The DISCOVERY of RHENIUM' LOREN C . HURDt The University of Wisconsin, Madison. Wisconsin
It is the purpose of this communication to extract from the maze of polenzic as to priority i n the discooery of rhenium the essential papers relahe to the issue and to report some of the work which has been fie7formed i n this laboratory and which has a direct bearing on the question.
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HE isolation of element 75, the dwimanganese of Mendel&&, was announced in 1925 by Walter Noddack and Ida Tacke.' Shortly after the appearance of the original work the claim was sharply contested by V. DolejSek and J. Heyrovskjrz and by F. H. Loring and J. G. F. D N C ~ . ~During !~ the ensuing two years there appeared in various journals over a score of papers relative to the validity of the discovery. Although persons acquainted with the chemistry of rhenium (element 75) readily recognize the absurdity of the contesting claims, the general confusion created by the mass of publication coupled with the more recent assertions of J. G. F. Druce5 to the effect that "the occurrence of dwimanganese in crude manganese compounds was discovered independently by Loring and Druce and by DolejSek and Heyrovskj." has created an erroneous impression in many minds. Inasmuch as the early work of Loring and Dmce was focused principally upon manganese compounds the statement by the lattere to the effect that all rhenium used in his research is being obtained from manganese salts has further confused the issue. That the early discussion has given rise to a distinctly incorrect understanding is evident from several recent casual references '7h9 to divalent rhenium. It is the purpose of this communication to extract from the maze of polernic the essential papers relative to the issue and to report some of the experimental work which has been performed in this laboratory and which has a direct bearing on the question.
CLAIMS OF LORING AND DRUCE
In one his earliest papers in this field, Druce3 reported the isolation of an appreciable amount of dwimanganese from manganese salts. In the case the sulfate the following procedure was employed: One hundred g. of entde M ~ S O was I dissolved in 600 cc. H,0 and 50 g. NH4Cl added. The solution was made ammaniacal and repeatedly saturated with HpSto precipitate all of the manganese as the sulfide (a). When the filtrate yielded no further precipitate with HIS, the excess of the gas was removed and the solution acidified with HCl. Following evaporation to dryness, the residue was strongly ignited, cooled, and dissolved in dilute nitric acid (b). Ammonium oxalate was added in slight excess, the precipitate filtered off, and the filtrate again evaporated and the residue ignited. In this way a 75 mg. residue of impure light brown oxide was obtained ( c ) . The oxide was soluble in dilute mineral acids. These solutions yielded a hydroxide of the element when treated with caustic alkalies (d).
In another paper a few weeks later Loring and Druce4 record the isolation of element 75 from impure manganese chloride and from pyrolusite. The essential details of the procedures reported do not differ from those outlined above for manganese sulfate. "Dwimanganese hydroxide" was found to be soluble in hot dilute KC1 and aqueous solutions of SOz. Caustic alkali solutions were without effect although alkaline oxidizing agents transformed the color of the precipitate to a deep brown. Alkali fusions were green. Although carbon dioxide when passed into solutions of the alkali fusion product failed to give rise to a permanganate pink, the solutions were reported to be unstable. HCl solutions of the oxide colored the Bnnsen flame bright green. The ignited higher oxide, after heating in a stream of oxygen, was reduced to a.low&roxide by heating in hydrogen. In the experiment reported, 0.2763 g. of the higher oxide yielded upon reduction 0.2364 g. of the lower oxide. Assuming the atomic weight of dwimanganese to he 188, Loring and Druce pointed out that their results indicated a rduction of DO3 to DO. X-ray examination of the precipitates indi* Contribution from the Division of Inorganic and An cated L, and LBL,lines in very close agreement with Chemistry, Department of Chemistry, The University calculated values. consin. t A portion of the experimental