Spectrophotometric study of the reactions of bromopyrogallol red with cationogenic tensides: Determination of antimony

MICROCHEMICAI.

JOLIRNAI.

30, 39-46

(1984)

Spectrophotometric Study of the Reactions of Bromopyrogallol Red with Cationogenic Tensides: Determination of Antimony 1. NBMCOVA, L)epartment

J. HRACHOVSKA,

J. MIKOVA,

AND

V. SUK

of Analytical Chemistry, Faculty of Natural Sciences, Charles University, Albertov 2030, 128 40 Prague 2, Czechoslovakia Received April 30, 1982

INTRODUCTION Bromopyrogallol red (5,5’-dibromopyrogallolsulfogalein, DG) was first used as a metallochromic indicator (7, 12) and later was widely used in spectrophotometry (8). It was found that this reagent yields complexes in acid medium with trivalent and quadrivalent cations and in alkaline medium with divalent cations. It was later noted that, in the presence of gelatine (10, 13) and especially in variously cationogenic tensides (3, 5, 6, I4), a marked change occurs in the absorption spectra of a number of these complexes, similarly as for complexes with other dyes of the same type (4). If bathochromic and hyperchromic shifts of the absorption bands used for the spectrophotometric determination occur, then the sensitivity of the determination is considerably increased. Simultaneously, a shift in the optimal pH value for complex formation was observed in a number of cases. It is well known that changes in the absorption spectra of complexes and shifts of the optimal pH values for complex formation result primarily from interactions of the tenside employed with the dye molecules. Consequently, this work considers the effect of several cationogenic tensides on the absorption spectra and dissociation constants of bromopyrogallol red. The favorable effect of cationogenic tensides e.g., cetylpyridinium bromide (CPB) was then utilized for development of new method for the spectrophotometric determination of antimony with bromopyrogallol red. This reagent has already been used for the determination of antimony by Christopher and West (f), who carried out photometric measurements on the binary Sb(III)-DG system at pH 6.6-6.8. In the ternary Sb(III)-DGCPB system studied here, the optimal pH value for complex formation is 0.9- 1.6, which considerably increases the selectivity of this determination. 39 0026-265X184 Cowwhf All rghrs

B1.SO

0 1984 by Academic Press. Inc. of reproducrmn in any form reserved.

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EXPERIMENTAL Instruments. The spectrophotometric measurements were carried out on a Unicam SP-800 spectrophotometer (Pye-Unicam Instruments Ltd., Cambridge, England) with l.OO-cm quartz cuvettes. The pH was measured on a PHM-52 pH-meter (Radiometer, Copenhagen, Denmark) with glass and saturated calomel electrodes. Solutions. The standard 0.1 M Sb(II1) solution was prepared by dissolving the required amount of SbCl, (Lachema, Czechoslovakia) in 3 M HCl. The metal content was controlled manganometrically. The stock solution of 2 x 10e4 M bromopyrogallol red was prepared by grinding the required amount of purified substance’ with ethanol, transferring it to a volumetric flask, and diluting with distilled water to a final ethanol content of 20%. The solution must be stored in a sealed vessel and must be less than 2 days old. The solutions of 1 x 10e2 M carbethoxypentadecyltrimethylammonium bromide (Spofa, Czechoslovakia) and of cetyltrimethylammonium bromide (Spolana, Czechoslovakia) were prepared by dissolving the required amounts of the substances in distilled water. The solution of 1 x 10 -2 M cetylpyridinium bromide (Spolana, Czechoslovakia) was prepared by dissolving the substance in 20% methanol. The pH was adjusted using Clark-Lubbs, McIlvaine, Walpole, Sorensen, and Kolthoff-Vleeschhouwer buffers, prepared according to instructions (2). The ionic strength was adjusted using a 2 M KC1 solution. RESULTS AND DISCUSSION I. The Interaction

of Bromopyrogallol

Red with Cationogenic

Tensides

1 Purification of bromopyrogallol red (Lachema, Czechoslovakia): DG was dissolved in a Na,CO, solution, filtered, and precipitated with HCl (1:l). The precipitate was heated to 8O”C, filtered hot, washed several times with hot water, and dried at 100°C. It was recrystallized several times from ethanol and dried to constant weight.

