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Spectrophotometric methods for the determination of osmium-I
SPECTROPHQTOMBTRIC DETERMINATION
METHODS F8R OF OSMIUM--I”
THE
EXTRACTION AND ULTRAVIOLET SPECTROPMOTOIiJETRIC DETERMJNATXQN OF OSMIUM TETROXEDE
&pUtment
of Chemistry,
university
of Tennessee,
KnoxviEe, Tt?nn@see, U.S.A.
&&PXKXX were desired for the ~~~~~rni~at~onof mifhgram and microgmm quantities of osmium in solutions of uranyl aulphate which contain copper3 nickel, iron, and chromium as minor components The existing methods4 are not sufficiently selective foiurthe direct determination of osmium without a prior separation. Moreover, osmium can exist in solution in many different valency states and therefore, a further prrrablemwas that of obtaining all of the osmium in a single valency state. Accordingly, an extremely selective separation procedure suggested by Sauerbrunn and SandelF was tested whereby osmium is o&i&cd to the octov&ent state md the osmium ~~~~~~~~ w&i& is formed, is thea &ramd &th chloroform. In the work presented in this paper the osmium is ~~~~~i~~ by measuring the ~~a~o~~t ~b~rb~y of the extracted osmium tetroxidc, A~t~o~~ the u&raviofet a~~~~~~ spectrum of osmium tetroxide in organic saXvents has been previously report& in studies of the vibmtianal frequencies of the osmium tetroxide moleculeSf the ahsarbartcy of osmium tetroxide has not been utilised previously for analytical purposes, In a second method’ the osmium is oxidised and extracted a,@before, then the extmct is added to an ethanolic solution of 1:5-diphenylcarbahydrazide and a blueviolet coloured complex is formed This method is also extremely selective for osmium aad is about 15 times more sensitive than the method described hero, A further methods involves the fossnation of the osmium complex with 1:5d~~h~~y~~bohydr~de in an aqueous medium rather than in the organic extract.
rc%%&E%T&Xr S&W&Tc%3&& Nir,~*~~~~~R~~~ finian
$
Et @Zk
Rid@?
?‘k&‘Sd
&&XRtOrJrjF, ~r&ed
Carbide Nu&ar CCL,dkisian of t3rtiorr C.&rbi&zCoq.~ For the Atomic Eneqj! Commition.
Regent
address:
?+&a~
Mate&& and &&me?% 296
~u~~ratiolI,
#q&o,
%?emqivania, tr.S.A_
&y
S~ro~hoto~tric
methods for the determination
of osmium-I
297
The complex is then extracted with chloroform and the absorbancy of the extract is measured. Although this method is not as selective for osmium as the two previousIy described methods, it provides an .extremely sensitive means of determining osmium. The molar absorbancy index is about 150,000. EXPERIMENTAL Reqplts Osmiumt&oxide, O-38 mg of osmium per ml, in O*lM sulphuric acid : A vial containing O-5 g of osmium tetroxide was broken under 500 ml of @tM sulphuric acid. After the osmium tetroxide had dissolved compietely, the solution was filtered into a I-Ii&e volumetric flask and diluted to volume with 0.1 M sulphuric acid. Thii solution was standard&d by the gravimetric benzotriazole method.B More dilute osmium scrtutionswere prepared by appropriate dilution of the standard solution.
Beckman Model DU s~trophotometer~
equipped with ultraviolet accessories.
