2819
Anal. Chem. 1984, 56, 2819-2822
Photoacoustic Spectra of Prussian Blue and Photochemical Reaction of Ferric Ferricyanide T o r u Ozeki,* Koichi Matsumoto, a n d Seiichiro Hikime Hyogo University of Teacher Education, 942-1 Shimokume, Yashiro-cho, Kato-gun, Hyogo, 673-14, Japan
Prussian blue and Its related compounds were formed on glass-fiber filter paper, and their absorption spectra were measured by means of photoacoustlc spectroscopy. A broad band was observed at 700 nm for two preclpltates, prusslan blue and turnbull's blue. They were stable agalnst light Illumlnation, but In the case of ferrlc ferrlcyanlde, photochemical coloration was observed. The reactlon was well expialned by the foliowlng rnechanlsm. The ferrlcyanlde ion acts as a photocenter, and d-d" transltlon absorption of the ferrlcyanlde ion causes liberation of a CN- ligand. The llberated active CN- acts as the reductant and reduces a coexisting Fe3+to Fez+. The reduced Fez+ reacts wlth Fe(CN):and forms Prussian blue.
In qualitative analysis, ferricyanide and ferrocyanide ions are the most popular reagenta to detect ferrous and ferric ions, respectively (1, 2). According to the greater sensitivity in coloration on filter paper than in a test tube, the coloration of Prussian blue or turnbull's blue has often been utilized as the spot test (3). It has been shown that the compositions of Prussian blue and turnbull's blue were identical by means of X-ray diffraction pattern (4)and electron diffraction pattern (5) methods. I t has also been reported that these precipitates have three-dimensional lattice structure of a face-centered cubic unit cell (6). On the other hand, a ferric ion forms a yellowish brown soluble species with a ferricyanide ion in aqueous solution. I t has been reported that in some cases ferric ferricyanide formed on filter paper shows blue coloration which sometimes interrupts the spot test analysis of the ferric ion. It has been explained that the phenomenon was based on the reduction of ferric ferricyanide to Prussian blue by the cellulose of the filter paper (2). This concept was supported by the fact that permanganate ions were reduced rapidly to manganese oxide on fiiter paper (7,8). On the other hand, it has been observed that yellowish brown ferric ferricyanides formed on a gold plate changed color to green and finally to blue when the plate was dipped in aqueous solution. The change of the color was explained as the decomposition of the complex ion with the solvent (9). In these years, Prussian blue and its analogous compounds have been investigated with a great deal of interest placed on their characteristic electrochromism, the mixed valence electron transfer, etc. (10-13). However, there are few investigations on the effect of light on the stability of these compounds. Recently it was observed in our laboratory that ferric ferricyanide formed on glass-fiber filter paper (GFFP) also changed color gradually from light brown to blue upon light illumination. This phenomenon was studied by means of photoacoustic spectroscopy (PAS), because it has been known that absorption spectra of opaque species could be observed by the PAS technique (14716). In this paper, the photoacoustic spectra of Prussian blue and its related compounds are shown, and the mechanism on the color change of ferric ferricyanide on GFFP upon light illumination is discussed.
