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Anal. Chem. 1984, 56, 1146-1151
Comparison of Flow Injection Analysis Configurations for Differential Kinetic Determination of Cobalt and Nickel Alfonso Fernandez, M. D. Luque de Castro, a n d Miguel Valcarcel* Department of Analytical Chemistry, Faculty of Sciences, University of Cdrdoba, Avda. Medina Azahara, CBrdoba, Spain
Three new F I A conflgurations are presented for slmutlaneous determinatlon based on differential klnetics by use of a single detector. The first of the manifolds used Is based on the splitting of fhe injected sample Into two reactors of different geometric characterlstlcs, whose confluence takes place before reachlng the flow cell. The other two configurations are based on the use of two flow cells In a double-beam spectrophotometer: In the series conflguratlon the cells are connected to each other by a reactor, and In the parallel conflguraflon each subbolus Into the sample Is dlvlded before passlng through one of the cells. I n the three cases a recorded trace of signal vs. time shows two peaks. These assemblies, characterized by thelr simpllclty, have been utillzed for the simultaneous determination of cobalt and nickel based on the different formatlon rat? of thelr complexes with 2-hydroxybenzaldehydethlosemicarbazone. The optlmlzatlon of variables has been performed by the modlfied simplex method.
One of the most advantageous features of kinetic methods of analysis with respect to equilibrium methods is the possibility of carrying out simultaneous determinations of species based on their different reaction rates with a common reagent. However, several major limitations restrict their applicability. First, it is not easy to find chemical systems in which, under specific experimental conditions, sufficient differences between two or more reaction rates can be established. Second, the differential kinetic methods described up to the present do not show a very high level of either accuracy or reproducibility (1, 2).
FIA is an important alternative in the development of differential kinetic methods, as it has technical advantages over manual procedures. It can be applied to chemical systems with a short half-life (1-10 s), which are inapplicable to traditional diffential kinetics without a stopped-flow arrangement. The basic principle of this type of determination is the measurement of a property of the reacting systems at two different times. The different contribution of each system to the monitored signal in each measurement time permits an easy calculation of the concentration of unknowns by the application of the method of proportional equations. Most of the differential kinetic methods developed by FIA have been devised by Jensen et al. and almost all of them are based on displacement reactions. The configurations used have been (a) two or more detectors ( 3 , 4 ) ,(b) a single detector and double synchronized injection (5),and (c) simple injection and stopped-flow (6). A fecent paper by Betterigde et al. (7) describes the simultaneous determination of nickel and cobalt by splitting up the sample and passing each subbolus through holes drilled in a Perspex block as flow cell. In a recent review we have summarized (8) the different types of simultaneous determinations developed by FIA and the generalized manifolds used in each case. The first of the new manifolds for differential kinetic determinations developed by FIA presented in this paper is 0003-2700/84/0356-1146$01.50/0
based on the splitting up of the sample bolus into two subboluses before reaching the flow cell in the detector. The residence (or travel) times of both subboluses are different, which allows one to obtain a two-peak recording in which the contribution of two species that react at a different rate with a common reagent contained in the carrier should also be different. In an earlier paper, we carried out a study of the behavior of both hydrodynamic and geometrical variables in the configurations with splitting (9). The second configuration presented is based on the use of a double-beam spectrophotometer. The sample is divided into two reactors with different geometric characteristics, each of which reaches one of the flow cells. A recording is obtained with two peaks inversed with respect to each other, corresponding to the passing of subboluses through the blank cell and the sample cell. In the third manifold presented, a double-beam spectrophotometer is also used