Anal. Chem. 1995,67, 729-734
Analyte Pulse Perturbation Technique: A Tool for Analytical Determinations in Far-from-Equilibrium Dynamic Systems Rafael JidneDPrieto, Manuel Silva, and Dolores Perez=Bendito*
Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, E- 14004 Cordoba, Spain
This paper reports a new methodological approach for obtaining simple and fast quantitative analytical information from oscillating chemical reactions in a straightforward, expeditious manner based on the system perturbation caused by a microvolume of an analyte solution. The system perturbation results in a change in the oscillation amplitude and period, which are both proportional to the analyte concentration. The oscillating, copper(n)-catalyzedreactionbetween hydrogen peroxide and sodium thiocyanate, implemented in a continuous-flow stirred-tank reactor perturbed by sodium thiosulfate as the analyte, was used as a model reaction to evaluate the performance of this technique. The proposed methods allow the determinationof the analyte with good precision and sample throughput. Some notions on the interaction of sodium thiosulfate with the oscillating chemical system are discussed. Oscillating chemical reactions are complex systems involving a large number of chemical species as reactants, products, and intermediates that interactvia unusual mechanisms.'b2 Oscillations in the concentrations of some reaction intermediates arise when the mechanism of a chemical reaction that is kept far from equilibrium involves coupled feedback steps (especially autocatalytic steps). The net effect is a monotonic decrease in the free energy of the overall reaction. Applications of oscillating reactions in chemistry have grown substantially in the last few years, particularly since the advent of the continuous-flow stirred-tank reactor (CSTR), which allows one to design and implement new chemical oscillator^.^-^ A well-known oscillating reaction is the Belousov-Zhabotinskii reaction,1s6-10which involves the oxidation of an organic compound (e.g., malonic acid) by bromate ion in a strongly acidic aqueous medium. This reaction is usually catalyzed by a oneelectron metal ion couple [e.g., Ce(IV)/Ce(III)l. A major group of oscillating reactions is based on the chemistry (1) Field, R J.J Chem Educ. 1972, 49, 308-311. (2) Field, R J.; Schneider, F. W. I. Chem Educ. 1 9 8 9 , 66, 195-204. (3) De Kepper, P.; Epstein, I. R; Kustin, IC/. Am. Chem. Soc. 1981,103,21332134. (4) Epstein, I. R]. Phys. Chem. 1984, 88, 187-198. (5) Epstein, I. R J. Chem Educ. 1989, 66, 191-195. (6) Field, R J. In Oscillations and Traveling Waves in Chemical Systems; Field, R J., Burger, M., Eds.; Wiley-Interscience: New York, 1985; p 55. (7) Gyorgyi, L.; Deutsch, T.;Koros, E. Int. J. Chem. Kinet. 1987,19,435-455. (8) Zhuravlev, A I.; Trainin, V. M. J. Biolumin. Chemilumin. 1990, 5, 227234. (9) Weigt, H.R Angew. Chem., Int. Ed. Engl. 1992, 31, 355-357. (10) Yoshimoto, M.; Yoshikawa, K; Mori, Y.; Hanazaki, I. Chem. Phys. Lett 1992, 189, 18-22. 0003-2700/95/0367-0729$9.00/0 0 1995 American Chemical Society
of oxyhalogen anions,11J2of which the Bray-Liebhafsky reaction (decomposition of hydrogen peroxide in the presence of potassium iodate and sulfuric acid) is the most representative example.l3-'6 On the other hand, oscillating systems involving no halogen compounds are still somewhat uncommon as chemical o s ~ i l l a t o r s , the ~ ~ -copper ~ ~ sulfate-catalyzed reaction of hydrogen peroxide with thiocyanate ion being the most representative example. Both the oscillations and the bistability of this halogenfree system have been characterized,21.22as well as its interaction with luminol. Relatively few oscillating chemiluminescent reactions have been reported so far.23,24 Despite the many reports of chemical oscillators, their reactions have scarcely been used for analytical purposes (the detection of trace quantities of iodide ion using the iodate-arsenite