work herein reported is from In an effort to isolate the product reported by the the senior theses of M. W. Bessert, J. K. Colehour, C. W. Eggert, above workers the procedure has been applied to a large C. R. Naeser, F. N. Pansch, and E. J. Seelig, The University of Wisconsin, 1929-32. number of manganese salts and minerals. In no case ' NODDACE ~ r TACKE, n Siizber. fleuss. Akod. Wiss., Physik: has a product been obtained in which rhenium could be math. Klasse, 1925, 400-5. spectroscopically identified. In some cases a product a DOLEJSEH AND HEYROVSKP, Nature, 116, 782 (1925). News, 131, 273 (1925). D ~ u c s Chem. , was obtained which answered in general the description "ORING AND DRUCE. ihid., 131, 337 (1925). of Loring and Dmce. In all such cases the alkali pre" DRUCE,Sci. Progress, 24, 480 (1930). cipitate was shown to consist for the most part of hyWRUCE, Chem. News, 144, 258 (1932). ' Pmlsn AND WAINER,3. Am. Chem. Soc., 53, 3818 (1931). drated manganese oxide. The absence of rhenium in ibid., 54, 8 A ~ ~ ~BISHOP, s o ~ ,SOWER. AND CHRISTENSEN, the end-product is not surprising, however, when one 613 (1932). considers the reactions which have taken place during ibid.. 54, 3074 (1932). YACODA, 605
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the course of the "concentration" (a) Rhenium sulfide may be precipitated from ammoniacal solutions. In an early communication Walter Noddack and Ida Tackelo reported that a solution of perrhenic acid yielded no precipitate when treated with ammonium sulfide or hydrogen sulfide. In a later paper the same authors ",'"indicated that dwimanganese sulfide was insoluble in ammonium sulfide and in acid solutions. This apparent contradiction was discussed by Druce13 in the following manner: They mention this (the insolubility of the sulfide in acid and alkaline solutions) apparently because my enrichment of dwimanganese depends in many cases on the fact that manganese could be largely removed from its new homolog by precipitation with H,S in alkaline solution.
In light of our present knowledge of the chemistty of rhenium the two apparently contradictory statements are quite in harmony with the facts. Ammonium sulfide as a reagent does not readily precipitate rhenium sulfide. It is only by the continued passage of hydrogen sulfide into an ammoniacal rhenium-containing solution that appreciable amounts of the sulfide are thrown down. Likewise Re& is, under ordinary circumstances, slowly and incompletely precipitated by H2Sfrom dilute acid solutions. It is, however, quantitatively precipitated from rather concentrated hydrochloric acid solutions of perrhenic acid. This was clearly indicated by the Noddacks, has since been demonstrated conclusively by Geilmann and Weibke,I4 and has been repeatedly verified in this laboratory. Rhenium, if present in a manganese salt, would probably be removed almost completely if subjected to the treatment described by Druce. Under ordinary circumstances the precipitation of rhenium sulfide from ammoniacal solutions is not complete. The presepce of a relatively large amount of manganese sulfide, however, greatly increases the amount of rhenium removed from the solution. A long-continued and careful process such as described is sufficient to remove practically all of the rhenium. Following removal of the dissolved hydrogen sulfide the solution was made acid with HC1, evaporated to dryness, and ignited (b). Perrhenic acid, if present, would be volatilized and such an evaporation and ignition would result in an appreciable loss of rhenium. Geilmann and Weibke14have shown that as much as 96% of the rhenium present in a solution of perrhenic acid may be volatilized as'a result of evaporation and gentle ignition. Although We "dwimanganese" solutions of Loring and Druce contained ammonium chloride, the volatility of perrhenic acid would not be greatly reduced. In fact under some conditions RezOi is more easily vaporized in the presence of a volatile chloride. ( c ) Ignition of a perrhenate or perrhenic acid in the presence of an oxalate produces, in general, metallic rhenium. If a "fixed base" is present the 10 NODDACK AND TACKE, Nal~rm'ssen~~haftefte11, 13, 567 (1925). 11
Nonnara. W. 16. 2129 11926). .. . A N D .T. ., Mdnllhiimr. ~~~. ..,--,---. ~
NODDACK, W. AND I., Chem. N m s , 133,241 (1926).
