Fresenius J Anal Chem (1990) 338 : 738 - 740 Fresenius' Journal of

© Springer-Verlag 1990

Fluorimetrie determination of beryllium with 4-methyl-6-aeetyl-7-hydroxyeoumarin Hiroyuki Yoshida 1, Taknshi Ito 2, and Akira Murata 1

1 Faculty of Engineering, Shizuoka University, Johoku, Hamamatsu, 432, Japan 2 College of Engineering, Shizuoka University, Johoku, Hamamatsu, 432, Japan

Summary. The acid dissociation constant of 4-methyl-6- acetyl-7-hydroxycoumarin, as determined by spectropho- tometry was found to be K, = 9.77 × 10 -9. Beryllium reacts with this reagent to form a water-insoluble complex that can be extracted into benzene. The maximum wavelengths of the excitation and emission spectra of the beryllium complex in benzene are 403 and 465 nm, respectively. Beryllium can be determined in the range 0.005 ~ 0.1 ~tg per 10 ml benzene when extracted from the solution at pH 7.5 ~ 7.8 into benzene.

Introduction

Hydroxyflavones have been used as photometric and fluorimetric reagents for metallic ions [1]. Hydroxy derivatives of chromone have been investigated as analytical reagents in our laboratory, because chromone is similar in structure to flavone which has a phenyl group in the 2- position of chromone. We have previously reported on the spectrophotometric determinations of beryllium [2] and pal- ladium [3] with 5-hydroxychromone, the spectrofluorimetric determinations of beryllium [4, 5], scandium [6] and titanium [7] with alkyl derivatives of 5-hydroxychromone, and the spectrofluorimetric determinations of zirconium [8], tin (IV) [9] and hafnium [10] with 3-hydroxychromone. As a result of these studies, it was found that alkyl derivatives of 5- hydroxychromone, and 3-hydroxychromone are useful as spectrofluorimetric reagents for metallic ions.

Hydroxy derivatives of coumarin (which is a constitutional isomer of chromone) have also been used as photometric reagents [11], but have so far not been investi- gated as fluorimetric reagents for metallic ions. We studied 4,5-dihydroxycoumarin as analytical reagent for that purpose. Consequently, we found that many metallic ions form complexes with this reagent and about a half of these ions form fluorescent complexes, and we reported on the fluorimetric determination of beryllium with this reagent [12]. Successively, we studied coumarin derivatives, in which is inserted an acetyl group at an orthoposition of the hydroxy derivatives of 4-methylcoumarin as analytical reagents and

Offprint requests to: T. Ito

found that these reagents react with many metallic ions to form intensively fluorescent complexes. This paper describes the fluorimetric determination of beryllium with 4-methyl- 6-acetyl-7-hydroxycoumarin.

It is thought that hydroxy derivatives of coumarin as well as hydroxy derivatives of flavone and chromone can be used as reagents for that purpose.

Experimental

Reagents

0.1388 g of beryllium oxide (Johnson Matthey Chemicals, Specpure) was dissolved by heating with potassium disul- phate and 1.39 ml of cone. sulphuric acid. After cooling, the resulting solution was diluted to 500 ml with water.

4-methyl-6-acetyl-7-hydroxycoumarin was synthesised by Desai's method [13]. The reagent was used in dioxane solution.

All other chemicals were of analytical-reagent grade.

Apparatus

Fluorescence spectra were recorded with a Hitachi Model 650-10S spectrofluorimeter fitted with a xenon lamp. The spectra were not corrected. A Hitachi Model 203 spectrofluorimeter fitted with a medium-pressure mercury lamp was used for quantitative measurements. An aqueous solution (0.5 gg m1-1) of sodium fluorescein was used to adjust the sensitivity of the spectrofluorimeter. Absorption spectra were recorded with a Hitachi Model 139 and 101 spectrophotometers. Quartz cells (I0 × 10 × 45 mm) were used for all measurements. A Toadenpa Model 1 M-20E pH- meter was used for pH-measurements.

