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Sequential determination of molybdenum and tungsten in silicate rocks by a spectrophotometric method
Analytica ChimicaActa, 270 (1992) 267-269
267
Elsevier Science Publishers B.V., Amsterdam
Sequential determination of molybdenum and tungsten in silicate rocks by a spectrophotometric method Z h e n - H a i Fan, Li-Xing Z h a n g and Hui-De Zhou Northwest Institute of Nuclear Technology, P.O. Box 69, Xian 710000 (China)
(Received 18th February 1992)
Abstract
A spectrophotometric method has been proposed for the sequential determination of molybdenum and tungsten in silicate rocks. The procedure proposed has been successfully used to determine molybdenum and tungsten in granite. The results show that this method can accurately determine molybdenum and tungsten down to the/~g g-i and sub-/xg g-i level with relative standard deviations of 1% and 3.5%, respectively. Keywords: Spectrophotometry; Molybdenum; Silicate rocks; Toluene-3,4-dithiol complexes; Tungsten
Toluene-3,4-dithiol, generally referred to as "dithiol", forms green complexes with both molybdenum and tungsten. These complexes are soluble in many organic solvents giving green solutions and have been used for the photometric determination of molybdenum and tungsten in silicate rocks. The wavelengths at which the maxim u m absorptions occur for the complexes of molybdenum and tungsten, are very close. Therefore a separation procedure is necessary for the quantitative determination of both elements. Earlier procedures [1] were based upon suppression of the formation of the tungsten complex by the addition of citric acid. Molybdenum was extracted into carbon tetrachloride and separated quantitatively from tungsten, which could be determined with dithiol after the destruction of the citric acid remaining in the solution. However, the destruction of the citric acid is troublesome and time-consuming. In the present p a p e r we propose a procedure which is based upon the fact that the molybdenum complex is formed under conditions of high acidity, in contrast to the tungCorrespondence to: L.-X. Zhang, Northwest Institute of Nuclear Technology, P.O. Box 69, Xian 710000 (China).
sten complex which is formed only in hot and weakly acid solutions. Therefore the molybdenum dithiol complex, which is formed in high acidity medium, can be extracted into carbon tetracfiloride and quantitatively separated from tungsten, which is not extracted, but can be determined with dithiol after adjusting the p H of the solution to a value of 1.5. The procedure proposed has been successfully used to sequentially determine molybdenum and tungsten in granite.
EXPERIMENTAL Reagents
To obtain the ot-benzoinoxime solution (2%), dissolve 1.0 g of a-benzoinoxime in 50 ml ethanol. To obtain the standard stock solution of molybd e n u m (500/xg ml-1), dissolve 0.9201 g of ammonium molybdate (G.R.) in 500 ml of 1% ammonium hydroxide. To obtain the standard stock solution of tungsten (500 /zg ml-1), dissolve 0.6305 g of tungsten oxide (S.P.) in 500 ml 1% sodium hydroxide. To obtain the toluene-3,4-dithiol solution (0.2%), dissolve 0.2 g of the reagent and 1 g of sodium hydroxide in 80 ml of water,
0003-2670/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
268
add 1 ml thioglycollic acid and dilute to 100 ml with water. This reagent is unstable, so the solution should be stored in a refrigerator and must be discarded after 14 days.
Apparatus A Model 72 spectrophotometer (Shanghai Analytical Instruments Factory, China) was used for all absorbance measurements. One-centimeter matched cells were used, and all readings were made against a carbon tetrachloride blank, pH measurements were made with a Model PHS-2 pH meter (Shanghai Second Analytical Instruments Factory, China).