EXPERIMENTAL SECTION PAS System. The block diagram of a handmade photoacoustic spectrometer is shown in Figure 1. The light source (L) is a 300-W xenon arc lamp (Varian VIX-300-F) which is modulated by a chopper (B) and then dispersed by a Czerny-Turner-type monochromator (M) (JASCO CT-10s; focal length 100 mm and relative aperture F:3). The modulation frequency of the light is 108 Hz, and the spectral band width is 15 nm at 700-nm light. Selection and movement of wavelength is controlled by a microcomputer system (CAS10 FP1100) and stepping motor driver (T) through an interface circuit. The sample cell in shown in Figure 2. Two small microphones (Sony ECM 150T) are used as acoustic detectors for the sample cell and the reference cell. Each photoacoustic signal is amplified by a lock-in amplifier (NF LI-570) and is digitized by an ADC circuit. Chemicals. Analytical-grade chemical reagents were used without further purification. The solution of potassium ferricyanide was prepared freshly before the use. Preparation of Samples. The samples for PAS were prepared as follows. One drop of a sample solution was spotted on a filter paper (cellulose or glass fiber) in a dark room, and the paper was dried for 5 min in a brown desiccator containing concentrated sulfuric acid. Precipitant solution was dropped just on the spot, and the paper was dried again for 2-3 h in the desiccator. A disk of 5 mm diameter surrounding the spot was cut from the filter paper as the sample. Measurement of Photoacoustic Spectra. The photoacoustic spectra of the sample and a reference carbon black were measured over the wavelength range from 800 to 350 nm. It takes 8 min to measure the PA spectra at 5-nm intervals over the wavelength region. By dividing the photoacoustic signal of the sample by that of the reference, the fluctuation of light intensity can be corrected. After each measurement, necessary correction treatments were carried out. Whenever light illumination is necessary, monochromic light can be irradiated into the sample cell. The total energy of the illuminated light may be adjusted to a constant by control of the light source independent of wavelength. RESULTS AND DISCUSSION Photoacoustic Spectra of Prussian Blue. The photoacoustic spectra of the Prussian blue and turnbull's blue formed on a filter paper are shown in Figure 3. The broad band at 700 nm is supposed to be the characteristic absorption of precipitate Prussian blue (or turnbull's blue). This band has been ascribed to the charge-transfer band of the F e W N-Fe"' structure expanded three dimensionally (6). The photoacoustic band corresponds well to the band obtained by a reflectance spectrometer (17, 18). The peak at 420 nm is observed in a higher concentration precipitate. This peak is ascribed to the excess Fe(CN)63unreacted because the absorption wavelength of this peak is identical with that found for K3Fe(CN), alone as shown in Figure 4. The photoacoustic spectra of K,Fe(CN)6 is also shown in Figure 4. Although the peak at 420 nm in Figure 3b is ascribed to the residual Fe(CN)63-,the peak may be also found in a case of mixing Fe3+ and Fe(CN)64-. It may be reasonable to consider that just after addition of ferric ion solution to the spot of ferrocyanide ions on the filter paper, the reaction Fe3+ + Fe(CN)64-
0003-2700/84/0356-2810$01.50/00 1984 American Chemical Society
-
Fe2+ + Fe(CN)63-
(1)
2820
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Figure 1. Block diagram of the photoacoustic spectrometer: L, xenon arc lamp; B, chopper; M, monochromator; N, mirror; R, reference cell; S, sample cell; P, preamplifier; As, Ar, lock-in ampllfier; C, micro-
'...
computer; T, stepping motor driver; and V; CRT/printer.
.'.........,
,
€60 '
500 WAVELENGTH
LOO
,
.... .. .....'....', ' ' .(
700 / nm
'
800
Flgure 4. Photoacoustic spectra of (a) 0.2 M K,Fe(CN), and (b) 0.2 M K,Fe(CN),.
-\e
I
L 4 6.9
Figure 2. Schematic illustration of the sample cell: a, brass cover; b, aluminum cyllnder; c, O-ring seal: d, fuzed quartz wlndow; e, paper sample tray; f , brass body; and g, microphone.
l
400
I l
I i
I l
I l
I l
I l
500 600 700 WAVELENGTH /nm
I l
i
800
Figure 3. Photoacoustic spectra of Prussian blue and turnbull's blue: (a)0.2 M FeCI, + 0.2 M K,Fe(CN),, (b) 0.2 M FeCI, 4- 0.2 M K,Fe(CN),, (c) 0.02 M FeCI, + 0.02 M K3Fe(CN),.
