1444
Anal. Chem. 1986, 58, 1444-1448
Effect of Nonionic Surfactant Micelles on the Formation of Beryllium Complexes with Chromazurol S and Chromazurol B and Their Use in the Spectrophotometric Determination of Beryllium Kenjiro Hayashi,* Yoshiaki Sasaki, Shoji Tagashira, and Eiji Kosaka Department of Chemistry, Faculty of Science, Yamaguchi University, Yoshida, Yamaguchi 753, Japan
micellar effects on the formation of sensitive complexes such as Be(CAS)2 are not well-understood, The electrostatic interaction, which is characteristic of only ionic surfactants, will be negligible in nonionic micellar systems. The unique properties exhibited by surfactant micelles relate to the solubilization and location of reagents in the systems (3). According to the classical Hartley model, nonionic micelles have a hydrophobic core and a hydrophilic mantle consisting of hydrated polyoxyethylene chains (9, 10). The triphenylmethane dyes have been considered to interact with these micelles, and spectra of the dyes have been changed in the micellar solutions (11, 12). On the other hand, we have investigated micellar solubilization of chelating reagents and metal chelates using spectrophotometric and kinetic methods for analytical purpose (13-15). The micellar and water phases were postulated in the surfactant solutions (16, 17), and solubilization of the reagents was determined in a manner similar to liquid-liquid extraction systems. The triphenylmethane dye CAB, which only differs from CAS by the absence of a sulfonic group, forms a water-insoluble complex with beryllium; however, the dye-rich complex Triphenylmethane dyes such as Chromazurol S (3-sulfoBe(CAB)2was formed in the micellar solutions. Hence, the 2,6-dichloro-3’,3”-dim~thyl-4’-hydroxyfu~hson-5’,5’’-dicar-partitioning of CAS, CAB, and their beryllium complexes was boxylic acid, CAS) react with many metal ions to form investigated between the micellar and water phases in several water-soluble, colored complexes. They have been used as nonionic micellar solutions. The formation of beryllium-CAS spectrophotometric reagents (1,2). The addition of surfaccomplexes was confirmed in various media. Evaluation of the tants frequently causes a bathochromic shift and increases enhanced local concentrations of the dyes enables us to explain the sensitivity of metal analysis along with the formation of the formation of dye-rich complexes in micellar systems. a dye-rich complex. The ratio of metal to dye changes from These complexes exhibiting high molar absorptivities were 1:l in an aqueous solution to 1:2 or 1:3 in surfactant-containing used for the spectrophotometric determination of beryllium solutions. The reviews of these beneficial effects of surfactants in the Triton X-100 micellar system. have been given by Hinze (3) and Cermakova (4),recently. EXPERIMENTAL SECTION These surfactants will form micelles when their concentration exceeds the critical micellar concentration (cmc). They Reagents. Water that had been deionized and distilled was used in the preparation of all solutions. A standard beryllium(I1) can then interact with dye and/or the metal-dye complex as solution was prepared by dissolving 8.9 g of beryllium sulfate an individual molecule or aggregates. For example, cationic tetrahydrate (Wako Pure Chemicals) in 250 mL of 2 M sulfuric surfactants, such as Zephiramine, react by ion pair formation acid. Water was added to make a standard 1-L solution. This with the anionic Be(CAS)2to form a ternary complex involving stock solution was standardized gravimetrically as beryllium oxide surfactant monomers ( 5 ) , and the ternary complex with (5.290 X M). A working solution was prepared by appropriate hexadecyltrimethylammonium cations was extractable into dilution of the stock solution. Both CAS and CAB (Dozin 1-butanol (6). Recently, Callahan and Cook (7,8) investigated Chemical Laboratory) were used without further purification. the surfactant-induced changes in the visible spectrometry Though most of the commercial preparations of the dyes were of the beryllium complex with CAS. Increases in molar abgenerally pure (18,19),the spectra of these reagents were tested by comparing with recrystallized reagents. The same spectra were sorptivity and bathochromic shifts in the wavelength of obtained. The solutions were prepared by dissolving a weighed maximum absorbance were observed in the cationic surfactant amount in water and used within 3 days. The nonionic surfac(hexadecyltrimethylammonium bromide) solution under the tants, Triton X-100 (Wako Pure Chemicals),Emulgen 120, and cmc, whose value was determined by measuring the surface Brij 58 (Kao Atlas Co.) were used without further purification. tension. The spectral changes were observed only a t the Their concentration was calculated assuming an average molecular surfactant concentration above the cmc in the case