Procedure Accurately weigh 1 g ( - 2 0 0 mesh) rock sample into a platinum dish, moisten with water and add 1 ml concentrated nitric acid and 10 ml concentrated hydrofluoric acid. Transfer the dish to a boifing-water bath and evaporate to near dryness. On cooling, add 5 ml of concentrated hydrofluoric acid and again evaporate to near dryness, repeat the treatment with another 5 ml of concentrated hydrofluoric acid. After cooling, add 2 ml of concentrated perchloric acid, evaporate on a hot plate to dryness, add another 2 ml of concentrated perchloric acid and repeat the treatment. Add 5 ml of concentrated hydrochloric acid and 20 ml of water, heat to dissolve the residue. Transfer the solution to a 100-ml beaker, adjust the pH of the solution to 1.5 with ammonia solution (1 + 1) and hydrochloric acid (1 + 3). Transfer the solution obtained to a 100-ml separating funnel. Add 2 ml of the a-benzoinoxime solution and mix by shaking. Add 10 ml of chloroform, shake for about 11 min. Allow the phases to separate, collect the organic layer in a platinum crucible and repeat the extraction twice each with a 5 ml portion of chloroform. Add 50 mg of sodium carbonate to the organic solution obtained, evaporate the chloroform to dryness at a temperature < 60°C, ignite and fuse over a burner. After cooling, extract the melt with 5 ml of water, transfer the solution to a 100-ml separating funnel, wash the crucible with 5 ml of 10 M sulphuric acid, then wash the crucible twice with a 5-ml portion of water, combine the washings in
Z.-H. Fan et al. ~Anal. Chim. Acta 270 (1992) 267-269
the separating funnel. Add 5 ml of the dithiol solution, mix and stand for 20 min. Add 5 ml of carbon tetrachloride and shake for 1 min. Allow the phases to separate, remove the organic layer into a 1-ml cell, measure the absorbance of molybdenum complex at a wavelength of 670 nm. Repeat the extraction with 5 ml of carbon tetrachloride and discard the organic layer. Run the aqueous layer into a 100-ml beaker, adjust the pH of the solution to a value of 1.5 with ammonia solution, add 3 ml of the dithiol solution. Place the beaker on a hot plate and heat to just below boiling. On cooling, transfer the solution into a 100-ml separating funnel, add 5 ml of carbon tetrachloride and shake for 1 min. Allow the phases to separate, collect the organic layer into a 1-ml cell, measure the absorbance of tungsten complex at a wavelength of 650 nm.
Calibration Transfer aliquots of the standard molybdenum working solution containing 0-10 ~g Mo to a series of 100-ml separating funnels, add 5 ml of 10 M sulphuric acid, dilute to 20 ml with ,water. Add 5 ml of the dithiol solution, mix and stand for 20 min. Extract the molybdenum complex and measure the absorbance as described above. Plot the relation of absorbance to molybdenum concentration. Transfer aliquots of standard tungsten working solution containing 0-10 /zg W to a series of 100-ml beakers, dilute the solution to 25 ml with water, adjust the pH to 1.5 with dilute sulphuric acid, add 3 ml of the dithiol solution, form and extract tungsten complex, and measure the absorbance as described above. Plot the relation of absorbance to tungsten concentration.
RESULTS AND DISCUSSION
Absorption spectra of the dithiol complexes of molybdenum and tungsten show that the maximum absorption occurs at wavelengths of 650 nm (tungsten) and 670 nm (molybdenum), respectively. Straight line calibration graphs indicate that the Beer-Lambert law is followed up to at least 20/zg tungsten and molybdenum.
Z.-H. Fan et al. ~Anal. Chim. Acta 270 (1992) 267-269
269
TABLE 1 Analytical results of No. 1 granite Element
Content (#g g - l )
Average (/zg g - l )
R.S.D. (%)
Standard recovery
(%) Mo
2.85 2.90 2.85 2.90 0.73 0.77 0.75 0.78
W
2.87 2.89 2.84 2.85 0.80 0.80 0.75 0.78
2.88 2.85 2.85 2.90 0.77 0.73 0.80 0.75
2.90 2.82 2.86
2.84 2.84 2.87
2.86
0.9
99.8
0.77 0.80 0.80
0.73 0.73 0.75
0.77
3.5
97.6
The effect of acidity on the formation of molybdenum and tungsten complexes with dithiol has been investigated in detail. The results shown in Fig. 1 indicate that molybdenum forms a stable complex with dithiol in 0.25-7.5 M sulphuric acid. Tungsten, however, forms a complex with dithiol when the acidity of solution is lower than 1.5 M, and the optimum acidity for the formation of tungsten dithiol complex is pH 1.5. The acidity of 2 M sulphuric acid is selected for the formation of molybdenum complex. Thus, after extraction of molybdenum complex with carbon tetrachloride, tungsten still remains in the aqueous layer. Therefore the molybdenum and tungsten are separated quantitatively. Heating is necessary for the formation of tungsten dithiol complex, but boiling should be avoided as experiments have shown that prolonged boiling will destroy the complex and re-
0.~
0.5
/
Itt°
0.,~
c
~ ~ <
0.~
0.3
0.2
0.2 ' ] o,
0.1 2.5 Concentration
i I 5.0 Z5 of H2SO4 , M
~
I
~H
Fig. 1. Effect of acidity on the formation of complexes: Mo 5 /zg, 670 nm ( ); W 20/zg, 650 nm ( . . . . . . ). For various sulphuric acid concentrations: 0.25-7.5 M (a), and for pH 0-4
(b).
