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Spectrophotometric determination of rhenium as perrhenate after extraction of its brilliant green ion-pair with microcrystalline benzophenone
Analytica Chimica Acta, 204 (1988) 359-363 Elsevier Science Publishers B.V., Amsterdam -
359 Printed in The Netherlands
Short Communication
SPECTROPHOTOMETRIC DETERMINATION OF RHENIUM AS PERRHENATE AFTER EXTRACTION OF ITS BRILLIANT GREEN ION-PAIR WITH MICROCRYSTALLINE BENZOPHENONE
D. THORBURN
BURNS* and N. TUNGKANANURUK
Department of Chemistry, The Queen’s University of Belfast, Belfast ET9 5AG (Northern Ireland) (Received 16th June 1987)
Summary Rhenium (O-40 pg) is determined as pherrhenate spectrophotometrically at 640 nm after its adsorptive extraction at pH 6.0 with Brilliant Green onto microcrystalline benzophenone and dissolution of the solid phase in benzene. The effects of pH and diverse ions are reported. The system is applied to the determination of rhenium on alumina and on carbon catalysts.
The commonest methods for the determination of rhenium are based on reduction of rhenium (VII) with tin ( II) prior to reaction with thiocyanate or cu-furildioxime to give extractable products [ 1,2]. Both methods have the disadvantage of requiring considerable time to achieve maximum colour development. Such delays can be avoided by direct liquid-liquid extraction of rhenium (VII) as perrhenate with onium cations [ 31 or basic dyes [ 41 such as Brilliant Green [ 51; the latter systems are directly compatible with sample preparation by oxidative fusion. To date, in common with other oxoanions [ 61, apart from phosphate as molybdophosphate [ 71 and perchlorate [ 81, no solid-liquid extractions have been reported for the determination of rhenium. The present communication extends earlier studies on rhenium (VII) [ 51 and of Brilliant Green as an ion-pairing reagent [ 4,5], including its use in solid-liquid systems [ 8,9], and describes the adsorptive ion-pair extraction of Brilliant Green perrhenate with microcrystalline benzophenone. The solid benzophenone can subsequently be dissolved in benzene and determinations completed spectrophotometrically at 640 nm. The system can be applied, after oxidation fusion, to the determination of rhenium on alumina or carbon.
0003-2670/88/$03.50
0 1988 Elsevier Science Publishers B.V.
360
Experimental Apparatus. Pye-Unicam SP8-400 and SP6-550 UV-visible spectrophotometers were used for recording absorption spectra and for routine measurements, respectively, with matched quartz l-cm cells. Reagents and solutions. Brilliant green (Aldrich Chemical Corn., declared dye content ca 95%) was used as supplied. A 0.05% (w/v) solution was prepared in absolute ethanol and stored in a dark brown bottle. A stock 5 ,ug ml-l rhenium (VII) solution was prepared by dissolving 0.1553 g of potassium perrhenate (Specpure, Johnson Matthey) in exactly 1 1 of distilled water and diluting 5 ml of this solution to 100 ml with pH 6 buffer solution in a volumetric flask. The pH 6.0 buffer was prepared by adding 56 ml of 0.1 M sodium hydroxide to 500 ml of 0.1 M potassium dihydrogenphosphate and diluting to exactly 11 with water. The benzophenone solution was 20% (w/v) in acetone. All other reagents were of analytical grade. Twice-distilled water was used throughout. Generalprocedure. Place a 1-8 ml aliquot of sample solution containing 5-40 ,ug of rhenium( VII) in a stoppered Erlenmeyer flask. Add 10 ml of pH 6.0 buffer and 1.0 ml of Brilliant Green solution. Swirl to mix, leave for 1 min, add 2.0 ml of benzophenone solution and shake vigorously for 30 s. Filter the blue solid formed through a sintered glass filter (No. 2 porosity). Wash with water, drain or suck dry, dissolve the solid in benzene and dilute to volume in a lo-ml volumetric flask. Dry the solution by addition of 1 g of anhydrous sodium sulphate. Measure the absorbance at 640 nm against a reagent blank prepared in the same way. Procedure for catalyst dissolution. For rhenium on alumina pellets, weigh one pellet and crush to a fine powder in a platinum dish. Add 1.0 g of sodium peroxide and 1.0 g of sodium hydroxide, mix, and fuse carefully over a Bunsen flame until reaction is complete. Allow to cool, add 30 ml of water, and heat gently to dissolve all the material. Cool, filter through a Whatman No. 40 filter paper into a 250-ml volumetric flask and dilute to volume with distilled water. Prepare a blank, using the same weight of pure alumina ( Anderman ) as the catalyst pellet. For rhenium on carbon (which is pyrophoric ) the sample must be handled as follows. Transfer the sample in a glove box to a preweighed aluminum sample pan (as supplied by Perkin-Elmer for use with CHN analyzers and for differential scanning calorimeters) and seal with a small press. Reweigh, place in a platinum dish and take into solution as described above. Prepare a blank with an empty sample pan. Extraction. Transfer a 5-ml aliquot catalyst solution to a lOO-ml beaker. Add 5 ml of 5% (w/v) citric acid solution. The pH should be 4.5-6.0. Adjust the pH by adding 2 M sodium hydroxide and transfer to a stoppered Erlenmeyer flask. Proceed as in the general procedure but omit addition of buffer.
