Colorimetric Determination of Traces of Osmium E. B. SANDELL, University of Minnesota, Minneapolis, Minn.
THE
work here described was undertaken to develop a colorimetric method for determining minute amounts of osmium after volatilization as the tetroxide, with special reference to the determination of the element in meteoric iron. GoldSchmidt and Peters (S) and I. and W. Xoddack (4) have determined the abundance of the platinum metals in meteorites by spectrographic methods. From theae studies it appears that the average osmium content of the nickel-iron phaee of meteorites lies somewhere in the range 3 to 10 parts per million. Thi. order of magnitude is such that osmium can be determined successfully A a colorimetrically by making use of the oenkitive thiourea reaction d i s c o v e d by Chugaev (I), after isoE l I lation of the tetroxide by distillation from a 1- to 2-gram sample. 2 0 4 0 6 0 8 0
I
TME, MINUTES
The procedure ordinarily used for the disFigure 1. Color Intensity of Osmiumtillation of decir ram Thiourea Solutions as a Function of (8) a n d milligramthe Time of Standing after Addition of centigram (6) quantities of osmium teThiourea troxide involves Green filter, Ceneo No. P passage of a stream of A. 1.47 0.p.m. of Or as Or04 in a roiution air through the boiling P N i n HCI and eonbinlng 0.9% thiourw room Iemperature !P7‘ C.) nitric acid solution and 8. A i in A excepI4 N HCl a b s o r p t i o n of t h e C. Ai in A exeepl6 N HCI tetroxide in several re0.0.5 mi. 01 Ot04 rolution conbinlng 0.0368 mg. of Os treated wllh 10 mi. ceivers of dilute hyof 6 N HCI uturaled with SO2 and drochloric acid satuallowed to rtand at room temseraturr rated with s u l f u r for PO minutrsi 0.5 mi. of 10% thiourea iolulion then added and whole dioxide; the solution is diluted to 25 mi. with water. then evaporated with E . 2.0 .p.m. of 01as chloroosrnate in P N hydrochloric acid and H& acd 0.9% thiourea rolullon at room tsrnoeraturr. osmium precipitated hvdrolvticallv as the h>dro;s dioxide. This proccdure has been modified for the present purpose. Distillation is made by boiling without prtssape of air through the solution. Hydrochloric-sulfurous acid is rctained as the absorbing solution, but the lattcr is not evaporated after the distillation, becausc this results in serious losses of osmium. The osmium in the hydrochloric acid-sulfur dioxide solution reacts readily with thiourea to form the red complex, which is stated ( I ) to have H20. in the solid state. the composition [Os(NHzCSh’H2)d,Cl~. Since conrentration of the absorbing solution by evaporztion after the distillation is not admissible, the volume of the distillate must be kept as small as possible to avoid undue loss in sensitivity. Fortunately, osmium tetroxide is readily volatilized, so that boiling off one fifth of the original solution gives a quantitative expulsion of small amounts of osmium, I n one experiment, 140 ml. of solution containing 12 microprams of osmium were distilled according t o the procedure described below, and i t was found that approximately 70YGof the osmium was prosent in tho first 10 ml. of distillate collected and 30% in the second 10-ml. portion; no osmium was detectable in the third 10 ml. of distillate. Usually the solution t o be distilled necd not have a volume greater than 50 ml., so that only 10 ml. of distillate need be collected. A sinplc portion of hydrochloric acid-sulfur dioxide absorbing solution having a volume of 10 ml. (or even 5 ml.) suffices for satisfactory collection of osmium tctroxide (Table I). The volume of the final solution in which the color has been developed can be kept down to 15 to 25 ml. With a photoclectric photometer the limit of detectability of osmium is then 1 or 2 microprams when a layer of solution 1 cm. thick is examined in green light (a solution containing 1 p.p.m. of osmium gives an extinction of ca. 0.015 in 1-cm. depth with a preen filtm). The use of a visual colorimetric method is less satisfactorv than a photometric method because thiourea gives a yellow color with a sulfur dioxide solution. The formation of this yellow substance is of no
.