of the weight of 625 for Triton X-100,759 for Emulgen 120, and 1124 nonionic surfactant (Triton X-100) solution. Therefore, they for Brij 58. A pyridine buffer solution was prepared from 250 assumed that both the ternary complex formation and micellar mL of 4 M pyridine. Its pH was adjusted to 5.40 with 3 M interaction were observed with the cationic surfactant system hydrochloric acid (pH meter control). Reagent grade sodium and strictly micellar interaction was involved in the case of chloride was used to maintain a constant ionic strength ( I = 0.1). All other chemicals were analytical grade reagents. the nonionic surfactant system. However, the mechanism of The triphenylmethane dyes, Chromarurol S (CAS) and Chromarurol B (CAB), react with berylllum to form the dye-rlch complexes Be(CAS), and Be(CAB), In nonlonlc mlcellar solutlons. The partition equlllbrla of the dyes and the complexes were spectrophotometrically studled, and the partltlon coefflclents between the mlceliar and water phases were evaluated In Trlton X-100, Emulgen 120, and Brlj 58 mlcellar systems. The mlcellar effects, Le., a local Increase In the dye concentratlon In the mlcelles, contribute to the formatlon of the dye-rlch complexes. Formatlon constants of the berylAt a low lium-CAS complex are p, = 104.78and f& = IO“” CAS concentratlon, the formatlon of Be(CAS), Is negllglble In aqueous solutions. The molar absorptlvltles of the dye-rlch complexes are about 4 times larger than those of the 1:l complexes. The spectrophotometrlc determlnatlon of beryllium uslng the hlgh molar absorptivltles of these complexes was performed In the Triton X-100 micellar system.
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0003-2700/86/03581444$0 1.50/0 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
and Binding
Table I. Molar Volume of Micelles ($m), Extraction Constants (IC,), Partition Coefficients Constants (ICbina) in Nonionic Micellar Systems
1.29 1.72 2.74
Triton X-100 Emulgen 120 Brij 58
12.76 12.78 12.84
5.56 5.60 5.65
15.48 15.50 15.48
3.1, 3.13 3.18
1445
4.04 4.18 4.10
360 40, 380" 530 d~ 40, 530" 960 f 50, 950," 920b
"Calculated from the values of partition coefficients (Pm,H2L) of CAS using eq 16. bReference 12. Apparatus. All spectrophotometric measurements were made with a Shimadzu Model UV-240 recording spectrophotometer. Most measurements were made by using a 1-cm-path-length quartz cell. When an absorbance exceeded 2.0 in the 1-cm cell, a 1-mm-path-length quartz cell was used. All partition measurements were carried out at a constant temperature of 25 f 0.2 OC. All pH measurements were made with a Toa type HM-5A pH meter and combination glass electrode. The electrode was calibrated with standard potassium hydrogen phthalate and phosphate buffer solutions. Quantification of Beryllium. A sample solution containing beryllium(I1) ion was transferred to a 10-mL ground-glass-stoppered test tube. One milliliter of 2.4 X lo-, M Triton X-100,1.5 M CAB, and 0.2 mL of 2.5 X M CAS or 1.0 mL of 1.3 X mL of pyridine buffer were successively added. The sample was diluted to the mark with water. After 20 min, the absorbance at 605 nm for CAS or at 608 nm for the CAB system was measured against a reagent blank. Derivation of the Partition Equations. The tetrabasic acid of CAS (H4L)and tribasic acid of CAB (H3L)are similar in their structures except for the sulfonic group. The partition equations of CAS are given below. For CAB, the same equations were used. The distribution ratio of CAS between the micellar and water phases is defined as
The subscripts m and w refer to the micellar and water phases, respectively. The acid dissociation constants obtained by spectrophotometric methods were pK, C 0, pK2 = 2.30, pK3 = 4.86, and pK4 = 11.72 in the aqueous solution (reported values are -1.2, 2.25, 4.88, and 11.75, respectively (20)). In general, the acid dissociation constants measured in the micellar solutions differ somewhat from those in the aqueous solution. The values pK, < 0, pK, = 2.94, pK3 = 5.77, and pK4 = 11.39 were obtained in the 0.15% Triton X-100 solution. Underwood (21)discussed the intrinsic pK, values in the sodium dodecyl sulfate micellar systems based on the electrostatic interactions. In nonionic micellar systems, we considered that the difference of solubilities between dissociated and associated species of reagents within micelles seemingly changes the pK, values, and acid-base equilibria of reagents in the water phase may be constant except near the micellar surface. Therefore, the acid dissociation constants measured in the aqueous solution were used for the K, values in the water phase in the micellar systems. At the pH 5-6 used in this study, CAS exists in the H2L2-and HL3- forms, which signify a species involving one and two dissociated carboxyl groups, respectively. The existence of H3L- and L4- can be neglected (