Procedure

To a sample solution containing 0.005 ~ 0.1 ~tg of beryllium, add 5.0ml of dioxane solution (1.6×10-2mol/1) of 4- methyl-6-acetyl-7-hydroxycoumarin (the final dioxane content should be 20% V/V), 5 ml of 0.5 tool/1 hexa- methylenetetramine solution and a sufficient amount of acetic acid or ammonia to adjust the pH to 7.5 ~ 7.8. Dilute the mixture to 25 ml with water. After 30 rain, extract the beryllium complex with 10 ml benzene by shaking vigorously

100 1 A C

= 0l// ~00 500 600

Waveteng th I n m

Fig. 1. Fluorescence spectra of the beryllium complex of 4-methyl- 6-acetyl-7-hydroxycoumarin in benzene. A excitation spectrum of the beryllium complex; B of the reagent; C and D corresponding emission spectra

739

> ,

®

¢b U C: O) U 1/1

0

r~

60

40

20 B

r_. A A .~, A

0 t . t t w ! v

6 7 8 9 PH

Fig. 2. Effect of pH on the extraction of the beryllium complex of 4-methyl-6-acetyl-7-hydroxycoumarin into benzene. A beryllium complex (5 x 10 - 9 tool Be); B reagent (8 × 10 -5 tool)

T a b l e 1. Effect of extraction solvents. Beryllium, 2.0 x 10 -7 tool/l; reagent, 3.2 x 10 -3 mol/1; pH, 7.5; solvent, 10 ml

Solvent Relative fluorescence intensity

Beryllium Reagent complex

Benzene 63.0 10.0 Chloroform 60.1 30.9 Carbon tetrachloride 0.7 6.7 4-Methylpentan-2-one 0 1200

for I rain. Separate the organic phase and dry over sodium sulphate. After 30 rain, irradiate the solution at 403 nm and measure the fluorescence intensity at 465 nm.

R e s u l t s a n d d i s c u s s i o n

The acid dissociation constant of 4-methyl-6-acetyl-7- hydroxycoumarin determined spectrophotometrically was Ka = 9.77 x 10-e. The effect o f p H of the reagent in aqueous solution containing 50% V/V dioxane was examined. The reagent in acidic medium does not fluoresce, but in basic medium it does. It is thought that a dissociated species in basic medium fluoresces.

Beryllium reacts with 4-methyl-6-acetyl-7-hydroxy- coumarin in basic medium to form a water-insoluble com- plex that is soluble in aqueous solutions containing alcohol or dioxane. Although these solutions fluoresce, the reagent solutions fluoresce in basic medium, too. On the other hand, a beryllium complex is easily extracted into organic solvents and exhibits intense fluorescence in organic solvents. The fluorescence intensities of the complex and the reagent blank extracted into organic solvents are shown in Table 1. Benzene is seen to be the most suitable solvent.

Fluorescence spectra

The excitation and emission spectra of the beryllium complex and the reagent in benzene are shown in Fig. 1. The excita-

tion and emission spectra have maxima at 403 and 465 nm, respectively.

Effect of reaction variables

The effect of the pH of the aqueous phase on the extraction of the beryllium complex into benzene is shown in Fig. 2. The maximum constant fluorescence intensity is obtained in the pH-range 7.5 ~ 7.8, although the fluorescence intensity of the reagent is weak. Phosphate cannot be used as the buffer solution, because beryllium reacts with phosphate to form a non-fluorescing complex. Hexamethylenetetramine solution is recommended for the determination of beryllium.

The effect of the standing time before extraction on the fluorescence intensity was examined. The fluorescence inten- sity increases gradually up to 30 rain, after which it remains constant up to 120 rain, The beryllium complex is readily extracted into benzene by shaking for 1 min. The fluores- cence intensity increases up to 30 min, after which it remains constant for at least 90 rain.

