Determination of Rhenium in Molybdenite
Minerals C. F. HISKEY'
AYD
I-, W. JIELOCHE
University of Wisconsin, Iladison, W'iS.
A method for the quantitative determination of 5-microgram amounts of rhenium in the presence of millionfold excesses of molybdenum has been developed, which combines a modified distillation and modified colorimetric technique. It has been applied to the analysis of 28 molybdenite samples.
COMPLETELY satisfactory method for the quant'itative determination of rhenium in molybdenite minerals has never been developed, because of a combination of several analytical difficulties. In the first place there is enough chemical similarity between rhenium and molybdenum to make clear-cut separations somewhat difficult. Secondly, although molybdenites contain the highest natural concentrations of rhenium that have been found, these concentrations are of the order of 0.0001 to 0.001 per cent. In addition, no colorimetric reaction for the determination of rhenium has yet been developed in which molybdenum does not interfere. Consequently, it becomes necessary to eliminate molybdenum before any rhenium estimation can be made, and i t is in this operation that most of the difficulties arise. Invariably, it happens that the small amounts of rhenium are either coextracted or coprecipitated with the molybdenum t'o such a degree as to make the procedure useless for the quantitative determination of rhenium. This has been the authors' experience as Yell as that of other writers. In the past, a variety of separations has been developed on a micro as well as a macro scale. Among these may be listed the distillation method of Geilmann and Weibke (Z), which utilized the technique of passing a stream of moist hydrogen chloride diluted with carbon dioxide through a hot concentrated sulfuric acid solution of the perrhenate, followed by a gravimetric determination of the rhenium which collected in the distillate. In another method advanced by these authors (3) the molybdenum was separated by precipitation from nearly neutral solutions with 8-hydroxyquinoline. Both methods work satisfactorily for the ranges in which they were intended, but when applied to the problem of separating 5-gram lots of molybdic anhydride from 0.1 mg. or less of rhenium are not at all satisfactory. Much the same may be said of the methods of Ilronman and his students (7-Q),as well as of the work of Mikhailova, Pevsner, and Archipova (IO) who used a combination of the distillation method with the 8-hydroxyquinoline method to effect the determination. The rhenium was generally determined by a nitron acetate precipitation. In all these variations on fundamental methods, none of the authors obtained the high degree of separation needed to make a simple and effective method for the determination of rhenium in a molybdenite mineral. Thus, the best that Mikhailova, Pevsner, and Archipova were able to do was to determine milligram amounts of rhenium with an accuracy of 0.4 to 3.0 per cent in the presence of not more than twice that amount of molybdenum. In an early paper by Hurd (j), a method was described whereby the molybdenum wis extracted from solutions containing rhenium by first treating with ethyl xanthate, folloa-ed by chloroform extraction of the molybdenum complex thus formed. The rhenium was then determined by reaction with stannous chloride and potassium thiocyanate (1). Although the method Present address, Cheinietry Departnient, University of Teiiiiesuee, Knoxville, Tenn. 1
was satisfactory for. qualitative work, it could not be used iix
quantitative determinations without additional refinements. During the past year, millard and Smith (11) have developed a method whereby the perrhenate may be precipitated from solutions with tetraphenyl arsonium chloride. From the work of these authors, it is apparent that' the molybdate does not interfere greatly but, unfortunately, it is not possible to precipitate the minute amounts of rhenium which are usually present in the molybdenites. Hoffman and Lundell (4)have used a differential reduction with mercury to effect, the required separation. Under certain conditions only the molybdate is reduced and this may be extracted with ether after treatment Lvith a thiocyanate. Separations of as much as 10 mg. of molybdenum from 0.001 mg. of rhenium have been claimed. This method would be satisfactory if it were not so detailed and if the order of separation were higher. Hurd and Hiskey (6) had previously announced a method which was capable of determining as little as one part of rhenium in twenty million of pyrolusite. In t'his method the rhenium was isolated by an extraction of the thiocyanate complex, followed by a modified distillation procedure. Since there was scarcely any molybdenum in these minerals, the difficulty of separation was not particularly great, and thus the method, although somewhat cumbersome, vias satisfactory.