precedes the Prussian blue formation reaction. The reaction in eq 1occurs spontaneously in aqueous solution because the standard redox potential of the Fe2+/Fe3+system is much more positive than that of Fe(CN)6k/Fe(CN)6s. The value of the equilibrium constant for eq 1 has been obtained as 8.7 x IO6by Weiser et al. (5). The spectra of these precipitates were stable against light illumination, and changes in the spectra were not observed. Photochemical Coloration of Ferric Ferricyanide. When 1drop of a solution of Fe3+and Fe(CN)z- were mixed on filter paper, light brown precipitates were formed at first but the color changed to blue (final color) through the color
1
green (intermediate color) upon light illumination. The color change, however, was negligible without illumination. Three repetitive scans of photoacoustic spectra of ferric ferricyanide on cellulose filter paper are shown in Figure 5. It is obvious that color changes are accelerated by light illumination. The photoacoustic spectrum after sufficient light illumination becomes the same as that of Prussian blue, implying that ferric ferricyanide should be reduced to ferrous ferricyanide or ferric ferrocyanide upon light illumination. It has been pointed out that the cellulose of the filter paper acts often as a reductant (2, 3). In order to check the mechanism, the following experiments were carried out from another point of view. It is well-known that Fe2+reacts with o-phenanthroline and forms a red complex (19),but Fe3+does not show red coloration with o-phenanthroline. When Fe3+ and o-phenanthroline were mixed on filter paper, the spot of the mixture was pale yellow and changed to red upon light illumination. However, the photochemical coloration did not proceed on a glass-fiber filter paper (GFFP). It is known that GFFP has no reducing ability. Therefore, it may be concluded that Fe3+ ions were reduced to Fe2+ions by the cellulose on the paper. On the other hand, it was observed that a light brown precipitate of ferric ferricyanide on a GFFP changed color to green, although the change was much slower than on cellulose filter paper. It is supposed that the ferric ferricyanide itself decomposed and formed Prussian blue upon light illumination. The following five reaction processes can be proposed for the photochemical reaction. Fe3+
+ Fe"'(CN):-
-% by the filter paper
-
Fe111(CN)63-
hu
-
Prussian blue (2-1)
-
-
[Fe111(CN)63-]* Fe11(CN)64-
+
Fe11(CN)64- Fe3+
Prussian blue
(2-2)
ANALYTICAL CHEMISTRY, VOL. 56,NO. 14, DECEMBER 1984
Fe3+
2821
+ FelI1(CN)$- 2Fe3+ + [Fe"'(CN)$-]* Fe2+
Fe3f Fe2+ Fe3+
-
+ Fe111(CN)63-
-Fe2++
Prussian blue
[Fe3+]*
+ Fe111(CN)63-
,
( )
(2-3)
Fez+
Prussian blue
(2-4)
-
+ Fe"'(CN)$- 2 [Fe3+]* + Fe"'(CN)$( ) + Fe"(CN):Fe"(CN):+ Fe3+ Prussian blue (2-5)
-
In the above equations, [ ]* denotes a species activated by the absorbing of light energy and ( ) denotes an unknown species to which an activated species is converted by the electrontransfer reaction. The reaction process in eq 2-1 may be eliminated because GFFP is used. The Fe"'(CN)63- ions in eq 2-2 and 2-3 and Fe3+ ions in eq 2-4 and 2-5 are the photoactive centers. Equations 2-2 and 2-4 show that the activated species undergoes self-decomposition and converts to a reductant form. This product reacts with the partner and forms the Prussian blue. Equations 2-3 and 2-5 show that the activated species reduce their partner to its reductant form. This product reacts with the residual particle of the partner species and forms the Prussian blue. In order to assert which of eq 2-2, 2-3, 2-4, and 2-5 is the true reaction, the following series of experiments were performed. Of FeC1, solution (or K3Fe1I'(CN),) 1 drop was spotted on a GFFP and illuminated with light at 400 nm for 40 min after drying. Then 1 drop of the partner solution K3Fe"1(CN)6(or FeC13) was added on the spot. The following results were obtained.
FeC13 added
K3Fe(CN), + hv(400 nm, 40 min) color change (yellow FeCl,
+ hv(400 nm, 40 min)
green) (3-1)
K,Fe(CN)B added
no color change (yellow) (3-2) The photoacoustic spectra of the GFFP spots are shown in Figure 6. In Figure 6b, the broad band is found at 700 nm in the absorption curve when Fe3+ was added to the K3Fe(CN), illuminated spot. This band is the same as that of Prussian blue. It may be concluded from this result that the photoactive center is Fe111(CN)63and not Fe3+. Therefore, reaction processes 2-4 and 2-5 are denied. It is known that ammonium molybdate reacts with Fe"(CN)64-selectively and forms a red precipitate under acidic condition but does not react with Fe111(CN)63-.After light illumination of 400 nm on the Fe111(CN)63-spot for 40 min, addition of acidic ammonium molybdate (pH 2) caused no change of color on the GFFP. On the other hand, in the case of the Fe11(CN)64-spot, the yellow color of the spot changed to red by addition of molybdate solution. K4Fe(CN)6 + (NH4),Mo04 color change (colorless red) (4-1)
-
K3Fe(CN)6
- -
+ hv(400 nm, 40 min) + (NH4),Mo04
no change (yellow) (4-2) If the reaction process in eq 2-2 is correct, the activated [Fe111(CN)B3-]* itself decomposes and forms the Fe11(CN)64species. In this case, a change of the color ought to be observed after addition of molybdate solution. No change of color was observed; therefore, it is concluded that the reaction process in eq 2-3 should be valid. Relationship between the Wavelength of the Illuminating Light and the Photochemical Coloration Reaction. The photochemical reaction process, eq 2-3, may be the most
YI 400
500 600 700 WAVELENGTH/nrn
800
Figure 6. Photoacoustic spectra of ferric ferricyanide prepared by two different ways upon GFFP (a)after light illumination (400 nm, 40 min) to 0.2 M FeCI,, 0.2 M K,Fe(CN), is added, (b) after light illumination (400 nm, 40 mln) to 0.2 M K,Fe(CN),, 0.2 M FeCI, is added.
