalyst and one or more reactants that can readily bind to the micellar surface. If the complex is sufficiently stable, the accelerator or catalyst concentration is effectively increased in the vicinity of the micelle. The reagent can be a substrate or activator of the reaction and, in theory, this approach could be extended to ligands not directly involved in the reaction provided they can readily bind to the micelles without altering the activity of the accelerator or catalyst they complex. In this work we investigated the influence of micelles on the reduction of 12-molybdophosphoric acid to the classic heteropoly blue compound.13 Antimony(II1) increases considerably the rate of this reduction reaction by incorporation into the phosphate-heptamolybdate complexin an Sb/P ratio of 1:1,14which was taken advantage of to develop a quantitative FIA technique for the determination of antimony.15 This reduction reaction was selected for the initial study for two reasons: first, antimony(II1) forms a well-defined stable complex with phosphate-heptamolybdate, a reagent than readily incorporates into nonionic and cationic micelles (12)Moroi, Y. J. Phys. Chem. 1980,84,2186-2190. (13)Meehan, E. S.In Treatise on Analytical Chemistry; Kolthoff, J. M.; Elving, P. J.; Eds.; John Wiley and Sons: New York, 1964;Part I, Vol. 5,pp, 2753-2803. (14)Murphy, J.; Riley, J. P. Anal. Chim. Acta 1962,27,31-36. (15)Lacy, N.; Christian, G. D.; Ruzicka, J. Anal. Chim. Acta 1989,224,. 373-381. 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 04, NO. 13, JULY 1, 1992
according to previous experiments; and second, the only method proposed so far for the determination of Sb(II1)based on this reaction15 featured a limit of detection of 0.5 mg/L, and a linear range between at least 0.5 and 20 mgIL, of antimony, but the absorbance increment in this range is comprised between 0.0 and 0.2, and arsenate and silicate interfere seriously with the determination. Therefore, this reaction was a good candidate for micellar catalysis. The results obtained in relation to the influence of the nonionic Triton X-100 micellar system on the reduction reaction show the advantages in terms of sensitivity, detection limit, and selectivity of using these micelles for the kinetic determination of Sb(II1). EXPERIMENTAL SECTION Apparatus. Kinetic measurements were made on a Philips PU 8625 UV/vis spectrophotometer fitted with a stopped-flow module16 (Quimi-Sur Instrumentation, Seville, Spain). This module,furnished with an observation cell of 0.3-cmpath length, was controlled by the associated electronics via a 640K Mitac computer for acquisition and processing of kinetic data. The solutions in the stopped-flowmodule and the cell compartment were kept at a constant temperature by circulating water from a thermostated tank. A classical stalagmometer was used for surface tension measurements in order to determine the critical micelle concentrations (cmc) of the surfactants.17 Reagents. Commercially availablehighest gradereagentswere used throughout, without further purification. Solutions were stored in polyethylene bottles. Because dissolved molybdate polymerizes to some extent, all Mo(V1) solutions were prepared at least 24 h prior to their first use in order to ensure that molybdate aggregateshad reached a stable equilibrium.lE The stock phosphate-molybdate-nitric acid solution referred to in the Procedure section was prepared by dissolving KHzPO4 (Merck) and (NH4)gMo,024*4H20(Merck) in nitric acid and diluting so that [KHzPOJ = 3.23 X M, [(NH4)6Mo&24]= 2.5 X M, and [HN03] = 0.2 M. An aqueous L-ascorbic acid (Merck) solution (0.284 M) was prepared fresh daily to avoid oxidation of the ascorbic acid. A 1.000 g/L stock antimony solution was prepared by dissolving0.2743 g of K(SbO)C4H40g0.5H~O (Merck) in 100 mL of doubly distilled water. Analyte solutions ([Sb(III))] = 10 mg/L) were prepared daily by appropriate dilution. A 10% aqueous solution of glycerine employed to prevent deposition of reaction products on the walls of the flow-through M) (Serva) was made cell.lBA triton X-100 solution (8.24 X by dissolving the appropriate amount of surfactant in doubly distilled water. The other surfactantstested, viz. Brij-35(Merck), cetyltrimethylammonium bromide (CTAB) (Sigma), dodecyltrimethylammonium bromide (DTAB) (Sigma),cetylpyridinium chloride (CPC) (Serva),sodium dodecylsulfate (SDS) (Aldrich), sodium dioctylsulfosuccinate (Aerosol OT) (Aldrich), and N dodecyl-Nfl-dimethylammonium3-propanesulfonate (SB-12, sulfobetaine)(Serva)were prepared in a similarway. Less readily soluble surfactants were dissolved with warming. Procedure. Two solutions (A and B) were used to fill the two 10-mLreservoir syringes of the stopped-flow module. Solution A contained between 0.1 and 1.8 mg/L antimony(III), 1 mL of 