Separation of Platinum, Palladium, Rhodium, and iridium by Paper Electrochromatography WILLIAM M. MacNEVlN and MYRON
L.
DUNTON'
McPherson Chemical laboratory, The Ohio State University, Columbus, Ohio Complexing of palladium(ll) and iridium(lV) with (ethylenedinitri1o)tetraacetic acid suggested the possibility of separating combinations of these metals and platinum(1V) and rhodium(ll1) by paper electrochromatography in more than micro quantities. Platinum(IV), palladium(ll), rhodium(lll), and iridium(lV) were shown to travel a t different rates in vertical paper chromatography and in horizontal paper electrochromatography. A continuous process based on apparatus of Durrum was therefore considered possible. The following combinations have been quantitatively separated on a continuous basis: platinum(1V) and palladium(ll), palladium(ll) and iridium(lV), rhodium(ll1) and iridium(lV), rhodium(lll) and platinum(lV), and rhodium(lll), palladium(ll), and iridium(1V). These separations are useful in the analysis and preparation of relatively pure forms of the mixtures noted.
P
separation of these metals by chromatography has been carried out in acid solution ( I ) , in order to prevent precipitation of palladium, rhodium, and iridium, the three metals of the group that precipitate easily as hydroxides. LlacSevin and Kriege (4, 6-9) have shown that palladiuni and iridium form soluble complexes with (ethylenedinitri1o)tetraacetic acid (Versene) in the presence of much chloride and in both acid and alkaline solution. I t was expected that (1) the formation of these complexes in alkaline solution would allow further differentiation of the migration behavior of these four ions, (2) the use of alkaline solutions rrould change the rate of platinum migration as conversion from PtC16-- to Pt(OH)e-- progressed with increasing pH, and (3) rhodium would not migrate a t all in alkaline solution, because it forms an insoluble hydroxide, does not complex with (ethylenedinitri1o)teti-aacetic acid during the experiment, and would therefore remain a t the point of application. Previous electrochromatographic separations have been limited to microgram quantities. One object of the Present address, Wright Air Development Center, iT-right-Patterson Air Force Base, Ohio. REVIOUS
1806
ANALYTICAL CHEMISTRY
present investigation \vas to develop a continuous separation providing for larger quantities. The principle described by Durrum ( S ) , in which descending chromatography and electrochromatography are combined, was selected for study. To predict the probable success of continuous paper electrochroniatographic separations it was necessary to know the behavior of a mixture of these four metals in simple descending paper chromatography and in horizontal paper electrochromatography, If the degree of separation was the same vertically and horizontally (under applied potential), continuous separation by the Durrum principle Fvould not be possible. As distinctly different degrees of separation were found, it was concluded that a continuous process of separation could be developed. The coninion oxidation states of these metals-namely, platinurn(IV), palladium(II), rhodium(III), and iridium(IV) nere used. REAGENTS
Stock platinum solution, 4.60 mg. of platinum per ml., was prepared by dissolving Alallinckrodt analytical reagent grade platinum(1V) chloride in 0.1-11 hydrochloric acid. Stock palladium solution, 2.82 mg. of palladium per ml., was prepared by dissolving hhtheson, Coleman and Bell palladium(I1) chloride in 0 . l N hydrochloric acid. Stock rhodium solution, 2.89 mg. of rhodium per ml., was prepared by dissolving American Platinum Works rhodium(II1) chloride in 0.1-11 hydrochloric acid. Stock iridium solution, 4.03 mg. of iridium per ml., was prepared by dissolving American Platinum Works iridium(111) chloride in 0.1.V hydrochloric acid. Stannous chloride solution was prepared by dissolving 10 grams of stannous chloride hydrate in 100 ml. of 6 S hydrochloric acid and adding 1 gram of granular tin to maintain the stannous condition. EDTA, disodium salt of (ethylenedinitrilo) tetraacetic acid, was obtained from the F. TIT. Bersworth Co. DESCENDING PAPER CHROMATOGRAPHY
Apparatus. Descending paper chromatography was conducted in a