W
I
+ Z I
l0
400 i W A 500 V 'E L E N 600 GTH
/700nrn
800
Figure 7. Relationship of the rate of photochemical coloration and an illuminating light wavelength. As a measure of the former, the intensity difference of the 700-nm photoacoustic band between before and after light illumlnatlon for 100 mln was used.
probable one for the reduction of ferric ferricyanide. In order to determine how photoactivated ferricyanide reduces the ferric ion to ferrous ion, the effect of the illuminating wavelength on the rate of photochemical reaction was investigated. Intensity differences of the photoacoustic bands at 700 nm, between after light illumination for 100 min and before the illumination, were measured. The results, shown in Figure 7, indicate that the intensity difference depends upon the wavelength of the illuminating light. The longer the light wavelength, the larger the intensity difference. That is to say that longer wavelength light affects the coloration more strongly. The absorption of 800-nm light by ferric ferricyanide is ascribed to the d-d* transition. As a rule, in photochemical reactions, when a complex absorbs a light the charge-transfer absorption causes a redox decomposition of the complex. The d-d* transition causes a liberation reaction or a substitution reaction of the ligands in the complex (20). The following mechanisms may be proposed:
- + - + -
Fe(CN)63- + hv [Fe(CN),3-]*
[Fe(CN),S-]* (5-1)
Fe(CN)52- CN-* (5-2)
+ CN-* Fe2+ 1/2(CN)2(5-3) Fe2+ + Fe(CN),3Prussian blue (5-4) Fe3+
In the above sequence of reactions, the activated [Fe(cN)e3-]*liberates CN-* which acts as a reducing agent. The product Fe2+and the residual Fe(CN)63-react together and form Prussian blue. The color of the spot turns from brown to green, accordingly. If this reaction mechanism is correct, it then explains the fact that molybdate solution added to the ferricyanide ion spot after 400-nm light illumination caused no observable color change. The photochemical reaction product is Fe(CN)52-,
2822
Anal. Chem. 1984, 56,2822-2825
not Fe(CN),", and the former does not react with molybdate ion so that no change of color is expected. In this case, it is suggested that prussian blue is formed by the reaction of Fe3+ with Fe(CN)63-as shown in eq 5-1-5-4. In Figure 5, an isosbestic point was observed near 520 nm, indicating the presence of an equilibrium reaction in the system. It is interesting that such a reaction is proceeding at the surface region of the spot under light illumination. This fact implies that the photoacoustic spectroscopy is a useful tool to trace and investigate photochemical and surface reaction.
ACKNOWLEDGMENT We thank S. Ikeda and I. Watanabe of Osaka University and Y. Yokoyama of Technological University of Nagaoka for the useful suggestions regarding the assembly of the photoacoustic spectrometer. Registry No. K,Fe(CN),, 13746-66-2;K4Fe(CN)G,13943-58-3; FeCl,, 7705-08-0; FeC12, 7758-94-3; Prussian Blue, 12240-15-2; Turnbull's Blue, 65505-26-2; ferric ferricyanide, 14433-93-3. LITERATURE CITED (1) Treadwell, F. P. "Analytical Chemlstry"; (translated by Hall, W. T.) Wiley: New York, 1935; Vol. 1.