Procedure Transfer a sample aliquot which contains between 0.4 and 3.3 mg of osmium to a &O-mlseparatory funnel. If the osmium is not in the octovalent state, dilute to 5 ml with 6N sulplmric acid and oxidise the osmium by the dropwise addition of a 5 % potassium permanganate solution until a permanent pink colour persists. Discharge the pink colour with .a few drops of a 2 % ferrous ammonium sulphate solution. Immediately add 3 ml of 15M nitric acid and 2 ml of water and extract the osmium tetroxide with two lo-ml portions of chloroform. Drain the extracts into a second 60”ml separatory funnel which contains 10 ml of 0.1M sulphuric acid, and wash to remove any traces of nitric acid. Drain the chloroform phase mto a 25-ml volumetric flask containing about I g of anhydrous sodium sulphate and dilute to vofume with chloroform. Measure the absorbancy of the solution against a chloroform blank at the appropriate wavelength using I-cm cells. Prepare a calibration curve by adjusting suitable aliquots of a standard osmium tetroxide solution to a IO-ml volume containing 30 volume per cent of nitric acid and extracting them by the same procedure. The oxidation step is not necessary. RESULTS
AND
DISCUSSION
Oxidation and extraction of osmium tetroxide These procedures are essentially the same as those proposed by Sauerbrunn and Sandells and they are discussed by Sandell. 4 Since osmium is extracted from a nitric acid solution, many elements besides osmium can also be extracted if the solvent is polar and has an oxygen-containing functional group. 2 Therefore a nonpolar solvent is necessary for the selective extraction of osmium. The dist~bu~on ratiu of osmium tetroxide into chloroform is slightly greater than into carbon tetrachloride. More than 99 % of the osmium is extracted into the chloroform phase. Ultraviolet
absorption spectrum of osmium tetroxide The absorption spectrum of osmium tetroxide in chloroform (Fig. 1) exhibits the same features as the spectra previously reported in carbon tetrachloride and in hexane.l Above 320 rnp the absorbancy of the osmium tetroxide solution is very small. Below 260 rnp the absorbancy increases rapidly. However, the absorbancy of the chloroform blank also becom.es appreciable below 260 rnp, Adhererwe to Beer’s law Calibration data for the dete~~tion of osmium were obtained for the 5 absorption bands which cents-e at 282,289, 297, 304 and 312 rnp= The molar absorbancy indexes are presented in Table I,
GERALDGOLDSTEIN,D. L. MANNING, OSCAR MENB and J. A. DEAN
298
The absorbancies at all of the wavelengths adhere to Beer’s law. In addition, the optimum concentration range for the determination of osmium was evaluated for each wavelength by the method of Ringborn and these are shown in Table I. A range of from O-4 to 3.3 mg of osmium can be determined by proper choice of the wavelength, with a coefficient of variation of about 3 %. 0.7
‘r
0.6
g 0.5 T P ,o $ 0.4
0.:
i WAVE LENGTH, m/i FIG. l.-Ultraviolet
absorption spectrumof osmium tetroxidein chloroform Osmium, M, 3.20 x IO-* Cells, cm, 1
Eflect of foreign substances The effect of various cations and anions was evaluated by treating solutions containing 1.14 mg of osmium and the foreign substance to be tested by the recommended procedure. The results of these tests are shown in Table II. Neither the uranyl ion nor the other cations which are normally found as minor coppeP, nickel,r’ chromiumvrcomponents in uranyl sulphate solutions-iron”‘, interfere appreciably in the determination of osmium. When more than 500 mg of chromiumvl was present high results were obtained. The platinum group elements, in their lower valency states, do not interfere. Ruthenium in the octovalent state is TABLE I-DATA
Wavelength, w
FOR THE ULTRAVIOLETSPECTROPHOTOMETRIC DETERMINATION OF OSMIUM
Molar absorbancy index --
Optimum concentration range*
Mx
10’
I
mgl2.5 ml
282
1870
0.8 -
3.7
0.4 -
289 297 304 312
1760 1640 1400 1000
0.9 0.9 1.1 1.5
4.0 4.3 5-O 7.0
0.4 0.5 0.5 @7
-
-
1.8 1.9 2.0 24 3.3
Spectrophotometric methods for the determination of osmium-I
299