and the sample, which undergoes no splitting at all, passes successively through the two flow cells separed by a delay coil. The recording obtained is similar to that of the second configuration: two inverse peaks. Precursors of FIA methods based on two peaks obtained in a similar way were presented in a paper by Stewart and Ruzicka (IO) on simultaneous determination of and NO3-by splitting the sample and passing the corresponding subboluses by photometric cells aligned in the same optical path in such a way as to bring about the splitting up of the sample, but avoiding the subsequent confluence of the two channels; the procedures used in ref 10 were by Jensen et al. (5) and by Betteridge and Fields (7). The usefulness of these configurations has been tested by using two new systems formed by cobalt and nickel with 2-hydroxybenzaldehyde thiosemicarbazone (2-OHsBAT). EXPERIMENTAL SECTION Reagents. Stock ethanolic solution of 2-OH-BAT(0.05%)was used. Stock standard Co(I1) and Ni(I1) nitrates were prepared and standardized by AAS. A carrier of pH 5.2 consisting of a 20% stock solution of 2OH-BAT and a 20% acetic acid-sodium acetate buffer solution of pH 5.1 was used. Apparatus. A Pye Unicam SP-500 spectrophotometer, equipped with a Hellma 178.10 flow cell (inner volume 18 ML) was used. A Perkin-Elmer575 spectrophotometer,equipped with flow cells of the same type was also empIoyed (Peltier system for the control of the temperature). Gilson Minipuls 2 and Ismatec S-840 perstaltic pumps, a Tecator L100-1 injection valve, Tecator TM I11 chemifold and the accessory instruments, Beckman 3500 pH meter, and Selecta-S 382 thermostat were used. A Hewlett-Packard HP-85 microcomputer with built-in tape cartridge drive, equipped with an HP-IB interface to which an HP-3478 multimeter was connected to the spectrophotometers was used. Manifolds. The configurations presented herein are shown in Figure 1,their optimum characteristicsfor the chemical systems studied being described in detail. The existence of a confluence point between the injected sample and an auxiliary channel of 2-OH-BATsolution favors the cation-reagent mixing and provides 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
::I::--
A
LI = 30cm
A
91 ~ 0 ' 3 m5 m h-LOOnm r - - - - -
R
I
;
-Y-
II-
0'6
--
I"' w
L2= 300
1147
o
cm
Tz
Ti
time
02=0'70 mrn
B
C R R
-
Ti T2
0'5
time
0'5
U
W
m L ,min-' Flgure 1. FIA manifolds with optimal physicochemical and FIA variables (R is the reagent carrier): (A) wlth splitting up and confluence points and a single-beam spectrophotometer;(B) with splitting up of the sample and two flow cells in a double-beam spectrophotometer;(C) in series configuration. The two typical FIA peaks obtained are also shown.
better reproducible results in all the cases.
RESULTS AND DISCUSSION A conventional kinetic study of the complexes of nickel and cobalt with 2-OHaBAT is essential for their subsequent application to the testing of the new configurations presented. The knowledge of the spectrophotometric characteristics of both complexes is required beforehand. Characteristics of the Ni-2-OHaBAT and Co-2-OH. BAT Complexes in Solution. In a weakly acidic medium (acetic acid-acetate buffer) 2-OH-BAT forms yelow chelates with Ni(I1) and Co(II), whose solutions are stable and show maximum absorption at 385 and 400 nm, respectively. The absorbance of both complexes, measured a t 400 nm is not affected appreciably by the temperature, the ionic strength, the dielectric constant, or the buffer concentration. Their molar absorptivities are 2.4 X lo3and 1.2 X lo4 L.mol-l.cm-l, respectively. The metal-to-ligand ratio, determined by the Job method is 1:l (Ni-2-OHeBAT system) and 1:2 (C0-2OH-BAT system). The conditional stability constants were calculated from the data obtained by the Job method (log K N ~ = 6.9 f 0.2 and log Kco = 10.9 f 0.2). In order to establish the oxidation stage of the cobalt ion in the complex, assays were run with samples prepared in an inert atmosphere and in the presence of air or chemical oxidizing agents. The records, shown in Figure 2, clearly prove that the complex corresponds to the cobaltic ion, since the sample prepared in the inert atmosphere shows only a slight absorbance, attributable to a partial oxidation occurring during the transfer to the photometric cell. Similar behavior has been reported for this element by Kitawa and Fujikawa, who have used this as a basis for the simultaneous kinetic determination
Cb.'12
OH BAT),
I 0'8 !
