Microdetermination of Osmium W. J. ALLAN
AND
F. E. BEAMISH
University of Toronto, Toronto, Ontario, Canada The investigation of the distribution of osmium in the fire assay required a method for its microdetermination. The hydrolytic precipitation of micro quantities of osmium could be made complete, but contamination of the precipitate, particularly by silica from glassware, and retention by this silica of small amounts of osmium introduced significant errors. -4n improved method for the colorimetric determination of osmium by thiourea has been applied to the determination of osmium in lead buttons. Precipitation of osmium by thionalide followed by ignition in hydrogen to the metal has been investigated. The results show t h a t microdeter-
T
HE determination of osmium has always been difficult.
Gravimetric, volumetric, potentiometric, colorimetric, spectrographic, and reaction velocity methods have all been used with varying degrees of success. The present investigations were made t o develop a satisfactory micromethod for osmium which could be used for the determination of its distribution in the fire assay. The methods thought most likely to yield acceptable microdeterminations were the hydrolytic precipitation used by Gilchrist ( b ) for macro amounts of osmium, and the colorimetric determination used for trace quantities by Sandell (IO). Gilchrist (6)evaporated solutions containing osmium tetroxide in 6 N hydrochloric acid saturated with sulfur dioxide on a steam bath prior to hydrolytic precipitation; however, Sandell ( I O ) reported serious losses v,Then these solutions containing traces of osmium were evaporated. H e avoided evaporation by using small volumes of absorbing liquid. I n the present work small quantities of sulfur dioxide-hydrochloric acid absorbing liquid could not be employed because as much as 5 mg. of osmium was t o be determined. It was found, however, that if the distillates were allowed t o stand 12 to 16 hours a t room temperature no loss whatever occurred eithrr during aging or subsequently when the solution was concentrated by boiling. It was concluded that the fresh distillate contained some dissolved osmium tetroxide which was lost if the solution were heated before reduction of the tetroxide was completed. This conclusion was supported by the fact that addition of thiourea to fresh distillates resulted in the development of the intense rose color of the osmium-thiourea I\ complex; distillates several hours old gave a less intense color, while distillates 24 hours old gave no color.
mination of osmium by hydrolytic precipitation is impractical. The results are of some significance, as similar difficulties may be encountered in other hydrolytic determinations where the same conditions exist. The colorimetric method described is useful for the determination of from 80 micrograms of osmium in 200 ml. to 5 mg. in 1 liter within a relative analysis error of 5%. The new gravimetric method provides the first application of organic reagents for the determination of osmium. It should be applicable to determination of other metals where ignition in air is not desirable and the metal has catalytic activity.
of the small flasks, were used for the introduction of reagents and flushing of the tubes after a distillation. Air was draTm through the apparatus by suction applied t o a bubbler which contained thiourea solution t o detect any osmium tetroxide escaping from the receivers. The distillation apparatus as described was used in. both the hydrolytic and colorimetric determinations of osmium. The transmittancies of colored osmium-thiourea solutions were determined Tvith a Lumetron 402 EF colorimeter. h narrow band filter centered at 480 mp ( 2 ) and two cells, one rectangular in shape and 50 mm. long, the other cylindrical and 150 mm. in length, n-ere used. The fire assays were made in a TT’illiams and Wilson 15 KVA Globar assay furnace. Chemicals. .hfiroNvar BROMO-OsaIaTE. ii sample of ammonium bromo-osmate was prepared by the method of Gilchrist (6‘). and analyzed for osmium content bv ignition of weighed quantities in hFdrogen. S o residue remained after volatilization of the osmium tetroxide by ignition in air. The average of five determinations showed an osmium content of 26.98%, with an average deviation of 0.02%. The theoretical osmium content was 26.95%. -imrowL\f CHLORIDE,C.P. Baker and .idamson, recrystallized from hot distilled water. HYDROBRO~IIC ACID, c.P., 48%. J. T . Baker Chemical Co., purified by distilling twice, discarding the first and last portions
r
hlICRODETERMIVATION OF OSMIUM
Apparatus. The apparatus shown in Figure 1 was constructed from borosilicate glass in individual parts, mounted on a steel frame and sealed together. S o rubber connections or grease was used. The distillation flask was of 1-liter capacity; the trap and receivers were each of 200-ml. capacity. A spray trap and reflux device prevented solids from being carried into the trap. A condenser between the trap and first receiver largely prevented the distillation of water and acid from the trap when i t was heated. The oxidizing acid was introduced via the removable reservoir. Centrifuge tubes, sealed t o stopcocks at the top of the inlet tubes