DRUCE.Chem. Weckblad. 23, 497 (1926). 'WEILMANN AND WEIBKE,Z.nnorg. oll&m. Chem., 195, 289 (1931). IS
metallic rhenium is not all lost on subsequent ignition. In the absence of such a base, however, the rhenium is immediately oxidized to volatile Rez07. That such is the case may be readily demonstrated by employing the r6st-rohr technic described by Geilmann and Wrigge.15 If the residue contained ammonium perrhenate the ignitions described would result in a total loss of rhenium. As Druce himself recently reported,16 ammonium perrhenate is decomposed upon heating. Since all of the products of decomposition are volatilized when the ignition is carried out in the air, i t is probable that any rhenium which escaped precipitation as the sulfide would be lost a t this point. (d) Perrhenate solutions do not yield hydroxide precipitates when treated with alkalies. The only known case of what might be called a hydroxide precipitation is the flocculation of rhenium dioxide. Quadrivalent rhenium is produced in solution by the action of strong reducing agents. From such reduced solutions hydrous rhenium dioxide is precipitated by the action of an alkali. Unless the acid concentration be moderately high, hydrolysis takes place immediately upon reduction. Under the conditions described by Loring and Druce, rhenium-containing solutions do not yield a precipitate. Furthermore all known oxides of rhenium as well as metallic rhenium are completely volatilized when ignited in the air. A total of nine different samples of manganese sulfate and chloride were carried through the Loring and Druce procedure in this laboratory during 1929-30. Although fifteen times the quantity of material used in the original work was a t times employed, no trace of rhenium could be detected in the concentrates. Likewise, samples of crude manganese iodide, manganese borate, and native manganiferous silicate failed to yield a rhenium-containing precipitate when treated according to the method described. A number of samples of pyrolusite, concentrated according to the published method, failed to yield a rhenium-containing hydroxide. In view of the splendid agreement between the observed and calculated values for the wave-lengths of the X-ray lines reported, it was thought that perhaps Loring and Druce had access to salts and minerals which contained such amounts of rhenium as would pass through the concentration process and by virtue of various unusual solubility and adsorption phenomena be present in the final hydrated manganese oxide precipitate. Accordingly, 1% of KReOl was added to several 100-g. samples of hydrated manganese sulfate and the solutions carried through the procedure as reported. Spectroscopic examination of the "dwimanganese" precipitates indicated the presence of large amounts of manganese but failed to indicate the presence of rhenium. Rhenium if present in the concentrate was in amount less than 10W5. The same process of enrichment was repeated with MnOz and rhenium was likewise shown to be absent in the concentrate.
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GEILMANN AND WRIGGE,Z. anorp. . allgem. Chem.. 199, 66 (1931). l6 DRUCE, C k m . & Ind., 51, 632 (1932).
With respect to the physical and chemical properties of element 75 as reported by Loring and Druce, the following points might be noted: (1) The only "brown oxide" of rhenium known a t the present time consists of a mixture of black ReOz and red ReOa. This product is but slightly soluble in dilute HCI. (2) Aqueous solutions of SOz are without appreciable solvent action on ReOz. -(3) Alkali fusions of rhenium or oxides of rhenium are, unless unusual precautions are taken, invariably yellow. I t is only when the fusions are carried out in an inert atmosphere that a color approaching green is obtained. (4) Perrhenate solutions are characterized by extraordinary stability. ( 5 ) Rhenium does impart to the Bunsen flame a green color. The color, due to the 4889 A.U. line, is not a bright green but a bluish green. The test is not particularly delicate. (6) As has been pointed out by Dr. Noddack, the weight conversion experiment of Druce might well be interpreted as the reduction of Mn02 to Mn20s. The discrepancy of 14.4 mg. is not surprising when one considers the difficulties encountered in preparing synthetic manganese dioxide. CLAIMSOF DOLEJSEK AND HEYROVSKY Concurrent with the publications of Loring and Druce there appeared during the period 1925-27 a series of publications by V. Dolejgek and J. Heyrovskj. relative to the isolation of dwimanganese. Although they first obtained spectroscopic evidence for element 75 in the alkali precipitate of an electrolytic concentration, the bulk of the work appears to have been centered around the intensification and interpretation of an irregularity in a number of polarographic curves. In the first papers of the series Dolejhk and Heyrovskl",'g reported that in May, 1925, they found evidence of dwimanganese in manganese salts. The method of concentration consisted of depositing upon zinc those impurities which might be precipitated from a concentrated solution of manganese sulfate. The deposit on the zinc was dissolved in concentrated HCl, evaporated to dryness, and from a solution of the residue a precipitate was obtained by adding NaOH. Although Xray examination of the precipitate indicated tife presence of 75, the evidence was admittedly weak. The auto-registering polarograph which was used through the work1=is essentially a very clever device for measuring a total deposition potential effect on a flowing mercury cathode. Although the data obtained are rather difficult to interpret they appear to be quite reproducible. Curves obtained during the course of an investigation carried out on certain manganese solutions possessed two "humus." The first of these could be elimil7 DOLEJSEKAND He~novsK~, Nature, 116, 782 (1925). DOLEJSEKAND HEYROVSK~, Chem. Listy, 20, 4 (1926). ~"HEYROVSK~, Rec, tram chim., 44, 488 (1925). la