TABLEH.-EFFECT OF FOREIGN SUBSTANCES Conditions :
Volume, ml 25 Wavelength, rnp 289 1 Cells, cm
I
Foreign substance Element uv
r
Fern Nirr Cu” Tht” Crvl Crln MO”’ Pdt ’ Au”’ Rip
ptr* Irlu Ru”’
w
.-
1000 1000 1000 1000 500 500 500 500 6 5 2 2 1 1
-
1.14
Osmium,mg
-
Osmium, rn,? Found
Error
1.19 1.22 1.18 1.19 1.17 1.19 1.16 1.14 1.20 1.12 1.10 1.17 1.16 1.19
0.05 0.08 0.04 0.05 0.03 0.05 0.02 O@O 0.06 -0.02 -o+l 0.03 0.02 0.05
-/
an interference since ruthenium tetroxide extracts along with osmium tetroxide4 and has a similar absorption spectrum. l However, osmium can be separated from ruthenium by a modification of the extraction procedure.4 Of the anions which were tested, perchlorate, sulphate and phosphate did not interfere. When the osmium was initially in a reduced state in the presence of chloride, low results were obtained, since chloride forms complex ions with osmiumIV and v1 and inhibits the complete oxidation and subsequent extraction of osmium .4 For this application, however, the chloride concentration in uranyl sulphate sample solutions is very small and therefore, chloride interference is not significant. CONCLUSION The recommended procedure provides a simple and rapid method for the determination of milligram amounts of osmium in solutions of uranyl sulphate. This method is probably applicable to many other types of sample solutions. Of the elements tested only chloride and octovalent ruthenium were found to interfere; however, procedures are available for elimination of both of these interferences.4 Acknowledgments-The authors acknowledge the assistance of H. P. House and M. A. Marler in the preparation of this manuscript. This work was abstracted from a thesis submitted by Gerald Goldstein in partial fulfillment of the requirements for the degree of Master of Science, 1959 Zusammenfassung-Eine uv-spektrophotometrische Methode zur Bestimmung von Milligrammengen Osmium in Uranylsulfatlosungen wird beschrieben. Osmium wird erst in die achtwertige Oxydationstufe gebracht und das Osmiumtetroxyd selektiv mittels Chloroform ausgezogen. Das UV-Spektrum von Osmiumtetroxyd hat mehrere Absorptionsmaxima: 282, 289, 297, 304 und 312 rnp mit den dazugehiirigen Absorptionskoeffizienten von 1870,1760,1640,1400 und 1000. Fur jede der genannten
300
GERALD GOLDSTEIN,D. L. MANNING, OSCAR MENI~ and J. A. DEAN
We~enl~ngen wurde der optimale Ko~entmtions~rei~h ermittelt. Fur Mengen von 0.4-3.3 mg kann die Bestimmung mit einem Variationskoeffizienten von 3 % ausgefttrt werden. Von den untersuchten Ionen St&en nur Ruthenium(III) und Chloride, jedoch sind diese Storungen auszuschaltbar. R&sum&-Les auteurs presentent une methode spectrophotom&rique ultra-violette de dosage de quantites d’osmium de l’ordre du milligramme en solution de sulfate d’uranyle. L’osmium est d’abord oxyde il petat octavalent, et le tetroxyde d’osmium formi: est extrait selectivement par le chloroforme. Le spectre ultra-violet du tetroxyde d’osmium dans le chloroforme a une serie de bandes d’absorption avec des maxima a 282,289,297,304 et 312 rnp, et des coefficients d’extinction molaires de 1870,1760, 1640, 1400 et 1000 respectivement Pour chaque longueur d’onde, le domaine de concentration le meilleur pour le dosage de l’osmium a ettCCvalue. De 0.4 Q 3,3 mg d’osmium peuvent etre doses avec un coefficient de variation de 3 pour cent. Panni les elements essay&s, seuls le chlornre et le ruthenium octavalent g&rent; ces deux interfbences peuvent cependant ttre &mine&. REFERENCES 1 A. Langseth and B. Qviller, Zphys. Chem., 1934, B 27,79. e G. H. Morrison and II. Freiser, Solvent Extraction in Analytical Chemistry. John Wiley, New York, 1957, p. 140. J A. Ringbom, Z. analyt. Chem. 1939,115, 322. rl E. B. Sandell, Calorimetric Determination of Traces of Metals. Interscience Publishers, Inc., New York, 1959,3rd ed. Chap. XxX1. 6 R. D. Sauerbrunn and E. B. Sandell, Aluriyt. Chim. Acta, 1953,9,86. 6 R. F. Wilson and L. J. Baye, Tafunta, 1958, 1,351. ’ Gerald Goldstein, D. L. Manning, Oscar Menis and J. A. Dean, Tulunta, 1961,7,301. s idem, i&id., 1961,7, 307.