\
Figure 1. Apparatus
1608
V O L U M E 24, NO. 10, O C T O B E R 1 9 5 2
1609
determinations averaged 40 micrograms, with a deviation of 0.03%. VVeight of Distillation. An aliquot of Residue Osmium after Volathis standard solution was disTotal Weight tilization of Difference Taken ,a6 Ammonium between Osmium tilled after addition of perof Reduced Osmium TeOsmium Taken and Volatilized Sample KO Bromo-osmate Volatilized Precipitate troxide chloric acid. The distillate was MQ. Ma. W e i g h t 5% Mg. Mo. collected by hydrobromic acid. 1 4.893 5,441 4.725 -3.5 0.716 2 5,193 6.092 0.594 -2.0 5.686 Hydrobromic acid was used 3 5.051 4.934 5,638 0.704 -2.3 rather than sulfur dioxide4 4.821 5.306 4.920 0,485 -2.0 5,236 5 1,372 -9.2 4.757 6.129 hydrochloric acid because it 6 6.252 1.042 -2.1 5.210 5.321 7 5.046 5.219 5.791 0.745 -3.3 was more efficient as an absorb5,337 8 5.072 -5 0 6.057 0.986 ing agent and easier to handle, and the resulting distillate did not require aging before evapoof distillates. This acid is referred t o in the text as hydrobromic ration to a small volume or evaporations to dryness to destroy acid. sulfite complexes ( 5 ) . Perchloric acid was used because nitric HYDROCHLORIC ACID,concentrated, reagent grade. Canadian acid and aqua regia were only partially effective in oxidizing the Industries, Ltd., diluted 1 t o 1 with distilled water. Purified in dissolved osmium complex. Sulfuric acid was not used, because the same manner as the hydrobromic acid. PERCHLORIC Xcrn, 72%. G. Frederick Smith Chemical Co. lead would eventually be present when the procedure XT-as apand J. T. Baker Chemical Co. This acid is referred t o in the text plied to lead button analysis. The results from the hydrolytic as perchloric acid, unless ot'hern-ise stated. precipitation lacked precision and were 5 to 8V0 high. T H I O N B L I D E OR THIOGLYCOLIC P - B ~ I I S O S . ~ P H T H a L I D E . PreBecause distillation is the only knoTm process m-hich quantipared by a method developed in this laboratory and recorded by TTelcher ( 1 4 ) . Melting point llO-lllo C. tatively isolates osmium, and hydrolytic precipitation of the osSULFURDIOXIDE-HYDROCHLORIC . ~ C I D SOLUTIOS. This solumium is the only recorded gravimetric method which might be tion was made by saturating 6 ,V hydrochloric acid with sulfur used on the evaporated distillate, these procedures were examined dioxide a t room temperature. for sources of error. OSMICMTETROXIDE IN SCLFURDIOXIDE-HYDROCHLORIC ACID. To save the time involved in the distillation procedure, a st'ock Because hydrated oxides are precipitated a t an acidity which solution similar t o an evaporated distillate was made up by encourages precipitation of such impurities as silica and iron, dissolving 2 grams of osmium tetroxide in 500 ml. of sulfur it seemed probable that muchof thecontaminationcould be caused dioxide-hydrochloric acid and allowing the solution to stand by corrosion of the glassware during the evaporation as ne11 as in a glass-stoppered bottle until no osmium tetroxide could be detected in the vapor above t,he solution. The solution vas then by impurities in the hydrobromic acid n hich could not be removed evaporated almost t o dryness six times with concentrated hydroby distillation. This possibility was examined bl- preparing a chloric acid, An aliquot was diluted viith distilled TTater so that special osmium solution which required only slight evapoiation a solution containing approximately 5 mg. of osmium in 25 inl. before precipitation of the osmium. The results were non only of 1 S hydrochloric acid was produced. O s u r n r TETROXIDE IS SULFUR DIOXIDE-HYDROCHLORIC ACID. 1yogreater than calculated and it was concluded that long evapoA solution similar t o the above was made by dissolving 1.009 rations caused excessive contamination of the distillates by corgrams of osmium tetroxide in 500 ml. of sulfur dioxide-hydrorosion of the glassn are. chloric acid. This solution xas not evaporated. It x a s hoped that the volatilization of osmium tetroxide from THIOUREA SOLUTIOX'.Made by dissolving 200 grams of thiourea in 2 liters of distilled water. the precipitate nould give a true measure of the blank. Hon: -411solutions were filtered before use. ever, spectrographic and microscopic examination of the ignited residues from the above precipitates revealed the presence of HYDROLYTlC PREClPlTATION AND DISTILLATION OF OSMIUhI osmium as ne11 as iron and silicon. To examine the distillation process, purified ammonium bromoPrecipitation. The solution of osmium tetroxide in sulfur osmate was used as the standard because its osmium content