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The bromopyrogallol red molecule, H,DG (I), contains four dissociabled hydrogens; the four color transitions correspond to stepwise dissociation of these hydrogens. Protonation with formation of the H,DG+ cation, which does not produce a color change, can be assumed to take place in concentrated strong acids. A dissociation scheme for bromopyrogall01 red was propsed by Suk, who also found the individual dissociation constants (II): pK, = 0.16 pK = 9.13 pK, = 4.32 H,DG + + H,DG B H,DG - + H,DG2 - W pK, = 11.27 HDG3- \ ’ DG4In this work the absorption spectra of bromopyrogallol red were measured in the presence of the cationogenic tensides cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPB), and carboxypentadecyltrimethylammonium bromide (Septonex) at various pH values. In alkaline solutions where DG solutions are rapidly oxidized by atmospheric oxygen with bleaching, these solutions were stabilized (at pH > 7) by addition of 0.4 ml of a 1% ascorbic acid solution to 50 ml of solution. The absorbance of these solutions is then constant for 20 min. The measurement was carried out at an ionic strength of 0.05. It was found that the shifts in the absorption maxima and absorbance changes produced by the individual tensides are very similar and consequently, further measurements were carried out with Septonex, which is available in a very pure form. The absorption spectra of the DG-Septonex system (coo = 2 x 10-j Mj CSept. = 5 x 10m4M) at various pH values are given in Figs. la-d. These spectra were compared with those of bromopyrogallol red given in the work by Suk (11), the individual absorption curves were assigned to the respective forms of DG and the effect of Septonex on these forms was noted. The presence of Septonex leads to bathochromic and hyperchromic shifts in the absorption maxima of the symmetric forms of DG, e.g., the H,DG*- anion has an absorbance maximum at a wavelength of 558 nm with a absorbance value of 1.0; in the presence of Septonex, the absorbance value is 1.2 at A,,, of 580 nm. Similarly, a shift of A,, from 598 to 630 nm was observed for the DG4- form. The dependences of the absorbance on the pH depicted in Fig. 2 for wavelengths of 580 and 550 nm were derived from the absorption spectra and the relationships given in Suk’s work (11) were used to calculate the apparent dissociation constants of bromopyrogallol red in the presence of Septonex. The pKi value, determinable in strongly acid media, was

NfiMCOVA

Wavelength

Wavelength

(nm)

(nm)

ET AL.

Wavelength

Wavelength

(nm)

d (nm)

FIG. 1. Absorption curves of bromopyrogallol red in the presence of Septonex. (a) (1) pH -0.5, (2) pH 0.5, (3) pH 1.4, (4) pH 2.4; (b) (1) pH 2.9, (2) pH 3.5, (3) pH 4.2, (4) pH 5.6; (c) (1) pH 7.6, (2) pH 8.4, (3) pH 8.8, (4) pH 9.2; (d) (1) pH 9.6, (2) pH 10.6, (3) pH 11.0, (4) pH 11.6. CDG = 2 x 1O-5 M; CSept. = 5 x 10m4M.

not found. The following values were determined: pKI = 3.01, pKI = 7.85, pK4 = 10.91. It is thus apparent that the interaction of cationogenic tensides with bromopyrogallol red increases the dissociation of all forms of this reagent. This tenside effect is decreased with increasing ionic strength of the solution (9). ZZ.Determination of Antimony with Bromopyrogallol Red in the Presence of Cetylpyridinium Bromide Absorption spectra and optimal reaction conditions. The absorption curves of the studied system and of its individual components in solutions

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8

lo

12

PH

Fig. 2. Dependence of the absorbance of bromopyrogallol red in the presence of Septonex on the pH value. (1) h = 580 nm, (2) A = 550 nm. CDG = 2 x lo-*M; ‘Sept. = 5 x 1O-4M.

with pH 1.2 are given in Fig. 3. It is apparent that the difference between the reagent absorbance (curve 1) and complex Sb(III)-DG absorbance (curve 2) is quite small and that the photometric determination of antimony under these conditions is not suitable. However, on addition of CTAB, Septonex, or CPB bathochromic and hyperchromic shifts appear which are largest for cetylpyridinium bromide, which was thus selected for further studies. Figure 3 depicts the absorption curve of DG in the presence of CPB (curve 3) and that of the Sb(III)-DG-CPB complex

0 400

500 Wavelength

600

700

(nm)

FIG. 3. Absorption curves of the Sb(III)-DG-CPB complex and its components. (1) DG; (2) Sb(II1) + DG; (3) DG + CPB; (4) Sb(II1) + DG + CPB; (5) differential curve (4) (3). CDG = 4 x 1O-5M; CSb(lII) = 2 x 10-5M; ‘CPB = 6 x 10-4M; pH 1.2.

