945
Anal. Chem. 1907. 59. 945-950
of systematic error associated with interferences either in the determination of metals in one or more of the fractions or in the sequential extraction procedure itself. Precision was assessed by performing sequential microwave extractions on replicate ( n = 31) sediment samples from California Gulch, CO (5CA). These samples were extracted in seven separate batches of four to six replicates by three technicians who had little or no experience working with this system. Data from these experiments are presented in Table I11 for major elements of Ca and Fe and minor elements Mn, Pb, and Zn. Excluding the exchangeable fraction, the average percent relative standard deviation (% RSD) for any particular fraction was about 11% . The average % RSD for total metals as determined from the sum of the metals in each of the fractions was about 5 % . In the exchangeable fraction Fe and P b average below the detectable limit for the determination of those metals by flame AAS, while the other three elements have rather large %RSD’s ranging from 60% to 100%. This large variation is due in part to the fact that concentrations of metals in the exchangeable fraction are frequently being measured near the detectable limit for this procedure. This work shows that microwave heating techniques can be valuable in the study of the speciation of metals within binding fractions of sediments. Use of the microwave technique reduces the time required for fractionation from about 24 h to about 4 h, while at the same time producing results that are comparable to the conventional procedure. Recovery of iron from the residual fraction is virtually complete if the aqua regia/HF digestion is used rather than HNO,/HCl alone. The procedure appears to give the best results for minor and trace elements since the extraction rate experiments suggest that the efficiency of separation of these elements from one fraction to another is much greater than for major matrix elements. This paper reports results obtained from metal determinations made by flame AAS using direct calibration after correction of the absorbance for the signal from an appropriate reagent blank. Other than the correction for reagent blank, no special effort was made to identify and compensate
for interferences. Accuracy and precision could possibly be improved by using the method of standard additions. ACKNOWLEDGMENT The authors thank the following undergraduate students at USC who also contributed to this work Cheryl Passarelli, Julie Boyce, Aklilu Dunlap, James Tafoya, Richard Sanchez, Wendy Hsieh, and Charles Davis. Registry No. Ca, 7440-70-2;Fe, 7439-89-6;Cr, 7440-47-3;Mn, 7439-96-5; Pb, 7439-92-1; Zn, 7440-66-6. LITERATURE CITED Jenne, E. A.; Luoma, S. N. Biological Implications of Metals in the Environment; Wiklung, R. E., Drucker, H., Eds.; National Technical Information Service: Springfield, VA, 1977; Conf. 750929, pp 110-143. Tessier, A.; Campbell, P.; Bisson, M. Anal. Chem. 1979, 5 1 , 844. Tessier, A.; Campbell, P.; Bisson, M. Can. J . Earth Sci. 1980, 17, 90. Forstner, U.; Saiomons, W. “Publication No. 248, Waterloopkundip Labaoratorium Delft Hydraulics Laboratory”,Jan 1981. Caimano, W.; Weliershaus, S.; Forstner, U. Environ. Techno/. Lett. 1982, 3, 199. Farmer, John G.; Gibson, M. J.; Loveli, M. A. Miner. Environ. 1983, 5 , 57. Brannon, J. M.; Engler, R. M.; Rose, J. M.; Hunt, P. G.; Smith, I., U.S. Corps of Engineers, Technical Report D-76-7, 1976; NTIS: Springfield, VA. Popp, C. J.; Laquer, F. Chemosphere 1980, 9 , 89. Gubaia, C. P.; White, J. R. Presented at the Eighth Rocky Mountain Regional Meeting of the American Chemical Society, Denver, CO, 1986. Tessier, A.; Campbell, P. S. C.; Bisson, M. J . Geochem. Explor. 1982, 16, 77. Nadkarni, R. Anal. Chem. 1984, 5 6 , 2233. Matthes, S. A.; Farreii, R. F.; Mackie, A. J. Technical Progress Report No. 120, 1983; US. Bureau of Mines. Lamothe, P. J.; Frles, T. L.; Consul, J. J. Anal. Chem. 1986, 5 8 , 1881. Ficher, L. B. Anal. Chem. 1986, 5 8 , 261. Caravajai, G.; Mahan, K.; Goforth, D.;Leyden, D. Anal. Chim. Acta 1983, 147, 133.