6 X 18 inch borosilicate glass jar fitted with glass lid, stainless steel support rack, and solvent assembly consisting of a stainless steel cradle, borosilicate glass solvent trough, glass anchor rod, and two glass antisyphon rods. Eaton-Dikeman paper No. 652, x 12 inch strips, was the cut into separation medium. Procedure. One hour prior to starting a n experiment, m s h liquid was placed in a Petri dish on the bottom of the chromatographic jar in order t o saturate the jar with wash liquid vapor. A measured amount of platinum group metal solution m s added 3 inches from the upper end of the paper. The paper was then folded and hung over the edge of the solvent trough, nith the end immersed in 15 to 20 ml. of O . l M (ethylenedinitri1o)tetraacetic acid. The p H of the solution was adjusted with dilute sodium hydroxide to be the same as that of the platinum group metal solution under study. Usually the wash liquid reached the bottom of the paper in 25 to 30 minutes. The strips were then removed and the position of the metal zones was detected. Detection of Zones. The stannous chloride-potassium iodide test s o h tion of Burstall ( 2 ) m-as used to detect the zones. Khen freshly mixed solution TT as sprayed on the paper, platinum appeared yellow to brownish yellow, palladium pink to dark purple, and rhodium orange to maroon. Iridium \vas detected after platinum, palladium, and rhodium by placing 1-inch successive lengths of the paper in successive test tubes with 1 ml. of concentrated nitric acid. Iridium was indicated by a brown color. Platinum, palladium, and iridium invariably moved with the solvent front. Rhodium behaved similarly, except a t pH values greater than 5, when it precipitated and did not move. It R as concluded that in acid wash solutions (pH < 5) absorption, partition, and ion exchange are not significant processes in descending chromatography, as all four metals keep up with the solvent front. HORIZONTAL PAPER ELECTROCHROMATOGRAPHY
Apparatus. The electrochromatographic cell was similar to t h a t of
Ward and Lcderer ( 11 ) . One-inch strips of Eaton-Dikeman KO.652 paper, 13 inches long, n ere held between two 9inch glass plates. The ends of the paper dipped into ieservoirs of electrolyte. Platinum electrodes led current into each end of the cell. A brass cooling plate n ith attached water coil supported the glass plates and kept the paper cool. The poiwr supply operated on 110 volts alternating current. It could supply 1000 volts direct current at zero current or 200 volts at 50 ma. The voltage was stable and the ripple did not exceed 1.5%. Procedure. The chromatographic paper n a s prenashed n i t h n a t e r and solvent and the excess n a s removed by blotting betn een filtei papers. The strip was placed on the glass plate and 0.1 nil. of sample added a t a marked point. The papei nas coveied with the second glass plate and a 15-pound weight placed on top of the assembly. V~'ashsolution was placed in the cells a t each end of the plate and the ends of the paper xcre foldc>d to dip into the solvent. A potential of 500 volts was applied for 30 to 4#5 minutes. The poiver n a q turned o f f , and tlie ends of the paper mere liftcd out of the solution and blotted to rcniove the cycess. The paper strip nas then removed from the cell, and sprayed Trith .tannous chloridepotaqsiuni iodidc solution. If iridium nas involwd, the center section was cut into 1-inch strips, placed in test tubes, and t e s t d n-itli nitric acid.
Iridium and rhodium were expected to show similar behavior. Solutions of platinum, palladium, iridium, and rhodium chlorides containing 0.111 (ethylenedinitrilo) tetraacetic acid were studied a t several pH values. A voltage of 500 volts v a s used in all experiments. The rate of electrochromatographic migration of palladiuni(I1) in (ethylenedinitri1o)tetraacetic acid toward the anode increased slightly between p H 4 and 9. This slight increase may be accounted for by the change from PdY-- to PdOHY---. Although the charge on the ion has increased by 1, the ion has increased considerably in size, partly because of the increase of 17 units in atoniic weight a i d partly because a n acetyl group bond has been broken, thus increasing the effectivc size of the molecule. Platinum moved toward the anode a t all pH values. At p H -1, fire zones appeared. At higher pH values, the more s l o d y moving zones disappeared, until at pH 9, as sho1Y.n in Figure 1, only one sharply defined zone appeared. Further increase in p H to 11 caused the appearance of a new and faster zone.