o0'4 l Ni"i2
OH BAT1
0
6 ' I Z OH BAT],
2(inert atmosphere)
10
20
30
k
llminl
Flgure 2. Absorbance-time curves for nickel and cobalt complexes. The presence of oxygen for the development of colored compound (prior oxidation of Co(I1)to Co(II1))is obvious.
of nickel and cobalt mixtures as well (11). Kinetic Study. Different techniques have been used for this study due to the different formation rate of the Ni(I1) and Co(I1) complexes. For the slower system (Co-2-OH.BAT) samples have been prepared by a traditional procedure. For the fast system (Ni-2-OH-BAT) the FIA stopped-flow method has been used with a monochannel system through which a reagent stream circulates. The nickel solution, containing an ethanol ratio exactly equal to that of the carrier, is injected into this stream in order to eliminate the refractive index phenomenon (22). In order to establish the partial orders of reaction with respect to the reagent concentration, the logarithm of the rate constant was plotted against the logarithm of the reagent
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
Table I. Optimization of Variables in Configuration A Univariate Method q t , mL.rnin-'
Lo, cm
L , , cm
L,, cm
1.25
50
30
300
~ ~ , m m 0.50
G,, mm 0.35
$2,mm 0.70
PH
[ 2-OH.BAT1, %
temp, "C
F(4
expts
5.2
0.01
22
3.1
227
Modified Simplex Method qt,
FIA variables
mL.min-'
LJL,
L o , cm
Vi, W L
F(i)
expts
cycles
16.5
123
91
3.1
21
11
1.25 temp,"C
pH
Lo,cm
22
5.2
30
FIA and physicochemical variables
concentration. The partial orders obtained were 1 for the Ni-2-OH-BAT system and 3 / 2 for Co-2-OH.BAT. The rate constant of the nickel complex is not affected by the change in pH, while that of cobalt increases noticeably, especially within the range of 4.5-5.5. This phenomenon is to be expected due to the occurrence of an oxidation process prior to the formation of the Cc-2-OH.BAT complex. The respective equations are d(Ni-2-OH-BAT)ldt = kNi[Ni2'] [2-OH.BAT] d(Co-2-OH*BAT)/dt = kco[Co2+][2-OH*BAT]3/2[H+]-1 where k N i and kco are the conditional rate constants. Under the conditions pH carrier 5.0,20% ethanol, 20 "C, large excess of reagent, the reactions are pseudo first order with respect to the nickel and cobalt ions, the values of the conditional rate constants calculated in the traditional way being kNi = 5.513 f 0.002 mh-l and k ~ ,=, 0.221 f 0.002 min-l. A. FIA Manifold with Splitting and Confluence Points. Optimizationof Variables. According to the above kinetic study, the following conditions are to be established for the optimization to be carried out: the ratio signal for both peaks (labeled hl and h,) should be as high as possible; the contribution of the Ni-2-OH-BAT complex should be maximum in hl and minimum in hz (the opposite for the Co-2OHeBAT system); high values for hl and h,; short travel time (high sampling rate). Therefore, the following variables must be optimized: Lo, &;, L1, &; L,, 4,; ql, q, (see Figure 1A); temperature; pH; 2-OHsBAT concentration; and ethanol percentage in the carrier. Thus, the selection of the response function has been on the morphology of the peaks obtained for each system. The response function selected was
F(i) = h1"hzCo/h2Nih1Co Le., the maximum contribution of the nickel complex in peak 1 and the maximum contribution of the cobalt complex in peak 2. For the application of the univariate method all but one of the n variables (which take k values, that yielding the maximum response function being chosen) were kept constant. The procedure, repeated r times, needs a number of experiments to be performed equal to knr. The results obtained by this tedious procedure are summarized in Table I, the response function being F(i) = 3.0. Modified Simplex Method (MSM) (13). In order to reduce the enormous number of experiments necessary to optimize the system, we used a less laborious one method suitable for systems with interacting variables. The MSM was chosen because it closely suited the system being studied. The
LJL,
13.5
qt,mL,min-' F ( i ) 1.20
3.1
expts
cycles
23
11