RECEIVED for review August 25,1986. Accepted November 26,1986. Financial support for this project was provided by the Minority Biomedical Research Support Program of the National Institutes of Health of DRR Contract No. RR08197.
Studies of Raman Spectra and Equilibria of Isopolymolybdate Ions in Aqueous Acidic Solutions by Factor Analysis Toru Ozeki,* Hiroshi Kihara, and Seiichiro Hikime
Hyogo University of Teacher Education, 942-1 Shimokume, Yashiro-cho, Kato-gun, Hyogo 673-14, Japan
Factor analysls Is a useful statlstkal method to get Information on spectral overlapping. By appllcatlon of the method to the analysls of overlapped Raman spectra, each spectrum based on the lndlvldual specles can be obtalned. The method was applied to the system of the lsopolymolybdate solutlons of pH range 7.2-2.1, and the presence of the followlng four specles was elucidated monomer, heptamer, protonated heptamer, and octamer. The heptamer species corresponds to the paramolybdate Ion and the octamer corresponds to the octamolybdate Ion. The equlHbrlum constants of these specles were also obtalned.
and the separation of each spectrum based on the individual component is generally difficult. In such cases, the amounts of each components have been usually estimated by measuring the intensity of the spectrum peaks much less overlapped. In such treatment, some limited spectrum peaks are only utilized for the analysis, and almost all the other parts of the spectrum are abandoned. If two chemical species have definite intensities at the selected peak position, wrong conclusion will be derived. In recent years, a statistical method, called factor analysis, has been introduced into the field of analytical chemistry and applied to the separation of the overlapped spectra to get each spectra based on the individual species (1-10).
Raman spectra of the solutions containing several Raman-active species are often measured as overlapped spectra
On the other hand, it has been reported that the molybdate ions change the dissolved states according to the coexisting ions in the solution and form various types of polymolybdate
0003-2700/87/0359-0945$01.50/063 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987
ions (11-12). Some isopolymolybdate ions have been also found in the acidic solutions (13-21). These isopolymolybdate ions are in equilibrium in the solutions and give overlapped spectra. When some species can be separated in the crystal form, their identification is easily carried out by comparison of the Raman spectra of the solution with that of the crystal (18-20). Identification of the species whose crystal cannot be isolated is consequently difficult. It is the case that the identification of the whole isopolymolybdate species is difficult, although the presence of the unknown species is implied from the distorted Raman spectrum. The factor analysis was applied to the analysis of the Raman spectra obtained with the isopolymolybdate solutions of pH range 7.2-2.1 and the identification of the dissolved chemical species was studied. EXPERIMENTAL SECTION Preparation of Samples. The reagents used were all analytical grade chemicals and were used without further purification. Sodium molybdate Na,Mo0,.2H20 was used for the preparation of the sample solutions. Prior to the preparation, the titration curve of the sodium molybdate solution was obtained. With reference to the curve, pH of the molybdate sample solution was adjusted to 11 different values from 7.2 to 2.1 with hydrochloric acid. Then, lithium chloride was added into the solutions to 3 M in order to keep a constant ionic strength, and potassium nitrate was also added to 0.1 M in order to get the reference peak intensity in the Raman spectra. After standing the solutions for 1 h, their Raman spectra were measured. The concentration of the molybdate ion was always 0.1 M. After the Raman measurement, pH was measured again and achievement of the equilibrium was checked. The amount of consumed acid per one molybdenum atom (2value) was calculated from the used amount of the acid and the changed value of the pH. The Raman spectra of solid powders of ammonium paramolybdate, (NH4)6M07024-4H20 (Wako Chemicals), and ammonium octamolybdate, (NH4),Mo8026-5H20, crystalized by Aveston's method ( I @ , were also measured. Measurement of Raman Spectra. Raman spectra were measured by a laser Raman spectrometer with double monochromators (JASCO RSOO) and an Ar+ laser of 514.5 nm was used as an excitation light source. One milliliter of the sample was used for each measurement. Slit widths of the spectrometer for entrance, intermediate, and exit were 300, 500, and 300 pm, respectively. Spectra from 700 to 1100 cm-' were measured and