I
are joined. The phenolic groups of the indicators are free to react with acid or base, and in strong base it is possible to produce the symmetrically resonating forms which give rise to the characteristic basic colors of the indicators. The transition between the acid and basic forms of phenolphthalein upon addition of acid and base is:
from alcoholic solution. If cationic ion exchange resins are used, the indicators are apparently adsorbed by the resins, in that the beads present their basic color upon addition of alkzli but both the color and the indicator are rapidly and completely desorbed from the resin even in water. Microscopic examination of phenolphthalein resin beads un-
0
HO
Na +-0
-
red have also been prepared in the manner described, but their color changes were not so distinct. SUMMARY
The prime adrantage of the technique described is the preparation of an indicator in a phase separate from the solution whose p H it indicates. The ability of an ion exchange resin to load up to the limit of its exchange capacity with the indicator effectively results in a highly concentrated “point” indicator.
NaOH
c
d
c
HCI
Similar transitions would occur for thymol blue and bromocresol green resins. Further evidence of chemical linkage of the indicator molecule to the ion exchange resin rather than mere physical adsorption is that the indicator is eluted from the resin by ethyl alcohol to only a slight extent, and not at all by n-ater, whereas the indicator-resin readily forms
der 100 power shows them to be uniformly colored throughout their bodies after immersion in 0.1N sodium hydroxide. I n subsequent conversion to the colorless form upon addition of acid, the radial disappearance of the pink color can be observed as the hydronium ions penetrate the beads. Anionic ion exchange resin forms of methyl orange, methyl red, and Congo
ACKNOWLEDGMENT
The author wishes to express his gratitude to Refining Unincorporated, New York, K. Y., for permission to publish this article. H e is also grateful to Saul Soloway of The City College for his helpful discussions during preparation of this article. LITERATURE CITED
(1) Kolthoff, I. PIT.,
“Acid-Base Indicators,” Macmillan, Xew York, 1937. ( 2 ) Miller, W. E., ANAL. CHEW 29, 1891-3 (1957).
RECEIVEDfor review June 5, 1957. Accepted April 21, 1958.
Separation of Rhodium and Iridium from Base Metals by Ion Exchange ALICE G. MARKS’ and F. E. BEAMISH University o f Toronto, Toronto 5, Canada The fire assay with lead as the collector is the one method universally used for determining the platinum metals in ores and concentrates. Generally, the lead button thus obtained is cupeled to form a silver alloy. The more insoluble platinum metals fail to alloy with silver and, to a degree, with lead, and result in an easily removed encrustment on the bead. No modification of the fire assay has been recorded to eliminate this mechanical loss. With a view to the production of a collecting alloy which would allow dissolution of all the platinum metals, a fire assay was devised, which permitted use of the siderophilic metals as collectors for platinum and palladium. However, the problem of collection and determination of rhodium and iridium remained. Now, by ion exchange processes, microgram amounts of rhodium and iridium can b e isolated from very large proportions of the associated base metals, iron, nickel, and copper.
1464
ANALYTICAL CHEMISTRY
T
extremely low content of precious metals in ores necessitates some method of concentration prior to determining precious metal values. The only practical procedure has been collection of the precious metals in molten lead, followed by further concentration by a cupellation process which removes lead. This method is not satisfactory for some of the insoluble platinum metals, as they are often found at the outer surfaces of the lead buttons and otherwise mechanically mixed. Losses are thus possible during subsequent cupellation or other operations. Rhodium, iridium, etc., may often be detected as a n excrescence at the surface of the silver bead resulting from cupellation. Because of the siderophilic nature of the platinum group of metals and the recognized ability of iron and nickel to form platinum metals alloys, it seemed probable that the platinum metals could be collected in molten iron or nickel. HE
Attempts were made to h d a satisfactory method of analyzing ferronickel assay buttons. As cupellation is impossible with iron, a wet chemical method was necessary for final concentration of the platinum metals. Coburn, Beamish, and Lexvis (4) developed a method for separating platinum and palladium from the base metals by cation exchange and supplied it to the analysis of ferronickel assay buttons. It is believed that these buttons will collect iridium more efficiently than the classical lead alloy in which iridium is relatively insoluble. Furthermore, ruthenium and osmium may be thus more efficiently collected from ores, etc. The behavior of these tn7o metals during fire assay, particularly osmium, is little understood, but ample data show that such assay with subsequent cupellation is an inadmissible process for the determination of osmium. The following report shows the 1 Present address, Canadian Industries, Ltd., Millhaven, Ontario, Canada.