METHODS F8R OF OSMIUM--I”
THE
EXTRACTION AND ULTRAVIOLET SPECTROPMOTOIiJETRIC DETERMJNATXQN OF OSMIUM TETROXEDE
&pUtment
of Chemistry,
university
of Tennessee,
KnoxviEe, Tt?nn@see, U.S.A.
&&PXKXX were desired for the ~~~~~rni~at~onof mifhgram and microgmm quantities of osmium in solutions of uranyl aulphate which contain copper3 nickel, iron, and chromium as minor components The existing methods4 are not sufficiently selective foiurthe direct determination of osmium without a prior separation. Moreover, osmium can exist in solution in many different valency states and therefore, a further prrrablemwas that of obtaining all of the osmium in a single valency state. Accordingly, an extremely selective separation procedure suggested by Sauerbrunn and SandelF was tested whereby osmium is o&i&cd to the octov&ent state md the osmium ~~~~~~~~ w&i& is formed, is thea &ramd &th chloroform. In the work presented in this paper the osmium is ~~~~~i~~ by measuring the ~~a~o~~t ~b~rb~y of the extracted osmium tetroxidc, A~t~o~~ the u&raviofet a~~~~~~ spectrum of osmium tetroxide in organic saXvents has been previously report& in studies of the vibmtianal frequencies of the osmium tetroxide moleculeSf the ahsarbartcy of osmium tetroxide has not been utilised previously for analytical purposes, In a second method’ the osmium is oxidised and extracted a,@before, then the extmct is added to an ethanolic solution of 1:5-diphenylcarbahydrazide and a blueviolet coloured complex is formed This method is also extremely selective for osmium aad is about 15 times more sensitive than the method described hero, A further methods involves the fossnation of the osmium complex with 1:5d~~h~~y~~bohydr~de in an aqueous medium rather than in the organic extract.
rc%%&E%T&Xr S&W&Tc%3&& Nir,~*~~~~~R~~~ finian
$
Et @Zk
Rid@?
?‘k&‘Sd
&&XRtOrJrjF, ~r&ed
Carbide Nu&ar CCL,dkisian of t3rtiorr C.&rbi&zCoq.~ For the Atomic Eneqj! Commition.
Regent
address:
?+&a~
Mate&& and &&me?% 296
~u~~ratiolI,
#q&o,
%?emqivania, tr.S.A_
&y
S~ro~hoto~tric
methods for the determination
of osmium-I
297
The complex is then extracted with chloroform and the absorbancy of the extract is measured. Although this method is not as selective for osmium as the two previousIy described methods, it provides an .extremely sensitive means of determining osmium. The molar absorbancy index is about 150,000. EXPERIMENTAL Reqplts Osmiumt&oxide, O-38 mg of osmium per ml, in O*lM sulphuric acid : A vial containing O-5 g of osmium tetroxide was broken under 500 ml of @tM sulphuric acid. After the osmium tetroxide had dissolved compietely, the solution was filtered into a I-Ii&e volumetric flask and diluted to volume with 0.1 M sulphuric acid. Thii solution was standard&d by the gravimetric benzotriazole method.B More dilute osmium scrtutionswere prepared by appropriate dilution of the standard solution.
Beckman Model DU s~trophotometer~
equipped with ultraviolet accessories.