all of the spectra data were recorded by a microcomputer. A data set of 100 wavenumber points of each spectra from 700 to 1000 cm-', with an interval of 3 cm-', was used for the analysis. Base-Line Correction of Raman Spectra. It was observed that the base-line intensity of the Raman spectra at lower wavenumber is a little larger than that at higher wavenumber. Three wavenumber points at which no Raman peak was appeared were selected and the base line was approximated by the second-order equation which passed through them. The base-linecorrected Raman spectra were used for the analysis. Program of Factor Analysis. The basic algorithm of the factor analysis which runs on a digital computer has been reported by Knorr (6) and Malinowski (8, 22, 23). Some improvements were carried out. The calculated results are displayed graphically on a CRT screen. The program was written in BASIC language and a microcomputer, NEC PC9801, was used. RESULTS AND DISCUSSION pH Dependence of Raman Spectra of Molybdate Aqueous Solutions. When the pH value of the molybdate solution was varied from 7.2 to 2.1, the Raman spectra from 700 to 1100 cm-' showed noticeable changes as shown in Figure 1. Spectrum intensities of Figure 1 were all normalized for the peak area of the nitrate ion at 1050 cm-' to have a constant value. When pH decreased, the peak intensity of 900 cm-I decreased and a new peak appeared near 950 cm-'. Under pH 4.7, the peak at 950 cm-' decreased again and an another peak appeared at a higher wavenumber region. It is suggested that there are at least three different Raman-active ions. Thus
j/
h
m
Y
1100
1000
'
900
800
10
c rn-1 Flgure 1. Raman spectra of 0.1 M sodium molybdate, 0.1 M potassium nitrate, and 3 M lithium chloride mixture solutions of various pH values (a.u., arbitrary unit).
the factor analysis was applied to this system to get further information. In application of the factor analysis, the sample solutions were divided to two groups: one group contains six solutions from pH 7.2 to 4.7 and the other six solutions from pH 4.7 to 2.1. Analysis of the Raman Spectra of Solutions of pH 7.2-4.7. A data matrix D was formed of Raman intensities of 100 wavenumbers and six sample solutions. Thus, the matrix D consists of 100 rows and six columns. Each column corresponds to the spectrum of each sample solution, and each row corresponded to the intensity dependence upon the sample composition. The multiplication of the transpose tD to D gives a covariance matrix Z, having six rows and six columns. Z = tDD (1) The covariance matrix is a kind of a correlation matrix, the elements of which express the degree of similarity between the sample solutions. The number of the independent ( b a n active) pure species can be obtained as the number of the independent base vectors forming the space of the matrix Z. The eigenvalue matrix and the eigenvector matrix for Z are obtained by the use of Jacobi method.
Z = tQEQ (2) Here, E is an eigenvalue matrix having six eigenvalues at diagonal elements and Q is a corresponding eigenvector matrix having eigenvectors a t columns. If the number of Ramanactive species is n, then n eigenvalues are large and any other values are negligible or small. When the eigenvectors corresponding to the former eigenvalues are extracted, matrix Q is newly constructed by lining them a t columns. On the other hand, a true spectrum matrix R, of which columns consist of individual Raman spectra for each n pure components, has dimensions of 100 rows and n columns. And a true composition matrix C , of which rows show the concentration distributions of the pure species a t each solutions, has dimensions of n rows and six columns. The relation of R and C with D can be expressed by a following equation. D = RC (3) The transpose of the eigenvector matrix, "&, has dimensions of n rows and six columns similar to matrix C and is one expression of the composition matrix C . The multiplication of D and Q gives the new matrix V. DQ = V (4) The matrix V consists of 100 rows and n columns. This matrix V has the same dimensions as R,so V is one expression of R. A new data matrix D' is obtained by the following calculation. D' = VtQ (5) If the selection of eigenvectors is correct, the new data matrix
ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987
-I
I
.'
' .
947
1
0
Id
v
c
% r-
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.-
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e
.
.c
C
-
0
c
0"
0:
4-
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\ Figure 2.
a
'.
E
pH!?
700
? ? ? ?
, 3 5 1 9 2
Figure 3. Raman spectra and composition profiles of two factors, A and B, in the isopoiymolybdate solutions of pH 7.2-4.7 (a.u., arbitrary unit).