clock reaction25and the use of coordination chain reactions26are two earlier precedents), with the sole likely exception of the Belousov-Zhabotinskii reaction. Ensuing determinations are generally based on changes in the cycling frequencies of the Belousov-Zhabotinskii oscillator arising from the presence of a chemical species; thus, ruthenium(IID27increases and mercury(ID and thallium(lII)28decrease the cycling frequency to an extent proportional to their concentrations. In other cases, the decrease in the oscillation amplitude is linearly related to the analyte concentration, which has been exploited for determining chlorideB (11) Melichercik, M.; Sodnomdordz, G.; Treindl, L. ]. Phys. Chem. 1991, 95, 4923-4924. (12) Pojman, J. A; Epstein, I. R; McManus, T. J.; Showalter, K. J. Phys. Chem. 1991, 95, 1299-1306. (13) Bray, W. C.; Liebhafsky, H. AI. Am. Chem. SOC.1 9 3 1 , 53, 38-44. (14) Anic, S.; Kolar-Anic, L. 1. Chem. SOC.,Faraday Trans. 1988, 84, 34133421. (15) Anic, S.; Kolar-Anic, L;Stanisavljev, D.; Begovic, N.; Mitic, D.React. Kinet. Catal. Lett. 1991, 43, 155-162. (16) Kolar-Anic,L.; Schmitz, G.1. Chem. Soc., Faraday Trans. 1992,88,23432349. (17) Druliner, J. D.;Wasserman, E.]. Am. Chem. SOC.1988, 110, 5270-5275. (18) Rabai, G.; Kustin, IC; Epstein, I. R]. Am. Chem. SOC.1989, 111, 38703874. (19) Nagy, A; Treindl, L. J Phys. Chem. 1989, 93, 2807-2810. (20) Keki, S.; Beck, M. T. React. Kinet. Catal. Left. 1991, 44, 75-77. (21) OrbPn, M.J Am. Chem. SOC.1986, 108, 6893-6898. (22) Luo, Y.; Orbh, M.; Kustin, IC;Epstein, I. R]. Am. Chem. SOC.1989,111, 4541-4548. (23) Amrehn, J.; Resch, P.; Schneider, R W. J Phys. Chem. 1 9 8 8 , 92, 33183320. (24) Sattar, S.; Epstein. I. R]. Phys. Chem. 1990, 94, 275-277. (25) Bognk, J.; SArosi, S. Anal. Chim. Acta 1 9 6 3 , 29, 406-414. (26) Espenson, J. M. Chemical Kinetics and Reaction Mechanisms; McGraw-Hill, Inc.: New York, 1981; Chapter 7. (27) Tichonova, L. P.; Zakrevskaya, L. N.; Yatsimirskii, K B. Zh. Anal. Khim. 1978, 33, 1991-1994. (28) Liang, Y.; Yu, R Gaodeng Xuexiao Huarue Xuebao 1 9 8 8 , 9, 881-885. (29) Zhang, Q.; Chen, J. Fenri Shiyanshi 1988, 7, 4-7.
Analytical Chemistry, Val. 67, No. 4,February 15, 1995 729
and hexacyanoferrates30 at the micromolar level. The most recent application of this type is the determination of manganese0 by its effect on some chaotic regimes in the Belousov-Zhabotinskii reaction.31 This work introduces a new methodology for using oscillating chemical reactions in analytical chemistry for routine quantitative analytical determinations. The proposed methodology is based on the effect of a fast pulse of analyte on an oscillating system; for this purpose, a volume of a few microliters of standard or sample solution is added to the oscillating system in a CSTR The resulting perturbation causes a change in the oscillation amplitude and period proportional to the analyte concentration and after that a rapid return to the unperturbed state, whence the system is ready for a new determination by use of another fast pulse of analyte. Methods thus developed offer a number of advantages, namely, (a) the oscillating system can be used over long periods in successive analyte determinations; (b) immediate, convenient, rapid acquisition of analytical information; (c) a relatively high sample throughput (analysis times are typically much shorter than those of other analytical applications of oscillating reactions); (d) precise measurements (e.g., relative standard deviations of ca. 1.5%)ensured by a control of experimental variables affecting the oscillating reaction in the C m and (e) good sensitivity (determination levels lie in the nanomole to micromole range, depending on the particular oscillating system and detection technique used). In order to validate the proposed methodology, we chose a halogen-free oscillating system, viz.the copper(IT)atalyzed reaction between hydrogen peroxide and sodium