Procedure Transfer a sample aliquot which contains between 0.4 and 3.3 mg of osmium to a &O-mlseparatory funnel. If the osmium is not in the octovalent state, dilute to 5 ml with 6N sulplmric acid and oxidise the osmium by the dropwise addition of a 5 % potassium permanganate solution until a permanent pink colour persists. Discharge the pink colour with .a few drops of a 2 % ferrous ammonium sulphate solution. Immediately add 3 ml of 15M nitric acid and 2 ml of water and extract the osmium tetroxide with two lo-ml portions of chloroform. Drain the extracts into a second 60”ml separatory funnel which contains 10 ml of 0.1M sulphuric acid, and wash to remove any traces of nitric acid. Drain the chloroform phase mto a 25-ml volumetric flask containing about I g of anhydrous sodium sulphate and dilute to vofume with chloroform. Measure the absorbancy of the solution against a chloroform blank at the appropriate wavelength using I-cm cells. Prepare a calibration curve by adjusting suitable aliquots of a standard osmium tetroxide solution to a IO-ml volume containing 30 volume per cent of nitric acid and extracting them by the same procedure. The oxidation step is not necessary. RESULTS
AND
DISCUSSION
Oxidation and extraction of osmium tetroxide These procedures are essentially the same as those proposed by Sauerbrunn and Sandells and they are discussed by Sandell. 4 Since osmium is extracted from a nitric acid solution, many elements besides osmium can also be extracted if the solvent is polar and has an oxygen-containing functional group. 2 Therefore a nonpolar solvent is necessary for the selective extraction of osmium. The dist~bu~on ratiu of osmium tetroxide into chloroform is slightly greater than into carbon tetrachloride. More than 99 % of the osmium is extracted into the chloroform phase. Ultraviolet
absorption spectrum of osmium tetroxide The absorption spectrum of osmium tetroxide in chloroform (Fig. 1) exhibits the same features as the spectra previously reported in carbon tetrachloride and in hexane.l Above 320 rnp the absorbancy of the osmium tetroxide solution is very small. Below 260 rnp the absorbancy increases rapidly. However, the absorbancy of the chloroform blank also becom.es appreciable below 260 rnp, Adhererwe to Beer’s law Calibration data for the dete~~tion of osmium were obtained for the 5 absorption bands which cents-e at 282,289, 297, 304 and 312 rnp= The molar absorbancy indexes are presented in Table I,
GERALDGOLDSTEIN,D. L. MANNING, OSCAR MENB and J. A. DEAN
298
The absorbancies at all of the wavelengths adhere to Beer’s law. In addition, the optimum concentration range for the determination of osmium was evaluated for each wavelength by the method of Ringborn and these are shown in Table I. A range of from O-4 to 3.3 mg of osmium can be determined by proper choice of the wavelength, with a coefficient of variation of about 3 %. 0.7
‘r
0.6
g 0.5 T P ,o $ 0.4
0.:
i WAVE LENGTH, m/i FIG. l.-Ultraviolet
absorption spectrumof osmium tetroxidein chloroform Osmium, M, 3.20 x IO-* Cells, cm, 1
Eflect of foreign substances The effect of various cations and anions was evaluated by treating solutions containing 1.14 mg of osmium and the foreign substance to be tested by the recommended procedure. The results of these tests are shown in Table II. Neither the uranyl ion nor the other cations which are normally found as minor coppeP, nickel,r’ chromiumvrcomponents in uranyl sulphate solutions-iron”‘, interfere appreciably in the determination of osmium. When more than 500 mg of chromiumvl was present high results were obtained. The platinum group elements, in their lower valency states, do not interfere. Ruthenium in the octovalent state is TABLE I-DATA
Wavelength, w
FOR THE ULTRAVIOLETSPECTROPHOTOMETRIC DETERMINATION OF OSMIUM
Molar absorbancy index --
Optimum concentration range*
Mx
10’
I
mgl2.5 ml
282
1870
0.8 -
3.7
0.4 -
289 297 304 312
1760 1640 1400 1000
0.9 0.9 1.1 1.5
4.0 4.3 5-O 7.0
0.4 0.5 0.5 @7
-
-
1.8 1.9 2.0 24 3.3
Spectrophotometric methods for the determination of osmium-I
299
