Spectrophotometric determination of microamounts of scandium with o-chlorophenylfluorone and cetyltrimethylammonium bromide

Talanta,Vol. 37, No. 6, pp. 641-644, 1990

0039-9140/90 $3.00 + 0.00 Copyright 0 1990 Pergamon Press plc

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SPECTROPHOTOMETRIC DETERMINATION OF MICROAMOUNTS OF SCANDIUM WITH o-CHLOROPHENYLFLUORONE AND CETYLTRIMETHYLAMMONIUM BROMIDE ZONG-MING Luo and WEI-MING HE Department of Chemical Engineering, Guangdong Institute of Technology, Guangzhou, People’s Republic of China (Receiued 28 September 1988. Revised 15 March 1989. Accepted 14 December

1989)

Summary-The reaction of scandium(II1) with ochlorophenylfluorone (o-ClPF) in the presence of cetyltrimethylammonium bromide (CTMAB) has been studied. In an acetate buffer at pH 4.4, a redpurple complex is obtained, with maximum absorption at 569 nm and a molar absorptivity of 1.31 x 1051.mole-‘.cm-‘. The composition of the complex is found to be 1:2:2 St-o-CIPFCTMAB. Beer’s law is obeyed over the range O-12 &25 ml scandium. The proposed method has been used for determination of trace scandium in tungsten ores after its prior separation by solvent extraction.

Xylenol Orange and Arsenazo III have been recommended’ as chromophoric reagents for the spectrophotometric determination of trace amounts of scandium, but as these have poor sensitivity several new methods have been developed.2-6 Recently it has been proposed’ that ternary systems containing scandium, triphenylmethane dyes and cationic surfactants have even greater sensitivity. The derivatives of phenylfluorone* are highly sensitive chromophoric reagents for the spectrophotometric determination of easily hydrolysed high-valence metals, e.g., o -chlorophenylfluorone (o-ClPF) can be used to determine Mo,~ Ta,” Al” and Ga,12 but there are no reports that it can be used to determine scandium in the presence of surfactants. In this work, the optimum conditions for the reaction of scandium(II1) with o-ClPF and cetyltrimethylammonium bromide (CTMAB) have been studied. The spectrophotometric method developed has been used for the determination of scandium in tungsten ores after solvent extraction of scandium. EXPERIMENTAL

Reagents All solutions were prepared with analyticalgrade chemicals and demineralized distilled water unless otherwise stated. Standard scandium solution, 5pglml. Dissolve 0.1534 g of pure scandium oxide in 10 ml of

12M hydrochloric acid, bring the solution to the boil to remove some of the acid, then dilute to volume in a lOO-ml standard flask. Dilute this solution (1 mg/ml) to 5 pg/ml. Dissolve o-ClPF solution, 1.0 x IO-‘M. 0.0886 g of o-ClPF in 125 ml of ethanol and dilute to 250 ml with water. Acetate buffer solution, l.O.M, pH 4.6. Dissolve 40.5 g of anhydrous sodium acetate in 250 ml of water, adjust the pH to 4.6 with 4M hydrochloric acid and dilute to 500 ml with water. CTMAB solution, 1.0 x IO-‘M. PMBP solution, 1.0 x IO-*M. Dissolve 1.4 g of I-phenyl-3-methyl-4-benzoylpyrazol-5-one in 500 ml of benzene. Sodium diethyldithiocarbamate (DDTC), 20% aqueous solution. General procedure Place 1.00 ml of the standard scandium(II1) solution in a 25-ml standard flask, add 5 ml of buffer solution, 5 ml of CTMAB solution, 1.5 ml of o-ClPF solution, dilute to the mark with water and heat the solution in a water-bath at 70” for 10 min. Cool, and measure the absorbance at 569 nm against a reagent blank in l-cm cells. Determination of scandium in tungsten ores Fuse a 0.5-g sample with 10 g of sodium peroxide in a nickel crucible at 500” for 45 min. Leach the melt with 80 ml of a solution

641

ZONGMING

642

Luo and

containing 8 g of sodium carbonate, 2 g of sodium chloride and 2 g of sodium hydroxide, in a 250-ml poly(viny1 fluoride) beaker and warm it for 1 hr. Cool the solution and let it stand overnight. Filter the solution and wash the precipitate with 2% sodium carbonate solution about 10 times. Place the paper in a 250-ml beaker and dissolve the precipitate with 30 ml of hot 6M hydrochloric acid. Transfer the solution to a lOO-ml standard flask, dilute to volume with water and mix. Pipette 20.00 ml of this solution into a loo-ml separatory funnel, add 1 ml of 10% hydroxylamine hydrochloride solution and 1 ml of 20% sulphosalicylic acid solution. Adjust the pH to about 4, add 5 ml of acetate buffer (pH 5.5) and 3 ml of DDTC solution, mix and let stand for 3 min. Then shake the mixture for 1 min with first 15 and then 10 ml of chloroform, discarding the organic phases. Add 20 ml of PMBP solution to the aqueous phase, shake the mixture for 1 min and discard the aqueous phase. Shake the organic phase for 1 min with 10 and 5 ml of 5% hydrochloric acid. Evaporate the combined hydrochloric acid phases on a hot-plate to near dryness. Cool, add 5 ml of 5% v/v hydrochloric acid and heat to take up the residue. Cool and transfer the solution into a 25-ml standard flask. Add 1 drop of 0.1% p-nitrophenol solution and 6M ammonia solution until the solu tion turns yellow. Then add 2% v/v hydrochloric acid dropwise with frequent shaking, until the yellow colour just disappears. Then determine scandium according to the general procedure.