0.284 M L-ascorbic acid, 0.5 mL of 10% glycerine,0.6 mL of 8.24 x lo4 M Triton X-100, and doubly distilled water to a final volume of 10 mL. Solution B contained 1 mL of 2 M HN03,4 mL of phosphate-molybdate-nitric acid solution, 0.5 mL of 107% glycerine solution, and doubly distilled water. After the two 2-mL drive syringes had been filled with the corresponding solutions from the reservoir syringes, 0.15 mL of each solution was mixed in the mixing chamber in each run. The reaction was monitored at 750 nm by recording the variation of the absor(16) Loriguillo,A,; Silva, M.;PBrez-Bendito,D. Anal. Chim. Acta 1987, 199, 29-40. (17)Mukerjee, P.; Hysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36; U S . Department of Commerce: Washington, D.C., 1971. (18) Crouch,S.R.; Malmstadt, H. V. Anal. Chem. 1967,39,1084-1089. (19) Ruzicka, J.; Hansen, E. H. Flow Injection Analysis; 2nd ed.; Wiley-Interscience: New York, 1988; pp 303-309.
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bance as a function of time while keeping the system at 25 f 0.1 "C. Absorbance data were collected and processed by the microcomputer using a linear regression program for application of the initial-rate method. The reaction rate was determined within 5 s. Blank solutions were prepared in the same way as the samples but with no antimony(III), and their signals were subtracted from those obtained for the samples.
RESULTS AND DISCUSSION Selection of the Micellar System. Influence on the Reduction of 12-Molybdophosphate a n d Antimony 12Molybdophosphate Complexes by Ascorbic Acid. In strongly acidic solutions molybdenum(V1) and phosphate form the 12-molybdophosphate anion (12-MPA). If antimony is added to the medium it incorporates into the 12-MPA structure, giving rise to the mixed heteropoly complex 12MPA/Sb. Since the 12-MPAISb complex is reduced considerably more rapidly than 12-MPA, if substoichiometric amounts of antimony are used, the observed initial rate of color development on addition of the reductant will be proportional to the rates for the simultaneous reduction of the two forms (12-MPAISb and 12-MPA). Once 12-MPAI Sb is completely reduced, the observed rate should be equal to the rate of the 12-MPA reduction reaction. In order to determine the effect of micelles on the rate of these reactions, one must consider the negative charge born by the complexes 12-MPA/Sb and 12-MPA since the accelerating effect of micelles arises essentially from electrostatic and hydrophobic interactions between the reactants and the micellar surface. Therefore, it seems logical to think that cationic and nonionic micelles will accelerate these reduction reactions. The results obtained in studying the effect of surfactants of a different nature [cationic (CPC, CTAB, DTAB), nonionic (Triton X-100, Brij-35), anionic (SDS, Aerosol OT), and zwitterionic (SB-12)] confirmed the above assumption. The surfactants were tested at three concentration levels, namely below, close to, and above their cmc, on both 12MPA/Sb 12-MPA and 12-MPA reduction reactions. The Sb(II1) concentration tested was 1.0 mg/L, a t which only a small accelerating effect by Sb(II1) on the reaction was observed in the absence of surfactant. Both the 12-MPAISb and the 12-MPA complexes apparently interacted strongly with cationic and nonionic micelles as judged by their insolubilization when the surfactant or complex concentration exceeded a certain limit. All cationic and nonionic surfactants tested had the same effect on the reduction reactions studied a t concentrations resulting in no precipitate. They slightly decreased the reduction rate of 12-MPA and considerably increased the reduction rate of 12-MPAISb. Because of the rapidity of the 12-MPAISb complex reduction in the presence of these surfactants, use of the stopped-flow technique was mandatory in order to determine its initial rate. Figure 1 shows the effect of the surfactants exerting the most significant micellar catalysis on the rate of the 12-MPAISb + 12-MPA reduction reaction as a function of their concentration. Triton X-100 showed greater micellar catalysis than Brij-35 and CPC and was thus chosen for subsequent experiments. Figure 2 shows typical kinetic curves obtained for the reaction in the presence (curve b) and absence (curve a) of Sb(II1) when Triton X-100 was added to the reaction medium. The effect of the Triton X-100 concentration on the reduction rate of the 12-MPA (curve a) and 12-MPAISb + 12-MPAcomplexes at different Sb(II1) concentrations(curves b-d) is shown in Figure 3. As can be seen in curve a, Triton X-100 does not accelerate the reduction of 12-MPA in the concentrationrange tested. The rate of reduction of 12-MPA/ Sb was maximal at surfactant concentrations above the cmc (1.9 X M, calculated under our reactions conditions).
