Spectrophotometric Determination of Osmium in Osmium Hexaf I uoride Using Thiourea SIR: We report a method for the determination of osmium in highly reactive, volatile osmium hexafluoride (m.p. 32.1’ C., b.p. 45.9’ C) and in mixtures of this compound and uranium hexafluoride. Details are given for preparing solutions of osmium hexafluoride for analysis and for converting the hydrolyzed compound into a species which reacts with thiourea a t a reasonable rate. Data obtained using this method have furnished corroborative evidence for the identity of osmium hexafluoride ( 7 ) and have been used to measure the composition of mixtures of osmium hexafluoride and uranium hexafluoride on which magnetic susceptibility measurements have been made (1). EXPERIMENTAL
Reagents and Apparatus. Osmium metal-finely powdered metal, reduced in hydrogen-was used for the preparation of solutions of known osmium content. This material was assayed by igniting a weighed sample (in a well ventilated hood) and measuring the loss in weight due to volatilization of osmium as the tetroxide, Os04. The material assayed 99.6% osmium. Modified Carius combustion tubes and steel protecting shell similar to those described by Gordon, Schlecht, and Wichers (4) were used to prepare standardized osmium solutions and to convert hydrolyzed osmium hexafluoride into a reactive species. Preparation of Sample Solutions for Osmium Determination. An accurately weighed quartz bulb containing osmium hexafluoride or an osmium hexafluoride-uranium hexafluoride mixture was frozen in -100 ml. of liquid nitrogen contained in a 250-ml. platinum beaker. The beaker was nested in a larger beaker of Teflon also containing liquid nitrogen. Both beakers were contained in a larger Dewar flask. When chilled to liquid nitrogen temperature, the bulb was broken by crushing it with a 15mm. diameter, fire polished borosilicate glass rod. About 50 ml. of water was carefully added drop by drop to the liquid nitrogen while stirring. Approximately 100 ml. more of water was added slowly. During this operation the water froze and the liquid nitrogen evaporated. The platinum beaker was then allowed to stand a t room temperature until the ice melted and a clear solution resulted. This solution was filtered through a weighed Munro type platinum sponge crucible directly into an appropriately sized volumetric flask. The crushed quartz fragments were quantitatively washed into the crucible with water. The filtrate was then 1430
ANALYTICAL CHEMISTRY
diluted to volume. The quartz fragments and crucible were dried and weighed to obtain the weight of the original quartz bulb and, by difference from the original bulb and sample weight, the net weight of hexafluoride sample. Osmium Determination. Osmium was determined by spectrophotometric measurement of the osmiumthiourea complex a t 480 mp. It takes about three weeks for the reaction between the hydrolyzed osmium hexafluoride species [mostly fluoroosmate, OSFG-~,. ( r ) ] and thiourea to go to completion a t room temperature. Dilute nitric acid oxidized fluoroosmate a t 300’ C. in a sealed tube to form an osmium species, probably the tetroxide, which reacts rapidly and quantitatively with thiourea to form the colored complex. Procedure. Approximately 500 pl. of concentrated nitric acid and a few ml. of water were added to a modified Carius tube, and the mixture was frozen by immersing the end of the tube in liquid nitrogen. An aliquot of the hydrolyzed hexafluoride sample solution containing approximately 1.8 mg. of osmium (0.36 mg. of osmium when 5-cm. cells were used for the absorbance measurements) was transferred into the modified Carius tube and the tube was sealed (See note below). The tube was then placed in a protecting steel shell, along with solid carbon dioxide to provide compensating pressure within the shell. After the shell was capped and tested for leaks, the shell and tube were heated a t 300’ C. for a t least 4 hours and then allowed to cool to room temperature. After removal from the protecting shell, the tube and its contents were frozen in liquid nitrogen. The tube was then opened (See note below) and placed in 50 ml. of a solution -0.4M thiourea and -4M sulfuric acid contained in a 500ml. glass-stoppered Erlenmeyer flask. After the sample solution had melted and reacted with thiourea, the glass fragments from the tube were separated by filtering the solution through a medium porosity, glass-sintered crucible directly into a 100-ml. volumetric flask. Water was used as the transfer agent. (Filter paper was not used for this step because it bleached the color of the complex.) The solution was diluted to 100.0 ml. The resulting color due to the [ O S ( N H ~ C S ? ~ ” ~ )ion J + ~(6) is quite stable and can be measured when convenient. Immediately before spectrophotometric measurement, however, a portion of the solution was again filtered through a clean, dry, fine porosity glass sintered filter to remove an insoluble thiourea decomposition product, probably sulfur, which formed on standing. The absorbance of the filtered solution was measured a t 480 mp us. water. One- or five-cm. cells,