efficiency of the cation exchange method for the separation of rhodium and iridium from base metals. APPARATUS A N D REAGENTS
Dowex 50 X 8, sodium form, cation exchange resin (20 to 50 mesh) was used in the absorption of the base metals. Absorbance measurements were made with a Beckman Model B spectrophotometer. Solutions. RHODIUM. A weighed amount of sodium rhodium chloride salt was dissolved in 10 ml. of concentrated hydrochloric acid and the required amount of water. T h e solution was filtered, diluted t o 1 liter, and standardized by thiobarbituric acid (5). INDIUM.A weighed amount of iridium ammonium chloride was dissolved in 10 ml. of concentrated hydrochloric acid and the necessary amount of water. After the solution was filtered and diluted to 1 liter, it was standardized by 2-mercaptobenzothiazole (1). SYNTHETIC BASE METAL. Because of the time and expense involved in preparing the buttons, experiments were performed on a synthetic base metal solution Khich simulated, as exactly as possible, the constituents resulting from the dissolution of a button. The stock solution contained 20 grams of iron in the form of ferric chloride, 2.5 grams of nickel, 1 gram of copper, in the form of their hydrated chloride salts, and 4 ml. of hydrochloric acid for each 400 ml. of the final volume. The salts were dissolved in the acid and a minimum amount of water, filtered, and diluted to the required volume. EXPERIMENTAL
The resin used in a tower contained 42 to 48% moisture and weighed 700 grams. The recorded capacity was 4.7 =t 0.3 meq. per dry gram. Capacity measurements showed that a bed of resin 70 em. deep and 4 cni. in diameter supported on a plug of glass wool absorbed 24 grams of the base metals. The resin is then operating a t a n efficiency of about 50%, as expected with this method of operation and rate of flow of solutions; some base metals escape through the column before the resin exchanges the maximum amount possible. The column was regenerated just before use TTith 3N hydrochloric acid until there ITas no indication of iron in the eluents, as shown by a spot test with potassium thiocyanate. The excess acid mas removed from the column by washing with water, about 1.5 liters, until the eluent was neutral to litmus paper. Initial experiments indicated that rhodium and iridium passed through the columns qualitatively. Procedure. The samples consisted of 400 ml. of the stock base metal solution and a known amount of rhodium and/or iridium in a 2-liter Erlenmeyer flask. These samples were diluted t o 1.5 liters and mixed
well. T h e p H of this solution should be 1.5 (4). The sample was added to a Miter separatory funnel supported above the column, and passed through the regenerated and washed resin bed a t a rate of 25 ml. per minute. The sample was washed with 1.5 liters of water in three portions; the entire inner surface of the separatory funnel was carefully rinsed with each portion. The effluent was evaporated to a very small volume in a 4-liter beaker, then transferred to a 50-ml. beaker, where it was taken just to dryness with salt present to prevent baking of the platinum metals on the bottom of the container. The sample was then diluted to 30 ml., 5 ml. of 2% sodium chloride solution were added t o maintain a sufficient chloride ion concentration, and the p H was adjusted to 1.5. Trace amounts of base metals, still present in the effluent sample, may have escaped through the resin or resulted from contamination from the resin itself. These traces were readily removed by passing the sample through another column of the same resin in a bed 4 cm. deep and 1 cm. in diameter. The column was regenerated and washed in the same manner as the larger ones. The sample was passed through a t a rate of 1 drop per second and then washed with 100 ml. of water. Other experiments showed that trace amounts of base metals can be removed by a small cationic exchange column when dealing with platinum and palladium. This method was used in preference to hydrolytic precipitation of the base metals (6) because erratic results were obtained with the latter, even when the precipitation was repeated. This conclusion is consistent with other findings by the authors' associates. A considerable amount of organic matter was dissolved from the resin and carried through in the effluent. It was easily removed a t this stage by treating with 3 drops of nitric acid and 2 drops of 30% hydrogen peroxide, repeating three times, and taking to dryness on a steam bath each time in the presence of salt. After this treatment, the sample was evaporated to dryness several times with a few drops of hydrochloric acid to ensure reconversion of the platinum metals t o the chloride. During this treatment the samples were covered tightly with a cover glass, and the cover was tilted slightly to allow the solution to evaporate. Separation of Rhodium and Iridium. The methods available for the separation of rhodium and iridium on a microgram scale were precipitation of the rhodium by antimony (11)) cation exchange (8),paper chroniatography (7), and an anionic exchange resin separation recently developed by Berman and McBryde ( 3 ) . The first method was rejected because it involved exceedingly difficult and lengthy technique. The second method was not used because the rhodium must be in the cationic form, which could be achieved only by elaborate modification.