dioxide-hydrochloric acid was standardized by the hydrolytic could be determined with considerable accuracy. The distillates precipitation method. were evaporated t o dryness, then taken up in 10 ml. of 0.1 AT After precipitation was complete, 10 ml. of filtered 95% ethyl hydrochloric acid and filtered. When hydrobromic acid was alcohol were added to aid coagulation and thus prevent small losses t o the filtrate. The precipitate was allowed t o coagulate used as the absorbing media, the silica thus removed \vas found on the steam bath for a t least 2 hours. The supernatant liquid to contain osmium. Surprisingly, when sulfur dioaide-hydrowas decanted t,hrough a weighed porous porcelain crucible of A2 chloric acid was used as the absorbing liquid the silica removed porosity. The precipitate in the beaker vias washed with 25 ml. did not contain osmium and for this reason hydiobromic acid of 1% ammonium chloride solution and 10 ml. of 95% ethyl alcohol. The beaker was placed on the steam bath for 15 minwas discarded in favor of sulfur dioxide-hydrochloric acid soluutes and the supernatant liquid decanted as before. The pretion. The results from several dibtillations for a hich sulfur cipitate was then washed four times and transferred to the filter dioxide-hydrochloric acid was used as absorbing solution are crucible, washed x i t h a few milliliters of alcohol, and covered given in Table I. The distillates from samples 5 and 6 viere with ammonium chloride. The crucible was placed in a quartz ignition tube through which hydrogen was allowed t o flow. After evaporated in a platinum dish. 5 minutes the Meker burner was lighted and the ammonium Spectrograms of the residue remaining after volatilization of the chloride slowly volatilized. The precipitate was then ignited for osmium revealed iron, silicon, and osmium. Oxidation of the 1 hour a t full heat, cooled in hydrogen for 5 minutes and in nitroresidues at high temperatures failed to remove the osmium. gen for 15 minutes, and placed in a constant humidity desiccator ( 1 2 ) for 10 minutes, and weighed after remaining in the balance Treatment of the residues nith nitric acid was also ineffective. case for 20 minutes. During precipitation and ignition all Fusion of a residue n-ith sodium carbonate resulted in removal of vapors were tested for osmium tetroxide by drawing them 10 t o 15 micrograms of oqmium as the tetroxide. Boiling of the through thiourea solution acidified with hydrochloric acid. The fused extract n i t h nitric acid yielded 40 micrograms of osmium. filtrate and washings were boiled separately, each with 20 ml. of 30% hydrogen peroxide and a few milliliters of concentrated hyThe latter amounts B ere determined colorimetrically. Undrochloric acid, and the resulting vapors tested for osmium. doubtedly osmium was somehon- retained even after ignition by the impurities incident to hydroli-tic precipitation. The exThe average of five determinations v a s 5.285 mg. of osmium planation for this unexpected phenomenon may lie in the forper 24.96 ml. of solution; the average deviation was 0.1%. There was no loss of osmium in these determinations. Blank mation of a n osmium-silicon alloy; a palladium-silicon alloy Table I. Hydrolytic Determinations of Osmium i n Sulfur Dioxide-Hydrochloric Acid Distilled from Ammonium Bromo-osmate Using - Nitric Acid
1610
ANALYTICAL CHEMISTRY
formed under conditions similar t o those used for the determination of osmium has been reported (8). The quantitative examination of the nonvolatile residue t o determine the osmium content was discontinued and the hydrolytic method for osmium wa~ rejected as a quantitative micro method where a high degree of accuracy was required. Conclusions. The sulfur dioxide-hydrochloric acid distillates containing osmium could be evaporated without loss of osmium tetroxide, if they were allowed to age 12 to 16 hours before evaporation. Hydrobromic acid distillates could be evaporated immediately without any loss of osmium tetroxide. Hydrobromic acid, although a better absorbing agent than sulfur dioxide-hydrochloric acid, was rejected because silica removed from the distillate before precipitation retained small amounts of osmium. Trace losses of osmium to the filtrate and washings were avoided by the use of alcohol as a coagulating agent. These losses frequently occur if the operator has not gained some experience in this particular type of precipitation. The difficulty lies in the narrow range of acidity required for complete hydrolysis and is a characteristic of certain other similar platinum metal precipitations. L-ncontaminated osmium solutions could not be prepared, owing to the consistent presence of silica and iron. httempts t o determine a true blank for each sample failed because about 3% of the osmium could not be removed from the residue by oxidizing ignitions. It is to be expected that the large positive error incident to this precipitation will be characteristic of “hydrolytic methods” for the platinum metals. This has been the experience of one of the authors and has been recorded in the determination of iridium ( 3 ) and ruthenium (7). The authors nest investigated the applicability of a colorimetric method COLORIMETRIC DETERMINATION OF OS~\IIUI\I