suitability of the MSM in FIA experiments has been earlier described by Betteridge et al. (14). Due to the high number of variables, the MSM was applied in the first place only for FIA variables, such as total flow rate, qt, length ratio, L2/L1,length of the premixing reactor, Lo,and injected sample volume, Vi, keeping the rest of the variables constant and also including in the respone function a factor accounting for the travel time of the second peak, T,, with the aim of obtaining an acceptable sampling rate. Table I shows the results of the optimization of these four variables. In a four-variable system, five points make up a simplex. After the first simplex a new one is generated by replacing one of the five points, the most unfavorable one, by a new point. The cycle for generating a new point usually involves one or two experiments. A total of 21 experiments (11 reflections, 4 contractions, and 1 expansion) was required to reach the maximum response function, 3.1. A criterion of convergence, in which the optimum value was defined as that of the simplex at which the same result was obtained within the limits of experimental security, was established. Having optimized these variables, we applied this method again, this time including physicochemical variables. The set of variables includes temperature, T,pH of the carrier solution, total flow rate, length of the premixing reactor, and length ratio, L2/L1,keeping the other variables constant. The results obtained are given in Table I. Only 23 experiments (11reflections, 5 contractions, and 1 expansion) were necessary in this case to obtain the same value of F(i). The results are in agreement, within a narrow margin, with those found by the univariate method, the response function being slightly higher. It has been possible to diminish to a surprising extent the number of experiments necessary for the optimization. Thus, the optimization by the MSM is reliable and faster than by univariate means. Each analysis requires 3 min to complete, since the travel times of the peaks 1 and 2 are 30 s and 2 min + 30 s, respectively. Under these working conditions and according to the theoretical expressions deduced for this type of configuration in which splitting up of the sample exists (9),this is divided in the following form when the reactor diameters are different 4
_--
= 0.6
(being Vi = VI
+ V,)
Logically the optimum working conditionsrequire subbolus 2 to have a higher percentage of the injected sample to compensate for the fact that it undergoes a higher dispersion which
causes the analytical signal to decrease. Nickel and Cobalt Determination. The height of the absorbance signal, h, at time t for a chemical species M of
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
2)
11
A
mol L-’.Io-~
A
I
1149
10.18
t
t
mol
L-’
31
A
170N1t169Co 511N1+509Co 8 51NitBL8Co 5 11NI t 6 79 CO 10 22Nit169Co 3 LO N1+3 39C0 6 81N1.6 79C0 170N1+1010Co 601N1t509
Figure 3. Calibrated standard of nickel (l), cobalt (2), and their mixtures for different ratios (3) in triplicate injection.
molar absorptivity tM in a cell length, 1, forming with a first-order rate constant, k , at temperature T, is related to the initial concentration [M’] of a reacting species if the rates show first-order dependence of the species, by the equation
ht = tMl[M’](l - e+Tt)
+ h’
(1)
h’ being the initial absorbance of the system, according to Betteridge and Fields (7). For measurements made on an individual species, M, at a fixed time, t , tMl(l - e-kTt) is a constant K M , ~Therefore . h, = K,,t[M’] h’ (2)
+
This is true for a static system but in the FIA system dispersion, D, of the sample zone occurs which is constant D, for a fixed point downstream, assuming a constant flow rate, thus
h, = D&M,,[M’] + h
(3)
Therefore, at the first peak (t = Tl) and for a given sample
hT1 = ~TIKNi,T1[Ni’] + DTIKCo,TIICO’l+ h’
(4)
hT2= &rzK~i,n[Ni’] DT~Kc~,T~[CO~] + h”
(5)
The terms D,and KM,, are determined for each element (cobalt and nickel) at each peak (2’1 and 2’2) and h’ and h” from the calibration curves obtained by using solutions of the individual metals for the experiments. They represent the slopes and intercepts, respectively, of these calibration curves. Then, for each sample, hT1and h, are obtained and eq 4 and 5 are then solved simultaneously for [Co’] and [Ni’]. Calibration curves for each ion were plotted from absorbances corresponding to the peak height of the standard solution of Co(I1) and Ni(I1). The reproducibility of this configuration is demonstrated in Figure 3, in which triplicate recordings for both nickel and cobalt and their mixtures are shown. In each case the absorbance a t the peak maximum minus the reagent absorbance was plotted. For this system, the background blank absorbance is always equal to h’ + h” and should therefore remain constant. The variations in this value reported below reflect instrument drift. The calibration curves were found to be linear.