Rotation of two-dimensional factor axes. pH 7.2
D' approaches to the true data matrix D. By analyzing Raman spectra along the above procedure, following six eigenvalues are obtained, 35 2269,45 681,1021, 278, 37, and 30 for pH 7.2-4.7 solutions. It is necessary to examine two or three factors for evaluation of the factor number. When the matrixes tQ and V were calculated and the data matrix D' was reproduced by eq 5, the square sum of the differences of each elements in D and D' decreased with an increase in the number of factors examined. No large difference was observed in the square sum between the case of two factors and that of three factors. Hence, it is suggested that these systems consist of two factors. Each row vector of the matrix V represents the intensity distribution at the respective wavenumbers upon the two factors. When the vectors in the rows are plotted in a plane consisting of two factors axes, Figure 2 is obtained. All points disperse around the A axis of the first factor. If new two axes A'and B'are taken at the farthest points from the A axis, all the wavenumber points are included within a plane between A' and B'. When the axes A and B are rotated to A'and B', respectively, the wavenumber points,which had negative values, come to have zero or positive values. This rotated matrix can be obtained operationally as follows. The elements of each row of the matrix V are normalized so that the square sum of them becomes unity. A vector having a farthest angle from the A axis is selected from 100 row unit vectors (namely, the dot product is calculated between the A-axis (1,O) and a tested vector of another row), and a vector giving the smallest value of the dot product is selected. The second base vector is selected by a similar method; then the dot product between the first base vector and a tested vector is examined. The rotation matrix thus obtained is expressed as A. A is a square matrix and has a square inverse matrix A-' (A-lA = I: unit matrix). The matrix V was multiplied by A-' and tQ was multiplied by A.
D = VtQ = (VA-')(AtQ) = RC
(6)
The true spectra matrix R and the true composition matrix
C are obtained in such a way. The Raman spectra and composition profiles of two factors, A and B, are shown in Figure 3. Here, each of the columns of R were multiplied by adequate constants so that the maximum value of the each column has a definite value, 100, and each of the rows of C were divided by the corresponding constants. In Figure 4, the Raman spectra experimentally obtained are shown in solid lines and those reproduced by eq 5 are shown in dotted lines. These two lines are overlapped very well, and it suggests that the Raman spectra of such solutions of pH range pH 7.2-4.7 are reproduced satisfactorily by two species, A and B.
2 factors
'I
5.7
0
v
E
5.5
4-
C
E 5.3
L
L
1000
)O
cm"
Comparison of the observed Raman spectra and the reproduced ones for pH 7.2-4.7.
Figure 4.
The operation, carried out to obtain the rotation matrix, is based on the criterion that each pure component has a unique wavenumber, called "pure point", at which only one component has a definite intensity but the other components have zero intensity (6,8).In the Raman spectra of this experiment, 826 cm-l is the unique wavenumber to the factor A and 961 cm-l is that to B. Analysis of t h e Raman Spectra of Solutions of p H 4.7-2.1. When similar operations were applied to the six solutions of pH 4.7-2.1, six eigenvalues, 42 9952,46 241, 4860, 277,54, and 42, were obtained. In this case, it seems that two or three factors should be examined for evaluation of the factors. When two factors were examined, the Raman spectra and composition profiles were obtained as shown in Figure 5. It was found that 871 and 980 cm-l were the wavenumbers corresponding to the pure points. In Figure 6, the spectra obtained experimentally were shown by solid lines and the spectra reproduced by the analysis were shown by dotted lines. Remarkable difference was observed between two curves of pH 3.1 solution. This suggests that an assumption of two factors is not sufficient to explain the experimental spectra. Then three factors were examined. Both pure points for the first and second factors were easily selected, but the third could not be selected, because the spectrum based on the third factor was completely overlapped with those of the first and
948
ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987 mi
I
i4 r
a
-
-
i
Flgure 5. Raman spectra and composition profiles of two factors, and D,in the isopolymolybdate solutions of pH 4.7-2.1.
pH4.7
D l
B
I
I
C
900
700
cm-’
Figure 8. Raman spectra and composition profiles of three factors, B, C, and D, in the isopolymolybdate solutions of pH 4.7-2.1. 1
1
pH4.7
3 factors
a
m
4.3
e>
.E
31
C
E
K
26
A
2.1
~
i Flgure 6. Comparison of the observed Raman spectra and the re-
produced ones based on two factors, B and D,for pH 4.7-2.1 solutions. S
Flgure 7. Rotation of three-dimensional factor axes. Balls denote the location of the wavenumber points.