+
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
.
-1
U 2
4
6
10
8
12
14
16
M Flgure 1. Influence of the concentration of (a) BriJ-35, (b) Triton X100, and (c) CPC on the reduction rate of 12-MPAISb 12-MPA complexes by ascorbic acid. Solutions were prepared as described in the Experimental Section. [Sb(III)] = 1.0 mg/L, expressed as lnltial concentration in the syringe. ISURFACTANTI x lo5
51
+
0
2
6
L
8
l O t 2 l i
[TRITON x-io01 x
io5
M
Figure 3. Influence of the Triton X-100 concentration on the rate of reduction of 12-MPA (curve a) and 12-MPA/Sb 12-MPA (curves b-d) by ascorbic acid [Sb(III)]: (b) 0.2 mg/L, (c) 0.6 mg/L, (d) 1.0 mg/L, as initial concentration in the syringe. The concentrationsof the other reactants are given in the Experimental Section.
+
2
I
2 L 6 volume%f phos hate molybdate nitric acid s o k t i o n ( m L )
0
-
[ Mo (VI)] / [ P I
-a c
TIME Is)
Figure 2. Kinetlc curves for the systems made up of (a) 12-MPAascorbic acid and (b) (12-MPA/Sb 12-MPA)-ascorbic acid in the presence of 4.95 X M Triton X-100. [Sb(III)] = 1.0 mg/L. These concentrations are initial concentrations in the syringes.
+
The final decrease in the rate, with increasing Triton X-100 concentration, observed in the curves is due to the dilution of the reactants on increasing the concentration of micelles in the solution. Triton X-100 enhanced the acceleratingeffect of antimony, as can be inferred from Figure 3 by comparing the reaction rate increments of the different curves at the Triton X-100 concentrations 0 and 4.95 x M. Therefore, the kinetic determination of Sb(II1) in the presence of this surfactant will be more sensitive. The anionic surfactants tested had no effect on the reactions studied. The zwitterionic surfactant SB-12 considerably decreased the rate of reduction of both the 12-MPA and the 12-MPAISb complex. Optimization of the Reaction Conditions. In order to achieve the best precision and sensitivity in the determination of Sb(III), two aims were considered in selecting the most appropriate reactant concentrations: namely the rate of reduction of 12-MPA/Sb should be maximal and that of 12-MPA should be decreased as far as possible during the initial reaction period. The effect of different variables affecting the reaction was studied by changing each variable in turn while keeping all others constant.
0
02
OL [HN03] ( M I
0 6 0
1 2 3 4 [Ascorbic acid] x I O 2 M
Figure 4. Influence of (A) the volume of phosphate-molybdate-nitric acid solution, (6) the Mo/P molar ratio, (C) the nitric acid concentration, and (D) the ascorbic acid concentration on the rate of reductlon of 12-MPA (curve I), 12-MPA 12-MPAISb (curve 2), and 12-MPAISb (curve 3) complexes by ascorbic acid [Sb(III)] = 1.0 mg/L, as initial concentration in the syringe.