The most promising colorimetric determination of osmium involved formation of the rose-colored osmium thiourea complex ( 4 ) . Sandell ( 1 0 ) added thiourea solution t o sulfur dioxidehydrochloric acid solutions into which osmium tetroxide had been distilled, and determined the oqmium colorimetrically with a photoelectric photometer. When aqueous thiourea solution was added to a few milliliters of a solution of osmium tetroxide in sulfur dioxide-hydrochloric wid, no color developed. However, a color developed immediately when a few drops of an alkaline solution of osmium tetroxide were added. A sample of ammonium bromo-osmate, containing 5 mg. of osmium, u-as oxidized with 40 ml. of perchloric acid in the distilling apparatus. The osmium tetroxide produced \ % a absorbed i in sulfur dioxide-hydrochloric acid. Samples taken from the first two receivers immediately after distillation and treated with aqueous thiourea solution gave an intense rose color. Thiourea added t o samples taken several hours after distillation gave only a weak color, while samples tested 24 hours later gave no color. I t appeared that only osmium present as osmium tetroxide in the sulfur dioxidehydrochloric acid solution would give a color with thiourea. and that it was necessary t o add the reagent as soon after distillation as possible. Because different amounts of osmium would require different lengths of time for distillation, the thiourea was added to the sulfur dioxide-hydrochloric acid just before distillation. In this way any osmium tetroxide produced reacted immediately with the thiourea.
Distillation of Osmium Tetroxide from Ammonium Bromoosmate. A sample of the osmium salt containing about 5 mg. of osmium was oxidized with perchloric acid and 20 ml. of loco aqueous thiourea solution were added t o the sulfur diouidehydrochloric acid solution in each of the receiving flasks. The trap, containing 100 ml. of water, and the first two receivers were cooled in ice until the distillation of the osmium tetrovide was
complete (-30 minutes). The ice bath for t,he trap was then removed and the contents of the trap were brought t o boiling. after which 20 ml. of perchloric acid were added and the boiling was continued for 20 t o 30 minutes. During this process the color of t,he solution in the first receiver changed from rose-pink to red-black and t’hen became much lighter and more orange in hue. No reason for these color changes has been found. The second receiver rarely became colored and the third never did. The residual liquids in the distillation flask and trap gave no test, for osmium tetroside. The receivers were emptied into a 2-liter volumetric flask and each was rinsed with a total of 75 ml. of 1 N hydrochloric acid. The delivery tubes were flushed and rinsed with 1 N hydrochloric acid and the washings added t o the solution in the flask. The solution was diluted with 6 N hydrochloric acid t o within 2 inches of the mark and allowed to stand overnight, during which time the orange-red color of the solution changed to the deep magenta of the osmium-thiourea complex. The solution wm made up t o 2 liters and its transmittancy was determined in the 150-mm. cylindrical cell. A blank of 6 N hydrochloric acid was used because i t had the same transmission as the sulfur dioxide-hydrochloric acid solution with thiourea, although the latter was slightly yellow. The transmittancy of the sulfur dioside-hydrochloric acid solution containing osmium and thiourea was checked for several days and was found t o increase rather rapidly. Sulfur which precipitated had been removed by filtering the solution through a large porcelain crucible of -42 porosity previous to determination of the transmittancy. Investigation showed that the fading was due to insufficient thiourea, the amount required being dependent not only on the total amount of osmium present but also on the total volume of the solution. Fourteen grams of thiourea were required for 5 nig. of osmium in 2 liters, 6 grams being used in the first two receivers and 2 grams in the third along with 100, 50, and 50 ml., respectively, of sulfur dioxide-hydrochloric acid. For 1 mg. or less of osmium, only two receivers were used, the first containing 100 ml. of sulfur dioxide-hydrochloric acid and 2 grams of thiourea, the second 50 ml. of sulfur dioxide-hydrochloric acid and the same weight of thiourea. These samples were diluted t o 200 ml. However, great difficulty was experienced because more sulfur precipitated during filtration and it was necessary to filter a number of times, with periods of standing between filtrations.