Effect of Concentration of (Ethylenedinitri1o)tetraacetic Acid in Solvent. It was expected that (ethylenedinitrilo) tetraacetic acid M ould have more effect upon palladium and iridium t h a n upon platinum and rhodium, as LllacSevin and Kriege shoned (4, 6,8,Q) that complexation occurs, readily with the first two. To determine the effect of concentration of the complexing agent, a pH of 7 was selected, for the experiments because the palladium complex is stable under these conditions. When 0.006J1 complexing agent \vas used at p H 7.0, palladium migrated toward the anode in a diffuse zone. At larger concentrations, the zone became sharper up to a concentration of 0.08JI. The platinum zone also became sharper with increasing concentration of complexing agent, which suggests that platinum complexes with (ethylenedinitri1o)tetraacetic acid under these conditions. Effect of pH on Electromigration Rates of Platinum Group Metals. The electromigration of the palladium coniplex n a s not expected to vary bet n e e n pH 4 and 9, as only one complex occurs. Platinum chloride anion, PtC&--, undergoes gradual hydrolysis with increasing p H to form a series of complexes ending in Pt(OH)6-- and hence may be expected to show variation in its migration behavior with pH.
p H 9
&
i
o
-
-
aa
0
]
PH 4
=em
1
PH7
p H II
Figure 1. Effect of pH on electromigration of platinum
Rhodium precipitated a t pH 5 and shou-ed no migration b e t w e n p H 5 and 10. At p H 4, one to three zones appeared, depending upon the pretreatment of the rhodium. The niajor zone in all cases moved tonard the anode. K h e n three zones appeared, one was anionic. The precipitation of rhodium a t pH 5 to 10 is not objectionable, as it results in the rhodium's remaining a t the point of application. Iridium behaved as an anionic complex and was the most mobile of the four metals. Increased p H caused more diffuseness of the zone. Only a slight increase in mobility of iridium occurred between pH 4 and 9. Further increase in p H led to precipitation of hydrated iridium oxide. Effect of Aging on Electromigration. Aging plays a dominant part in electromigration of solutions of platinum group metals. Stepnise reactions, often slowv,occur when complexing agents are added or the pH of the system is changed. These effects are troublesonie if not understood, but may be used to advantage. The fol-
lowing sloiv reactions have been ohserved.
PLATIKIUM. T o a freshly prcparcd chloroplatinic acid (1 gram in 100 nil. of 3 9 hydrochloric acid) n a s added a threefold excess of (ethylenedinitr11o)tetraacetic acid and the pH n a s adjusted t o 4 with sodium hydroxidc. Khen applied t o paper moistened n ith (ethylenedinitrilo)tc,traaceticacid a t p H 4, and a potential of 500 volts applied for 30 minutes, nearly all the platinum rcmained a t the point of application. A small amount migrated toiiard the anode. After wvcral hours. more of the platinum moved toward the anode, although in a diffusc zone. Karming thc solutions containing chloroplatinic acid antl (ethylencdxnitri1o)tetraacetic acid to 80" t o 90" caused the zones to hcconir more diffusc antl inove more slonly. This iudicateq partial conversion of chloride coniplexes.to more slon.ly moving complex ions, involving replncenient of chlorine atoms by hydroxy, aquo, or (ethyleiiedinitri1o)tetraacetic acid groups. Although Kriege and AIacTcrin could find no spectrophotometric evidence of thc existence of complexm of platinum(1V) and (eth~-lencdiiiitrilo)tetraa~etic. acid in a chloride solution, the pronouncd effect of this comple\ing agent upon the electromigration of platinuin(1T') indicates probable complcxntion. Frcshly prepared platiriuni pci cahlorate solutionq adjusted to p H 7 n ith sodium hydroxide showed almost no dcctroniigration. A small amount of anodic bcliavior occurred, siniilar to that ohs aged platinum chloride a t pH 7 . This suggests the rcplacc,ment of the chloridc ions in the complex, probably by OHions. with subsequent dehj dration of the platinum hydroxy complex. PALLADIUM. The electrochroniatographic behavior of palladium(I1) in an excess of (ethylenedinitri1o)tetraacetic acid is not influenced by heating or aging of the solution. RHODIUM. The electrochromatographic behavior of rhodium chloride depends upon the pretreatment given the solutions. A rhodium(II1) chloride solution after standing for several months was treated with the complexing agent and the p H adjusted to 4 and separated into three zones. One zonr moved rapidly t o the anode, one slowly to the cathode, and a third very slowly to the cathode. 