depending on the amount of osmium present, were used for the measurements. The reagent blank was negligible. When necessary a correction was made for the absorbance of any uranium present; this correction is quite small. The osmium concentration was calculated from the net sample absorbance and the absorptivity of the osmium thiourea complex. The molar absorptivity of the complex was estimated to be 4.19 x lo3 by reacting known quantities of osmium (VIIT) with thiourea and measuring the absorbance of the resulting solution. Solutions of accurately known [Os(NH&SNHz),] + 3 concentration were prepared by dissolving weighed amounts (-0.1 gram) of osmium metal in -10 ml. of -0.8M nitric acid a t 275’-300° C. for a t least 4 hours using the sealed tube technique as above. Upon removal from the steel protecting shell the tube containing the dissolved osmium metal was frozen, opened, and the tube and contents were placed in 600 ml. of -6N sulfuric acid containing about 15 grams of thiourea. After formation of the osmium-thiourea complex the solution was quantitatively decanted into a 1-liter volumetric flask and diluted to volume with water. Aliquots of the standard osmiumthiourea solution were diluted to contain about 20 p g . of osmium per ml. and the absorbance of the solution was measured a t 480 mp using I-cm. cells. Note: Articles by Gordon (3) and Gordon, Schlecht, and Wichers (4) describe the development of and uses for modified Carius combustion tubes of the type employed herein for the oxidation of hydrolyzed osmium hexafluoride and for the preparation of standard osmium solutions. These articles should be consulted before carrying out the procedure. Their recommendations regarding details of equipment and its use were followed with but minor variations. However, a modified technique for sealing and for opening the tubes was used. To seal a tube the stem was rotated in a small oxygen-gas flame until the opening was almost closed. The tube was temporarily sealed by touching a drop of molten glass (from a borosilicate glass rod heated simultaneously) to the almost closed stem. The solution and tube were chilled by immersion in liquid nitrogen for 30 to 45 seconds. This cooling created a negative pressure within the tube. The end of the tube was then heated with rotation until a molten bulb of glass formed, part of which sucked back into the stem. This resulted in a strong, well rounded seal free from the presence of a minute capillary. Certain sealing techniques can leave a minute capillary in the seal. Gordon et al. (4) have noted that this may result in a pin-hole leak which can allow a possibly dangerous
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pressure to develop during heating and remain within the tube after it has been cooled to room temperature. The tubes were opened only after any possible remaining pressure was released. The tube was placed in a simple shell made from a 6-inch length of ordinary 1-inch 0.d. threaded black pipe with a cap a t one end. The cap had a a/,-inch hole drilled through it as did a second cap which was placed over the stem of the tube and screwed into place, thus locking the tube in the shell. The protecting shell containing the tube and its contents was placed in a small Dewar containing liquid nitrogen. Only about 2 inches of the stem of the tube remained exposed out of the shell. After the shell and tube had reached the temperature of the liquid nitrogen, possible pressure within the tube was released by heating a spot near the end of the stem with a small oxygen-gas flame until the glass melted and a hole in the stem was either blown out or sucked in depending on the final pressure within the tube. Possible pressure within the tube thus released, the cap was unscrewed and the tube removed from the protecting shell. The tube was broken a t the desired places by making a deep scratch with a file and touching the scratch with a molten hot borosilicate