N e w Fire Assay Method for Rhodium BHARAT R. SANT and
F. E. BEAMISH
Department of Chemistry, University of Toronto, Toronto 5, Ontario, Canada
b Rhodium is shown to b e efficiently collected in a copper-nickel-iron button b y the new fire assay method. After parling the button with hydrochloric acid, the base metals are separated by cation exchange and the rhodium is determined either gravimetrically or spectrophotometrically. Ten micrograms to 10 mg. of rhodium have been determined. Losses of rhodium to slag or pot walls were found to b e negligible.
A
for the efficient collection of platinum and palladium from ores and concentrates in a copper-nickel-iron button was recently reported by Beamish and coworkers (7, 8). This work was later extended to milligram amounts of osmium and ruthenium (4) wherein these metals were quantitatively dissolved and distilled in a single operation. As a part of the general program to study the behavior of other members of the precious metals, this investigation was undertaken to examine the applicability of the above method for the collection and subsequent determination of rhodium. Earlier researches on the lead collection ( I ) have given evidence concerning the inherent weaknesses of this procedure for rhodium. Invariably reassaying of slag is necessary and cupellation is unsatisfactory (IO) because of the difficulties in removing lead from the high melting point rhodium-rich alloys. The customary addition of silver fails as rhodium is insoluble in silver. Rhodium may be lost during cupellation because of oxidation and subsequent mechanical loss of the oxide particles. Allen and Beamish ( 1 ) therefore recommended met method for the button analysis. In the present method, collection of micro and macro amounts of rhodium is shown to be complete. On dissolution of the button, rhodium is separated from the base metals by cation exchange and is determined either gravimetrically or spectrophotometrically depending upon its quantity. That rhodium can be isolated from the base metals by cation exchange and subsequently determined has been shown by Marks and Beamish (6),who worked with synthetic base metals solution salted with known quantities of the precious metal. Along with rhodium, work to this NEW FIRE ASSAY PROCEDURE
i
304
ANALYTICAL CHEMISTRY
sulfide, 7 . 5 grams; nickel(I1) sulfide, 9.3 grams; silica, 5.1 grams; lime, 2.0 grams; alumina, 4.5 grams; magnesia, 2.0 grams. STAKD.~RD RHODICM SoLurIox. A weighed amount of sodium rhodium chloride (reported to contain 21.3% Rh) was dissolved in 10 mi. of concentrated hydrochloric acid and water and diluted t o 1 liter. The rhodium content in an aliquot portion was determined by the thiobarbituric acid method ( 3 ) . More dilute solutions of rhodium were obtained bv successive dilution and maintaining the acidity a t about the 0.12-7V level. Procedure. PREPARATION OF CuXI-FE BUTTON. The earlier procedure of Plummer et al. (7? 8) .was EXPERIMENTAL followed with some modifications. Ninety-five grams of the artificial Apparatus. Furnace and Pyro sulfide concentrate were roasted in a optical pyrometer described previously 6-inch porcelain dish a t 980' C. with (4). Beckman Model H pH meter. frequent stirring for 2 hours. On Reagents. Sodium carbonate and cooling, the calcine was ground to pass borax were of technical grade. Grapha 45-mesh screen and mixed with 42.4 ite powder, 200-mesh, mas supplied grams of sodium carbonate, 27 grams by A. P. Green Co., Weston, Ontario. of borax glass, and 11 grams of graphite. Unless otherwise specified all other About one third of the charge was chemicals were of reagent grade. ARTIFICIALSULFIDECONCENTRATE. placed in a 4-inch evaporating dish lined with cellophane and salted with a The following mixture, closely resemknown volume of rhodium solution. bling the natural concentrate of the This mixture was air-dried either in a Sudbury district of Ontario, was used: steam cabinet for 6 to 8 hours or a t room iron(I1) sulfide, 64.6 grams; copper(I1) temperature for several days. The dried material was carefully powdered in a mortar. Three to 5 grams of the unsalted portion were Table 1. Determination of Rhodium by first placed a t the bottom of a 30-gram a Fire Assay with Iron-Copper- Nickel assay crucible, followed by the cellophane lining and the salted portion, and Buttons finally covered with the remaining unButton Rhodium Rhodium salted material. The crucible n - ? ~ Wt., Taken Recovered Error placed for 45 minutes in the gas-ar Grams Milligrams furnace preheated to 1260' C. and the 10.32 - 0.06 31.6 10.38 temperature was raised gradually to - 0.10 31.2 10.38 10.28 1480" C. At this stage the gas and air 10.32 - 0.06 10.38 24.8 were turned off and the crucible was 10.38 0.00 10.38 28.5 removed from the furnace to cool. 0.00 10.38 10.38 27.5 The crucible uas broken open and the - 0.05 5.19 5.14 26.8 small amount of slag adhering to the 4.92 4.98 0.06 25.5 - 0.04 4.88 4.92 27.2 bottom n.as removed by tapping. 