1612
ANALYTICAL CHEMISTRY
alkali metal oxides as well as the heavy alkaline earth oxides will react with bromine trifluoride, but that the Group 111 and IV oxides will be inert, except for those metals which form volatile fluorides. Several metal oxides which form insoluble, nonvolatile fluorides react with bromine trifluoride. Whether this reactivity is due to the formation of a nonadherent fluoride film, or to some other cause, has not been determined. A number of possible changes in procedure are possible in an attempt to increase the number of metal oxides that can be analyzed. Some qualitative experiments reported by Andrews and Katz ( 1 ) indicate that beryllium, mercuric, ceric, zirconium, nianganese(IVj, cobaltic, and nickel oxides can be completely converted to the fluorides by bromine trifluoride a t 300” C. in a high-pressure vessel. S o attempt n’as made to measure oxygen evolution and the extent of reaction was calculated from weight change and crystal structures. A mixed bromine trifluoridehydrogen fluoride reagent caused complete fluorination of aluminum, lanthanum, and calcium oxides a t 200” C. Modification of equipment to permit collection of oxygen generated by high pressure, high temperature exposure to bromine trifluoride would materially increase the number of oxides which can be analyzed. The authors’ work t o date has been concerned principally with obtaining a quantitative evolution of oxygen from oxide samples,
and very little with reaction mechanisms, or characterization of final products or of intermediate products when they appear. They hope to investigate these fields as well as the reactions of bromine trifluoride with a number of other groups of compounds including nitrides, carbides, and borides. LITERATURE CITED
(1) Andrews, H. C., and Katz, J. J., Division of Physical and In-
organic Chemistry, 118th Meeting, AM.CHEM.Soc., Chicago, Ill., 1950. (2) Banks, A. A., Emelkus, H. J., and Woolf, A. A., J . Chem. Soc., 1949,2861. (3) Eggertsen, F. T., and Roberts, R. SI., ~ N A L .CHEM.,22, 924 (1950). (4) EmelQus,H. J., and Woolf, A. A., .J. Chem. Soc., 1950, 164. (5) Roberts, L. E. J., and Harper, E. A., Harwell Report, AERE, CIR-885 (May 5, 1952). (6) Scott, W.W., “Standard IIethods of Chemical Analysis,” Vol. I, p. 669, New York, D. Van Nostrand Co., 1939. (7) Sharpe, A. G., and Emeleus, H. J., J . Chem. SOC.,1948, 2136. (8) Sheft, I., Hyman, H. H., and Kats, J. J., J . Am. Chem. Soc., in press. (9) Simons, J. H., “Fluorine Chemistry,” Vol. I, p. 191, S e w Tork, Academic Press, 1950. (10) Wartenberg, H. von, 2. anorg. allgem. Chem., 251, 161 (1943). (11) Woolf, d.A, and EmelQus,H. J., J . Cheni. Soc., 1949, 2865. RECEIYEO for reviev Aiigiist 17. 1933. Accepted August 24, 1953
6th Annaal Summer Symposium-Less Familiar Elements
Present limitations of Platinum Group Analysis Introduction to the Symposium on the Platinum Metals WILLIAM M. MAcNEVIN, Ohio State Unicersity, Columbus, Ohio
T
HE analytical chemistry of the platinum group metals (os-
mium, rubidium, palladium, platinum, rhodium, and iridium j was thoroughly reviewed in 1943 by Gilchrist ( 2 ) of the Sational Bureau of Standards. He observed that the extensive literature was difficult to organize except under individual authors because there was often little relationship between ideas from different laboratories. Consequently the description of the analytical chemistry of the platinum group metals under a few general chemical reactions is inadequate. He did, however, point out that the problems of platinum group analytical chemistry can be grouped under the headings: (1j solution of ores and metals, (2) separation of platinum group metals from other metals, and (3) separation of platinum group metals from each other. To this must be added a fourth, determination of platinum group metals. The trend of research in platinum group chemistry since the appearance of the Gilchrist review in 1943 is indicated by the distribution in subject matter of papers shown in Table I. The trend in all this work has been toward specific methods for detection and determination, w-ithout the necessity for separation if possible. Methods used have been largely empirical, so that little conclusion can be drawn as to the specific properties involved. During the period 1900 to the present, platinum group analysis has been of prime interest in connection with the development of Canadian ores. At Toronto University, Beamish ( 1 ) and his coworkers have been concerned with the dissolving of ores and metals containing platinum group metals and with separations and determinations of individual metals. Their experience is summarized in the first paper of this symposium. -4 widely used method of analysis for mixtures of platinum group metals is that of Gilchrist and Wichers (3) of the Bureau of Standards, published in 1935. The unique experience of the
Bureau of Standards in applying these methods over a 20-year period is represented in the paper by Raleigh Gilchrist. rlmong the newer developments in platinum group chemistry are the measurement and evaluation of specific properties of platinum group metal compounds which are useful in analysis. The photometric study of complex ions, their formation and stability, and subsequent use in analysis is the subject of the paper by Gilbert hyres of the University of Tews , Although there has been a trend toiv-ard discovery of specific reagents for detection and determination for strictly analytical purposes, there have also occurred an increased interest in and need for quantitative separations of the platinum group metals.
Table I.