second factors. Hence, the pure point for the third factor was operationally selected. This pure point has no physical meaning, giving negative intensities at several wavenumbers. Three factors thus obtained were named X, Y, and Z and each wavenumber point of the three columns of matrix V were plotted in the three-dimensional space X-Y-Z, as shown in Figure 7. Some of the wavenumber points were excluded from the cube denoted by OXQY-ZPSR. Those V matrix elements had negative intensities. As the two factor axes, X and Y , seemed to be correct, the direction of the third axis was moved from Z to Z’so that a new cube OXQY-ZPSR’included all
*.l
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 7 , APRIL 1 , 1987
Flgure 10. Dependence of the fraction of molybdenum atoms contributing to each factor on pH of the solution.
where niA, ..., n i , D are the number of molybdenum atoms contained in species A, ..., D of the ith sample. When the polymerization degree of molybdenum atoms in A is expressed as P A and the constant used to normalize the intensity of the spectra is expressed as fA-', the element C ~ , Aof the matrix C is related with ni,Aby a following equation:
=
(8) The coefficient f A / P A is a constant unique to species A. There are 11 equations corresponding to eq 7 by varying i. Four coefficients ( ~ A I P A ..., , fD/pD)were obtained by the application of the least-square method to them. Then the atomic fraction, ni,A/nMo, was also obtained. For the molybdate ions, equilibria between the monomer and the heptamer and that between the monomer and the octamer were expressed as follows (18): Ci,A
(fA/pA)%A
7 M 0 0 ~+ ~ -8H+ = Mo70246-+ 4H,O
+
8 M 0 0 ~ ~ -12H+ = M080264-+ 6H20
(9) (10)
1.143 (817) and 1.5 (1218) protons per one molybdenum atom are consumed to form the heptamer and the octamer, respectively. These amounts of proton are shown as 2 value in the abscissa of Figure 10. When the 2 value of the solution is equal to 1.143 or 1.5, then the corresponding isopolymolybdate ions become the major species. For instance, as shown in Figure 10, the amount of factor B becomes maximum at 2 = 1.1 and that of the factor D increases toward 2 = 1.5. It is suggested that the factors A, B, and D correspond to the monomer, heptamer, and octamer, respectively. Raman spectra based on the individual four factors are shown in Figure 11. The spectra with oblique lines are those of the isolated crystals monomer, Na2Mo04.2H20;heptamer, (NH4)6M07024'5H20; and octamer, (NH4)4M08026.5Hz0.The Raman spectrum of the monomer corresponds well to that of factor A and that of the octamer to D. The wavenumber of the peaks of factor B, however, shifts to a higher wavenumber region than that of the heptamer solid. It has been reported in the crystal structure analysis of isopolymolybdate salts that the monomer is a symmetrical tetrahedral ion (19) and the octamer consists of eight octahedral molybdates having a center of symmetry as a whole (24). On the other hand, the heptamer has a structure consisting of three octahedral molybdates on four octahedral molybdates, but it has no center of symmetry as a whole (25, 26). It is supposed that the ion having better symmetry is more stable and the ion having poor symmetry is liable to suffer effects from the surroundings. Both the monomer and
949
$0
Flgure 11. Raman spectra of each factor and those of the corresponding salt crystals.
the octamer might not be affected from surroundings in either solution or solid, because of their good symmetry. The heptamer, however, would be liable to suffer effects from surroundings and the structure would be distorted between both states. It is the present conception to explain the shift of heptamer peak wavenumber. As shown in Figure 10, factor C shows the presence of a maximum near 2 = 1.3. Several equilibrium reactions were examined and the following two reactions seemed to be adequate for the 2 value.