361
Examination of the main experimental variables Naphthalene, diphenyl, 1,4-dichlorobenzene and benzophenone were examined for their adsorptive-extraction properties in microcrystalline solid formation from acetone solution. Benzophenone showed the highest apparent molar absorptivity (3.3 x lo-* 1 mol-’ cm-’ at 640 nm, which is 2.2 x that of naphthalene and 4 x that of diphenyl or 1,4-dichlorobenzene); each solid gave acceptably low blanks (0.03-0.05 absorbance units measured against water). Solvents with a range of functional group types were examined for dissolution of the ion-pair and the solid extractant. Benzene was chosen for routine use because the ion-pair there gave the greatest apparent molar absorptivity. The ion-pair was stable in all solvents except dimethylformamide and dioxan. The effect of pH was examined for 1Opg of rhenium ( VII) by careful addition of 1 M hydrochloric acid or 1 M ammonia prior to extraction. The absorbances were measured as above and were almost independent of pH in the range 4.0-7.0, decreasing outside these limits. In all subsequent work, the aqueous phase was buffered at pH 6.0. The extent of extraction was constant over the range 8.0-14.0 ml of buffer; 10 ml of buffer solution was adopted for routine use. The effect of varying the amount of Brilliant Green solution was examined for 10 ,ugof rhenium ( VII). The absorbance of the extract increased with increasing volume of reagent up to 1.0 ml, remained constant up to 1.5 ml and decreased slowly thereafter. The reagent blank increased slowly with increasing amount of reagent, thus 1.0 ml of Brilliant Green solution was used as a compromise. The extent of extraction was unaffected by ionic strength, phase-volume ratios up to 7.51 of water-/benzophenone in acetone, sequence of addition of reagents, shaking time or digestion time. The absorbance of the dissolved extracts was stable in diffuse daylight for more than 2 h but faded slowly in direct daylight. The composition of the complex was established by Job’s method of continuous variations [lo] and by the mole ratio method [ 11,121 to be [ C,,H,,N,] ReO,. Curvature of the Job’s plot indicated that the ion-pair is appreciably dissociated in solution, similarly to the perchlorate ion-pair [ 81. Results and discussion A linear calibration graph was obtained over the range O-40 ,ug of rhenium (VII) in 10 ml of the final benzene solution. For the determination of 10 pg of rhenium (VII), the relative standard deviation was 0.6% (10 results). The possible interferences of various ion’s were checked (Table 1) . Most of the ions tested were tolerated in moderate amounts. Under the conditions of the general procedure, there was no interference from 150 mg of chloride or from 1.5-mg amounts of ammonium, sodium, calcium, magnesium, manganese ( II), vanadyl, acetate, oxalate, carbonate, citrate, tartrate or thiosulphate. Results for the determination of rhenium in the catalyst materials (Table
362 TABLE 1 Effect of other ions on the determination of rhenium (10 pg) Ion”
Ratio to Re (VII) (w/w)
uo;+
200 80 50 50 100 20 40 20 10 4 5 100 4
co2+ Ni2+ cuz+ Fe3+ MnOc MOO:wo:IO,
Absorbance changeb ( % )
Ratio to Re (VII) (w/w)
Iona
-18
IO, I-
0 0 0
BrO;
0
Br-
60 100 10 200 60 80
-15
Absorbance changeb ( % ) 0
-36 0 -9 0 0
-14 ClO,
0 0 0 0 + 106 0
200 10 1 15 000 20
ClO, ClNO,
-62 0 0 0 0
“Cations added as chloride, anions added as sodium salts. bOindicates < 3% change in absorbance. TABLE 2 Analysis of rhenium-based catalysts Sample”
Rhenium on l/8-in alumina pellets (0.04 g) Rhenium on carbon
Rhenium content ( % ) Specified
Foundb
0.5 5.0
0.49 + 0.06 4.92 f 0.06
“From Aldrich Chemical Co. bMean + 95% confidence
intervals
for 5 replicates.