Publications on Platinum Group Chemistry,
1943 to 1952 Subject Solution of samples Qualitative detection Separations Chromatography (column) Chromatography (paper) Electrolysis Extraction Fluxing Organic reagents Volatilization N e t inorganic methods Zinc sulfide column Determinations Colorimetry Fluorophotometry Gravimetric methods Polarographic Potentiometric titration Spectrographic Volumetric methods X-ray diffraction X-ray fluorescence
Number of Publieations 3 10 10 2
8 1 2 120 7 65 5
38 3 10 3 16 38 9
18 2
V O L U M E 25, NO. 1 1 , N O V E M B E R 1 9 5 3 Methods of separation are naturally important in the industrial preparation of these metals. Radiochemistry has emphasized the need for chemical separations of these metals. It has also placed a special demand-that of speed-upon the chemist concerned with the operations of solution and separation. An unusual example of difficulty occurring in radiochemistry, in the author’s laboratory, is the greatly increased resistance to chemical attack of iridium which has been bombarded in the cyclotron. Consequently it becomes necessary to look for more rapid chemical methods. The fourth and last paper of the symposium is concerned with separation by means of ion exchange resins. The principal limitation in platinum group analysis a t present therefore is that the methods, excellent as they are as to results, are too slow to fit the needs of radiochemistry. More rapid meth-
1613 ods of quantitative separation are especially needed. Platinum group chemistry has also been largely empirical. Systematic quantitative studies of solubilities of precipitates, of adsorption phenomena, of complex formation and stability, of photometric behavior, and of electrochemical behavior are needed. The systematic application of such information should lead to an improved analytical scheme for the platinum group metals. LITERATURE CITED
(1) Beamish, F. E., et aZ.;“Applied Inorganic Analysis,” Hillebrand and Lundell, 2nd ed., Chap. 20, New York, John Wiley &
Sons, 1953. (2) Gilchrist, R., Chem. Revs., 32, 277 (1943). (3) Gilchrist, R., and Wichers, E., J . Am. Chem. SOC.,57, 2565 (1935).
6th Annual Summer Symposium-Less Familiar Elements
Concentrating and Dissolving Platinum Metals F. E. BEAMISH AND W. A. E. McBRYDE Department of Chemistry, University of Toronto, Toronto, Ont., Canada This is an interim report of the efficiency of the fire assay for ruthenium, osmium, rhodium, and iridium. Reference is made to the losses caused by cupellation and to the methods of effecting quantitative corrosion of platinum metals and their alloys. Data are provided to proFe that certain types of fluxes may “slag off” some of the platinum metals. Of particular significance is the fact that prefusion of ore mixtures may result in verj- unsatisfactory assay recoveries. This observation suggests the possibility that similar phenomena may be found in natural occurrences. Furthermore, these researches provide the first experimental data on the efficiency of lead collection.
T
HE treatments ahich go to make up the fire assay for pre-
cious metals go back a great many years. In discussing the history of assaying, Smith ( 1 7 ) mentions that many of the procedures such as cupellation and parting are recorded as early as the 12th or 14th century. The refining of silver by fire in the manner of cupellation is recorded by Jeremiah in the Old Testament. The operations are by no means well known to many chemists. At universities, assaying is taught in schools of mines or of mining engineering, and the average chemistry student hears very little or nothing about it. Severtheless, the operations involved when an assay is required for the platinum metals call for a high degree of chemical skill. Thus, Bugbee ( 7 ) states, in connection with the platinum metals, “their correct determination is considered the most difficult anal) sis in the realm of inorganic chemistry.’’ The success of the crucible assay depends on several factors. One is having the ore reduced to a very fine powder and intimately mixed with the flux. The choice of composition for the flux is governed by the composition of the gangue to be slagged off. For instance, an ore rich in silica requires a compensating amount of soda ash and litharge to form a suitable slag. The composition of the flux and the temperature of the furnace must be controlled so that reduction of lead occurs while the slag is still quite viscous. In this way the extraction of the precious metal occurs throughout the body of the charge, Later, the temperature may be raised, and as the slag becomes more fluid the minute droplets of lead fall and collect together in the base of the crucible. Some knowledge of the approximate composition of the gangue is necessary t o predict the amounts of acidic or basic oxides to be added in order to form a suitable slag and to predict the amount of reduction of litharge which may take place by the action of sulfides. The characteristics of a good slag are numerous. I t
should remain rather viscous early in the fusion while the lead drops are forming, but become more fluid a t somewhat higher temperatures; it should separate cleanly from the lead button after cooling; and its composition must not be so basic as to corrode out the clay crucible. Ideally, about 25 grams of lead should be produced and form the so-called button on cooling. For ores containing but little sulfide, the quantity of lead in the button is regulated by the amount of flour or other reducing agent added with the charge. Sulfide ores themselves will reduce litharge, and it is customary to express the reducing power of an ore as the number of grams of lead produced by the reducing action of 1 gram of ore. Since it is desired to limit the size of the buttons and also t o leave some unreduced litharge in the slag, the custom is t o add potassium nitrate as an oxidizing agent in the assay of sulfide ores; this is the socalled niter assay. Another practice, somewhat rarer and certainly more time-consuming than the foregoing, is to roast the ore with free access of air in order to convert sulfides to oxides. hlention should also be made of the iron nail assay. I n this method, the reduction of litharge is accomplished by an excess of iron, usually in the form of nails. These are removed before the melt is poured. This type of assay is applied to sulfide ores and usually gives rise t o a ferrous sulfide matte. With‘this treatment a large amount of soda ash is added to the flux and much of the iron matte dissolves i n the slag in the form of sodium ferrous sulfide. A large proportion of base metals is found in the button in the case of the iron nail assay. The contents of the crucible or pot are poured into a conical mold, and on cooling the slag breaks aB-ay or can be removed from the lead button. The scorification assay consists of heating together ore and lead, plus a small amount of borax, in an oxidizing atmosphere. Thiq is carried out in a muffle furnace on a shallow saucerlike