In developing a method for molybdenit'e ores two possibilities presented themselves for effecting the separation. From concurrent studies of the properties of quinquevalent molybdenum solutions it seemed reasonable to expect extraction of this compound under conditions which would not favor the removal of the rhenium. If t,his would not work it seemed likely that a modified distillation technique, extended from the pyrolusite method, might give the desired results. Accordingly, the investigation x a s carried out along these two lines. Attempted Separation by Extraction Quinquevalent molybdenum formed by the reduction of molybdate xit,h sbannous chloride is amber in color in 2 M hydrochloric acid solutions and emerald green in 8 M hydrochloric acid. At intermediate points between these two concentrations the color varies from a dark amber, through a brown, an olive green, and a chrome green. It was observed in a qualitative way that at the higher acid concentrations the molybdenum developed such a solubility in butyl acetate as to suggest extraction under these conditions. Accordingly, separation of the molybdenum from the rhenium was attempted by reducing a mixed solution of molybdate and perrhenate with an excess of stannous chloride in 2 N hydrochloric acid. The excess stannous chloride was then oxidized by adding small amounts of concentrated bromine water until it was apparent that more than t'hat required for the excess stannous chloride had been added. It was necessary to destroy the excess stannous chloride before the acid concentration was increased; otherwise the molybdenum would be reduced t o the pink trivalent form which is not soluble in the butyl acetate. To this solut,ion, which was formed in a separatory funnel, a calculated amount of concentrated hydrochloric acid was added to bring the concentration to 8 N . The 8 N solution thus formed was extracted with butyl acetate until all the molybdenum seemed to be removed. The solution remaining was then analyzed for rhenium.
It was found, however, that despite the fact that large amounts of molybdenum could be removed from the rhenium, it was impossible to prevent some of the rhenium from being
INDUSTRIAL AND ENGINEERllG CHEPIISTRI
504
extracted along with the molybdenum. I n view of these results a number of additional experiment's were performed with the purpose of elucidating the cause of the difficulties encountered. Experiments were designed to test the effect of repeated extraction on the recovery of the rhenium as well as the effect of the hydrochloric acid concentration on the distribution of the rhenium between the two solvents. For Table I the data were obtained by taking 100 niicrograms of rhenium measured from a stock solution containing 50 micrograms per ml., diluting iyith 2 N hydrochloric acid, adding 3 ml. of 20 per cent stannous chloride solution, and after reduction adding 3 ml. of a concentrated bromine water solution. The solution thus formed was diluted to 100 ml. with concentrated hydrochloric acid in such a way that the samples were 8 N when the dilution was complete. The solution was then extracted with 25 ml. of butyl acetate a varying number of times. The solution remaining was cautiously evaporated to near dryness, the rhenium oxidized with a few drops of Super-0x01, and the rhenium determined in the usual way.
LOL. 12, NO. 9
the rhenium occur. This fact has been demonstrated for macro amounts of rhenium by Hurd (6) and for micro amounts by Hurd and Hiskey (6). Thus with a millionfold excess of manganese, the apparent recoveries of the rhenium were of the same order as the precision of t,he colorimetric method of analysi R . TABLE111. DISTILLATION OF RHENIUM HzSOd in Solution
'?u 100 100 100 100 100
100
100 100 100 100 1 on
0 0 7.0 13.0 19.0 25.0 30.0 35.0 40.0 44.0 48.0
52.0
Rhenium Founcl Micrograms 100 100 100 102 101 103 101
107 106 66 4i
For the molybdenite samples, solution was first effected with concentrated nitric acid fortified a-ith a small amount of fuming nitric. At no time during the solution process was the temperaNo. of Extracture permitted to go above 80" C. After all the molybdenite Extractions R e Taken R e Found tions R e Taken RE Found had been oxidized to t'he trioxide, the excess nitric acid was reMicrograms Micrograms Micrograms Micrograms moved by repeated evaporation with small amounts of hydro105 4 100 58 1 100 chloric acid. I t was found necessary to remove all the nitric 100 100 100 55 98 100 53 100 acid, because otherwise it m-ould interfere later in the colorimetric 100 97 100 50 determination of the rhenium. Following complete removal of 3 100 97 2 250 236 the nitric acid, the hydrochloric acid solution was permitted to 250 236 100 96 evaporate to a small volume but not to incipient dryness. Con250 216 100 89 centrated sulfuric acid was now added to the concentrated molybdenum solution and the sample was ready for distillation. DISTILLATIOS. The distillation of rhenium in 100-microgram quantities has been made a quantitative