A least-squares linear regression procedure was used to obtain the equations of the liner calibration curves from six standards in the range 10-60 gmL-’, run in triplicate for each element. The equation for the contribution of each cation in the two peaks and the proportional equations for the mixtures are shown in Table 11. The results of the analysis of Ni(I1) and Co(I1) mixtures, in 1:6 to 6:l ratios, calculated from these equations are also given in Table 11. In order to minimize the errors in the determination of these species in their mixtures, a program in BASIC language for collecting and analyzing absorbance-time points was prepared. With the calculation made in each experiment the following parameters were calculated: (a) maximal coordinates and minimal coordinates; (b) absorbance at predefined times; (c) independent semiareas; (d) areas. The process of the results was carried out by four different methods: Multiple regression analysis of the absorbance data (20 points for each experiment) and proportional equations from: the maximum absorbances, the areas, and the semiareas (first semiarea of peak 1 and second semiarea of peak 2) of the peaks. The errors obtained with this program were of the same order as those calculated by the simple measuring system of the absorbance at the maximum for each peak in a manual manner. In view of these results a calculation method avoiding the use of a microprocessor is presented, taking into account that the results show that the errors found are due to the intrinsic characteristics of the chemical systems rather than to the errors made in the measurements of the analytical signal. B. FIA Manifold with Splitting up of the Sample and Parallel Flow Cells in a Double-Beam Spectrophotometer. In this configuration the sample is divided into two reactors of different geometric characteristics, each of them reaching one of the flow cells in a double-beam spectrophotometer. It is, therefore, a simplified form of the detectors in parallel. In 1965 Hicks and Blaedel (15) described equipment for the determination of GOT activity in a continuous system with splitting of the sample and two cells in doublebeam spectrophotometer. In our configuration, the base line is a starting point for each of the two peaks, which appear inversed with respect to each other, as shown in Figure 1B. The optimization of variables for this new manifold was
1150
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
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8
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+
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
carried out by the simplex modified method, the variables taken into account being qt, Vi, Lo,$0, L 1 , h L2, and 92, while the chemical variables were kept at the optimum values obtained for the above configuration, taking into account that the same chemical system (Ni-Co-2-OH-BAT) was used. The response function employed was also the same, the height ratios mentioned above. The optimum values of the FIA variables obtained after the application of the MSM were very similar to those corresponding to configuration A, as should be expected if the partial similitude between both configurations is taken into account. In this case also the presence of a confluence point between the injected sample and an auxiliary channel of 2-OHeBAT solution favors the cationreagent mixing and provides more reproducible results. The optimum values of the FIA variables were as follows: qt (mL-min-l), L25; Vi (pL), 100; Lo (cm), 50; 4o (mm), 0.50; L1 (cm), 20; q51 (mm), 0.35; L2 (cm), 300; (mm) 0.70. Nickel and Cobalt Determination. Using the method described for configuration A, we prepared the individual standards of Ni(I1) and Co(I1) and those of their mixtures. The equations established from their measurements are given in Table 11. From these, the nickel and cobalt concentrations in the mixtures were calculated. Thb results obtained are shown in th8 same table. C. FIA Manifold with Flow Cells in Series in a Double-Beam Spectrophotometer. As opposed to the two above configurations, this is Ellr assembly connected in series without splitting the sample. The insertion of a reactor with suitable geometric characteristics between cells gives the best delay time for the peaks appearing at the optimum time intervals. A similar configuration had beeh previously reported by Blaedel and Hicks in 1962 (16) for an enzymatic glucose determination in A continuous flow system. This last configuration proposed is simpler than A and B and therefore the FIA variables to o timize are less numerous (the chemical variables have alrea y been studied in configuration A since the chemical system was the same). The optimization method used was MSM, the variables taken into account being qt, Vi, L1, &, L2,and 42. Although this result is logical, it is noteworthy that for the optimum values of these variables, residence times for peak 1 and 2 (T, and T2) practically equal to those found for manifolds A and B are obtained. The following optimum values of the FIA variables were found: qt tmL-min-l), 1.02; Vi (PL), 100; L1 (cm), 80; 41 (mm), 0.50; L2 (cm), 400; 4~~(mm), 0.70. Nickel and Cobalt Determination. By the proportional equation method described above, corresponding systems of equations were established from the measurements carried
6,
B
1151
out separately for Ni(I1) and Co(I1) standards. These equations and the results found in the determination of nickel and cobalt mixtures are summarized in Table 11.