+
Figure 4 shows the variation of the initial rate as a function of different variables [volumeof phosphate-molybdate-nitric acid solution (A), molybdenum/phosphorus molar ratio (B), nitric acid (C), and ascorbic acid (D)concentrations] for the reduction of 12-MPA(curve 1)12-MPA+ 12-MPAISb (curve 2), and 12-MPA/Sb (curve 3) by ascorbic acid. Curve 3 was obtained by subtracting the experimental values obtained from curve 1and 2. All concentrations given here are initial concentrations in the syringes(twicethe actual concentrations in the reaction mixture at time zero after mixing). Each kinetic result was the average of three measurements. The initial rate increased with increasing amount of phosphomolybdate in the reaction medium (Figure 4A). A
ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
constant effect was obtained for the reduction of the 12-MPAI Sb complex a t volumes of the 12-MPA solution above 4.0 mL (curve 3). Additional molybdate-phosphate had no further effect than to increase the blank correction required in each case. The molybdenum-phosphorus ratio had a marked effect on the rate of reduction of 12-MPA and 12 MPA + 12-MPA/ Sb (Figure 4B; curves 1and 2, respectively). The initial rate of reduction of 12-MPAISb hardly changed for a molybdenum-phosphorus ratio between 20:l and 55:l. A blank of minimum rate was obtained by using a 55:l ratio, which was thus chosen for subsequent experiments. The hydrogen ion concentration was found to affect the formation of 12-MPA and 12-MPA/Sb complexes and their reduction step. The formation and reduction of the antimony heteropoly, like that of 12-molybdophosphoric acid, was markedly pH-dependent; however, the overall rate of the reaction remained unchanged over a somewhatwider pH range than that of 12-molybdophosphoricacid, as shown in Figure 4C (curves 1and 3). A 0.28 M HNO3 concentration, which resulted in the maximum possible rate of reduction of the 12-MPAISb complex and the minimal blank signal (curve l), was chosen for subsequent experiments. Ascorbic acid influenced the initial rate of reduction of both the 12-MPA and 12-MPAISb complex (Figure 4D) and the shape of the calibration graph used for the determination of Sb(II1). Thus, the slope of the calibration graph obtained by using a reductant concentration of 1.7 X M changed at Sb(I1I)concentrations higher than 0.4 mg/L, the slope was (7.5 f 0.3) X loT3s-l m g l L for 0.1-0.4 mg/L and (1.31 f 0.08) X loT2s-l mg-l L for 0.4-1.8 mg/L of Sb(II1). A linear graph, of slope (1.37 f 0.04) X 10-2 s-l mg-' L was obtained for 0.11.8 mg/L at an ascorbic acid concentration of 2.84 X M. Therefore, a 2.84 X 10-2 M ascorbic acid concentration was chosen for further experiments. As noted earlier, the presence of glycerine in the reaction medium was required in order to avoid precipitation of the reaction products. A glycerine concentration between 0.5 and 4% did not apparently affect the initial rate of reduction of the lBMPA/Sb complex, although it did influence that of 12-MPA. Concentrations of glycerine above 4% caused a gradual decrease in the initial rate of reduction of 12-MPA/ Sb. Because of problems arising from viscosity differences in the mixing jet during injection of reactants, the two reactant solutions (A and B) used to fill the drive syringes of the stopped-flow module contained the same concentration of glycerine. Temperatures values between 20 and 30 "C did not apparently influence the rate reduction of the 12-MPAISb complex, while values above 30 OC caused a gradual decrease in this rate. The influence of the ionic strength on the initial rate of the reactions studied was examined up to about 0.2 M and was found to depend on the electrolyte used to adjust it. Sodium chloride had only a slight influence, while potassium nitrate decreased the initial rate of reduction of 12-MPAISbby about 40% at an added concentration of 2.5 X M. Higher concentrations had no further effect. The reactants involved in the reaction were mixed in various sequences to determine their optimum distribution in each syringe of the stopped-flow module. 12-MPA and Triton X100produced turbidity that increased with time, so they could not be mixed in the same syringe. Likewise, 12-MPA and ascorbic acid should be kept in separate solutions. According to the results, the initial rate did not depend on the way in which reactants were distributed in the syringes. Features of the Analytical Method. A calibration graph for the determination of Sb(II1)in the Triton X-100 micellar