Table 1. Determination of Rhodium or Iridium in Base Metal Solutions
Rhodium, hlg. Added Recovered 5.55 5.05"
Y
505
5.48 5.50 5.01 5.04 508
Iridium, Mg. Added Recovered 4.55 4.55 4.55 4.55 4.55 4.55
Y
506 Y
101 25
5
50 1 509 101 102 101 24 24 25 6
5 5
0
4.56 4.33 4.52 4.62 4.50 4.52
455" 455" 455. 91 91 91 46 46 46 46 11 11 11
428 442 45 1 88
90 89 43 43 45 44 12 12 10
Analyzed colorimetrically by aliquots.
Table II. Determination of Rhodium and Iridium Present Together in Base Metal Solution
Rhodium, y Iridium, y Added Recovered Added Recovered 101
98 100 97
46
9s
100 105 97
47 48 43 44 48 48 45
With the paper chroniatographic procedure difficulty arose in removing the iridium from the paper quantitatively and severe trailing was sometimes observed. The method reported by Berman and McBryde (3) mas readily adapted, This procedure involved the separation of rhodium and iridium as chlorides by columns of the anion exchanger Amberlite IRA-400. The separation depended upon the production of quadrivalent iridium by cerium (IV). The rhodium(II1) passed through the column quantitatively. Iridium \vas eluted by Soxhlet extraction with 6M hydrochloric acid. Initial experiments showed that for the quantities of iridium used it was not necessary to grind the resin. A column 3 em. deep and an extraction time of 1 hour were used. This increased period of extraction was required, as it was felt that the iridium mould be more strongly held in the centers of the large particles than in the fine resin. The resin was initially extracted in a Soxhlet with 6N hydrochloric acid to remove any extractable contaminating ions and to prevent absorption of color from the resin into the sample. Determination of Rhodium. The milligram samples of rhodium were analyzed gravimetrically by thioVOL. 30,
NO. 9, SEPTEMBER 1958
* 1465
barbituric acid ( 5 ) . For microgram quantities, stannous chloride in 2N hydrochloric acid (10) was used, with a wave length of 470 mp, as recommended by Maynes and McBryde ( 9 ) . Total volumes and path lengths were varied, depending upon the size of sample. This method could be used efficiently for samples containing as little as 2 y of rhodium in 25 ml. with a 5-cm. path length, if the sample mas sufficiently acidic during the heating period. Determination of Iridium. For milligram amounts, precipitation by 2-mercaptobenzothiazole ( I ) was used. The microgram samples were analyzed colorimetrically by stannous chloride dissolved in concentrated hydrobromic acid ( 2 ) . Prior to analysis, and folloxing the digestion to remove organic matter, the samples were boiled with 5 ml. of hydrobromic acid until the volume was reduced to 1 ml. During boiling the beaker was covered tightly with a cover glass, so that fumes were allowed to escape only a t the lip of the beaker. This boiling seemed necessary, as it was difficult to reconvert iridium from the nitro form. In the analysis, a heating period of 90 seconds was adopted. Because the reproduction of the solution composition was
uncertain, the standards and samples were analyzed simultaneously. I n all cases, the reference blank and standards were carried through the entire procedure. For the standards, the eluent from the column operations was salted with known amounts of the elements under consideration. Where rhodium or iridium was determined independently, these standards agreed with those determined on solutions taken directly from the stock bottles. When rhodium and iridium were mixed together in the procedure and a separation was performed, the standards appeared to be slightly lower than those determined directly, although 100% recovery was obtained for the separation on standard solutions directly. The reason for these low results has not been determined, but they may be due to any one of several concealed factors. The results of the analyses are recorded in Tables I and 11.