importance in a photometric method, since it absorbs green light to a negligible extent (the transmittancy of a blank solution, I cm. in thicknesq, under the conditions recommended below is 99.97, or more with a Cenco KO.2 green filter). The rcaction between thiourea and osmium in the hydrochloric acid-sulfur dioxide solution is rapid even at room temperature and full color intensity is attained in less than 5 minutes, In the preparation of known solutions for the construction of the standard curve, osmium must be added as the tctroxide. Osmium as chloroosmate gives no nppreciable color with thiourea at room temperature in hydrochloric acid medium, even after several days’ standing. The red color appcars only on heating (more rapidly if stannous chloride is added). This behavior indicates that osmium is not prcsent to any extent as chlorooqmate in the absorbing solution. The osmium-thiourea color system obey6 Beer’s law. When an attempt was made to determine osmium in the presence of metallic iron by dissolving thc latter in 5 N nitric acid in a distilling flask and thcn distilling the solution, the results were markedly low. The rewon for this was not further investigated. but thc cvolution of nitric oxide is apparently to blame. The difficulty was avoided by dissolving the iron in sulfuric acid, oxidizing the ferrous salt with potassium permanganate, destroying the excess of permanganate and mangancse dioxide with a small amount of ferrous salt, then adding nitric acid, and distilling. The excess permznganate and any higher oxides of manganese must be destroyed, elre ruthenium will didill as the tetroxide with osmium and interfere by giving D blue color with thiourea. APPARATUS
There is required an all-glass distilling apparatus consisting of a round-bottomed flask (provided with an inlet tube for addition of reapcnts) which is connected by means of a pround-glass joint to a water-rooled condenser. The distilling apparatus used in the present work was essentially the same as that described by Robinson, Dudley, Williams, and Bvcrs (5) for distilling selenium and arsenic from soil samples. In this apparatus the thistle tube for addition of reagent solutions is fused into a pound-glass connection, so that i t is easy to remove any insoluble material from tho flask a t the end of the distillation and examine i t for osmium. The distilling flask may have a volume of 250 to 500 ml. Table I. No.
Colorimetric Determination of Osmium after Distillation ol Tetroxide Iron
Addition
Cram
Ru**l
Os Taken
Os Found
Y
^I
7
7.5 18.8
1
2 3
4 5 6 7 8 9 10
37.5 25 50 50
25
7
17
7.5 18.8
36 7 18
18.8 18.8 18.8 3.8
18 17 18 3
37.5
34
SPECIAL SOLUTIONS
OSMIUMTETROXIDE,0.00501, osmium in 0.1 N sulfuric acid.
This solution is prepared by dilution of a stronger one, which may be obtained as follows: Make a number of scratches with a file on a 0.5-yram ampoulc of osmium tetroxidc and weigh the ampoule. Drop the ampoule into a glass-stoppered bottle (200 ml.) containing about 50 ml. of watcr. Break the nmpoulc by shaking the bottle, and when the osmium tetroxide has dissolved, dcrant off most of the supcrnatant liquid into a volumetric flask (250 ml., for example). Rinse the bottle well with successive portiona of water and transfer these to the volumetric flask, taking care to leave all the glass fragments in the bottle. Then transfer t,he 342
ANALYTICAL EDITION
May, 1944
ghss fragments to a weighed filter crucible and obtain the weight of the whole after drying. The w e i g h t of t h e osmium tetroxide used to pre are the stronger soktion is thus obtained by difference. THIOUREA,10% aqueous solution. POTASSIUM PERMANGANATE, 5 % solution. HYDROCHLORIC ACID-SULFUR DIOXIDE SOLUTION, 1 F! F! M. OSMIUM to 1 hydrochloric acid freshly eatuFigure 2. Extinction-Concentration rated with sulfur Curves for Osmium Tetroxide-Thiourea dioxide. Solutions
PROCEDURE
Green fiitw Cenco. No P A. Solutions contained ' 8 mi. of '6 N HCI and 0.5 ml. of 10% thiourea in 95 ml.1 toom lemperatuce 8 . 0 1 0 4 solutions belted with 10 mi. of 6 N HCI sattmled with SO, allowed lo stand 15 minutes at room tem erlture 0.5 mi. of 10% thiourea solution adbed, and whole diluted 40 95 mi. with water.