CONCLUSIONS The accuracy obtained with configuration A is similar to that obtained by Betteridge and Fields in the resolution of the same mixture by combination FIA differential kinetic methods (7). It is also comparable to the results obtained by Jensen et al. for several mixtures of alkaline-earth ions (3-5). Nevertheless, the precision of the results achieved by configuration B and C is not so good. Hence we propose the configuration of splitting up and confluence point to resolve the nickel and cobalt mixtures with 2-OHeBAT. This paper provides further evidence of the possibilities and versatility of flow injection analysis. The methods presented herein are simple, handy, and fast and show the importance of FIA in developing new differential kinetic methods. Other possibilities such as the application of the stopped flow when the first peak is obtained and the injection of a large sample volume providing two reaction zones (17)at the carrier-bolus interphases are currently under study in our laboratory. ACKNOWLEDGMENT We are grateful to M. A. Gomez-Nieto for the elaboration of the program in BASIC. Registry No. Cobalt, 7440-48-4; nickel, 7440-02-0; 2hydroxybenzaldehyde thiosemicarbazone, 5351-90-6. LITERATURE CITED Mottola, H. A. CRC Crit. Rev. Anal. Chem. 1975, 229. Perez-Bendito, D. Analyst (London), in press. Dahi, H. J.; Jensen, A. Anal. Chim. Acta 1979, 105, 327. Espersen, D.;Jensen, A. Anal. Chlm. Acta 1979, 108 241. Kagenow, H.; Jensen, A. Anal. Chim. Acta 1980, 774, 227. Kagenow, H.; Jensen, A. Acta Chim. Acta 1983, 145, 125. Betteridge, D.; Fields, E. Z . Anal. Chem. 1983, 314, 386. Luque de Castro, M. D.; Valcarcel, M. Analyst (London), in press. Fernandez, A.; Gomez-Nieto, M. A., Luque de Castro, M. D.; Valcarcel, M. Anal. Chim. Acta, in press. Stewart, J. W. E.; Ruzicka, J. Anal. Chim. Acta 1978, 82, 137. Kitagawa, T.; Fujikawa, K. Nlppon Kagaku Kaishl 1977, 7,998. Betteridge, D.; Dagless, E. L.; Fields, 6.; Graves, N. F. Analyst (London) 1978, 703, 897. Nelder, J. A.; Mead, R. Comput. J . 1985, 78,308. Betteridge, D.; Sly, T. J.; Wade, A. P. Anal. Chem. 1983, 55, 1292. Hicks, G. P.; Blaedei, W. J. Anal. Chem. 1965, 37,354. Blaedel, W. J.; Hicks, G. P. Anal. Chem. 1962, 34,388. Fernandez, A.; Linares, P.; Luque de Castro, M. D.; Valcarcei, M. "Kinetics in Analytical Chemistry"; First Symposium, Cbrdoba, Spain, 1983.
RECEIVED for review November 23,1983. Accepted February 1, 1984.