1.6

2

WEI-MINGHE

A ::p--\ 0.2 t

o3.9

4.1

4.3

4.5

4.7

4.9

PH

Fig. 2. Etrect of pH on the formation of the scandium complex. Absorbance measured at 569 nm against a reagent blank.

RESULTS AND DISCUSSION

Absorption spectra Figure 1 shows that in the presence of CTMAB the scandium complexes with o-chloro (o -ClPF), o -hydroxy (o -HPF) and o -nitrophenylfluorone (o-NPF) have maximum absorption at 569, 559 and 560 nm with apparent molar absorptivities of 1.31 x lo’, 1.22 x lo5 and 1.01 x lo5 l.mole-‘.cm-’ respectively. The o-CIPF complex shows the greatest sensitivity, so was used in the following experiments. EfSect of pH The effect of pH on the scandiumo-CIPF-CTMAB system was studied over the pH range 3.9-4.9. Figure 2 shows that the absorbance is constant and maximal from pH 4.2 to 4.7, so subsequent determinations were performed at pH 4.4. Acetate buffer is used to maintain the optimum pH and improves the stability of the colour without affecting the absorbance. Effect of surfactants

300

520

540

560

560

600

X/nm Fig. 1. Absorption spectra: (1) o-ClPF and (2) o-ClPFCTMAB, us. water; (3) St-o-CIPF-CTMAB, (4) Sc-oHPF-CTMAB, (5) SC-~-NPF-CTMAB, vs. reagent blank. [SC]= 4.45 x 10-6M, [o-ClPF] = [o-HPF] = [o-NPF] = 6.0 x IO-‘M, [CTMAB] = 2.0 x lo-“M.

The absorbance maximum and apparent molar absorptivity of the binary complex of scandium and o-ClPF are 520 nm and 3.60 x 104 l.mole-’ .cm-‘, respectively. If certain cationic and non-ionic surfactants, for example CTMAB, cetylpyridinium bromide (CPB), Tween-80, Triton X-100 and OP are added as sensitizers to the scandium-o-CIPF system, significant bathochromic and hyperchromic shifts are observed. The experimental results are given in Table 1. The results in Table 1 show that the sensitizing efficiency is greatest for CTMA so this surfactant was chosen as sensitizer.

Spectrophotometric

determination

Table 1. Characteristics of scandium complexes with o-ClPF in the presence of surfactants Surfactant*

I llUX, nm

CTMAB CPB Tween-80 Triton X-100 OP

569 570 565 565 570

104l.?no~-‘.cnl-’ 13.1 11.3 8.33 5.63 4.95

*Volume of surfactant added is 3 ml of 0.5% CTMAB or CPB solution, 5 ml of 3% Tween80 solution, 3 ml of 1% Triton X-100, OP or PVA solution, respectively, in the 25-ml standard flask.

Eflect of reagent concentrations When the general procedure was followed with varied amounts of the reagents, maximum and constant absorbance was obtained with 0.5-2.0 ml of o-ClPF solution and 3.0-7.0 ml of CTMAB solution. Therefore, 1.5 and 5.0 ml respectively were selected as the optimal volumes of these reagents. The addition of ethanol is necessary to stabilize the SC-o-CIPF-CTMAB system. The maximum absorbance is obtained with 2-7% v/v ethanol. A higher level of alcohol than 7% breaks up the micelles. Effect of temperature The colour will not develop at room temperature but is completely formed in 10 min at 70”. Once the colour has developed, the absorbance remains constant for at least 24 hr. Calibration graph and sensitivity The calibration graph is linear over the range O-12 pg/25 ml scandium. The apparent molar absorptivity calculated from the calibration graph is 1.31 x 10S1.mole-‘.cm-’ at 569 nm. Composition of the complex The molar ratio of scandium to o-ClPF in the complex was determined by the continuous variations and molar ratio methods and found to be 1:2. The molar ratio of scandium to CTMAB in the complex was established as 1: 2 by the Asmus linear method.13