procedure by heating the sulfuric acid t,o 260" to 270" C., while passing a mixture of Much the same procedure was followed in studying the efsteam and air through the solution at such a rate that in the fect of the hydrochloric acid concentration on the extraction course of 2 hours 250 ml. of distillate are recovered. This distillate contains, in addition to the rhenium, varying amounts of of the rhenium. The procedure was modified so that when sulfuric and molybdic acids. the solutions were finally diluted to 100 ml. the normalities In the past, it had been customary t o determine the specific of the hydrochloric acid would have different values. Extracgravity of the distillate in order to estimate the amount of sultion was then made with three 25-ml. portions. Data obfuric acid needed in preparing the standard solution for comparison. Because this is a very bothersome procedure, a series of tained in this way are listed in Table 11. studies TT-as made in which the amount of sulfuric acid in the was varied while the rhenium concentration was held ACID COSCENTR~TIOS distillate TABLE 11. EFFECTOF HYDROCHLORIC constant. To the sulfuric acid solutions of varying concentraNormality tions, 100 micrograms of rhenium as perrhenate were added. The of HCI Rhenium Taken Rhenium Found total volumes were 125 ml. To each of these solutions 50 ml. Jizcrograwis Mtcrograms of concentrated hydrochloric acid, 5 ml. of potassium thiocyanate 100 96 5 solution (20 per cent), and 5 ml. of stannous chloride solution 100 92 (20 per cent) were added in the above-named order. The solu86 100 tions were permitted to stand for about 7 minutes and then 98 100 compared in long Nessler tubes. With the solution containing 80 100 77 100 no sulfuric acid being taken as a standard, the results listed in Table I11 were obtained. 100 GO
TABLE I. EFFECTO F REPEATED EXTRACTIOS \-". "f -.
100 100
58 45
In view of the unpioinising tenor oi these results, it beenled clear that no approach to the separation of these two elementq could be satisfactorily based on this type of extraction. Recourse was then had to a modified Geilmann method of distilling in a n attempt to effect separation. Although it was known that the molybdenum did distill in the method proposed b y Hurd and Hiskey ( 6 ) , this was not especially troublesome in pyrolusite analysis because these samples seldom contained more than traces of the interfering element. In the case of the molybdenite minerals, the difficulty becomes of paramount importance, but by a series of changes in manipulative technique i t became possible to develop a n effective separation of the elements as well as a method for determining the rhenium content
Experimental
SOLUTIOX OF SAMPLE.T h e n pyrolusite samples are tlissolved or digested with hydrochloric acid in the presence of small amounts of hydrochloric acid, no appreciable losses of
From this it is evident that so long as not more than 35 per cent of sulfuric acid is present in the distillate, its effect can be ignored. Inasmuch as such high sulfuric acid concentrations as those listed in Table I1 were never obtained, the aubhors have generally tended to stop taking specific gravity determinations of the distillate. Their next concern was with the interference of the molybdic acid. -4series of distillations was made in which varying amounts of molybdenum as molybdic oxide were placed in the distilling flask and the distillation was performed in the usual way. The amount of the oxide in the flask was varied from 500 micrograms to 10 grams. As the concentration in the distilling flask increased, so also did the concentration of the molybdenum in the distillate increase. It can be said in general that the amount varied from a few micrograms where the distilling flask contained a few milligrams to a few milligrams when the flask had several grams of the molybdic oxide in it. What seems to be happening here is that the molybdenum is carried over mechanically by the fine spray formed when the steam comes in contact with the hot concentrated sul-
SEPTEMBER 15, 1940
.%SALYTICAL EDITION
furic acid. This was strikingly demoustratetl by filtering the gases, as they left, the flask, through a tube packed with glass thread. I n this way the misty appearance of the gas mas overcome and, at the same bime, the amount of molybdenum distilling was reduced to a very small amount. Unfortunabely this tlevice altered t,he distillation rate of the rhenium as well as the conditions under which it was obtained, in such a way as to be unsatisfactory. After t,rying a large nuniber of different experiments in which most of the variables were studied, it was observed that the molybdenum carry-over could be minimized if certain empirical conditions were maiiit'ained. Of t'hese condit'ions the most important were:
leaves the rhenium thiocyanate complex relatively unaffected. I n this way one can establish with certainty that the faint color observed is due to rhenium and not to traces of unbleached molybdenum thiocyanate.