1.04" - 0.01 1.05 26.5 The slag was retained for lead assay to 0.01 1.05 1.06" 27.9 test for losses of rhodium. ANALYSIS OF BUTTON. The button, Micrograms 25 to 30 grams, was placed in a 600-ml. -26 518 492 27.1 beaker and treated with concentrated 27.1 518 542 +24 hydrochloric acid. When parted, small 500 - 18 28.0 518 408 -11 419 25.3 volumes of nitric acid were added two 209 - 3 24.8 212 or three times until the reaction ceased, 61 0 26.5 61 followed by repeated treatment Bith 50 - 2 27.8 52 hydrochloric acid to expel the nitrous 0 21 21.0 25.0 fumes. After dilution, the solution n-as 26.1 9 10.0 1 9 30.2 10.0 1 filtered through a 9-cm. Whatman NO. 42 filter paper and washed well with Analyzed spectrophotometrically by water. The siliceous residue was found aliquots. free of rhodium. The filtrate was stage indicates that platinum, palladium, osmium, and ruthenium would also be collected in the copper-nickeliron button. The extent of recovery of iridium has not been determined, but on collection it would appear along with rhodium. While osmium and ruthenium can be separated through distillation of their tetroxides, the remaining four platinum metals may be separated by established procedures (11). Specifically, rhodium may be separated from iridium by an anion exchange method ( 2 ) or by copper powder (9).
+ +
++
0
evaporated and diluted t o 1500 ml. so that the final p H was 1.5. This solution was passed through Dowex 50 x Q, 20- to 50-mesh cation exchange resin, 70 cm. deep and 4 em. in diameter, and the effluent was collected in a 4liter beaker. The column was aashed with acidu!ated water whose p H was adjusted to 1.5 with hydrochloric acid. The solution was evaporated to about 25 ml., transferred to a 150-ml. beaker and again evaporated in presence of 5 ml. of 2% iYaC1. The dry residue ivas treated with nitric acid-hydrogen peroxide mixture to destroy any organic matter coming through the column and the nitrates were then converted to chlorides by hydrochloric acid. The residue nas dissolved in 1 or 2 drops of €IC1 and about 30 ml. of water and the p H adjusted to 1.5. The solution was passed through a small cation exchange column, 4 cm. deep and 1 em. in diameter, to remove traces of base metals, and washed with 80 ml. of pI-1 1.5 water. The effluent was taken directly for the gravimetric determination of rhodium by the thiobarbituric acid method. For the microdetermination of rhodium by the stannous chloride procedure, the effluent from the small exchange column was evaporated to a very small volume and transferred into a 30-ml. borosilicate glass test tube. The absorbance measurements were made at a wave length of 470 mp (6). One
hundred to 500 pg. of Rh in 50 ml. mere determined with 1-em. path length while less than 100 pg. of Rh in 25 nil. were determined using a 5-cm. path length. RESULTS
Experimental data obtained for rhodium concentrations ranging from 10 pg. to 10 mg. are given in Table I. These results show essentially complete recovery of rhodium. I n the gravimetric analysis a positive error was occasionally noticed, due to traces of silica associated with the button. This error was eliminated by subjecting the residue t o dry chlorination and reprecipitating rhodium by thiobarbituric acid. Dry chlorination was carried out in the presence of excess sodium chloride in a borosilicate glass tube heated to about 650' C. for 6 to 7 hours. The chlorinated product was leached with dilute hydrochloric acid, the solution was filtered, and the rhodium in the filtrate determined gravimetrically as described above. This difficulty with silica was not experienced when determining rhodium spectrophotometrically. The slag and pot walls from the 10mg. rhodium sample were analyzed by the classical fire assay with lead as the
collector. No rhodium was thus detected. ACKNOWLEDGMENT
This work was supported by a grant from the National Research Council of Canada. The authors express their appreciation to Suseela B. Sant for the spectrophotometric analyses. LITERATURE CITED
( 2 ) Berman, S. S., V ~
Cruikshank, A. J., Bkamish, F. E.; IND.EKQ.CHEM.,ANAL. ED. 18, 120 (1946).