duce the absorbance; heating to just below boiling is optimum. Under the conditions selected, molybdenum and tungsten are separated and determined quantitatively. The standard recovery of molybdenum (5 Izg) and tungsten (2 /zg) is 99.8% and 97.6%, respectively. For some silicate rocks it is possible to extract the molybdenum and tungsten complexes with dithiol directly after the acid decomposition procedure of samples. However, titanium, chromium, vanadium, iron etc. may interfere with the determinations. Therefore prior to the determination of molybdenum and tungsten for more complex rocks an extraction stage with a-benzoinoxime may be applied to separate these interference elements [2]. Thus, 5 mg chromium oxide, titanium oxide, vanadium pentoxide and 50 mg iron trioxide do not interfere with the determination of molybdenum and tungsten at the txg level. The procedure proposed has been successfully used to sequentially determine molybdenum and tungsten in granites. Typical results are shown in Table 1. The results show that this method can accurately determine Mo and W down to the ~zg g - 1 and sub-~g g - 1 level, and relative standard deviations are 1% and 3.5%, respectively.
REFERENCES 1 P.G. Jeffery and D. Hutchison, Chemical Methods of Rock Analysis, 3rd edn., Pergamon Press, 1981, p. 325. 2 Hu Shi-Fu and Zhang Bao-Chuan, unpublished work.
267
Elsevier Science Publishers B.V., Amsterdam
Sequential determination of molybdenum and tungsten in silicate rocks by a spectrophotometric method Z h e n - H a i Fan, Li-Xing Z h a n g and Hui-De Zhou Northwest Institute of Nuclear Technology, P.O. Box 69, Xian 710000 (China)
(Received 18th February 1992)
Abstract
A spectrophotometric method has been proposed for the sequential determination of molybdenum and tungsten in silicate rocks. The procedure proposed has been successfully used to determine molybdenum and tungsten in granite. The results show that this method can accurately determine molybdenum and tungsten down to the/~g g-i and sub-/xg g-i level with relative standard deviations of 1% and 3.5%, respectively. Keywords: Spectrophotometry; Molybdenum; Silicate rocks; Toluene-3,4-dithiol complexes; Tungsten
Toluene-3,4-dithiol, generally referred to as "dithiol", forms green complexes with both molybdenum and tungsten. These complexes are soluble in many organic solvents giving green solutions and have been used for the photometric determination of molybdenum and tungsten in silicate rocks. The wavelengths at which the maxim u m absorptions occur for the complexes of molybdenum and tungsten, are very close. Therefore a separation procedure is necessary for the quantitative determination of both elements. Earlier procedures [1] were based upon suppression of the formation of the tungsten complex by the addition of citric acid. Molybdenum was extracted into carbon tetrachloride and separated quantitatively from tungsten, which could be determined with dithiol after the destruction of the citric acid remaining in the solution. However, the destruction of the citric acid is troublesome and time-consuming. In the present p a p e r we propose a procedure which is based upon the fact that the molybdenum complex is formed under conditions of high acidity, in contrast to the tungCorrespondence to: L.-X. Zhang, Northwest Institute of Nuclear Technology, P.O. Box 69, Xian 710000 (China).
sten complex which is formed only in hot and weakly acid solutions. Therefore the molybdenum dithiol complex, which is formed in high acidity medium, can be extracted into carbon tetracfiloride and quantitatively separated from tungsten, which is not extracted, but can be determined with dithiol after adjusting the p H of the solution to a value of 1.5. The procedure proposed has been successfully used to sequentially determine molybdenum and tungsten in granite.