+ 9H+ = + 4H2O; 2 =1.29 (11) 7 M 0 0 ~ ~+- 10H+ = H2Mo7O244-+ 4H20; 2 = 1.43
7M004'-
(12)
It is also possible that one protonated and two protonated isopolymolybdate ions both participate in factor C. As a whole, it is suggested that the four factors are monomer, heptamer, protonated heptamer, and octamer. This conclusion is consistent with the suggestion of Murata et al. (13). After transforming the atomic fraction into the molar fraction, equilibrium constants K(Mo0,2-,H') for eq 9,11, and 10 were calculated as follows: log K(7,8) = 53.18 f 0.25, log K(7,9) = 56 f 0.14, and log K(8,12) = 69.73 f 0.23, respectively. These values agree well with the values reported by Aveston (18), 52.88, 57.39, and 71.75, respectively. ACKNOWLEDGMENT We express our deep gratitude to Professor Shigero Ikeda of Osaka University and Dr. Katuo Murata of Naruto University of Teacher Education for their useful suggestions on the dissolved states of isopolymolybdate ions. We also express our appreciation to Professor R. E. Malinowski for his kind information on the treatise of the factor analysis. Registry No. M070246-,12274-10-1; M080264-,12346-58-6; HMo,O~~&, 12371-76-5;Mood2-,14259-85-9. LITERATURE CITED Wold, S.; Anderson, K. J . Chromatogr. 1973, 80, 43. Slezer, R. 6.; Howery, D. 0.J. Chromatogr. 1975, 775, 139. Rozett, R. W.; Petersen, E. M. Anal. Chem. 1976, 4 8 , 817. Burgard, D. R.; Perone, S.P.; Wiebers, J. L. Anal. Chem. 1977, 4 9 , 1444. (5) Warner, I. M.; Davidson, E. R.; Christian, G. D. Anal. Chem. 1977, 49. 2155. (6) Knorr, F. J.; Futrell, J. H. Anal. Chem. 1979, 5 7 , 1236. (7) Cox, R. A.; Haldna, U. L.; Idler, K. L.; Yates, K. Can. J . Chem. 1981, 5 9 , 2591. (1) (2) (3) (4)
950
Anal. Chem. 1907, 59, 950-954
(8) Malinowski, E. R. Anal. Chim. Acta 1962, 734, 129. (9) Malinowskl, E. R.; Cox, R. A.; Haldna, U. L. Anal. Chem. 1984, 56, 778. (10) Ozeki, T.; Kihara, H.; Hikime, S.BunsekiKagaku 1986, 3 5 , 885. (11) Murata. K.; Ikeda, S . Anal. Chim. Acta 1963, 757,29. (12) Murata, K.; Ikeda, S . Po/yhedron 1983, 2, 1005. (13) Murata, K.; Ikeda, S . Spectrochim. Acta, Part A 1983, 3 9 A , 787. (14) Sasaki, Y.: Lindqvist, I.; SillBn, L. G. J . Inorg. Nucl. Chem. 1959, 9 , 93. (15) Sasaki, Y.; SillBn, L. G. Acta Chem. Scand. 1964, 78,1014. (16) Sasaki, Y.; SillBn, L. G. Ark. Kemi 1967, 29, 253. (17) Freedman, M. L. J . Inorg. Nucl. Chem. 1963, 25, 575. (18) Aveston, J.; Anacker, E. W.: Johnson, J. S. Inorg. Chem. 1984, 3 , 735.
(19) Griffith, W. P.; Lesniak, P. J. B. J . Chem. Soc. A 1969, 1966. ( 2 0 ) Tytko, K. H.; Schonfeld, B. 2.Naturforsch., B 1975, 308, 471. (21) Tytko, K. H.; Baethe, G.; Hirschfeld, E. R.; Mehmke, K.; Stellhorn, D. Z . Anorg. Allg. Chem. 1983, 503, 43. (22) Malinowski, E. R.; Howery, D. G. Factor Analysis in Chemistry; Wiiey: New York, 1980. (23) Malinowski, E. I?.,private communication, 1985. (24) Lindqvist. I . Ark. Keml 1950, 2, 349. (25) Lindqvist, 1. Ark. Kemi 1950, 2 , 325. (26) Shimao, E. Bull. Chem. SOC.Jpn. 1967, 4 0 , 1609.
RECEIVED for review August 4,1986. Accepted November 24, 1986.