It was believed that the sulfur dioxide in the absorbing liquid might have been the source of the sulfur precipitated in the colored solutions. It had also been noted that when thiourea was added to the rweiving solutions, collection was made almost’ entirely in the first flask, even when the flow rate of air through the system was high. \Vithout thiourea a very slow stream of air had to be used and every precaution taken against losses of osmium tetroxide from the end of the train. For these reasons another distillation was made, similar to the previous one, but employing 6 N hydrochloric acid only and the usual amount of thiourea. The collection was just as efficient as before and much superior to sulfur dioxide-hydrochloric acid alone. Some sulfur from decomposed thiourea still deposited from the solution but was easily removed by one filtration. The color of the solution remained stable indefinitely. Samples containing 5 mg. of osmium in 2 liters gave rather low transmittancy readings (-15%) in the 150-mm. cylindrical cell. To have a reading near 37% for which the error would be close to minimum, samples of this size r e r e therefore diluted to 1 liter, and the transmittancy (-30%) LTas determined in a 50mni. rectangular cell. Samples containing 0.5 t o 2 mg. were diluted t o 500 ml. and the transniittancy was determined as above. Samples of less than 0.5 mg., containing 4 grams of thiourea, were diluted t o 200 ml. and their transmittancy was determined in the cylindrical cell. Calibration curves of the concentration in micrograms per milliliter us. log transmittancy were made for each of the cells by distilling osmium tetroside from weighed quantities of ammonium bromo-osmate. Accuracy of Method. The curves of concentration us. log transmittancy conformed to Beer’s law. The concentration a t 36.8%, corresponding to minimum relative analysis error, was 4.28 micrograms per ml. for the 50-mm. rectangular cell, and 1.44 micrograms per ml. for the 150-mm. cylindrical cell. The transmittancy of replicate samples could be determined t o within 0.270 and the limits corresponding to a 5Oj, relative analysis error
V O L U M E 2 4 , NO. 10, O C T O B E R 1 9 5 2
1611
Conclusions. The intensity of the color of sulfur dioxidehydrochloric acid solutions conIn 2nd gas Total Difference taining thiourea and a Y .My. / amount of osmium was de1 0.195 -1 pendent on the proportion of 0 0 -E osmium present as osmiuni 0 0 .. 12 71 14 -1 6 0.507 -1 tetroxide. The osmium te5 0.478 -4 0 0.524 -11 troxide, present in the fresh 0 1.98 - 20 tlietillate, was reduced slowly! 0 2.09 - 10 a n d distillates 12 to 16 hours old -pave no color on addition of t.hiourea. These difficultie-c could be circumvented by addit,ion of thiourea to the sulfur dioxide-hydrochloric acid absorbing solution before distillation. Thiourea dissolved in sulfur dioxide-hydrochloric acid wac a much more efficient absorbent for osniium tetroxide than sulfur dioxide-hydrochloric acid alone. The presence of sulfur dioxide in the absorbing solution n-a found unnecessary for the collection of osmium tetroxide ir thiourea and G 117 hydrochloric acid were used. When sult‘ur dioxide was eliminated as a constihent of the absorbing solution. the difficulty of removing sulfur precipitated in the colored solutions was overcome. The n-eight of thiourea required for a fixed amount of osmium depended on the volume of bhe solution. The color was stable indefinitely if sufficient thiourea n-as present. The method developed has been successfully applied to the determination of osmium in lead buttons and acid solutions obtained from the fire assay.
Table 11. Colorimetric Determination of Osmium in Lead Buttons from a Zeutral Flux Sample No.
Osmium Taken M g
; 3 4
5 6
7 8
.