Takagi, S."Quantitative Analysis"; Nankodo: Tokyo, 1981: Vol. 11; p 345. Feigl, F. "Chemistry of Specific, Selective and Sensitive Reactions"; Academic Press: New York, 1949. Levi G. Chim. Ind. Appl. 1925, 7, 410. Weiser, H. B.; Miliigan, W. 0.; Bates, J. B. J . Phys. Cbm. 1942, 46, 99. Ludi, A.; Gudel, H. U. Struct. Bonding (Berlin) 1973, 14, 1. Felgl, F. "Spot Tests in Inorganic Analysis"; Elsevier: Amsterdam, 1958. Feigl, F; Suter, H. A. Chemlst-Analyst 1943, 32,4. Mortlmer, R. J.; Rosseinsky, D. R. J. Electroanal. Chem. 1983, 151, 133. Ellis, D.; Eckhoff, M.; Neff, V. D. J. Phys. Chem. 1081, 85, 1225. Itaya, K.; Shibayama, K.; Akahoshi, H.; Toshima, S. J . Appl. Phys. 1982, 53, 804. Itaya, K.; Akahoshi, H.; Toshima, S.J. Nectrochem. SOC. 1982, 129, 1498. Itaya, K.; Ataka, T.; Toshima, S. J . Am. Chem. SOC. 1982, 104, 4767. Kawamoto, S.;Yokoyama, Y.; Ikeda, S.Bull. Chem. SOC.Jpn. 1980, 53, 391. Ikeda, S.; Murakami, Y.; Akatsuka, K. Chem. Lett. 1981, 363. Rosencwalg, A.; Hall, S.S.Anal. Chem. 1975, 47,548. Robin, M. B. Inorg. Chem. 1062, 1 , 337. Robin, M. B.; Day, P. Adv. Inorg . Chem. Radiochem. 1987, 10, 247. Wenger, P.;Duckert, R. Helv. Chim. Acta 1044, 27, 757. Voglar, A.; Adamson, A. W. J. Phys. Chem. 1070, 74,67.
RECEIVED for review April 25,1984. Accepted July 30, 1984.
Solute- Induced Circular Dichroism: Complexation of Achiral Drugs with Cyclodextrin Soon M. Han and Neil Purdie* Chemistry Department, Oklahoma State University, Stillwater, Oklahoma 74078
The formation constants for the association of eight achiral drug molecules with P-cyclodextrln in aqueous media have been determined by using data from the induced circular dichroism spectra for the drugs. The data have also been used in the determlnatlon of meperidine (Demerol) In a dlspensary product.
The ability of @-cyclodextrinto act as a host in complexing a wide variety of other molecules in aqueous solutions has been recognized for many years and recently reviewed ( I , 2). The nature of the interaction is fairly well understood, and the process has been successfully applied to enhance the aqueous solubility of organic molecules (I,2), as an enzymatic model system (3),and to effect the chromatographic separation of structural isomers (2, 4 ) . The interactions have been the subject of a number of thermodynamic studies ( I , 2 , 5 , 6 ) and at least one indepth theoretical study (7). From this wealth of information it is understood that the center of the cyclic oligosaccharide is hydrophobic, making it accessible to nonpolar, usually aromatic, molecular moieties. It is generally believed also that the structure of the guest molecule external to the interaction site has little to no influence on the process or the physical properties of the complex (1-3). As a potential analytical reagent @-cyclodextrinand its aqueous solutions are stable, although the solubility is somewhat limited. It is available in relatively high purity and
is inexpensive. The property that we wished to exploit in this work is the ability of the molecule to induce chirality into an achiral guest molecule. If the guest contains a chromophore, the interaction produces a complex which will give a circular dichroism (CD) spectrum. The spectra could be used for drug identification and as a method to determine the formation constants for the complexation equilibria. Accordingly CD could be used in the determination of achiral compounds ((0. Examples of induced CD activity were previously reported from this laboratory using first a cholesteric liquid crystalline solvent (9),which could not be exploited for quantitative studies, and second for L-cocaine and phencyclidine (PCP) using @-cyclodextrin(10). In this work the complexation with PCP has been reexamined, and seven other inherently achiral drug molecules are included for comparison.
EXPERIMENTAL SECTION P-Cyclodextrin was obtained from Eastman Kodak and used without further purification. The eight guest molecules were PCP and the pyrrolidine (PCPy) and morpholine (PCM) hydrochloride analogues (Applied Science),P-phenethylamine and phenobarbital (Sigma Chemical Co.), meperidine hydrochloride (Sterling-Winthrop), and diazepam and dilantin (Drug Enforcement Administration). Solution concentrations of the guest compounds ranged from 5X M to be consistent with a solution absorbance to which is within the dynamic range of the CD instrument. The first five compounds were dissolved in distilled water. Diazepam was dissolved in 0.1 M hydrochloric acid and dilantin and meperidine in 0.1 M sodium hydroxide because of their limited solubilities in water. @-Cyclodextrinis stable in dilute base but is
0003-2700/84/0356-2822$01.50/00 1984 American Chemlcal Society