TABLE 17.
TABLE IV. EFFECT OF MOLYBDEXVUM 110 Taken
--1Iicrogranis o i Rhenium Fouiid after:20 min. 40 min. 60 min. 75 min.
90 nlin.
.irg. 0.0 0.1 0.1 0 2 0.2 0 5 0.5 1.0 1.0 2.0 2.0 5.0 10.0 10.0
100 117 116 130 129 127 12i
Id7
134 152 150 163 167 170
100 115 114 121 117 120 123 122 126 122 127 124 119 127
100 109 110 1Oi 107 10.5 107 10 i 108 100 10s 110 112 112
100 106 106 106 105 102 103 104 105 105 loti
10T 109 1Oi
100 101 100 102 102 103 102 100 102 100 102 104 101 105
Since ES~IXWI~> . it n a s impoisible to efiect a complete separation even n ith these modifications of the distillation method, it became necessary to alter the colorimetric technique so that the small amounts of molybdenum coming over would not interfere as they had done heretofore, As it had been shown previously that high concentrations of hydrochloric acid had a n appreciable fading effect on the thiocyanate complex of the molybdenum, wheieas its effect on the rhenium complex was insignificant, it mas decided to see whether the interference of the molybdenum could be eliminated by allowing a longer time interval for the fading effect of the hydrochloiic acid to operate before the compari-.oris were made. In order to du thib, a series of samples \$as prepared containing a fixed amount of rhenium (100 micrograms), a fixed amount of all the other reagents necessary for this proceSc, but various amounts of added molybdenum. These weie permitted to react and the color intensity was determined by comparing them with one of the samples that contained no molybdenum. In Table IV are listed the data obtained. It thus becomes apparent that the small amounts of molybdenum, always less than 1 to 2 mg., which are carried over in the distillation process can be prerented from interfering, if the solutiolis stand sufficiently long t o permit bleaching of the molybdenum complex to take place. K h e n little or no rhenium is present but one observes the prebence of molybdenum, it is aclrisable to extract the thiocyanate complex n i t h a few nullilitel; of ether in order to concentrate the color and then to a(ld a few milliliters of concentrated hydrochloric acid to the extract. This serves to bleach the molybdenum comple\: i n the ether layer but
DETER~lIXATIOXO F 1 < H E i i I l ~
(&grain sample of 1IoOaj SO.
1 2
1
t
ti
i S
1. The volume should be kept at a minirnuni in the distilling flask. For the 300-ml. flask used, it \vas not de.cira,ble to have more than 100 ml. of acid. 2. The temperature of the aolution in t,he distilling flask should be held near 260' to 270" C. without going above, in order to avoid mist formation attendant on the decomposition of t,he sulfuric acid. 3. The steam used should be kept ad dry as possible, and especially should one avoid the explosions due to drops of condensed ateani coming in contact with the hot acid.
505
0 10 11 12 13 14 1 .j
16 17 18
19 20 21 22 23 24 25 26 27 28 29
Rhenium Addrd P. p . ? t i . 1. 0 1. 5 o n
1, 2 3
0.25 1.25 0.0 1 5 1.0 2.5 3.0 0.0 ti.25 8.75 4.5 11.25 1.5 7.75 5.25 31,25 31,25 25.0
37.5
13.75 15.0 26.0 13.76 26.25 27,50
Rhenium Found P. p . 711. 1 .5 2.0 II n 1.23 I).75 1.75 0 0 1.25 p , 7.5 ,3.2.5 2.5
_ _
0.0 ,5.2 5 p.0 .3.75
11,7S 3.5 i.0. .i 2.1 .
:31,75 30.50 24 5 36,2*5 13.25 14.75 54.26 15.00 26.00 27.50
Error
P. p .
?a.