TABLEH.-EFFECT OF FOREIGN SUBSTANCES Conditions :
Volume, ml 25 Wavelength, rnp 289 1 Cells, cm
I
Foreign substance Element uv
r
Fern Nirr Cu” Tht” Crvl Crln MO”’ Pdt ’ Au”’ Rip
ptr* Irlu Ru”’
w
.-
1000 1000 1000 1000 500 500 500 500 6 5 2 2 1 1
-
1.14
Osmium,mg
-
Osmium, rn,? Found
Error
1.19 1.22 1.18 1.19 1.17 1.19 1.16 1.14 1.20 1.12 1.10 1.17 1.16 1.19
0.05 0.08 0.04 0.05 0.03 0.05 0.02 O@O 0.06 -0.02 -o+l 0.03 0.02 0.05
-/
an interference since ruthenium tetroxide extracts along with osmium tetroxide4 and has a similar absorption spectrum. l However, osmium can be separated from ruthenium by a modification of the extraction procedure.4 Of the anions which were tested, perchlorate, sulphate and phosphate did not interfere. When the osmium was initially in a reduced state in the presence of chloride, low results were obtained, since chloride forms complex ions with osmiumIV and v1 and inhibits the complete oxidation and subsequent extraction of osmium .4 For this application, however, the chloride concentration in uranyl sulphate sample solutions is very small and therefore, chloride interference is not significant. CONCLUSION The recommended procedure provides a simple and rapid method for the determination of milligram amounts of osmium in solutions of uranyl sulphate. This method is probably applicable to many other types of sample solutions. Of the elements tested only chloride and octovalent ruthenium were found to interfere; however, procedures are available for elimination of both of these interferences.4 Acknowledgments-The authors acknowledge the assistance of H. P. House and M. A. Marler in the preparation of this manuscript. This work was abstracted from a thesis submitted by Gerald Goldstein in partial fulfillment of the requirements for the degree of Master of Science, 1959 Zusammenfassung-Eine uv-spektrophotometrische Methode zur Bestimmung von Milligrammengen Osmium in Uranylsulfatlosungen wird beschrieben. Osmium wird erst in die achtwertige Oxydationstufe gebracht und das Osmiumtetroxyd selektiv mittels Chloroform ausgezogen. Das UV-Spektrum von Osmiumtetroxyd hat mehrere Absorptionsmaxima: 282, 289, 297, 304 und 312 rnp mit den dazugehiirigen Absorptionskoeffizienten von 1870,1760,1640,1400 und 1000. Fur jede der genannten
300
GERALD GOLDSTEIN,D. L. MANNING, OSCAR MENI~ and J. A. DEAN
We~enl~ngen wurde der optimale Ko~entmtions~rei~h ermittelt. Fur Mengen von 0.4-3.3 mg kann die Bestimmung mit einem Variationskoeffizienten von 3 % ausgefttrt werden. Von den untersuchten Ionen St&en nur Ruthenium(III) und Chloride, jedoch sind diese Storungen auszuschaltbar. R&sum&-Les auteurs presentent une methode spectrophotom&rique ultra-violette de dosage de quantites d’osmium de l’ordre du milligramme en solution de sulfate d’uranyle. L’osmium est d’abord oxyde il petat octavalent, et le tetroxyde d’osmium formi: est extrait selectivement par le chloroforme. Le spectre ultra-violet du tetroxyde d’osmium dans le chloroforme a une serie de bandes d’absorption avec des maxima a 282,289,297,304 et 312 rnp, et des coefficients d’extinction molaires de 1870,1760, 1640, 1400 et 1000 respectivement Pour chaque longueur d’onde, le domaine de concentration le meilleur pour le dosage de l’osmium a ettCCvalue. De 0.4 Q 3,3 mg d’osmium peuvent etre doses avec un coefficient de variation de 3 pour cent. Panni les elements essay&s, seuls le chlornre et le ruthenium octavalent g&rent; ces deux interfbences peuvent cependant ttre &mine&. REFERENCES 1 A. Langseth and B. Qviller, Zphys. Chem., 1934, B 27,79. e G. H. Morrison and II. Freiser, Solvent Extraction in Analytical Chemistry. John Wiley, New York, 1957, p. 140. J A. Ringbom, Z. analyt. Chem. 1939,115, 322. rl E. B. Sandell, Calorimetric Determination of Traces of Metals. Interscience Publishers, Inc., New York, 1959,3rd ed. Chap. XxX1. 6 R. D. Sauerbrunn and E. B. Sandell, Aluriyt. Chim. Acta, 1953,9,86. 6 R. F. Wilson and L. J. Baye, Tafunta, 1958, 1,351. ’ Gerald Goldstein, D. L. Manning, Oscar Menis and J. A. Dean, Tulunta, 1961,7,301. s idem, i&id., 1961,7, 307.