2) are in good agreement with the specified values. The method is faster and more convenient than methods based on reduction of rhenium (VII) prior to chromogenic complex formation.
REFERENCES F.D. Snell, Photometric and Fluorometric Methods of Analysis: Metals, Wiley, New York, 1978. Z. Marczenko, Separation and Spectrophotometric Determination of Elements, Horwood, Chichester, 1986. A.J. Bowd, D.T. Burns and A.G. Fogg, Talanta, 16 (1969) 719. A.G. Fogg, C. Burgess and D.T. Burns, Talanta, 18 (1971) 1175. A.G. Fogg, C. Burgess and D.T. Burns, Analyst, 95 (1970) 1012.
363 6 7 8 9 10 11 12
D.T. Burns, J.M. Jones and N. Tungkananuruk, Trends Anal. Chem., 4 (1985) VI. N. Ichinose, S. Yamada, N. Sakurai, T. Fujiyama and N. Masuda, Fresenius’ Z. Anal. Chem., 293 (1978) 23. D.T. Burns and N. Tungkananuruk, Anal. Chim. Acta, 199 (1987) 237. D.T. Burns and N. Tungkananuruk, Anal. Chim. Acta, 189 (1986) 383. P. Job, Ann. Chim. (Paris), 9 (1928) 113. J.H. Yoe and A.L. Jones, Ind. Eng. Chem., Anal. Ed., 16 (1944) 111. K. Momoki, J. Sekino, H. Sato and N. Yamaguchi, Anal. Chem., 41 (1969) 1286.
359 Printed in The Netherlands
Short Communication
SPECTROPHOTOMETRIC DETERMINATION OF RHENIUM AS PERRHENATE AFTER EXTRACTION OF ITS BRILLIANT GREEN ION-PAIR WITH MICROCRYSTALLINE BENZOPHENONE
D. THORBURN
BURNS* and N. TUNGKANANURUK
Department of Chemistry, The Queen’s University of Belfast, Belfast ET9 5AG (Northern Ireland) (Received 16th June 1987)
Summary Rhenium (O-40 pg) is determined as pherrhenate spectrophotometrically at 640 nm after its adsorptive extraction at pH 6.0 with Brilliant Green onto microcrystalline benzophenone and dissolution of the solid phase in benzene. The effects of pH and diverse ions are reported. The system is applied to the determination of rhenium on alumina and on carbon catalysts.
The commonest methods for the determination of rhenium are based on reduction of rhenium (VII) with tin ( II) prior to reaction with thiocyanate or cu-furildioxime to give extractable products [ 1,2]. Both methods have the disadvantage of requiring considerable time to achieve maximum colour development. Such delays can be avoided by direct liquid-liquid extraction of rhenium (VII) as perrhenate with onium cations [ 31 or basic dyes [ 41 such as Brilliant Green [ 51; the latter systems are directly compatible with sample preparation by oxidative fusion. To date, in common with other oxoanions [ 61, apart from phosphate as molybdophosphate [ 71 and perchlorate [ 81, no solid-liquid extractions have been reported for the determination of rhenium. The present communication extends earlier studies on rhenium (VII) [ 51 and of Brilliant Green as an ion-pairing reagent [ 4,5], including its use in solid-liquid systems [ 8,9], and describes the adsorptive ion-pair extraction of Brilliant Green perrhenate with microcrystalline benzophenone. The solid benzophenone can subsequently be dissolved in benzene and determinations completed spectrophotometrically at 640 nm. The system can be applied, after oxidation fusion, to the determination of rhenium on alumina or carbon.
