Spectrophotometric determination of chromium(III) with bromopyrogallol red in the presence of Septonex

MICROCHEMICAL

31, 233-236 (1985)

JOURNAL

Spectrophotometric Bromopyrogallol

Determination of Chromium(lll) with Red in the Presence of Septonex

I. NI~MCOVA, J. HRACHOVSK&AND Department

of Analytical

V SUK

Chemistry, Charles University, 128 40 Prague 2, Czechoslovakia

Albertov

2030,

Received March 23, 1983 A new spectrophotometric method has been developed for the determination of chromium(II1) with bromopyrogallol red in the presence of the cationic tenside, carbethoxypentadecyltrimethylammonium bromide (Septonex) at pH 4.0 and within a concentration 0 1985 Academic Press, Inc. range from 0.1 to I.4 pg Cr(III) In-‘.

INTRODUCTION

Bromopyrogallol red (5,5’-dibromopyrogallolsulfophthalein, DG) is one of the dyes from the sulfophthalein series that has found wide application in the spectrophotometric determination of metals (5), after the original use in chelometry (4). The determinations have been improved, especially from the point of view of the sensitivity and sometimes also the selectivity, by employing ternary systems with gelatin (9), l,lO-phenanthroline (2), diphenylguanidine (7), and later cationic tensides (3) as the third component. The reaction of chromium(II1) with bromopyrogallol red has not yet been studied in the presence of tensides. The use of ternary systems involving other sulfophthalein dyes of this type, except for Eriochromcyanine R (ZO), for the determination of Cr(II1) has also not been published, even though these determinations are among the most sensitive ones. Therefore, we studied the reaction of Cr(II1) with DG in detail, with emphasis on the effect of the cationic tenside, carbethoxypentadecyltrimethylammonium bromide (Septonex) and developed a new spectrophotometric determination of chromium(II1) on this basis. EXPERIMENTAL Apparatus. The spectrophotometric measurements were carried out on an SP 800 instrument (Pye-Unicam Instruments Ltd., Cambridge, England), using l.OO-cm quartz cuvettes. The pH was measured using a PHM-52 pH-meter (Radiometer, Copenhagen, Denmark) with a glass and a saturated calomel electrode. Solutions. A 8 x lop4 M solution of bromopyrogallol red (Lachema, Brno, Czechoslovakia) was prepared by grinding the required amount of the purified substance (6) in ethanol, transferring to a volumetric flask and diluting with water to obtain a resultant ethanol content of 20% (v/v). The solution must be stored in a closed vessel and prepared fresh every 2 days. A solution of 2 x lop2 M’Cr(II1) was prepared by dissolving the required amount of Cr(NO,), * 9H20 in distilled water. The metal content was determined 233 0026-265X185$1SO Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved

234

Nl?MCOV&

HRACHOVSKA,

AND SUK

0.6

500

550

600 Wavelength

650 (nm)

7

FIG. 1. Absorption curves of the Cr(III)-DC binary complex (1) DC; (2) Cr(III)-DG; (3) difference curve: (2) - (1). Cm = 2 X 1O-5 M; Ccr(IIIJ = 1 x 10e5 M; pH 4.0, heated to 100°C.

gravimetrically as barium chromate. The working solutions with a concentration of 2 x 10e4 M were prepared by diluting this stock solution. A solution of 5 x lop3 M carbethoxypentadecyltrimethylammonium bromide (Septonex) (Spofa, Czechoslovakia) was prepared by dissolving the substance in distilled water. The acetate buffer solutions (Walpole) for adjustment of the solution pH were prepared according to the literature (I). RESULTS Absorption

AND DISCUSSION

Spectra

It is well known that Cr(III) ions form stable hydroxocomplexes in aqueous solutions, leading to very difficult formation of Cr(II1) chelates with organic dyes in most cases. We have found that the formation of a binary complex between Cr(II1) and bromopyrogallol red takes 2 days at laboratory temperature and in a weakly acidic medium. The reaction rate increases with increasing temperature; a maximum coloration of the complex is attained after 20 min at 60°C and after 5 min at 100°C. The absorption curve of a solution prepared in this way is given in Fig. 1 (curve 2). The absorbance maximum of bromopyrogallol red (curve 1) at 555 nm thus somewhat decreases on the formation of a complex with Cr(II1) and a very small maximum appears on the absorption curve at greater wavelengths. The difference curve (curve 3) exhibits a maximum at 610 nm that obviously corresponds to the Cr(III)-DG binary complex, but the absorbance values are very small. In the presence of gelatin (8), the absorption maximum of the solution shifts to greater wavelengths on a 30-min heating of the solution to 80°C and the absorbance increases; the difference curve has a maximum at 610 nm. In the presence of Septonex the heating of the solution (see procedure below) causes a change in the coloration of the Cr(III)-DG complex to a strong blue-purple; the maximum at 580 nm, corresponding to bromopyrogallol red in the presence of Septonex

SPECTROPHOTOMETRY

500

554

600 Wavelengrh

235

OF Cr(III)

650

7 (nm)

FIG. 2. Absorption curves of the Cr(III)-DG-Septonex ternary complex (1) DC + Septonex; (2) Cr(II1) + DG + Septonex; (3) difference curve: (2) - (1). C, = 2 x 10m5M; C,-,o,,, = I x 10e5 M; pH 4.0, heated to 100°C.