Calibration graph

Under the recommended conditions, the calibration graph is linear over the range 0.005 ~ 0.1 gg of beryllium per 10 ml of benzene. The coefficient of variation obtained from five measurements of 0.05 gg of beryllium is 1.2%. The sensitivity of the proposed method is about 10 times that of the 4, 5 -dihydroxycoumarin method [ 12] and is about 2 times that of the 2-ethyl-3-methyl-5-hydroxychromone method [4]. Further, the sensitivity of this method is approximately the same as that of the morin method [14] which is one of the representative methods for the fluorimetric determination of beryllium.

Effect of foreign ions

The effect of foreign ions on the determination of 0.05 gg of beryllium was examined. The tolerance limit was defined as the amount of diverse ion causing an error of not more than 5%. The diverse ions that interfere below 20000-fold amounts (by mass) are summarised in Table 2. Citrate,

740

Table 2. Effect of foreign ions on the determination of 0.05 [.tg of beryllium

Tolerance Ion limitAtg

~0

100 1000

0.l

1

Citrate, EDTA, Ti, Zr, Hf, Cr(III), Zn, Sn(IV), Sb(III), Bi(III) La, Mn(II), Fe(III), Pd, Cu(II), As(V), Se(IV), Te(IV), Hg(II), A1, Ga, In, T1 Phosphate, Sc, Y, Ce(III), Co(II), Ni(II), Au(III), As(III) Oxalate, Tartrate, Mg, W, Cd, Pb, NH2 Li, Ca, Sr, Ba, V, Cr(VI), Mo, Pt(II), Ag, Ge, Se(VI)

EDTA, titanium, zirconium, hafnium, chromium, zinc, tin, antimony and bismuth cause serious negative errors. No ions give positive errors. 100 mg of chloride, nitrate sulphate and acetate have no effect. Various cations give negative errors probably by formation of non-fluorescing complexes or adsorption of the beryllium complex on the hydroxide precipitates of diverse ions.

Comparison of substituent effects

Beryllium reacts with 4-methyl-5-hydroxy-6-acetyl- coumarin, 4 - methyl - 6 - acetyl - 7 - hydroxycoumarin and 4 - methyl- 7-hydroxy-8-acetylcoumarin to form water-insoluble complexes, which are extracted into benzene. The excitation and emission spectra of the beryllium complexes with these

three reagents are quite similar, but the fluorescence intensity varies markedly, that is, when the fluorescence intensity of the 4-methyl-6-acetyl-7-hydroxycoumarin complex is 100 (relative value), the intensity of the 4-methyl-5-hydroxy-6- acetylcoumarin complex is 67 and that of the 4-methyl-7- hydroxy-8-acetylcoumarin complex is 9. 4-Methyl-6-acetyl- 7-hydroxycoumarin was selected for the detailed study de- scribed above, taking into account the fluroescence intensity of the beryllium complex among these three reagents.

References

1. Sandell EB, Onishi H (1978) Photometric determination of traces of metals, part 1. Wiley, New York, pp 323-331

2. Murata A, Suzuki T (1967) Bunseki Kagaku 16:248-250 3. Murata A, Mizoguchi K (1969) Bunseki Kagaku 18:1471-

1477 4. Ito T, Murata A (1971) Bunseki Kagaku 20:335-340 5. [to T, Murata A (1971) Bunseki Kagaku 20:1422-1427 6. Nakarnura M, Murata A (1973) Bunseki Kagaku 22:1474-

/480 7. lto T, Murata A (1980) Anal Chim Acta 113:343- 349 8. lto T, Murata A (1974) Bunseki Kagaku 23 : 274- 280 9. Nakamura M, Murata A (1980) Mikrochim Acta 1:301 -306

10. Ito T, Murata A (1981) Anal Chim Acta 125:155-159 11. Katyal M, Singh HB (1968) Talanta 15:1043-1054 12. Ito T, Omura H, Suzuki T, Murata A (1988) Bunseki Kagaku

37: 327- 330 13. Desai RD, Hamid SA (1937) Proc Indian 6-A:185-190 14. Sill CW, Willis CP (1959) Anal Chem 31:598-608

Received April 23, 1990