0.194 0.222 0.172 0.508 0.482 0.535 2.00 2.10
In 1st button MV. 0.193 0.212 0.169 0.493 0.470 0.623 1.98 2.09
Osmium Found In In 2nd button 1st gas /
/
1
0
7
0 3 0 1 0 0
; 5
3 0 0 0
were 8 and i i %transmittancy (1). Thus any solution containing less than 1 microgram per ml. of osmium determined in the rectangular cell \vas beyond the 5 % error limit, as was any solution containing less than 0.4 microgram per ml. of osmium determined in the cylindrical cell. The lowest transmitt’ancy measured in the analyses n-as 20 to 30% and therefore all were well n-ithin the lower error limit. Colorimetric Determination of Osmium in Lead Buttons and Acid Solutions. PROCEDURE. A number of samples of a flux representing the slag from a neutral ore were “salted” with ammonium bromo-osmate and assayed. The lead buttons obtained were parted with perchloric acid ( 1 3 ) and analyzed for osmium by the method described below. Any lead tetrachloride produced during distillation was hydrolyzed in the water trap and therefore did not interfere in the determination. The first button, weighing approximately 40 grams, was placed in the distilling flask and 100 ml. of water was put into the trap. If the button contained approximately 500 micrograms of osmium, only two receivers were used, the first holding 100 ml. of 6 Ar hydrochloric acid and 2 grams of thiourea and the second 50 ml. of G S hydrochloric acid and 2 grams of thiourea. If more than 500 micrograms of osmium were present in the button, three receivers were used, containing 100, 50, and 50 ml. of 6 S hydrochloric acid and 6, 6, and 2 grams of thiourea, respectively. The r.rap and first two receivers were cooled in ice. Air was drawn through the apparatus by suction applied to the last receiver a t 4 to 5 bubbles per second. Eighty milliliters of perchloric acid were allowed to flo~vinto the distillation flask from the reservoir and the burner was then lighted. When the acid became hot the butt’on started to dissolve rapidly with the evolution of much gas. Dissolution was complete in 45 to GO minutes. The stopcock on the reservoir of the trap was opened, the ice bath removed, and the burner placed under the trap. The contents of the trap were boiled gently for 5 to 10 minutes, then 20 ml. of perchloric acid were added and boiling was continued for 20 to 30 minutes more, during n-hich time the various color changes mentioned above took place. The osmium content was determined as described, the samples containing less than 500 micrograms being diluted to 200 ml. and the others to 500 nil. The slag from the first assay was ground and reassayed. Buttons obtained on reassay invariably contained very small quantities of osmium and \\-ere therefore analyzed in t,he same manner as the first buttons containing less than 500 micrograms of osmium. The gases over the molten flux n-ere collected by drarving through towers containing sulfur dioxide-hydrochloric acid. When the assay was completed the solutions in the towers were rinsed into a 750-ml. Erlenmeyer flask with 1 3 ’ hydrochloric acid, 50 ml. of 12 iY hydrochloric acid were added, and after aging 12 to 16 hours the solution 13-3s evaporated. Because this solution contained carbonaceous material, it was oxidized in the distilling apparatus rvith 30 ml. of 50% perchloric acid. The boiling perchloric acid gradually concrntrat,ed to 72%, destroying the most easily oxidized substances first. Only substances very difficult to oxidize remained when the concentration of the acid reached 727,. I t was found necessarj- to oxidize all the organic material in the solution before the osmium tetroxide could be completely distilled, and x-ith the use of this technique no esplosion occurred during many distillations. Except for the modification described above, the osmium content of the solution was determined in the manner described for buttons containing less than 500 micrograms of osmium. The results of the analyses of several buttons and acid solutions are given in Table 11.