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Table I. Effect of Foreign Ions on the Determination of 6.56 X 10" M Sb(II1) ~~
ion tested
tolerated mole ratio of ion to Sb(II1)
anion
c1sio32-,AsOz-, S042-
VOs-,NOa-
Fw0a2-, cation Fe(III), Al(II1) Hg(II), Ni(II), Co(I1) Ti(IV), Cr(III), Fe(I1) Cu(I1) Bi(II1)
5000 1000 500 100 10 2500 100 10
1
medium was run under the optimal conditions described above. The determination of this ion was feasible over the range 0.1-1.8 mg/L. The standard error of the estimate (3.5 X 10-4 s-l) and the correlation coefficient obtained was 0.999 (n = 10). The sensitivity was (1.37 f 0.04) X s-l mg-1 L. The detection limit (3a) was 0.07 mg/L, i.e. about 7 times lower than that of the earlier method based on this reaction,15 which was 0.5 mg/L. The precision of the proposed method, expressed as the relative standard deviation, was 1.8% [n = 11,for 0.8 mg/L Sb(III)]. Comprehensive interference studies have already been conducted on mixed heteropoly chemistries,20-22and because of inadequate selectivity, most of the methods based on them have a limited utility. Therefore, it was interesting to determine whether micellar catalysis results in increased selectivity toward Sb(II1). We tested most of the ions considered to be serious interferents with the determination of Bi,21Th,20Nb,22and Sb15 based on their acceleration of the phosphomolybdate reduction. Thus, the major interferents tested included arsenate and silicate which form heteropoly complexes with heptamolybdate; tungsten and vanadium, which yield heteropoly acids analogous to heteropoly molybdic acids; titanium, bismuth and nickel, which form ternary heteropoly acids with phosphomolybdate; iron(II1) and mercury(II), which are reduced in the presence of ascorbic acid; iron(II),which reduces the heteropoly acid of interest; fluoride, which complexes Mo(VI), etc. We found no explanation in the literature for the interference of other major ions such as chloride, copper(II), aluminum(III), cobalt(II), chromium(III),and nitrate. Table I summarizesthe effects of these ions on the analytical reaction. The maximum mole ratio of anion to Sb(II1)tested was 5000. For cations, the maximum mole ratio tested was 500 in order to avoid interferences from the anions contained in the added salt as counterions. A given ion was considered not to interfere with the determination if the interferent plus analyte mixture yielded a signal comprised in the range S, f a, where S, is the signal provided by the analyte in the absence of interferent and a is the standard deviation of the method. The selectivity, compared to the results in the absence of surfactant, was substantially improved with respect to the ions tested. It is significant that arsenate and silicate were tolerated at concentrations 10- and 1000-fold, respectively, that of antimony without the need for a masking agent [concentrations &fold that of antimony cause errors of -20 % and +23 % for arsenate and silicate, respectively, in the earlier method based on the same reactionl51. It is also worth noting that chloride and fluoride were tolerated at concentrations 5000- and loo-fold, respectively, that of antimony, while they (20) Madison, B. L.; Guyon, J. C. Anal. Chem. 1967, 39, 1706-1708. (21) Hargis, L. G. Anal. Chem. 1969, 41, 597-600. (22) Norwitz, G.; Codell, M. Anal. Chem. 1954,26, 1230-1234.
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08 -
a 02 -
300
320
3LO
obtained by using a series of solutions prepared in 10-mL standard flasks by mixing a nonionic surfactant concentration M, 0.28 M nitric acid, 1.0 X between 0 and 3.71 X M ammonium heptamolybdate, 1.3 x M potassium dihydrogen phosphate, and doubly distilled water to the mark. That of 12-MPA/Sb was determined by using solutions prepared in the same way but containing 1.3 X M antimony as well. A P:Sb ratio of 1:l was used in order to ensure complete formation of the complex. An aliquot was transferred to a 1-cm quartz that was thermostated at 25 “C, and the absorbance was measured at 320 nm. Binding constants were calculated from the expre~sion2~~25
A inml Flgure 5. Absorption spectrum of 12-molybdophosphate in (a)water, (b) 1.29 X 10-5MSb(III),(~)2.47X 10-5MTrRonX-100,and(d) 1.29 X M Sb(II1) and 2.47 X M Triton X-100. [heptamolybdate] = M; [KH2P04] = 1.29 X M.