643

Ba(II), 2000; Cd(II), 510; Zn(II), 160; lanthanides(III), 140; Pb(II), 65; Co(II), 55; Nb(V), 40; Y(III), 32; Ni(I1) and Mo(VI), 25; Bi(III), 23; Ta(V), 18; Be(II), 8. Suitable amounts of certain metal ions that react with o-ClPF may be masked by the addition of masking reagents, e.g., 30 pg of Cu(I1) by 3.2 x 10e2M thiourea, 8 pg of Fe(II1) or 2 pg of Cr(V1) by reduction with 3.5 x IO-*M hydroxylamine hydrochloride, and 20 pg of Cr(III), 8 pg of Th(IV), 6 pg of V(V), 3 pugof Mn(II), 2 pg of Zr(IV) or 1 ,ug of W(V1) by 1.3 x 10m3M tartaric acid. Under the same conditions, Ga(II1) and In(II1) form intensely coloured complexes with o-ClPF in the presence of CTMAB, and thus interfere in the general procedure. Small amounts of Al(II1) and Ti(IV) can be masked with sulphosalicylic acid. Scandium must therefore be separated from these ions if it is to be determined when they are present in the sample. The tolerance levels of anions and reagents were at least 100 mg of sulphate, 50 mg of nitrate or chloride, 80 mg of hydroxylamine hydrochloride, 60 mg of ascorbic acid, 6 mg of tartaric acid and 1 ml of 30% hydrogen peroxide. Fluoride, oxalate and citrate decrease the reactivity of scandium with o-ClPF and therefore must be absent. Separation of scandium In addition to the matrix elements such as W, Fe, Mn, Ca and Mg in tungsten ores, Al, Sn, V, U, Nb, Ta and lanthanides may also be present. To eliminate the interference of diverse ions the separation of scandium in the solution prepared from a fused sample is required. When the sample fused with sodium peroxide is dissolved in water, several elements such as W, MO, V, Mn and Al form anions and pass into solution; these can be removed by filtering. The precipitate, which contains scandium and other hydroxides, is dissolved in hydrochloric acid and this solution is extracted first with Table 2. Results for determination of scandium in tungsten ores sc,o,,

%

Found by proposed method

E#ect of diverse ions To assess the usefulness of the method, the effect of diverse ions which often accompany scandium was studied. The following pg amounts of the ions were found to give an error of less than & 5% in the determination of 5 pg of scandium: La(III), 2400; Ca(II), Mg(I1) and

of scandium

Sample

Certified value*

Average?

Standard deviation

WO-A WO-k wo-1

0.0018 0.0039 0.0030

0.00178 0.00382 0.00307

6.2 x 1O-5 7.6 x 1O-5 6.2 x 1O-5

*Certified value provided by the Guangzhou Institute of Non-ferrous Metals. tAverage values of six determinations.

Research

ZCBNGM~NG Luo and WEI-MINGHE

644

4. I. Mori, Y. Fujita, K. Fujita, A. Usami, H. Kawabe, sodium diethyldithiocarbamate to remove Fe, Y. Koshiyama and T. Tanaka, Bull. Chem. Sot. Japan, Cu, Ni, Bi, U and Sn etc.14 Scandium and the 1986, 59, 1623. lanthanides are then extracted with PMBP.‘5*‘6 5. C. D. Sharma, S. G. Nagarkar and M. C. Eshwar, Scandium can then be stripped with 0.6M ibid., 1986, 59, 1662. hydrochloric acid. ‘w’ To avoid possible inter6. Wan-Ru Chen, Jiao-Mai Pan, Chung-Gin Hsu and Sheng-Song Ge, Mikrochim. Acta, 1985 III, 417. ference by residual traces of aluminium and 7. M. Jarosz and Z. Marczenko, Anal. Chim. Acta, titanium, which are also extracted into the 1984, 159, 309. PMBP organic phase, sulphosalicylic acid is 8. V. A. Nazarenko, V. P. Antonovich and N. A. added to mask them.ls This separation of scanVeschikova, Talanta, 1987, 34, 215. dium may also be applied to other ores.16*‘7 9. Zhen-qing Wang, Quang-hui Xu and Han-xi Shen, Yankuang Ceshi, 1986, 5, 8. The results for determination of scandium in 10. Zong-ming Luo and Wei Shen, ibid., 1987, 6, 210. some tungsten ores by the proposed method 11. Zong-ming Luo and Zeng-wen Lin, ibid., 1988, 7, 104. are shown in Table 2, and are in reasonable 12. Ying-lu He, Ying Liu, Jin-duan Zhao, Zhong-yi Zhao agreement with the certified values. and Fu-peng Wang, Fenxi Huaxue, 1988, 16, 341.

REFERENCES Z. Marczenko, Spectrophotometric Determination Elements, Chap. 45, Horwood, Chichester, 1976.

of

M. A. H. Hafez, I. M. M. Kenawy and M. A. Kabil, Anal, Lett., 1985, 18, 2043. G. V. Rathaiah and M. C. Eshwar, Indian J. Chem., 1986, 25A, 101.

13. E. Asmus, 2. Anal. Chem., 1960, 178, 104. 14. Z. Holzbecher, L. DiviS, M. Kdl, L. Sucha and F. Vlatil, Handbook of Organic Reagents in Inorganic Analysis, pp. 230-231. Horwood, Chichester, 1976. 15. A. Roy and A. Nag, J. Inorg. Nucl. Chem., 1978, 40, 331. 16. You-yun Hu, Fenxi Shiyanshi, 1985, 4, No. 12, 57. 17. Jian Shen, Xiyou Jinshu, 1987, 11, 107. 18. D. D. Perrin, Masking and Demasking oj” Chemical Reactions, pp. 42-44. Wiley, New York, 1970.