glass rod. Uranium Determination. Uranium was determined in a separate sample aliquot after volatilization of osmium (in a well ventilated hood) as the tetroxide from a mixture of sulfuric and nitric acids. After complete expulsion of osmium and nitric acid the uranium, in hydrochloric acid-sulfuric acid solution, was determined by reduction to uranium(1V) in a lead reductor and reoxidation with excess standard ceric sulfate solut,ion. The remaining ceric sulfate was then titrated with standard ferrous sulfate solution. The titrations were made using weight burets (6). Fluoride Determination. Fluoride was separated from osmium as fluosilicic acid, H2SiFs, by use of the Wil!ard and Winter distillation (9) from sulfuric acid to which ferrous sulfate was first added to prevent distillation of osmium tetroxide. The fluoride in the distillate was titrated with 0.025M thorium nitrate using sodium alizarin sulfonate as indicator. I
h
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RESULTS AND DISCUSSION
The results obtained using the methods described are shown in Table I. The materials analyzed were very pure hexafluoride samples. Details of the techniques used to prepare and purify hexafluoride compounds have been reported elsewhere (7, 8). With pure hexafluoride samples, estimating the relative standard deviation for the sample weight to be O.lyo and using the published value of 0.08% (5) for the relative standard deviation in the uranium analysis, the mass balance recovery data in Table I indicate the relative standard deviation VOL. 37, NO. 1 1 , OCTOBER 1965
1431
in the osmium determination to be 0.6%. An independent measure of the error in the osmium determination was made by determining fluoride in addition to the uranium and/or osmium content. Total elemental determinations on five samples yielded a mass balance recovery averaging 99.3 f 1.7y0 as also shown in Table I. This recovery is consistent with the aforementioned estimated error in the osmium determination since the relative error in the fluoride determination is estimated to be f 2 % ($)-much higher than the error in the other determinations. Preliminary dissolution experiments showed that if capsules containing osmium hexafluoride were simply broken under chilled water a vigorous reaction ensued with osmium hexafluoride rising so rapidly to the surface that some of the compound was lost by volatilization. Freezing the compound prevented any Also, the compound hysuch loss. drolyzed a t a controlled rate while the frozen solution melted. Dilute sodium hydroxide has been used as a solvent for osmium hexa-
fluoride (7) but could not be used for mixtures containing uranium hexafluoride since a sodium diuranate precipitate resulted. The aqueous sample solutions were stable for a t least one week although a black residue, probably hydrated osmium dioxide, OsOz XH20, did appear in some of the sample solutions after standing for longer periods. Sample aliquots were taken immediately after dissolution of the sample. The sealed tube oxidation technique quantitatively converts fluoroosmate into a reactive osmium species thus solving the problem inherent in any osmium determination-namely, to make sure that all of the osmium is present as a species which will react a t a reasonable rate with the reagent used for the determination. No osmium can be lost by volatilization in the process. The technique also allows for a convenient preparation of solutions of accurately known osmium(VII1) concentration.
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ACKNOWLEDGMENT
Grateful acknowledgment is made to Alice h1. Essling and Irene M. Fox, both
of this laboratory, who were responsible for the uranium and fluoride determinations reported. LITERATURE CITED
(1) Bromberg, J.. P., Ph. D. Thesis,
University of Chicago, December, 1964. (2) Fox, I. M., Argonne National Labora-
tory, Argonne, Ill., personal communication, April 1965. (3) Gordon, C. L., J . Res. Natl. Bur. Std. 30,107 (1943). (4) Gordon, C. L., Schlecht, W. G., Wichers, E., Ibid., 33, 457 (1944). (5) Patterson, J. H., U. S. At. Energy Comm. Reat.. ANL-5410 (195.5). (6) Sauerbrinn, R. D., Sand& E. B., J . Am. Chem. SOC.75,3554 (1953). (7) Weinstock, B., Malm, J. G., J . Am. Chem. SOC.80 4466 (1958). (8) Weinstock, b., Malm, J. G..' J . Inoro. Nucl. Chem. 2, 380 (1956). (9) Willard, H. H., Winter, 0. B., ANAL. CHEM.5,7 (1933). KENNETH J. JENSEN Chemistry Division Argonne National Laboratory Argonne, Ill. 60440 Work erformed under the auspices of the U. S.k)tomic Energy Commission.