Doubly Stopped Flow: A New Alternative to Simultaneous Kinetic Multideterminations in Unsegmented Flow Systems Fernando Ldzaro, M. D. Luque de Castro, and Miguel ValcBrcel*
Department of Analytical Chemistry, Faculty of Sciences, University of Cbrdoba, Cbrdoba, Spain
We have deslgned a new assembly for simultaneous multldetermlnatlons by the stopped-flow technique. I t involves both the kinetk and the nonlclnetlc modes commonty used in flow Injection analysts. The device Includes a dual Injection valve that Inserts the sample Into two channels: bolus, Is retalned at the detector as the flow Is halted throughout the system for the flrst t h e and Is used to monHor the development of the reaction of Interest; meanwhlle, bolus, Is kept In a reactor, where a step prior to the Indicator reaction takes place. The start of the pump allows the second bolus to reach the detector after merging with the reagent, the slgnal corresponding to the evolution of the reaction of this second analyte being measured durlng the second halting of the flow. The device has been successfully used in the determination of free and bound SO2 in wines (bound SO2 requlres a prlor hydrolytic release).
The high cost and sophistication of the instrumentation used in the application of the conventional stopped-flow technique limits its scope of application to fast reactions (half-lives less than 10 s). Flow injection analysis (FIA) (I) is a novel way to implement this technique involving the use of a timer synchronizing the start and stop of the peristaltic pump with the injection and the stopping of the reacting bolus either in the reactor or in the flow cell according to a nonkinetic mode, that makes possible the use of slow reactions in FIA and provides increased sensitivity of the method in all cases, or a kinetic mode. The FJA/stopped-flow methodology can be applied to reactions with half-lives in the range 5 s to 10 min. The methods described so far deal with the determination of individual species and, occasionally, of two analytes (I). The design presented here is aimed to the resolution of complex mixtures in which the determination of one of the analytes requires a step prior to the indicator reaction and involves a simultaneous dual injection with parallel valves and stopping the flow twice sequentially. The first halting coincides with the passage of one of the reacting boluses (bolusl) through the detector and the monitoring of its evolution. The other bolus (bolusz) is kept in a reactor, where reaction preceding that of the interest takes place. In the second halting, bolus2 is kept in the detector, where the development of the
corresponding indicator reaction is monitored. One of the salient applications of this assembly is the determination of free and bound SOz in wines. Sulfur dioxide is added to wine during its elaboration to avoid undesirable oxidation processes along the different steps involved. It is usually found in wine either bound to carbonyl or unsaturated compounds and/or phenol derivatives or free, as HS03- and SOz, this last being the sole species with antiseptic properties. The interest in the determination of SO2 arises from (a) the differences in the maximum allowed levels established by legislation in each country, (b) the monitoring of the disappearance of SO2from wine during aging (diffusion, oxidation, and binding) to determine the amount of SOz to be added, and (c) the need to avoid the disagreeable aroma and taste that SOz gives wine (2). The analysis of free and total SO2is conventionally carried out directly or with the prior separation of the analyte. The direct determination is carried out titrimetrically with iodine (3-5) and calls for the use of one aliquot to determine free SOz,another for total SO2 (after a prior alkaline hydrolysis), and a third one to measure the blank after addition of a chelating agent for SOz. The prior separation step (dragging by N2 (61, air (7), vacuum (8), or distillation (9))requires the use of two aliquots. Measurements are either titrimetric (6, 10)or gravimetric (these last are made after oxidizing SO2 to H2S04with HzOz (9)). The FIA methods proposed so far for the determination of this analyte in different samples and with different types of detection (11-22) show the potential of FIA for analysis of SO2. The interferents in the determination of this analyte in wines are eliminated by using a gas-diffusion cell (15,22) or the stopped-flow technique (21).However, only free SO2 (15,21)or free and total SOz individually (22)can be determined in this way. We have developed a method for the simultaneous determination of free and total SOz by use of the new assembly and a doubly stopped flow mode based on the formation of a colored compound (ArnB 578 nm) between the analyte, formaldehyde, and p-rosaniline (23).Simultaneous determinations carried out with the proposed assembly (Figure 1)involve the following: Simultaneous injection of the sample, which merges at point b in bolusl with the p-rosanilinelformaldehyde mixture, and halting of the stream containing this bolus as it reaches the detector, where the indicator reaction is mon-
0003-2700/87/0359-0950$01.50/00 1987 American Chemical Society