thiocyanate, implemented in a CSTR, which has a high potential for analytical determinationsby virtue of the chemical properties and concentration level of copper(II). Oscillations,monitored by a means of Pt electrode, were perturbed by adding a few microliters of a standard sodium thiosulfate solution (a number of other inorganic and organic species with redox or chelating properties could also have been used for this purpose). EXPERIMENTALSECTION Chemicals. Stock solutionswere prepared kom commercially available reagent-grade (Merck) sodium thiocyanate, 30%hydre gen peroxide, sodium hydroxide, copper sulfate pentahydrate, sodium chloride, and sodium thiosulfate without further purification. Bidistilled water was used throughout. Apparatus. The instrumental setup used to implement the oscillating chemical reaction (depicted in F m r e 1) consisted of a glass CSTR (50.0 mL) wrapped water recirculation jacket connected to a Selecta 6000383 thermostat. The oscillating reaction in the CSlX was monitored with a Pt electrode (Metrohm EA 202) and a SCE (Amel 303) and a PGMultilab PCL812PG 12 bit analog-to-digitalconverter (ADC) installed in a Mitac PGAT 12 MHz compatible computer. A Metrohm Dosimat 665 autoburet was used to dispense the small volume of perturbing analyte into the oscillating system. The flow of reagent solutions through the CSTR was driven by a Gilson Minipuls-3 peristaltic pump, one channel of which was used to keep the reaction volume in the CSTR constant. Poly(viny1 chloride) pumping tubes and Teflon tubing were used. (30) Jm, M.;Li,Y.;Zhou, X, Zhao, 2.;Wang,J.; Mo,J. A d . Chinr. Acta 1990,
236,411-416. (31) Yatsimirskii, K B.;Sbizhak, P. E.; Ivaschenko, T.S. Tulonta 1993,40, 1227-1232.
730 Analytical Chemistry, Vol. 67, No. 4, Febnrary 15, 1995
mumin
Saturated calomel electrode
t
-
11i mh
-v Thermostatic jacket
Figure 1 Experimental setup for implementation of oscillating reactions in a CSTR. The composition of the reagent solutions is as follows: (1) 1.3 M hydrogen peroxide and 0.2 M sodium chloride; (2) 0.15 M sodium thiocyanate, 0.3 M sodium hydroxide, and 0.2 M sodium chloride; and (3) 6 x M copper(l1) and 0.2 M sodium chloride.
D
E
W L
.-0
-60
-
a, 0 -70
-
-80
-
Y
C U
a
Time
Figure 2. Temporal evolution of oscillations for the H202/NaSCN/ NaOH/CuS04 system, in the absence and presence of a sodium thiosulfate perturbation. Rows indicate the time at which oscillations are perturbed. Input concentrations and flow conditions as given under Procedure. The meaning of zones A-E is explained in the text.
Optimbhg the instrumental performance entails several technical aspects, such as where and how injection should be carried out and what flow rates should be delivered by the peristaltic pump in order to keep the CSTR in a steady state so as to ensure a stable, reproducible oscillating system. The solution zone at which injection was performed was indifferent, as was use of a manual micropipet or an autoburet, since the stirring ensured efficient homogenization. ?he autoburet was employed (see Figure 1). On the other hand, the timiing of the injection relative to the normal oscillating cycle was crucial. Thus, when the perturbation was introduced at the maximum of the cycle or in the rising potential zone preceding it, the oscillating system did not respond, while it did when the perturbation was introduced at the minimum of the cycle (see rows in Figure 2). The flow rates delivered by the peristaltic pump were crucial in order to preserve oscillations. The flow rate of the waste stream (Figure 1) should be at least as high as the summation of the other three; in practice, an even higher flow rate should be used since the fluid level in the CSTR is kept constant by means of the attached aspiration probe. Also, constancy in the flow rates of the other three channels should be maintained as far as possible, otherwise betweenday variations entail readjusting the peristaltic pump to preserve it, which is rather cumbersome. Instead, we