+0.5 +0.5 n n 0.0 +0.5 f0.5 0.0 -0.25
-0.25
f0.75 -0.5 0.0 -1.0 f0.25
-0.73 +0.5 -1.0 -0.75 0.0 +0.5 -0.76 -0.5 -1.25 -0.5 -0.25
-0.75 fl.25 -0.25 0.0
PROCEDERE. The ooniplete procedure as finally developed v a s as follows:
Four grams of the finely pulverized molybdenite are transferred to a 250-ml. Erlenmeyer flask and thoroughly mixed into 20 ml. of concentrated nitric acid. After any initial frothing ceases, about 5 ml. of fuming nitric acid are added. The reaction is allowed to proceed with occasional stirring. After the main reaction subsides, the flask is placed on a hot plate and heated to just below the boiling temperature. When all the red fumes have cleared from the flask, about 50 ml. of concentrated hydrochloric acid are added with sufficient care to prevent the hot' solution from foaming out. The solution is slowly evaporated with occasional replacement of the hydrochloric acid until there is no trace of chlorine being evolved. This usually requires a total of about 125 to 150 ml. of the hydrochloric acid. The volume is then reduced to about 25 ml., but never to dryness or even incipient dryness, and 75 ml. of concentrated sulfuric acid are carefully added, care being taken to avoid excessive frothing due to escape of the hydrogen chloride gas. The solution as well as any precipitated molybdic oxide is noy transferred to a distilling flask similar to that described in a previous communication (6), and separation is effected by distilling n-ith a mixture of t\vo parts of steam and one part of carbon dioxide or air, meanwhile keeping the sulfuric acid solution at about 260" to 270" C. Distillation is continued for about 2.5 hours at such a rate that in that time 250 ml. of distillate are collected. The distillate is collected in a receiver immersed in an ice bath. The 250-ml. distillat,e is nox treated with a few d r o p of a concentrated solution of bromine dissolved in an aqueous bromide solution, in order t o destroy any sulfur dioxide which may have formed during the distillation process. Only enough bromine should be added to produce a faint yellow color i n the solution. A series of standards is now prepared containing 10, 50, and 100 micrograms of rhenium. Then to each of t,hese, as well as the unknown, 100 nil. of concentrated hydrochloric acid are added. '4fter cooling, 10 ml. of 20 per cent sodium thiocyanate are added, followed by 10 ml. of a 20 per cent stannous chloride solution. The solutions are permitted to stand until the color due to the molybdenum complex has faded out, usually about half an hour, although occasionally more time must be allowed, and then comparisons of the unknonns with the proper standards are made in 100-ml. Nessler tubes. It is advisable to check the aolutions a second time after a lapse of some 20 minutes, to be sure that the molybdenum has faded completely. From the ratio of the height of the column of the liquids in the unknown and in the standard tube, the amount of rhenium in the original sample can be estimated.
INDUSTRIAL AXD ENGINEERING CHEMISTRY
306
This method was tested uaing 1lieiiium-free molybdenum trioxide to which various amounts of rhenium were added. I n Table V the results are listed as obtained in the analysis of a series of samples whose composition was unknown to the operator. Examination of Table V reveals that while the general percentage accuracy is not very high, especially on the smaller amounts, a t no time was rhenium reported when none was present. Thus, while the accuracy of the rhenium determination is low, it is nevertheless possible to estimate the order of the rhenium content to a degree which has not hitherto been possible.
I n the application of this method to naturally occuiring molybdenum minerals, no difficulty was encountered except in a few cases where the minerals contained selenium. I n these instances a pronounced red coloration due to precipitated selenium was obtained, making colorimetric estimation impossible. The course taken by the selenium in this method seems relatively clear. During the solution process the selenium is oxidized to the selenate, but during distillation the sample is partially reduced, forming selenite and elemental selenium. both of which distill into the receiver. While it is possible to remove all the elemental selenium b y filtration, this does not eliminate the selenite. Consequently on addition of the stannous chloride solution, the selenite is reduced to elemental selenium under such conditions that some sol formation takes place. DETERMINATION OF RHENIUMIN PRESENCEOF MOLYBDENUM AND SELENIUM
(4 grams of ammonium molybdate in each sample)
0.1 M HrSeOa
M1. 0.1 0.3 0.4 0.5 0.6 0.8
1.0 1.2
Rhenium Taken Mtcrogramu 12 23 21
Rhenium Found Micrograms 13 2ti 30
71 35
77 3:
9 40 123
121)
a
47
DISTRIBUTIOS OF
KHENICX I?: MINER.4LS
MOLYBDENITE
1-0.
Arizona, Copper Creek Region 1 Sample 1 3 Sample 3 3 Sample 3 1 2 3 4
1
1
Interference from Selenium
TABLEVI.
TABLE VII.