Complexes of IJO-Phenanthroline and 2,2'-Bipyridine with Copper(l1) as Adsorbents for Gas Chromatography SIR: Silica and similar adsorbents have been used quite successfully for the separation of low molecular-weight substances such as CHa, CzHc, He, CO, COz, etc., but have not found much application for the separation of larger compounds because strong adsorption and unsymmetrical peaks are usually obtained. However, pure substances which have small specific surface areas (on the order of 1-10 sq. meters per gram) can be used as adsorbents for gas chromatographic separations provided very small samples (around 0.1 pg.) are injected (1, 4). Adsorbents prepared by removal of a volatile substance (pyridine, ammonia, and water) from an inorganic salt separated a variety of organic compounds including polar compounds, like ketones and alcohols, which are difficult or impossible to elute from the usual adsorbents. Volatile compounds were eluted, usually as symmetrical peaks, in less than one hour a t column temperatures as much as 150' C. below their boiling points. Several of these adsorbents afforded separations comparable to those obtained using liquid-phase columns and, in some cases, superior to them. Many separations by these adsorbents appeared t o be governed largely by an interaction between the metal ion of the 1432
ANALYTICAL CHEMISTRY
electronegative center of the sample molecule (1, 4). The fact that a wide variety of compounds could be eluted rather easily from columns packed with some of these materials seemed sufEciently unusual to warrant other studies. In addition, high selectivity was unexpectedly found for certain pairs of substances such as the 2- and 3-heptanones. The adsorbents in the present study were also capable of the same types of separation to varying degrees. The chief disadvantage of the copperamine adsorbents studied earlier was a low thermal stability which limited the range of volatile compounds that could be separated. At relatively low column temperatures (around 100' C.), the adsorbents began to decompose further. This led both to a noisy base line (when an organic amine was being liberated) and a change in the characteristics of the adsorbent. Therefore, the objective of the present work was to prepare useful adsorbents which were stable a t higher temperatures. Because di-amines were known to form much more stable complexes with metal ions than monoamines, complexes of l,l0-phenanthroline (Phen) and 2,2'-bipyridine (Bipy) with copper(I1) ion were investigated as column packings.
EXPERIMENTAL
Apparatus. The gas chromatographic equipment and procedure of sample injection have been described previously (4). One-eighth inch 0.d. copper tubing was used for the columns. Prepurified nitrogen obtained from the Air Reduction Co., Inc., was employed as the carrier gas. Sample sizes from about 0.1 pg. to 5 pg. were employed. The longer a compound was retained, the larger the sample required. The samples of volatile compounds were assumed to be sufficiently pure because each gave only one chromatographic peak. The 1,lO-phenanthroline monohydrate was obtained from Columbia Organic Chemicals Co., Inc., Columbia, S. C. and the 2,2'-bipyridine from the G. Frederick Smith Chemical Co.. Columbus, Ohio. A duPont Model 900 Differential .~~ ... Thermal Analyzer was used to record differential thermograms of the adsorbents. Preparation of Adsorbents. The adsorbents [Cu(Phen)dNO&, Cu(Phen)Cl*, Cu(Phen)SOd. HzO, and Cu(Bipy) (NO& J were prepared by adding 0.01 mole of the complexing agent to an aqueous solution containing 0.01 mole of the appropriate copper salt. This mixture was then heated on a bot plate until the complexing agent had dissolved. For Cu(NO&, about 25 ml.