(Fig. 2, curve l), decreases and the absorption curve substantially broadens toward greater wavelengths (curve 2). The maximum of the difference curve (curve 3) lies at 640 nm and corresponds to a Cr(III)-DG-Septonex ternary complex; ail further measurements were carried out at this wavelength. Optimum Conditions for the Reaction The effect of the pH on the complex formation was studied from pH 2 to 5; it has been found that the absorbance is constant from pH 3.9 to 4.5 and then decreases. Therefore, the value, pH 4.0, was selected for the measurement. It has further been found that the maximum solution absorbance is attained when the excesses of DG and Septonex over the Cr(II1) concentration are 4- and 50fold, respectively; at lower Septonex concentrations, turbidity is formed in the solution and at higher concentrations the solution absorbance slightly decreases. It has been found when the effect of the temperature on the complex coloration was studied that, to attain the maximum coloration of the solution in the shortest possible time, the binary Cr(III)-DG complex should first be heated for 5 min on a boiling-water bath, then Septonex be added and the solution heated for another 5 min. The complex is then stable for 5 days. If all the components of the ternary complex are mixed simultaneously and only then is the solution heated, it is necessary to heat to 100°Cfor 20 min to obtain a maximum coloration and the absorbance is still about 25% lower. Stoichiometric Composition of the Complex It has been found by the Job method of continuous variations and by the method of molar ratios that chromium(II1) and bromopyrogallol red form a binary complex with a ratio of the components of 1:2; the same ratio has also been found in the presence of Septonex.

236 Calibration

Nl?MCOV/i,

HRACHOVSKA,

AND

SUK

Curve

Under the optimum conditions (X = 640 nm, pH 4.0; the solution is heated to 100°C) and at the concentrations, C,, = 8 x lop5 M and CSept,= 1 x 10e3 M, it has been found that the Lambert-Beer law is obeyed from 0.1 to 1.4 Fg Cr(II1) ml-‘. The relative standard deviation amounts to 0.65 to 4.2%. The sensitivity according to Sandell is S = 1.22 x lop3 pg cm-*. Interfering

Ions

No interference has been observed from ions Ca*+, Ba*+, Cd*+, SO:-, halides, and CH,COO- up to a ratio of l:lOOO, Zn*+, Mn*+, up to a ratio of 1:lOO. Even small excess of ions A13+, Ga3+, In3+, Fe3+, Sn4+, Ti4+, Zr4+, and Th4+ interferes, as do fluoride, tartrate, EDTA ions.

S?+, NO,, Co*+, Ni*+ Bi3+, Sb3+, oxalate, and

Procedure

An amount of a chromium(II1) solution is pipetted into a 50-ml volumetric flask, so that the resultant concentration does not exceed 70 pg. For pH adjustment of the solution to 4.0, methyl orange, whose color transition lies between pH 3.1 and 4.4, can be used (4 drops of a 0.01% solution of the indicator are added and the pH is adjusted with dilute ammonia or nitric acid to the required coloration corresponding to pH 4.0; the use of a reference solution with the same pH is most advantageous). Then 10 ml of the acetate buffer and 5 ml DG (8 x lop4 M) are added and the solution is heated to 100°C for 5 min. After the addition of 10 ml of 5 x low3 M Septonex solution, the solution is heated for another 5 min, cooled, and diluted with distilled water to the mark. The solution absorbance is measured at 640 nm against the blank solution. REFERENCES 1. Clhahk, J., Dvorak, J., and Suk, V., “pH Measurement Handbook.” SNTL, Prague, 1975. 2. Dagnall, R. M., and West, T. S., A selective and sensitive colour reaction for silver. Talanta 11, 1533-1.541 (1964). 3. Hinze, W. L., in “Solution Chemistry of Surfactants” (Mittal, K. L., ed.). Plenum, New York/ London, 1979. 4. Jenlckova, A., Suk, V., and Malat, M., Komplexometrische Titrationen (Chelatometrie). XIX. Brompyrogallol rot als komplexometrische Indikator. Collect. Czech. C/tern. Common. 21, 1257-1261 (1956). 5. Malat, M., “Absorption Inorganic Photometry.” Academia, Prague, 1973. 6. MatouSkova, E., Nemcova, I., and Suk, V., The spectrophotometric determination of gold with bromopyrogallol red. Microchem. J. 25, 403-409 (1980). 7. Shitareva, G. G., Poluektova, E. N., and Nazarenko, V. A., Bromopyrogallol red as a reagent for photometric determination of tellurium. Zh. Anal. Khim. 30, 1166-l 169 (1975). 8. Smetanova, M., Bromopyrogallol red as a calorimetric reagent. Thesis, Charles University, Prague, 1964. 9. 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). 10. Xia Dao - Pei, Spectrophotometric determination of chromium with Eriochrome cyanine R and cetyltrimethylammonium bromide. Fen Hsi Hua Hsue 9, 196- 197, 210 (1981); Chem. Abstr. 95, 143461(1981).