EXPERIMENTAL Reagents
To obtain the ot-benzoinoxime solution (2%), dissolve 1.0 g of a-benzoinoxime in 50 ml ethanol. To obtain the standard stock solution of molybd e n u m (500/xg ml-1), dissolve 0.9201 g of ammonium molybdate (G.R.) in 500 ml of 1% ammonium hydroxide. To obtain the standard stock solution of tungsten (500 /zg ml-1), dissolve 0.6305 g of tungsten oxide (S.P.) in 500 ml 1% sodium hydroxide. To obtain the toluene-3,4-dithiol solution (0.2%), dissolve 0.2 g of the reagent and 1 g of sodium hydroxide in 80 ml of water,
0003-2670/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
268
add 1 ml thioglycollic acid and dilute to 100 ml with water. This reagent is unstable, so the solution should be stored in a refrigerator and must be discarded after 14 days.
Apparatus A Model 72 spectrophotometer (Shanghai Analytical Instruments Factory, China) was used for all absorbance measurements. One-centimeter matched cells were used, and all readings were made against a carbon tetrachloride blank, pH measurements were made with a Model PHS-2 pH meter (Shanghai Second Analytical Instruments Factory, China).
Procedure Accurately weigh 1 g ( - 2 0 0 mesh) rock sample into a platinum dish, moisten with water and add 1 ml concentrated nitric acid and 10 ml concentrated hydrofluoric acid. Transfer the dish to a boifing-water bath and evaporate to near dryness. On cooling, add 5 ml of concentrated hydrofluoric acid and again evaporate to near dryness, repeat the treatment with another 5 ml of concentrated hydrofluoric acid. After cooling, add 2 ml of concentrated perchloric acid, evaporate on a hot plate to dryness, add another 2 ml of concentrated perchloric acid and repeat the treatment. Add 5 ml of concentrated hydrochloric acid and 20 ml of water, heat to dissolve the residue. Transfer the solution to a 100-ml beaker, adjust the pH of the solution to 1.5 with ammonia solution (1 + 1) and hydrochloric acid (1 + 3). Transfer the solution obtained to a 100-ml separating funnel. Add 2 ml of the a-benzoinoxime solution and mix by shaking. Add 10 ml of chloroform, shake for about 11 min. Allow the phases to separate, collect the organic layer in a platinum crucible and repeat the extraction twice each with a 5 ml portion of chloroform. Add 50 mg of sodium carbonate to the organic solution obtained, evaporate the chloroform to dryness at a temperature < 60°C, ignite and fuse over a burner. After cooling, extract the melt with 5 ml of water, transfer the solution to a 100-ml separating funnel, wash the crucible with 5 ml of 10 M sulphuric acid, then wash the crucible twice with a 5-ml portion of water, combine the washings in
Z.-H. Fan et al. ~Anal. Chim. Acta 270 (1992) 267-269
the separating funnel. Add 5 ml of the dithiol solution, mix and stand for 20 min. Add 5 ml of carbon tetrachloride and shake for 1 min. Allow the phases to separate, remove the organic layer into a 1-ml cell, measure the absorbance of molybdenum complex at a wavelength of 670 nm. Repeat the extraction with 5 ml of carbon tetrachloride and discard the organic layer. Run the aqueous layer into a 100-ml beaker, adjust the pH of the solution to a value of 1.5 with ammonia solution, add 3 ml of the dithiol solution. Place the beaker on a hot plate and heat to just below boiling. On cooling, transfer the solution into a 100-ml separating funnel, add 5 ml of carbon tetrachloride and shake for 1 min. Allow the phases to separate, collect the organic layer into a 1-ml cell, measure the absorbance of tungsten complex at a wavelength of 650 nm.