aid in the form of a research scholarship which enabled them to carry out this research. LITERATURE CITED
( 1 ) Barefoot, R. R., McDonnell, W.J., Beamish, F. E., ANAL.CHERI. 23, 514 (1951). ( 2 ) Berman, S. S., illCBr!.de, TI-. A. E., Analyst 81, 566 (1956). ( 3 ) Berman, S. S., RicBryde, W.A . E., Can. J . Chem. 36, 835-52 (1958). ( 4 ) Coburn, H. G., Beamish, F. E., Lewis, C. L., ASAL. CHEN. 28, 1297 (1956). ( 5 ) Currah, J. E., &IcBrq.de, JT. A . E., Cruickshank, A. J., Beamish, F. E., ISD. EKG.CHEM.,ASAL. ED. 18, 120 f1946). (6, Gilchris,t, R., J . Researcn S a t l . Bur. Stan,?a-ds 30,89 (1943). Analyst 1 7 1 K ember, ?rT. F., Wells, R. -4.> 80, T35 (1955).
( 8 ) >I [ac,Uevin, W. AI., McKay, E. S., ANAL.CHI211. 29, 1220 (1957). ( 9 ) Maynet4 , A. D., McBryde, \I7.A. E., .4na/yst 79, 230 (1954). (10) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., DD. 523-5, Interscience, Xew York,
ibo.
ACKNOWLEDGMENT
The authors are grateful to the Ontario Research Foundation for financial
(11) Westland, A. D . , Beamish, F. E., Mikrochtm. Acta 10, 1474 (1956). RECEITEDfor review October 14, 1957. hccepted Ma!. 9. 1958.
Cholesteryl Esters of Long-chain Fatty Acids Infrared Spectra and Separation by Paper Chromatography J. A. LABARRERE, J. R. CHIPAULT, and W. 0.LUNDBERG The Hormel Institute, University of Minnesota, Austin, Minn. Pure cholesteryl laurate, myristate, palmitate, stearate, oleate, linoleate, and linolenate were prepared and used to develop analytical procedures for the separation and identification of these compounds. Melting points, specific rotations, and infrared and near-infrared spectra were obtained. Infrared analysis can be used for identification of only a few milligrams of pure, unknown samples. Model mixtures containing both saturated and unsaturated esters were separated by ascending reversed-phase paper chromatography using two solvent systems (acetone-ethanol-formic acidwater, and chloroform-methanol-formic acid-water) consecutively on the same paper strip. The amount of each compound required for this purpose is approximately 10 to 20 y.
T
present interest in the relationship between cholesterol metabolism and atherosclerosis, and in the physiological role of cholesteryl esters HE
1466
ANALYTICAL CHEMISTRY
of long-chain fatty acids, has resulted in numerous attempts to separate and identify these compounds in model mixtures and in lipides obtained from animal tissues. Methods for characterizing the individual esters, however, generally are based upon analysis of the mixed fatty acids obtained by saponification. Direct analysis by separation of the esters is preferable in all cases, and becomes a necessity when xorking with esters isotopically labeled in the cholesterol moiety. I n previous experiments on direct identification of mixtures of cholesteryl esters by chromatographic separation (4,11, 19, 20, 22, 26), difficulties 11 ere met with cholesteryl esters of longchain fatty acids, and also with esters of saturated and unsaturated fatty acids when present together. Michalec (20) and Zimmerman (26) have obtained nearly identical R , values for the cholesteryl esters of oleic acid and long-chain saturated fatty acids. Paper chromatography of cholesteryl linoleate has been mentioned recently (18),
but the linolenate ester apparently has not been examined previously by this technique. The infrared spectra of cholesterol and of the cholesteryl esters of a fen short-chain fatty acids hare appeared in the literature (5, 14, 23), and Holman and Edmondson ( I S ) have obtained the near-infrared spectrum of cholesterol. Our knowledge of the infrared spectra of cholesteryl esters of long-chain fatty acids, however, has been limited to the partial spectrum of cholesteryl laurate, in carbon disulfide, published by Freeman et al. (6, 7 ) , and to these authors’ observations that the fatty acid composition did not appreciably alter the solution spectra of cholesteryl esters. I n the present study, the cholesteryl esters of lauric, myristic, palmitic, stearic, oleic, linoleic, and linolenic acids have been prepared ; infrared spectrometry and paper chroma tography were used for identifying and separating these compounds.