0003-2670/88/$03.50
0 1988 Elsevier Science Publishers B.V.
360
Experimental Apparatus. Pye-Unicam SP8-400 and SP6-550 UV-visible spectrophotometers were used for recording absorption spectra and for routine measurements, respectively, with matched quartz l-cm cells. Reagents and solutions. Brilliant green (Aldrich Chemical Corn., declared dye content ca 95%) was used as supplied. A 0.05% (w/v) solution was prepared in absolute ethanol and stored in a dark brown bottle. A stock 5 ,ug ml-l rhenium (VII) solution was prepared by dissolving 0.1553 g of potassium perrhenate (Specpure, Johnson Matthey) in exactly 1 1 of distilled water and diluting 5 ml of this solution to 100 ml with pH 6 buffer solution in a volumetric flask. The pH 6.0 buffer was prepared by adding 56 ml of 0.1 M sodium hydroxide to 500 ml of 0.1 M potassium dihydrogenphosphate and diluting to exactly 11 with water. The benzophenone solution was 20% (w/v) in acetone. All other reagents were of analytical grade. Twice-distilled water was used throughout. Generalprocedure. Place a 1-8 ml aliquot of sample solution containing 5-40 ,ug of rhenium( VII) in a stoppered Erlenmeyer flask. Add 10 ml of pH 6.0 buffer and 1.0 ml of Brilliant Green solution. Swirl to mix, leave for 1 min, add 2.0 ml of benzophenone solution and shake vigorously for 30 s. Filter the blue solid formed through a sintered glass filter (No. 2 porosity). Wash with water, drain or suck dry, dissolve the solid in benzene and dilute to volume in a lo-ml volumetric flask. Dry the solution by addition of 1 g of anhydrous sodium sulphate. Measure the absorbance at 640 nm against a reagent blank prepared in the same way. Procedure for catalyst dissolution. For rhenium on alumina pellets, weigh one pellet and crush to a fine powder in a platinum dish. Add 1.0 g of sodium peroxide and 1.0 g of sodium hydroxide, mix, and fuse carefully over a Bunsen flame until reaction is complete. Allow to cool, add 30 ml of water, and heat gently to dissolve all the material. Cool, filter through a Whatman No. 40 filter paper into a 250-ml volumetric flask and dilute to volume with distilled water. Prepare a blank, using the same weight of pure alumina ( Anderman ) as the catalyst pellet. For rhenium on carbon (which is pyrophoric ) the sample must be handled as follows. Transfer the sample in a glove box to a preweighed aluminum sample pan (as supplied by Perkin-Elmer for use with CHN analyzers and for differential scanning calorimeters) and seal with a small press. Reweigh, place in a platinum dish and take into solution as described above. Prepare a blank with an empty sample pan. Extraction. Transfer a 5-ml aliquot catalyst solution to a lOO-ml beaker. Add 5 ml of 5% (w/v) citric acid solution. The pH should be 4.5-6.0. Adjust the pH by adding 2 M sodium hydroxide and transfer to a stoppered Erlenmeyer flask. Proceed as in the general procedure but omit addition of buffer.