THION-ILIDE 4s A POSSIBLE MICROGRAVIRIETRIC REAGEZT FOR OSMIIIp1zI
Although several organic reagents can be used for the quant.itative precipitation of osmium, none has been recommended for gravimetric determinations. The explanation lies in the persistent contamination of the precipitate 1))- the organic precipitan and the volatility of osmium tetroxide. The successful use of t.hionalide as a precipitant for ruthenium has been reported (9). I n this determination the organic materia‘! is removed by ignition in air. Investigation showed that precipitation of osmium by thionalide v a s complete from solution0.25 .V to 1 N hydrochloric acid. All attempts to remove escess thionalide from the precipitate failed, as they did in the case of ruthenium precipitation. The answer to the problem of gravimetric recovery of osmium was found in the observation that palladium dimethylglyoxime could be ignited in hydrogen t o produce the metal (11). Presumably this phenomenon is due to the catalytic activity of palladiuni. As osmium is a similarly active catalyst, there seemed some hope that osmium-organic complexes could be ignited to the metal. The experiments \\-ere successful and there was an additional advantage over hydrolytic precipitation in that ignition of osmium in air subsequent to ignition in hydrogen resulted in an osmium-free residue. This permitt,ed the determination of individual blanks. Procedure. In order to compare this determination with the hydrolytic precipitation, the osmium solution already analyzed by the hydrolytic method was used. The concentration of this solution determined by hydrolytic precipitation was 5.285 mg. per 24.96 ml., wit,h a blank of 40 micrograms. An aliquot of the osmium solution, approximately 1iV in hydrochloric acid, was placed in a 250-ml. beaker and diluted to 100 ml. with distilled water. The beaker was placed on a hot plate and the solution n-as heated to a gentle boil. Three or four drops of a solution containing 80 mg. of thionalide in 15 ml. of 95% ethyl alcohol were added and the boiling was continued. ,411 the thionalide solution was added slorrly over a period of 1 hour. The contents of the beaker vere gently boiled for 2 hours, and then placed on the steam bath for 2 hours t o coagulate the precipitate. The supernatant liquid was decanted through a porcelain crucible of A2 porosity previously ignited in hydrogen to
A N A L Y T I C A L CHEMISTRY
1612 Table 111. Precipitation of O s m i u m by Thionalide Sample S o .
Reduced Precipitate
Residue
Osmium Volatilized
Deviation
Mg.
Mg.
Mg.
72
5.232 5.170 -0.3 0.062 5.239 0.042 5.197 +0.3 5.245 5.173 -0.3 0.072 5.310 0.116 5.194 +0.2 AV. 5,257 0.073 5.184 0.3 The result obtained by hydrolytic precipitation was 5.245 mg. with a deviationof0.170. 1
2 3 4
constant weight. The filtrate was tested for osmium in the same manner as the filtrate from the hydrolytic precipitation. Twentyfive milliliters of 0.2 A’ hydrochloric acid were added t o the precipitate in the beaker and the beaker was placed on the steam bath for 15 t o 30 minutes. This process was repeated four times and the washings were tested for osmium. After the fourth n-ashing the precipitate was transferred to the crucible. The crucible was placed in the ignition tube and hydrogen allowed t o flow through the tube for 5 minutes before the burner was lighted. The precipitate wl-asdried with a low flame. As the temperature was increased the precipitate turned from brown t o black, and a yellow liquid appeared on the cool part of the tube. +4very disagreeable odor was also noted. The precipitate appeared as a small sphere assuming the gray color of osmium sponge. The precipitate was then ignited in hydrogen a t full heat for about 2 hours, cooled, and weighed as usual. KO osmium was detected escaping from the tube during ignition. after ignition t o constant weight, the osmium was ignited
strongly in air. The residue from this oxidation was ignited in hydrogen until its weight was constant.
A spectrogram of the residue revealed iron, silicon, and magnesium, but no osmium. Magnesium \vas used as a catalyst in the preparation of thionalide (14). The results are recorded in Table 111. Further investigations are being made to determine the optimum conditions for this precipitation and the applicability of the method to other organic precipitants. LITERATURE CITED
(1) Ayies, G. H., dx.4~.CHEY.,2 1 , 652-7 (1949). ( 2 ) Xyres, G. H., and Wells, TT’. K., Ibzd., 2 2 , 317-20 (1950). (3) Barefoot, R. R., hIcDonnel1, IT.J., and Beamish, F. E., I b i d . , 2 3 , 514-16 (1951). (4) Chugaev, L., Compt. rend., 1 6 7 , 2 3 5 (1918). (5) Gilchiist, R., B u r . Standards J . Research, 6 , 421-48 (1931). ( 6 ) Ibid., 9 , 2 7 9 (1932). (7) Hill, M. A., and Beamish, F. E., ANAL.CHmf., 2 2 , 590-4 (1950). (8) Lebeau, P., and Jolibois, P., Compt. rend., (17 146, 1028 (1908). (9) Rogers, W.J., Beamish, F. E., and Russell, D. S.,ISD. EXG. CHEU.,AKAL.ED., 1 2 , 5 6 1 - 3 (1940). (10) Sandell, E. B., Ibzd., 1 6 , 3 4 2 - 3 (1944). (11) Teeter, K. G., unpublished research report. (12) Thiers, R., and Beamish, F. E., ANAL.C H E l r . , 1 9 , 4 3 4 (1947). (13) Thiers, R., Graydon, I T , and Beamish, F. E., Ibzd., 2 0 , 8 3 1 - 7 (1948). (14) Welchei, F. J., “Organic Analytical Reagents,” Vol. IV, p. 165, New Tork, D. Van Nostrand Co., 1948. RECEIVED for review January 28, 1952.