The sample solution should have a volume such that when i t is ready for distillation, after addition of nitric acid and Dcrmanganate, the'total volume is less than 50 or 60 ml. Chlorides must be absent, and if permanganate oxidation is necessary the solution should be about 1 N in sulfuric acid. Transfer the solution to the distilling flask, and if ferrous iron or other reducing substances are present, add potassium permanganate solution until an excess of a drop is present as indicated by the color change; avoid getting permanganate on the neck of the flask. Next add approximately 50 mg. of ferrous ammonium sulfate hexahydrate to destroy permanganate and higher oxid'es of manganese. The volume of the solution a t this point should be 35 to 40 ml. Add a few small grains of pumice, connect the flask to the condenser, and heat the solution slowly to near the boiling point to make i t certain that higher manganese oxides have been brought completely into solution. Dip the end of the condenser into 10 mi. of hydrochloric acid-sulfur dioxide solution contained in a 100-ml. praduate, the upper half of which has been cut off (a large vial or test tube marked to indicate 20 ml. may be substituted). Add 15 ml. of concentrated nitric acid through the inlet tube of the flask and distill at such a rate that 10 ml. of distillate are collected in 10 to 15 minutes. Transfer the distillate mixture to a 25-m1. volumetric flask, rinsing the condenser and receiver with a few milliliter3 of water, add 0.50 ml. of thiourea solution, and make up to the mark with water. Determine the transmittancy of the solution after 5 minutes (longer standing does no harm), using preen light. In constructing the standard curve add 0, 25, and 50 micrograms of osmium as the tetroxide to distillates obtained from osmium-free nitric acid mixtures as already described. If the amount of osmium is likely to be less than 10 micrograms, use 5 ml. of hydrochloric acid-sulfur dioxide solution contained in a 25-ml. graduate for collecting 10 ml. of the distillate. Add 0.3 ml. of thiourea solution, read the volqme of the solution in the graduate (which has bccn checked for accuracy), and determine the tronsmittancy as described above. DETERMINATION OF OSMIUM IN METEORIC IRON
The following procedure waa used in determining osmium in the Cation Diablo siderite. A 1-gram Rzmple was heated near the boiling point with 10 ml. of 6 N sulfuric arid in an Erlenmeyer flask until there was practically no further action. The solution was decanted from the unattmked samplo and reserved. The remainder of thc metal was dissolved in 10 ml. o f hot 6 N hvdrorhloric arid. The solution was then treatcd with 10 ml. of 6 N sulfuric acid and evaporated to furnos of sulfuric acid. The evaporation to fumes was rcpeated after dissolving the salts in watcr. The residue was then heated with about 10 ml. of water to bring all but a small amourit of insoluble material into solution. This solution 'and the reserved sulfuric acid solution were transferred to the distilling flask and the ferrous iron was oxidized with permanganate. After
343
addition of nitric acid, the solution was distilled and osmium was determined as described above (5 ml. of hydrochloric acid-sulfur dioxide solution were used to collect 10 ml. of distiljate). The small amount of insoluble material remaining in the solution after distillation was collected in a small porous porcelain filter crucible, the bottom of which had been covered with a thin b y e r of quartz powder to farilitate the subsequent removal of the insoluble material. The collected material was dried by washing with acetone, transferred t o a nickel rrucible, mixed with 1 gram of sodium peroxide, and heated a t low redness for 30 minutes. The melt was extracted with 20 ml. of water and the solution heated near the boiling point to decompose peroxide. The solation was transferred to the distilling flask and treated with 10 ml. of 6 N sulfuric arid. Approximately 50 mg. of ferrous ammonium sulfate were added and the solution was heated to destroy nickelic oxide, Nitric acid was then added and the distillation made as already described. No osmium was detected in this dis-
tillate.
The osmium content of the C d o n Diablo meteorite thus found is 2.5 p.p,m. Since the method tends to give slightly low results, this value may as well be rounded off to 3 p.p.m. The Noddacks found 3 p.p.m. of osmium in this meteorite, and GoldSchmidt and Peters reported an approximate osmium content of 5 p.p.m. LITERATURE ClTED (1) Chugaev, L.A.,C m p t . rend., 167,235 (1918); Z . anorg. d l g e m . Chem., 148,65 (1925). (2) Gilchrist, R., Bur. Standards J. Research, 6,421(1931). (3) Goldsrhmidt, V. M.,and Peters, C., Nachr. Ges. wi8S. GBttingen. Math.-physik. Klasse, 1932,377. (4) Noddack, I., and Noddack, W., Naturwissenschujten, 18, 757 (1930);Z.physik. Chem., 154A. 207 (1931);Zbid., BodasteinFestband, 1931,890; Svenqk Kem. Tid.. 46,173 (1934). ( 5 ) Robinson, W. O., Dudley, H. C., Williams, K. T.. and Byera H. G., IND.ENG.CHEM.,ANAL.ED.,6,274 (1934). (6) Russell, J. J , Beamish, F. E., and Seath, J., Ibid., 9,475 (1937)
A
Funnel for Filling Capillaries
ALFRED 0. WALKER, National Aluminate Corp., Chicago, 111.
F
ILLING the capillary tubes used for supporting samples in x-ray diffrsction cameras is a tedious and time-consuming procedure. The funnel described and illustrated here greatly simplifies this operation. It is constructed of brass, although other materials could be used. The dimensions of the taper are determined by the size of the capillaries being filled. Plastic capillaries can be wedged into the tapered hole of the funnel firmly enough to stay in place during the filling operation. Glass capillaries must be held in with a slight pressure of the little finger, with the funnel held between the thumb and forefinger. The ground sample is placed in the funnel, and the top edge of the cap is rubbed with a serrated surface such as the side of a pair of tweezers or a dull file in order to shake the powder down into the capillary. If the hole plugs up, a wire ran be used to clear it. Hygroscopic samples can be dried in the funnel in an oven, broken up with a wire, and introduced into the capillary before they have a chance to pick up moisture.