VOL. 12, NO. 9
2 3 4
5 11
1 2 3
1 2 ,3
f I)
ci
1 2
Rhenium P. p . m. 40.
27.5 Nons
Colorado, Climax Region Powdered molybdenite sample Solid molybdenite sample Cottrell dust sample from rich gas dry precipitator Sludge sample from the lead Cottrell Ken. Mexico, Hurley Region Finished molybdenite concentrates recovered hy flotai,ion from copper flotation concentrates Questo Region Raniple 1 Sample 2 Sample 3 Sample 4 Sample 5 Flue dust sanipie Utah, Bingham Region Fissure specimen Fissure specimen Typical monazite porphyry with finely disseminated m o l y h denite Utah, Garfield Region Sample I Sample 2 Sample 3 Sample $ Sample ,> Sample 6 Brazil5 JIolihdeno Xanganese
.5 . l.% 6.2 tj.5
17 i
-< . . .)
7.5 7.5 9. 12..j
‘0 :3 1 44 I
.,
Norway, Knaben hlolybdaengruber, Iinaben Gruvor, Flekkrfjord 1 Sample containing approximately 95% molybdenite, 0.8%
2
5
copper pyrite, 3.0% quafts, and traces of pyrite and mica Sample of dust from drying furnace containing approximately 90.0% molybdenite, 1.80YG copper pyrite, the balance quartz, pyrites, iron, and organic substances
.j
,
.
Legacion de la Republica Dominica.
Significant among these results are the high rhenium coilcentrations found in the fissure specimens. Several concentrates showed analyses of 70, 325, and 597 parts of rhenium per million of ore, although for the two higher values i t was necessary to reduce the size of the sample to 0.1 gram in order to get reproducible results. These three samples represent the highest natural concentrations of rhenium ever recorded.
-4clinowledgments Since the selenium formed under these conditions cannot Lt, removed by filtration, it becomes impossible to make ail:. quantitative colorimetric estimation of the rhenium. To eliminate this difficulty the selenium should be removed immediately following the solution of the sample: The solution is made at least 8 or 10 N with hydrochloric acid, about 1 gram of sodium sulfite is added, and after it is thoroughly dissolved the solution is allowed t o stand for about 10 minutes. It should then be filtered, followed by a thorough washing of the residue. The volume of the filtrate is reduced to about 25 ml. by slow evaporation on the hot plate, and the determination may he continued in the regular way.
In Table VI are listed some typical determinations where various amounts of selenium dioxide were added. From an examination of these data it is apparent that the effect of the selenium has been eliminated, although the accuracy of the determinations is less than when the selenium separation is not included. Application The method, as developed, was applied to approximately 28 samples of molybdenite ores obtained from mines in Arizona, Colorado, Kew Mexico, Utah, Brazil, and KorTvay. The analyses of 23 of theqe qamples are listed in Table VI1
The authors would like to acknowledge the generous financial support of the Works Progress Administration and, in particular, the laboratory assistance of Harry Klawitter, illax Wolf, and Narvin Lubin. I n addition they would like to express appreciation for the many samples sent by the molybdenum producers which made the study of rhenium distribution in molybdenites possible.
Literature Cited (1) Geilman, Z. unorg. allgem. Chem., 208, 217 (1932). (2) Geilmann and Weibke, Ibid., 199, 120-8 (1931). (3) Ibid.. 199, 317-53 (1931). (4) Hoffman and Lundell, J . Research Nutl. Bur. Stardard8, 23,497(1939). (5)Hurd, IXD.ENG.CHEY..Anal. Ed.. 8, 11 (1936). (6) Hurd and Hiskey, Ibid., 10, 623 (1938). (7) Kronman, Bibikova, and .Iskenova, J . Applied C‘hern. (U.5 . S.R.), 7, 47 (1934). (8) Kronman, Bibikova, and Askenova, 2. anorg. allgem. C h e m . , 214, 143 (1933). (9) Kronman and Students, 2. anal. Chem., 90, 31 (1932). ( I O ) Mikhailova, Pevsner, and Archipova, Ibid., 91, 25 (1932). (11) Willard and Smith, IND.ENG. CHEX, Anal. Ed.. 11, 305-6 (1939). PHESENTED before the Division of Physical and Inorganic Chemistry 3t t h e ‘39th Meeting of the Americzn Chemical Society, Cincinnati, Ohio.