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(curve 4). The greatest difference between curves 3 and 4 lies at 570 nm (curve 5); this wavelength was thus selected for further measurements. The effect of the pH on the formation and coloration of the complex was studied at pH O-3.5 (at higher pH values a different acid-base form of the indicator predominates). It was found that, in the absence of surface-active substances, the absorbance of a solution of the complex is constant at pH 1.8-2.2; in the presence of cetylpyridinium bromide the absorbance is constant at pH 0.9- 1.6. It was also found that at least a twofold excess of the reagent with respect to the metal concentration is necessary for attainment of maximum absorbance; this amount is independent of the presence of a surface-active substance in solution. The cetylpyridinium bromide must be present in at least a 15fold excess over the bromopyrogallol red concentration. Study of the time dependence of the complex coloration indicated that in the absence of tenside the maximal absorbance value is attained immediately after solution mixing, after which the absorbance slowly decreases; this decrease is 3% after 10 min and 14% after 1 hr. In the presence of CPB the absorbance value is practically constant for 10 min and decreases by 4.5% after 1 hr. Stoichiometric composition of the complex. It was found by the mole ratio method and by the Job method of continuous variations that antimony and bromopyrogallol red primarily form a complex with a component ratio of 1:1; the solution apparently also contains a complex with ratio 2:l. Calibration curve. The region of applicability of the Lambert-Beer law was found for analytical applications and it followed that the determination of antimony in the presence of CPB can be carried out in the range 0.40-3.60 kg/ml. The standard deviation of a single determination for a probability level of 85% in this region is at most 0.89% and for probability level 95% is at most 1.23%. The effect of other ions on the determination of antimony. It was found that the following ions do not interfere in the determination: Zn2+, Cd2+, Ca2+, Sr2+, Mn2+, SO:-, PO:-, NOj, halides, and CH,COO- up to a ratio of 1:1000, Fe2+, In3+, and UOZ’ up to a ratio of l:lOO, Cr3+, Co2+, Ni2+, and Cu2+ up to a ratio of 1:50, and Ce4+ and Ga3+ up to a ratio of 1:lO. Ions which also react with DG in the given medium interfere, e.g., Sn4+ Bi3+ Fe3+ A13+ Th4+. Prkeduie. An ‘acid iolution in a 25-ml volumetric flask containing a maximum of 90 pg Sb(II1) is mixed with 1.5 ml of a 1 x 10m2A4 CPB solution. The solution pH is adjusted using a 0.5 M sodium acetate solution and 2 drops of malachite green indicator (it was found that this indicator does not absorb in the wavelength region employed) to a green

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color of the solution (a reference solution of the same pH should be used). Then 10 ml of pH 1.2 buffer and 5 ml 2 x 10A4M DG are added and the solution is diluted with distilled water to the mark (strong shaking should be avoided as the solution foams markedly in the presence of the tenside) and the absorbance is measured immediately at 570 nm vs a blank.

SUMMARY The effect of cationogenic tensides on the absorption spectra and apparent dissociation constants of bromopyrogallol red was studied. The pK values for the individual dissociation steps in the presence of Septonex indicate that the presence of cationogenic tenside increases the dissociation of all forms of this dye, which is of practical importance in the formation of complexes with many metals. Spectrophotometric study of the reaction of antimony(II1) with bromopyrogallol red in the presence of cetylpyridinium bromide formed a basis for development of a new method for the spectrophotometric determination of antimony(II1) in the range 0.40-3.60 pg Sb . ml-‘.

REFERENCES 1. Christopher, D. H., and West, T. S., Spectrophotometric determination of antimony with bromopyrogallol red. Talanfa 13, 507-513 (1966).

2. Clhallk, J., Dvorak, J., and Suk, V., “pH Measurement Handbook.” SNTL, Prague, Czechoslovakia, 1975 (in Czech). 3. Deguchi, M., and Mamiya, T., Spectrophotometric determination of tungsten (IV) with bromopyrogallol red and zephiramine. Buns& Kagaku 25, 60-62 (1976). 4. Hinze, W. L. In “Solution Chemistry of Surfactants” (K. L. Mittal, ed.) Plenum, New York/London, 1979. 5. Hausenblasova, Z., Nemcova, I., and Suk, V., Spectrophotometric study of the reaction of titanium with bromopyrogallol red in the presence of cetylpyridinium bromide. Microchem. J. 26, 262-268 (1981). 6. JaroS, M., “Spectrophotometric study of complexes of molybdenum and tungsten with bromopyrogallol red.” thesis, Charles Univ., Prague, Czechoslovakia, 1972. 7. JenIEkova, A., Suk, V., and Malat, M., Komplexometrische Titrationen (Chelatometrie) XIX. Brompyrogallol rot als Komplexometrische Indikator. Col/ect. Czech. Chem. Commun. 21, 1257-1261 (1956). 8. Marczenko, Z., “Spectrophotometric Determination of the Elements.” HorwoodWiley, New York, 1976. 9. Matouskova, E., Nemcova, I., and Suk, V., The spectrophotometric determination of gold with bromopyrogallol red. Microchem. J. 25, 403-409 (1980). 10. Nemcova, I., “Bromopyrogallol red as a calorimetric reagent. Photometric determination of antimony, tin and titanium.” thesis, Charles Univ., Prague, Czechoslovakia, 1963.

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II. Suk, V., Chemical Indicators. VI. The acid-base properties of pyrogallol and bromopyrogallol red. Collect. Czech. Chem. Commun. 31, 3127-3139 (1966). 12. Suk, V., Malat, M., Kbrbl, J., and Nedeljak, M., “Several New Metallochromic Indicators for Complexometric Titrations,” p. 3. Chemapol, Prague, 1959. 13. Suk, V., Nemcova, I., and Malat, M., Bromopyrogallol red as a calorimetric reagent. II. Photometric determination of titanium. Collect. Czech. Chem. Commun. 30, 2538-2543 (1965). 14. Sucmanova-Vondrova, M., Havel, J., and Sommer, Z., Spectrophotometric study of the reaction of uranium (IV) with bromopyrogallol red and determination of uranium (VI). Collect. Czech. Chem. Commun. 42, 1812-1829 (1977).