Kcomplex = f / W -f)([Dl - cmc) -f(l-f)[complexI) The fraction f of micelle-bound 12-MPA or 12-MPAlSb at various surfactant concentrations [D] was estimated from
are only tolerated at 25- and 5-fold concentrations in the determination of bismuth.21 The most serious interference was posed by Bi(II1). Research is currently being conducted for a simultaneous determination of Bi(II1) and Sb(II1). Antimony(V) concentrations of the same order as those of Sb(111)resulted in errors of +16% owing to the reduction of the former to the latter on mixing the A and B solutions (see Procedure). A prior reduction of Sb(V) to Sb(II1)by adding the analyte and interferent to solutions A was not possible since no contact with the acid medium was then established. Some Observations on t h e Action of Triton X-100 on the Reaction. In order to elucidate the mechanism via which Triton X-100 acts on the Sb(II1)-accelerated 12-MPAascorbic acid reaction we must first consider the net changes that this surfactant induces on the reaction features. The only evident effect of Triton X-100 was to accelerate the formation of the reduction product when Sb(II1) was present in the reaction medium. On the other hand, no acceleration or even a slight decrease in the rate of the blank reaction at the 12-MPA concentrations studied was observed. Since the “local concentration” of reactants on the micellar surface through hydrophobic and electrostatic (ion-ion, ion-dipole, and dipole-dipole) interactions23 is generally responsible for micellar catalysis, we investigated 12-MPAmicelle, 12-MPA/S&micelle, and ascorbic acid-micelle interactions in order to determine the origin of the alteration that Triton X-100 causes on the reaction. Also, the heteropoly blue-micelle interaction was studied to ascertain whether Triton X-100 modifies the molar absortivity of the reaction product, which would result in an apparently modified reaction rate since reaction development was monitored via the formation of the heteropoly blue compound. A. 1%-MPA-and 12-MPA/Sb-Micelle Interactions. Evidence of interaction between 12-MPA and 12-MPA/Sb complexeswith Triton X-100 is provided by precipitation on mixing of these reagents in the absence of ascorbic acid under the conditions used for the determination of Sb(II1). In order to determine the partition or “binding” constants of these complexes to the nonionic surfactant system, the concentration of 12-MPA used was 10-fold lower than that recommended for the determination of Sb(II1) so as to avoid precipitation. These constants were determined spectrophotometri~ally2~~~5 through the change that Triton X-100 induces in the absorption spectra of the 12-MPA and 12-MPAISb complexes (Figure 5 ) . The 12-MPA “binding” constant was
where A,, is the absorbance of the complex in pure water, A the absorbance in the presence of added surfactant, and A, the limiting absorbance resulting from complete incorporation of 12-MPA or 12-MPA/Sb into the micellar pseudophase. The plots of f/(1- f ) against the Triton X-100 concentration are linear up to a 2.47 X M surfactant concentration, with a slope O f K~Z-MPA = 6.5 X lo3 M-’ and KlZ-MpApb = 7.8 x lO3M-1, respectively. The abscissaintercept yieldeda cmc% of ca. 1.6 X 10-5 M, i.e. much lower than the cmc of Triton X-100in pure water (3.0X lo-*M) but similar to that obtained from the surface tension measurements made with the stalagmometer (2.2 X 10-5 M). Since the maximum rate of reduction of 12-MPA/Sb was obtained at surfactant concentrations above ca. 4 X 10-5 M, micelleswere indeed formed under the working conditions used. Because of the negative charge borne by 12-MPA and 12-MPAISb and the nonionic character of the surfactant,the interaction of these complexes with the micelle is probably of the ion-dipole type. B. Ascorbic Acid- a n d Heteropoly Blue-Micelle Interactions. The potential binding of ascorbic acid to the micelles was also investigated spectrophotometrically. No changes in the spectrum of ascorbic acid were found