chose to correct the reactant concentration delivered through a given channel as a function of the changes its flow rate might experience between days. Procedure. In a CSTR thermostated at 25 "C were placed 5.0 mL of 1 M sodium chloride, 1.0 mL of 0.625 M sodium thiocyanate, 2.0 mL of 0.625 M sodium hydroxide, 0.5 mL of 33% (m/v) hydrogen peroxide, and bidistilled water up to a final volume of 24.5 mL, and the mixture was homogenized by magnetic stirring. The oscillating reaction was started by adding 0.5 mL of M copperGI) solution;without delay, the peristaltic pump delivered the three reactant streams (see Figure 1) at a constant flow rate of 0.8 mL/min; steady-state volume and concentration conditions were achieved by using a waste channel at a flow rate higher than 2.5 mL/min (e.g., 7 mWmin). After the oscillation amplitude and period had stabilized, variable volumes (a few microliters) of sample or standard containing variable amounts of sodium thiosulfate from 1.5 to 18 pmol were sequentially injected. Data were acquired at a rate of 1.0 point/s, and changes in the oscillation amplitude and period following perturbation were used as the measurement parameters to construct the calibration plot. All data were acquired and processed with software written in QBasic version 4.0. RESULTS AND DISCUSSION The reaction between hydrogen peroxide and sodium thiocyanate in alkaline medium,z1
+
4H202 SCN-
- HSO, + NH,'
+ HC0,- + H,O
(1)
is catalyzed by copper(II) and is first-order in each reactant. The rate-determining step is
H20,
+ SCN- - HOSCN + OH-
(2)
However, more recent studiesz2have revealed the occurrence of other intermediates such as cyanosultite, -OS(O)CN, peroxocyanosulfte, -OOS(O)CN, hypothiocyanite, -OSCN, and peroxohypothiocyanate ions, -0OSCN. The formation of a yellow superoxide-cuprous complex, HOZ-CUO, which only exists at alkaline pH values, plays a crucial role in the copper catalytic pathway, as shown below. Figure 2 shows typical oscillation profiles obtained for the proposed system in the absence and presence of a sodium thiosulfate perturbation under the experimental conditions described above. Thus, the potential (zone A) initially decreased as copper(II) was added to the CSTR in order to start the oscillating reaction, after which the system began to oscillate and evolve (zone B) to a stable amplitude and period. After the oscillating reaction stabilized-the oscillation period was then ca. 2 min-the addition of sodium thiosulfate (zone C) caused a perturbation that decreased the oscillation amplitude and period and was followed by a gradual return to the initial oscillating conditions. Once the system had restabilized, several successive determinations could be carried out (zone D). It should be noted that high analyte concentrations may give rise to a brief induction period after the perturbation but before the normal oscillating trend is regained (zone E), which has no adverse effect on the determination. As can be seen, the sodium thiosulfate perturbation decreases the oscillation amplitude and period. The amounts
u
o
1
2
4 "-1,5
3
[Copper(ll)], x lo" M
1
3
5
7
2,5
3,5
4,5
[NaSCN],x l o 2 M
9
'
0
0.1
0,2
0,3
0.4
0,5
[NaOH],x l o 2 M [H202I1M Figure 3. Influence of the (A) copper(ll), (B) sodium thiocyanate, (C) sodium hydroxide, and (D) hydrogen peroxide concentration on the oscillating reaction. (U) Amplitude and (A)period ratio. Conditions: (A) 5 pmol of sodium thiosulfate, 0.25M hydrogen peroxide, 2.5 x M sodium thiocyanate, and 2.5 x M sodium M copperhydroxide; (B) 17.5 pmol of sodium thiosulfate, 2.5 x (II), 0.25M hydrogen peroxide, and 2.5 x M sodium hydroxide; (C) 17.5 pmol of sodium thiosulfate, 2.5 x 1 0-4 M copper(ll), 0.25M hydrogen peroxide, and 2.5 x M sodium thiocyanate; and (D) 17.5pmol of sodium thiosulfate, 2.5 x M copper(ll), 2.5x M sodium thiocyanate, and 5.0 x lo-' M sodium hydroxide.