Calibration Transfer aliquots of the standard molybdenum working solution containing 0-10 ~g Mo to a series of 100-ml separating funnels, add 5 ml of 10 M sulphuric acid, dilute to 20 ml with ,water. Add 5 ml of the dithiol solution, mix and stand for 20 min. Extract the molybdenum complex and measure the absorbance as described above. Plot the relation of absorbance to molybdenum concentration. Transfer aliquots of standard tungsten working solution containing 0-10 /zg W to a series of 100-ml beakers, dilute the solution to 25 ml with water, adjust the pH to 1.5 with dilute sulphuric acid, add 3 ml of the dithiol solution, form and extract tungsten complex, and measure the absorbance as described above. Plot the relation of absorbance to tungsten concentration.
RESULTS AND DISCUSSION
Absorption spectra of the dithiol complexes of molybdenum and tungsten show that the maximum absorption occurs at wavelengths of 650 nm (tungsten) and 670 nm (molybdenum), respectively. Straight line calibration graphs indicate that the Beer-Lambert law is followed up to at least 20/zg tungsten and molybdenum.
Z.-H. Fan et al. ~Anal. Chim. Acta 270 (1992) 267-269
269
TABLE 1 Analytical results of No. 1 granite Element
Content (#g g - l )
Average (/zg g - l )
R.S.D. (%)
Standard recovery
(%) Mo
2.85 2.90 2.85 2.90 0.73 0.77 0.75 0.78
W
2.87 2.89 2.84 2.85 0.80 0.80 0.75 0.78
2.88 2.85 2.85 2.90 0.77 0.73 0.80 0.75
2.90 2.82 2.86
2.84 2.84 2.87
2.86
0.9
99.8
0.77 0.80 0.80
0.73 0.73 0.75
0.77
3.5
97.6
The effect of acidity on the formation of molybdenum and tungsten complexes with dithiol has been investigated in detail. The results shown in Fig. 1 indicate that molybdenum forms a stable complex with dithiol in 0.25-7.5 M sulphuric acid. Tungsten, however, forms a complex with dithiol when the acidity of solution is lower than 1.5 M, and the optimum acidity for the formation of tungsten dithiol complex is pH 1.5. The acidity of 2 M sulphuric acid is selected for the formation of molybdenum complex. Thus, after extraction of molybdenum complex with carbon tetrachloride, tungsten still remains in the aqueous layer. Therefore the molybdenum and tungsten are separated quantitatively. Heating is necessary for the formation of tungsten dithiol complex, but boiling should be avoided as experiments have shown that prolonged boiling will destroy the complex and re-
0.~
0.5
/
Itt°
0.,~
c
~ ~ <
0.~
0.3
0.2
0.2 ' ] o,
0.1 2.5 Concentration
i I 5.0 Z5 of H2SO4 , M
~
I
~H
Fig. 1. Effect of acidity on the formation of complexes: Mo 5 /zg, 670 nm ( ); W 20/zg, 650 nm ( . . . . . . ). For various sulphuric acid concentrations: 0.25-7.5 M (a), and for pH 0-4
(b).
duce the absorbance; heating to just below boiling is optimum. Under the conditions selected, molybdenum and tungsten are separated and determined quantitatively. The standard recovery of molybdenum (5 Izg) and tungsten (2 /zg) is 99.8% and 97.6%, respectively. For some silicate rocks it is possible to extract the molybdenum and tungsten complexes with dithiol directly after the acid decomposition procedure of samples. However, titanium, chromium, vanadium, iron etc. may interfere with the determinations. Therefore prior to the determination of molybdenum and tungsten for more complex rocks an extraction stage with a-benzoinoxime may be applied to separate these interference elements [2]. Thus, 5 mg chromium oxide, titanium oxide, vanadium pentoxide and 50 mg iron trioxide do not interfere with the determination of molybdenum and tungsten at the txg level. The procedure proposed has been successfully used to sequentially determine molybdenum and tungsten in granites. Typical results are shown in Table 1. The results show that this method can accurately determine Mo and W down to the ~zg g - 1 and sub-~g g - 1 level, and relative standard deviations are 1% and 3.5%, respectively.
REFERENCES 1 P.G. Jeffery and D. Hutchison, Chemical Methods of Rock Analysis, 3rd edn., Pergamon Press, 1981, p. 325. 2 Hu Shi-Fu and Zhang Bao-Chuan, unpublished work.