to arise in the presence of Triton X-100 so no interaction could be proved. In order to determine whether Triton X-100 micelles increased the molar absorptivity of the heteropoly blue compound, some experiments were carried out by mixing all the reaction ingredients except Triton X-100. Once the reaction was complete, addition of the surfactant to the solution resulted in no net increase in the absorbance under the reaction conditions used. Therefore, no sensitization of the heteropoly blue compound by Triton X-100 micelles occurred. The results obtained in the above experiments allow some considerations to be made on the effect of Triton X-100 on the reaction between 12-MPA or 12-MPAISb and ascorbic acid, even though further research is required in order to elucidate the real action of this surfactant and to assess the intrinsic reactivity of the systems studied in the micelles. Since most models for micelle-modifiedreactions assume that the overall rate of reaction is the sum of the rates in each pseudophase and at least 12-MPA and 12-MPA/Sb are concentrated on the micellar surface, the overall rate of the reaction could be the sum of the following reactions: (12MPA/Sb), + (ascorbic acid), (I); (lL-MPA/Sb), + (ascorbic acid),, (11); (lP-MPA/Sb),, + (ascorbic acid),, (111); (12MPA), + (ascorbicacid), (IV);(12-MPA), + (ascorbicacid), (V); (12-MPA), + (ascorbicacid), (VI),where the subscripts m and aq denote micellarand water psueodphase,respectively. The contribution to the overall rate of the reaction from I,
(23) Sudholter, E. J. R.; Langkruis, G. B.; Engberts, J. B. F. N. J. Royal Netherlands C h e n . SOC.1980,99, 73-82. (24) Sepulveda, L. J . Colloid Interface Sci. 1974, 46, 372-379. (25) Bunton, C. A.; Rivera, F.; Sepulveda, L. J. Org. Chem. 1978,43, 1166-1173.
f = (A - Aa,)/(Am- Aa,)
ANALYTICAL CHEMISTRY, VOL. 84, NO. 13, JULY 1, 1992
Sbllll) (mg/L)
Figure 8. Dependence of the initial rate of reaction on the Sb(II1) concentration In the absence (a) and presence (b) of 4.95 X M Triton X-100, as lnltialconcentrationin the syringe. The concentrations of the other reactants are given in the Experimental Section.
11, IV, and V reactions will be significant or not, depending on whether or not ascorbic acid concentrates on the micellar pseudophase. As far as the 12-MPA reduction is concerned, the rate constant (k,) estimated in water (i.e. in the absence of surfactant) is 9 X s-l, and the apparent rate constant, k, for the reaction in the presence of 4.94 X 10-5 M Triton X-100 is 8.7 X s-l. The standard deviations for k , and k were 1.1 X and 0.8 X 10-3 s-l, respectively, for n = 7. Accordingly,concentration of 12-MPA on Triton X-100 does
1495
not increase the apparent rate constant, which seemingly indicates that the intrinsic reactivity of the reactants is lower in the micellar medium. Similar results were obtained in the presence of other surfactants (SB-12, CPC, CTAB, DTAB, and Brij 35))which evidences the interaction with 12-MPA. The behavior of other heteropoly acids such as that formed between Mo(V1) and Sn(I1) was similar to that exhibited by 12-MPA in the presence of the aforementioned Surfactants. The apparent rate constants for the 12-MPA/Sb system, at a Sb(II1) concentration of 4 mg/L, in the micellar and aqueous medium were 62 X s-l and 31 X s-l, respectively. The standard deviations for k , and k were 1.5 X 10-sand 1.1X lO-3s-l, respectively, for n = 7. Concentration of Sb on Triton X-100 increased this apparent rate constant, which redounded to a more sensitive determination. This "local Concentration" effect only affected the reaction rate, which is logical, when the effective Sb(II1) concentration in the bulk solution was lower than in the aqueous medium (Figure 6). Further kinetic studies are required in order to elucide the complete mechanism of the reaction in Triton X-100 micelles.
ACKNOWLEDGMENT We gratefully acknowledge financial support from the CI-
n-m
byl.
RECEIVED for review December 18, 1991. Accepted March 25, 1992.