by which these two variables were diminished were used as the measured analytical parameters for studying the effect of experimental variables and making quantitative measurements on calibration graphs. Intluence of Experimental Variables. In order to be able to cany out a large number of determinations with the maximum possible sensitivity, conditions were optimized with three criteria in mind, namely, (a) achieving the maximum possible stability in the oscillating system over time; (b) ensuring that the oscillation period allowed the effect of the perturbation on it to be accurately determined; and (c) accomplishing the maximum possible oscillation amplitude. The initial working conditions used were similar to those previously employed by Epstein et al.,2z with some alterations, based on the results of preliminary experiments. Thus, we used three separate reactant channels to feed the CSTR (1-3 in Figure 1) in order to avoid potential interactions between the reactants. The concentration of each reactant in the mixed stream was initially the same as that in the CSTR and hence one-third that in each individual stream. The different experimental variables were studied in the presence and absence of perturbation by using the ratio between the amplitudes or periods obtained under the two types of condition as the measured parameter. The copper(II) concentration in the CSTR was varied between 1.0 x 10-4and 3.0 x M ((3.0-9.0) x M in the flowing stream). As the copper concentration was raised over the range studied, the oscillation period increased and the oscillation amplitude decreased, yet the drift was never suppressed. A copper concentration of 2.0 x M in the CSTR was chosen for subsequent experiments as it resulted in the maximum possible response of the system to the perturbation (Figure 3A). Analytical Chemistty, Vol. 67, No. 4, February 15, 1995
731
The sodium thiocyanate concentration was investigated over the range (2.0-4.5) x M (6.0 x 10-2-0.135 M in the flowing stream). The perturbation affected the oscillation period and amplitude to a similar extent (see Figure 3B). A sodium M in the M (7.5 x thiocyanate concentration of 2.5 x flowing stream) was chosen as a compromise between maximum sensitivity (amplitude-dependent) and minimum analysis time (perioddependent). In fact, higher concentrations of this reagent resulted in greater variations in the amplitude but also in a considerably increased period. The sodium hydroxide concentration, and thus pH, had strong effects on the oscillating reaction, which were studied from 2.5 x to 0.21 M in the flowing to 7.0 x M NaOH (7.5 x stream). Overall, the oscillation drift gradually decreased with increasing NaOH concentration;also, the system oscillated more uniformly and responded better to the perturbation. As can be seen from Figure 3C, both measured parameters decreased with increasing NaOH concentration. We chose a concentration of 5.0 x loT2M (0.15 M in the flowing stream) as optimal in order to ensure precise measurements (the system oscillated quite uniformly prior to the perturbation and responded quite well to it) with the minimum possible detriment to the sensitivity. Hydrogen peroxide was investigated from 0.15 to 0.45 M (0.45-1.35 M in the flowing stream). The perturbation affected both measured parameters similarly (see Figure 3D): the response was degraded at high oxidant concentrations. We thus selected an HzOz concentration of 0.215 M (0.65 M in the flowing stream) as optimal for further experiments. Small changes in this concentration resulted in no appreciable errors in the measured parameters. The study of experimentalvariables revealed that the measured potential was subject to increasing background noise as time elapsed. This can be ascribed to the likely formation of a copper hydroxide precipitate on the Pt electrode since the noise disap peared after the electrode was cleaned. In order to avoid this interference, sodium chloride was tested as electrolyte at concentrations up to 1.0 M in the CSTR, No surface adsorption on the Pt electrode was observed in the presence of this electrolyte, which, in addition, considerably reduced the induction period of the oscillating system, thereby significantly increasing the potential of the proposed methodology for routine analyses. On the other hand, the oscillation period decreased and the amplitude increased somewhat as the NaCl concentration used was raised; however, such a concentration had no effect on the measured parameters used to quantdy the perturbation above 0.2 M. We thus chose to employ 0.2 M sodium chloride in the CSTR and each of the feed reactant solutions in subsequent experiments. Temperature significantly influences the behavior of the oscillating systems but scarcely affects the perturbation. Thus, increasing the CSTR temperature from room temperature (25 OC) resulted in a significant decrease in the oscillation period; this, unlike other cases, was not accompanied by a decrease in the oscillation amplitude, which remained virtually constant. Above 40 "C, the system stopped oscillating. No appreciable changes in the oscillation amplitude or period were observed at the different temperatures studied. We thus selected 25 "C as the working temperature, which ensured a suitable amplitude and period for the perturbation and facilitated its application and maintenance in the CSTR 732 Analytical Chemistry, Vol. 67, No. 4, February 15, 1995
-10
-
,
l 4
200 s
A
I
I
-60 Time
10 200 s
B
-30
-j I
-40 Time
Figure 4. Temporal evolution of oscillations in the copper(l1)catalyzed reaction between hydrogen peroxide and sodium thiocyanate as a function of the composition of the reagent streams: (A) in the 1:l:l:l state and (8)under the selected experimental conditions (2:2:2:1state). For details, see text.