Accepted August 22, 1952.
Titanic Chloride as an Intermediate in Coulometric Analyses P.AUL ARTHUR AND JOHN F. DONAHUE’ Department of Chemistry, Oklahoma Agricultural and Mechanical College, Stillwater, Okla.
THE feasibility of using the titanium couple as an intermediate in coulometric reductions was tested by using it in the analysis of a series of iron samples. The use of standard solutions of such reducing agents as titanous chloride, chromous chloride or sulfate, stannous chloride, and cuprous chloride in direct titrations of oxidizing agents has always been seriously limited by theinconvenience involved in handling, storing, and using these reagents. The difficulty of avoiding air oxidation and the consequent change in titer is so great that most chemists use indirect methods of analysis rather than employ such reagents. It has been pointed out by Szebelledy and Somogyi (3) and demonstrated by Swift and coworkers ( 2 ) and Cooke and Furman ( 1 ) that such reagents, if they have suitable properties, can be generated electrolytically, and the unknown determined by application of Faraday’s laws. Thus Swift was able t o analyze solutions containing chromates and vanadates by means of electrolytically generated cuprous copper, while Cooke and Furman determined chromates and ceric salts through the medium of electrolytically generated ferrous iron. To be suitable for use in the constant-current coulometric determination of a given oxidizing agent, an intermediate must have the follov,ing Characteristics: Its oxidized form must be reducible a t the electrode with 100% current efficiency; and its reduced form must be capable of reducing the unknown stoichiometrically and with reasonable rapidity. It was t o increase the number of substances known t o have the first of these characteristics and to make available more powerful reducing couples that this research was undertaken. APPARATUS AND REAGENTS
Electrolysis Cell. The electrolysis cell consisted of a 150-ml. 1
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beaker covered by a wooden block, 100 X 100 X 18 mm., with a circular cavity in the underside, 5 to 6 mm. deep and just big enough to fit around the top of the beaker. Through this cover holes were drilled as follows: a 25-mm. diameter hole in the center t o accommodate the anode compartment; a 7-mm. hole t o admit the salt bridge of a saturated calomel electrode; and two 12-mm. holes fitted with rubber stoppers. Through one of these stoppers the glass tubing support for a platinum wire indicator electrode was inserted; through the other the heavy wire, x-hich supported and provided electrical connection t o the cathode, was passed. The anode compartment was a glass tube 25 mm. in outer diameter and 100 mm. long. I n the lower end there was a sintered-glass disk of medium porosity. Diffusion through this disk proving t o be too rapid, a suspension of finely powdered glass was draxn through it by suction until the pores were sufficiently clogged to permit only a very small seepage. The electrolysis anode n-as a platinum gauze electrode taken from a set of ordinary electrodeposition electrodes. The cathode first used was made by spot welding a No. 18 B. and S. platinum wire 100 mm. long to the back of a 50 X 50 mm. sheet of 0.005inch platinum. Later this electrode was converted to a gold electrode by heavily electroplating it with gold from a cyanide bath. The indicator electrode was a No. 22 B. and S. platinum wire, 3 em. long, sealed through the bottom of a soft-glass tube. About 2.5 em. of the wire extended outside the glass, and this part of the wire was bent upward and coiled around the glass t o form the electrode surface, Electrical contact t o the external lead was made in the usual manner by means of a globule of mercury. The saturated calomel reference electrode was the conventional type with a Bide-arm salt bridge fitted with a stopcock and a capillary tip, the latter passing through a 7-mm. hole in the beaker cover and dipping into the solution. The solution was stirred by means of a magnetic stirrer. Electrical Circuit. Eight 6-volt storage batteries connected in series were used as the power supply. Two 50-watt, 5OO-ohm variable resistors and one 50-watt, 10-ohm variable resistor, all connected in series, n-ere made into a resistance box which served