(4j Kavanagh, J. PII., Beamish, F. E., ANAL.CHEM.32,490 (1960). (5) Marks, A. G., Beamish, F. E., Ibid., 30, 1464 (1958). (6) Maynes, A. D., McBryde, W. A. E., Analyst 79,230 (1954). (7) Plummer, hl. E. V., Beamish, F. E., ANAL.CHEM.31,1141 (1959). (8) Plummer, hI. E. V., Lewis, C. L., Beamish, F. E., Ibid., 31,254 (1959). (9) Tertipis, G. G., Beamish, F. E., Zbid., 32, 486 (1960); cf. Zachariasen, H., Beamish, F. E., Talanta 4,44 (1960). (10) Truthe, W., 2. anorg. Chem. 154,413 (1926). (11) Westland, A. D., Beamish, F. E., Mikrochim.Acta 5, 625 (1957). \----,-
RECEIVED for review October 3, 1960. Accepted November 14, 1960.
Quantitative Microdetermination of Cholesterol Using Tomatine as Precipitating Agent JON J. KABARA, JAMES T. McLAUGHLIN, and CAROL A. RIEGEL Biochemistry Section, Department of Chemisfry, University of Detroit, Detroit, Mich.
b A routine quantitative method for isolating and assaying radioactive and nonradioactive cholesterol (free and ester) is described. The method involves the replacement of digitonin by the glycoside tomatine as a precipitating agent. The new reagent is more specific for cholesterol than is digitonin. While the glycoside does not possess absolute specificity, the sterol isolated from mice 15 minutes after methyllabeled tritium acetate administration is free of higher counting companions.
w
HILE a great deal of effort is being directed to the study of cholesterol metabolism, most of the analytical procedures used in these studies are not entirely specific for cholesterol. The most popular procedures, with appropriate modification, incorporate the use of digitonin as a precipitating agent as introduced by Kindaus (9) and use the Liebermann-Burchard color reaction
employed by Grigaut ( 2 ) . Because of the lack of specificity their use has been criticized even when combined into a single procedure. This short-coming becomes especially significant in experiments involving the isolation and assay of radioactive cholesterol. Precipitation with digitonin resulted in the isolation of various products along with the principal sterol so that after further purification (via the dibromide) much of the samples' original radioactivity was lost (8). Consequently it was necessary for each investigator to prove either that the cholesterol isolated a t the time interval soon after isotope injection (in vivo) or addition (in vitro) was free of higher counting companions or that the tedious dibromide purification method was a necessary additional step to ascertain purity. Since incorporation experiments involving radioactive precursors are most meaningful, during the early time inter-
vals when the concentration of higher counting companions is highest, a procedure was sought which would be relatively specific for cholesterol and sufficiently practical to permit the handling of large numbers of samples on a routine basis. Early success with tomatine, a glycosidal alkaloid isolated from the leaves and stems of tomato plants ( I ) as a specific precipitating agent was sufficiently encouraging for us to concentrate on its analytical use. As a modification of an earlier procedure (6) the present microtechnique using tomatine for either radioactive or nonradioactive cholesterol determinations represents a considerable increase in specificity and a concomitant decrease in work, time, and cost. APPARATUS AND REAGENTS
A Bausch & Lomb colorimeter was used with a 630-mp filter. VOL 33, NO. 2, FEBRUARY 1961
305