The selected optimal values for the experimental variables allow oscillating reactions to be used for analytical purposes; however, while the oscillation amplitude and period are relatively constant, the system is subjected to considerable drift (see Figure 4A). Such drift arises from deficient feeding to the CSTR, which prevents it from reaching a fully steady state. In order to circumvent this shortcoming, we altered the concentrations of the different reactant streams fed to the CSTR from the previously established values (1:l:l:l state). The system oscillated with no drift at a constant amplitude and period (see Figure 4B) in a 2:2: 2:l state and thus is at the established copper level and a 2-fold concentration of the other reactants. Under these conditions, the system oscillated for at least 7 h, even if subjected to as many as 10 perturbations/h. We thus selected this proportion between the reactant concentrations used to feed the CSTR (see caption to Figure 1). Finally, we studied the influence of the reactant flow rates to the CSTR from 0.5 to 1.5 mL/min (overall flow rate, 1.5-4.5 mL/ min). No appreciable changes in the oscillating system as such or after perturbation with sodium thiosulfate were observed. We thus selected an intermediate flow rate corresponding to an overall flow rate of 2.4 or 0.8 mL/min in each individual stream. Approaches to the h a l y t e Determination. Under the selected experimental conditions, the oscillating curve was perturbed by using arbitrary amounts of analyte (sodium thiosulfate). Figure 5 shows typical amplitude (potential) and period responses thus obtained. As can be inferred from the smooth baseline in both graphs (unperturbed system), the system recovered in full after the perturbation was applied. These responses were analy-
Table I.Regression Data and Analytical Figures for Merit of the Determination of Sodium Thiosulfate by Various Methods
parameter linear range, pmol slope intercept std error of estimate regression coeff (n = 25) detection limit, pmol precision, RSD, %
determinative method corr amplitude
amplitude 1-18 0.521 3= 0.006 mV/pmol -0.85 f 0.08 mV 0.196 mV 0.9986 0.31 f1.68
-10
1
I *
4
-15
1-18 0.537 f 0.004 mV/pmol -0.47 f 0.06 mV 0.136 mV 0.9993 0.28 f0.71
I
period
corr period
2-18 3.20 f 0.04 s/pmol -13.1 f 0.6 s 0.99 s 0.9985 0.64
2-18 3.33 f 0.04 s/pmol -15.9 f 0.7 s 1.07 s 0.9982 0.73 f1.33
f1.90
perturbations of 10pmol of sodium thiosulfate each, are also given in Table 1 for the four methods. As can be seen, the corrected methods provide slightly better regression; the differences are even more favorable in terms of precision, as the corrected methods offset small fluctuations in the oscillating system. The corrected amplitude and corrected period methods are therefore the optimal choices for quantitative analytical determinations based on oscillating reactions. The two proposed approaches were subjected to least-squares fitting, obtaining
+
CA = (0.02 zk 0.03) (0.998 & 0.018)CP -30
-20
I
I
I
I
I
0
20
40
60
80
100
Number o f O s c i l l a t i o n s
B
120 100
40
20
1-1’
S, = 0.220
Y
= 0.9992
where CA and CP denote the corrected amplitude and corrected period method, respectively, and S, is the standard error of the estimate. These statistical parameters coniirm the observed high correlation between the two methods. Finally, the sample throughput, an analytical feature of great significance in routine analyses, was determined. Under the selected experimental conditions, the system oscillated in the CSTR for at least 7 h, during which time as many perturbations as required to implement quantitative analytical determinations could be applied without oscillations being stopped. Based on the time needed for the system to recover after each perturbation, the throughput can be estimated to be 10-12 samples/h, which is quite acceptable, taking into account how slow most oscillating reactions are. This sample throughput clearly exceeds those of the few reported quantitative determinations based on oscillating r e a c t i o n ~ ~as ~ -a~result l of the operational mode employed. In such determinations, the analyte is added jointly with the reagents to the oscillating system from the start, which entails preparing a fresh sample for each analysis; this obviously detracts from expeditiousness and reduces throughput to less than 1 sample/h in some cases. Of the methods available for the determination of thiosulfate, spectrophotometric method^^^-^^ are the most frequently used and rely on the oxidation of this anion by inorganic oxidants. The proposed method exhibits dynamic range and sensitivity similar to these spectrophotometric methods and those based on indirect atomic absorption ~pectrometry;~~ however, its limit of detection is higher by almost 1 order of magnitude than that afforded by
1I -20
0
20
40
60
80
100
Number of O s c i l l a t i o n s
Figure 5. Variation of the (A) amplitude and (B) period of the oscillating reaction resulting from various perturbations (solid lines) with arbitrary concentrations of sodium thiosulfate. Dashed lines represent potential and period recovery following perturbation. Conditions as described in the Experimental Section.