361
Examination of the main experimental variables Naphthalene, diphenyl, 1,4-dichlorobenzene and benzophenone were examined for their adsorptive-extraction properties in microcrystalline solid formation from acetone solution. Benzophenone showed the highest apparent molar absorptivity (3.3 x lo-* 1 mol-’ cm-’ at 640 nm, which is 2.2 x that of naphthalene and 4 x that of diphenyl or 1,4-dichlorobenzene); each solid gave acceptably low blanks (0.03-0.05 absorbance units measured against water). Solvents with a range of functional group types were examined for dissolution of the ion-pair and the solid extractant. Benzene was chosen for routine use because the ion-pair there gave the greatest apparent molar absorptivity. The ion-pair was stable in all solvents except dimethylformamide and dioxan. The effect of pH was examined for 1Opg of rhenium ( VII) by careful addition of 1 M hydrochloric acid or 1 M ammonia prior to extraction. The absorbances were measured as above and were almost independent of pH in the range 4.0-7.0, decreasing outside these limits. In all subsequent work, the aqueous phase was buffered at pH 6.0. The extent of extraction was constant over the range 8.0-14.0 ml of buffer; 10 ml of buffer solution was adopted for routine use. The effect of varying the amount of Brilliant Green solution was examined for 10 ,ugof rhenium ( VII). The absorbance of the extract increased with increasing volume of reagent up to 1.0 ml, remained constant up to 1.5 ml and decreased slowly thereafter. The reagent blank increased slowly with increasing amount of reagent, thus 1.0 ml of Brilliant Green solution was used as a compromise. The extent of extraction was unaffected by ionic strength, phase-volume ratios up to 7.51 of water-/benzophenone in acetone, sequence of addition of reagents, shaking time or digestion time. The absorbance of the dissolved extracts was stable in diffuse daylight for more than 2 h but faded slowly in direct daylight. The composition of the complex was established by Job’s method of continuous variations [lo] and by the mole ratio method [ 11,121 to be [ C,,H,,N,] ReO,. Curvature of the Job’s plot indicated that the ion-pair is appreciably dissociated in solution, similarly to the perchlorate ion-pair [ 81. Results and discussion A linear calibration graph was obtained over the range O-40 ,ug of rhenium (VII) in 10 ml of the final benzene solution. For the determination of 10 pg of rhenium (VII), the relative standard deviation was 0.6% (10 results). The possible interferences of various ion’s were checked (Table 1) . Most of the ions tested were tolerated in moderate amounts. Under the conditions of the general procedure, there was no interference from 150 mg of chloride or from 1.5-mg amounts of ammonium, sodium, calcium, magnesium, manganese ( II), vanadyl, acetate, oxalate, carbonate, citrate, tartrate or thiosulphate. Results for the determination of rhenium in the catalyst materials (Table
362 TABLE 1 Effect of other ions on the determination of rhenium (10 pg) Ion”
Ratio to Re (VII) (w/w)
uo;+
200 80 50 50 100 20 40 20 10 4 5 100 4
co2+ Ni2+ cuz+ Fe3+ MnOc MOO:wo:IO,
Absorbance changeb ( % )
Ratio to Re (VII) (w/w)
Iona
-18
IO, I-
0 0 0
BrO;
0
Br-
60 100 10 200 60 80
-15
Absorbance changeb ( % ) 0
-36 0 -9 0 0
-14 ClO,
0 0 0 0 + 106 0
200 10 1 15 000 20
ClO, ClNO,
-62 0 0 0 0
“Cations added as chloride, anions added as sodium salts. bOindicates < 3% change in absorbance. TABLE 2 Analysis of rhenium-based catalysts Sample”
Rhenium on l/8-in alumina pellets (0.04 g) Rhenium on carbon
Rhenium content ( % ) Specified
Foundb
0.5 5.0
0.49 + 0.06 4.92 f 0.06
“From Aldrich Chemical Co. bMean + 95% confidence
intervals
for 5 replicates.
2) are in good agreement with the specified values. The method is faster and more convenient than methods based on reduction of rhenium (VII) prior to chromogenic complex formation.
REFERENCES F.D. Snell, Photometric and Fluorometric Methods of Analysis: Metals, Wiley, New York, 1978. Z. Marczenko, Separation and Spectrophotometric Determination of Elements, Horwood, Chichester, 1986. A.J. Bowd, D.T. Burns and A.G. Fogg, Talanta, 16 (1969) 719. A.G. Fogg, C. Burgess and D.T. Burns, Talanta, 18 (1971) 1175. A.G. Fogg, C. Burgess and D.T. Burns, Analyst, 95 (1970) 1012.
363 6 7 8 9 10 11 12
D.T. Burns, J.M. Jones and N. Tungkananuruk, Trends Anal. Chem., 4 (1985) VI. N. Ichinose, S. Yamada, N. Sakurai, T. Fujiyama and N. Masuda, Fresenius’ Z. Anal. Chem., 293 (1978) 23. D.T. Burns and N. Tungkananuruk, Anal. Chim. Acta, 199 (1987) 237. D.T. Burns and N. Tungkananuruk, Anal. Chim. Acta, 189 (1986) 383. P. Job, Ann. Chim. (Paris), 9 (1928) 113. J.H. Yoe and A.L. Jones, Ind. Eng. Chem., Anal. Ed., 16 (1944) 111. K. Momoki, J. Sekino, H. Sato and N. Yamaguchi, Anal. Chem., 41 (1969) 1286.