nated by passing HLSthrough the acidic or sodium acetate solution. The precipitate gave rise to no X-ray lines which could be attributed to 75. The second irregularity in the curve was not affected by passing H S through the acid or alkaline solution. When a solution displaying this second hump was treated with NaOH a hydroxide precipitate was obtained which, upon examination in a Siegbahn spectroscope, indicated element 75. Because of the possible interference of the zinc K,, line with the L,, line of 75, the workers deemed i t advisable to develop a method of concentration which would provide against the introduction of zinc. This contamination was inherent in the original concentration. Accordingly the following procedure was developed. Into a nearly saturated solution of MnS04, a small crucible containing manganese amalgam, prepared electrolytically, was introduced together with a large platinum foil connected externally to the amalgam. After several days the platinum foil was removed, rinsed with water, and the deposit washed off with concentrated HCI. The solution thus obtained was diluted with water, neutrdized with NaCOa, and slightly acidified with acetic acid. HIS was passed into the solution to remove Zn, Co,and Ni and the metals of the first two analytical groups. The remaining solution contained manganese contaminated with about 2% dwimanganese. The figure for the dwimanganese content was apparently arrived a t from the intensity of the lines resulting from Rontgenograpbic study of the NaOH precipitate. The acidic chloride solutions were reported to have a greenish color. The dry green chloride rapidly darkened and turned black on standing. Neutral solutions deposited a yellowish brown precipitate on standing in air. This phenomenon was taken as an indication that higher valency compounds of 75 were more stable than those of manganese. A rather brief note by DolejSek, Dmce, and Heyr~vskj.'~ indicated that the concentrates obtained in the two laboratories were similar. Failure of the Noddacks to detect 7.5 i h MnSOr or KMnOa according to the DolejSek-Heyrovskj. method of concentration was ascribed2' 2 2 to the possible lack of 75 in the salts. In this communication the workers emphasized that, contrary to the Noddacks, they helieved that dwimanganese was not precipitated by HzS from a HC1 solution and that it was precipitated from a manganese solution by Hz02. DISCUSSION
The two concentration methods described have been repeated on several different samples of crude manganese sulfate in this laboratory. In no case has an endproduct been obtained in which rhenium could he identified. Manganese sulfate containing 1% KRe04 was concentrated according to the published procedure. Spectroscopic examination of the end-product indilo DOLBJSEK. DRUCE, AND HEYROVSRG. Nalure, 117, 159 (1926).
DOLEJSEK AND HEYROVSK~, Rec. Irov. chim., 46,248 (1927). HEYROVSK~, Chem. News, 135, 229 (1927).
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cated the total absence of rhenium. In the case of the first method of concentration it might be noted that rhenium is not precipitated from neutral solutions by the addition of zinc. It is only when the solution is acidified that reduction takes place. Hydrogen sulfide precipitates rhenium sulfide from hydrochloric acid solutions and hydrogen peroxide does not precipitate Re, Re02, ReOa, or Re2O7. In fact all known oxides of rhenium are converted to very soluble HReO4 by the action of this reagent. Acidic chloride solutions of HRe04 are colorless or slightly yellow. The only green rhenium salt known a t the present time is the complex chloride produced by the action of strong reducing agents on HCl solutions of KReOa or by the action of dry chlorine on a hot mixture of rhenium and KC1. From a study of the several methods of concentration
and extraction published by Loring and Druce and by DolejSek and Heyrovskj. it is apparent that only by an amazing sequence of incomplete precipitations, complete reductions, and tremendous adsorptions could rhenium be isolated by following the procedures. Whether or not conditions could be adjusted as to result in an appreciable concentration is problematical. It seems fairly certain, however, that if ordinary manganese salts contain rhenium, the element is present in such infinitesimal amounts as to escape detection. Repeated attempts in this laboratory to isolate rhenium from widely divergent types, kinds, and brands of manganese salts have proved fruitless. Likewise the vast majority of pyrolusites either contain no rhenium, or rhenium, if present, is in amounts far below those reported.