sed by using (a) the oscillating amplitude of the perturbed system; (b) the corrected amplitude obtained by using a factor given by the ratio between the amplitude of the perturbed system at the start and immediately before the perturbation was applied, which allowed small fluctuations in the system with time to be offset; (c) the period for the perturbed system; and (d) the corrected period obtained by using a factor similar to that of used in (b). Table 1summarizes the figures of merit of such calibration graphs. The detection limit, calculated as the analyte concentration yielding an analytical signal equal to 3 times the standard deviation (n = 30) of the oscillation amplitude or period of the oscillation in the abscence of perturbation, which was taken as the blank, and the precision, relative standard deviation, calculated from 11
(32) Ma, Z.; Nie, L.; Yao, S. Anal. Chim. Acta 1991, 246, 425-428. (33) Sonne, IC; Dasgupta, P. IC Anal. Chem. 1991, 63, 427-432. (34) Koch, S.; Peisker, S. Z. Chem. 1990,30, 261-262. (35) Keitlin, I. M.; Petrenko, V. V.; Artemchenko, S. S.; Nichvoloda, V. M. Zavod. Lab. 1990, 56, 19-21. (36) Miura, Y.; Koh, T. Anal. Sci. 1990, 6, 695-700.
Analytical Chemistry, Vol. 67, No. 4,February 15, 1995
733
ionchromatographicdeterminations.%-@ In general, the precision of the proposed method is similar to or even higher than that of such methods, as is its throughput on account of the expeditiousness with which the proposed methodology allows one to obtain analytical information from oscillating reactions. Interaction of Thiosulfate with the Oscillating System. According to Epstein et al.,22 the mechanism by which the oscillating reaction studied takes place consists of 30 kinetic steps involving 26 independent variables and thus requires some refining.24Obviously, elucidating the type of interaction whereby thiosulfate ion interacts with the oscillating system was beyond the scope of this work. In any case, we can put forward a tentative interpretation based on some of our experimental results and the previous findings of Epstein et al. Thus, the centerpiece of the mechanism of this reaction, which contains the key of the oscillation, is the positive and negative feedback loops on which the autocatalytic process reliesF2 Thus, the positive feedback loop produces the yellow superoxy copper0 complex, which disap pears in the negative feedback loop: the concentration of species Cu+{SCN-}, is crucial for this feedback network. Based on the oscillation zone where the perturbation with sodium thiosulfate was introduced and on complementary photometric experiments, the oscillating system can be assumed to respond to such a perturbation at the maximal concentration of the yellow superoxy copper0 complex. Under such conditions, ion thiosulfate may interact with the negative feedback loop, which, according to (37) Yao, Y.;Shen, H.; Su,J.; Tao, W. Liuhafianya, Huaxue-Fence 1990,26, 209-210. (38)Obrezkov, 0.N.;Shpigun, 0.A; Zolotov,Y. A; Shlyamin,V. I.]. Chromotop. 1991,558, 209-213. (39) Maki, S. A; Danielson, N. D.]. Chromotogr. 1991,542, 101-113. (40)Obrezkov, 0.N.;Shlyamin,V. I.; Shpigun, 0. A; Zolotov, Y. A Mendeleeu Commun. 1991,I , 38-39.
734 Analytical Chemistry, Vol. 67,No. 4, February 15, 1995
Epstein et al.,22essentially involves the following steps:
+ H02* - SO,*- + HOCN + Cu+{SCN-), - SO-: + Cu2++ {nSCN-} -OS(O>CN
SO,*-
(3)
(4)
If the sulfite radical ion interacts with the thiosulfate ion, then intermediate -OS(O)CN will be depleted more rapidly-which will result in the experimentally observed decrease in the oscillation period. Also, the concentration of species Cu+{SCN-}, will not be so low as in the absence of thiosulfate, so this species may be able to start the positive feedback loop. Because such a species originates in the following reaction, H02-Cu0
+ {nSCN-)
-
Cu+{SCN-},
+ HO2*
(5)
the oscillating amplitude will be decreased-as experimentally observed-as a result of the concentration of yellow superoxy copper0 complex at the start of the positive loop being higher in the presence of thiosulfate. ACKNOWLEWMENT
The authors gratefully acknowledge Gnancial support from the Direccion General Interministerial de Ciencia y Tecnologia (DIGIC y 3 for the realization of this work as part of Project PB91-0840. Received for review July 5, 1994. Accepted November 2, 1994.a AC9406726 Abstract published in Advance ACS Abstracts, December 15,1994.