1IacSevin and 1\IcIiay (10)have shown by ion exchange experiments that rhodium(II1) chloride may exist as cation or anion or a mixture of the t1y.o. The cationic form was produced when rhodium was precipitated as hydroxide and immediately redissolved in acid. Upon standing, a mixture of anionic and cationic forins developed, nhich accomts for the dual behavior in migration. Freshly prepared rhodium(lI1) chloride does not precipitate completely a t p H 7 . K h e n such a mixture is subjected to migration, part of the rhodium remains stationary, part moves to the anode. If the solution is first made strongly basic and the p H is then adjusted to 7, precipitation is complete and VOL. 29, NO. 12, DECEMBER 1957
1807
no migration occurs. No evidence has been found for complexation between rhodium(II1) and (ethylenedinitri1o)tetraacetic acid in chloride solution. IRIDIUM. Iridium(1V) has been shown (4) to complex with (ethylenedinitri1o)tetraacetic acid, with the probable formation of H21r2YCls-- and IrSYCIl2--- where k' represents the complexing agent. The mobilities of these ions decreased, if the solutions were heated to 80" to 90" C. a t pH 4 t o 9. This indicates a probable formation of hydroxy or aquo complexes with lower mobilities. SEPARATIONS BY HORIZONTAL PAPER ELECTROCHROMATOGRAPHY
Chemically complete separations of platinum, palladium, rhodium, and iridium could be achieved in 45 minutes a t 500 volts and 25 ma. The solvent was 0.1Ji (ethylenedinitri1o)tetraacetic acid a t pH 9. A typical sample consisted of 0.100 nil. of a solution containing 0.092 mg. of platinuni(IV), 0.062 mg. of palladium(II), 0.058 mg. of rhodium(III), 0.080 nig. of iridium, and 0.1X (ethylenedinitri1o)tetraacetic acid, adjusted to pH 9. Cnder these conditions, rhodium(II1) was precipitated a t the point of application, and iridium(1V) moved most rapidly toward the anode. Platinum(1V) moved more sloivly toward the anode and palladium(I1) appeared between rhodium(I1) and platinum(1V). Figure 2 shows the location of the zones. The sharpness of separation of the zones indicated a quantitative separation.
r-
A
a-0
9h
Pd
Pt
+I
lr
Figure 2. Electrochromatographic separation of rhodium(Ill), palladium(ll), platinum(lV), and iridium(lV)
To check this point, the paper u-as cut into sections containing individual zones and the content of each metal determined. The paper was cut into small pieces and ignited in porcelain. The residue was taken up in 1 ml. of 1 to 1 hydrochloric acid, through which chlorine gas bubbled slowly. After the residue had dissolved, each solution was transferred completely to a 1 x 11/2 inch piece of Eaton-Dikenian KO.652 paper, and the solvent evaporated and compared with aliquots of the original solutions by x-ray fluorescence analysis as described by MacNevin and Hakkila (6). The spectra obtained in this way also showed the purity of metal in each zone. No evidence of stray metals was found in neighboring zones within the limits of sensitivity-about 1 in 1000 under these conditions. Emission spec1808
ANALYTICAL CHEMISTRY
tra were also observed for each of the zone residues; in no case did the impurity exceed an estimated 0.01%. The zone separations are therefore reasonably quantitative. Table I shon-s recovery data for the mixture of four metals described above.
Table 1. Recovery of Platinum Metals from Zones Measured by X-Ray Fluorescence Analysis
Pt Pd Rh Ir
0.092 0.062 0.058 0.080
0,092 0.063 0,058 0.079
0 +1.6 0 -1.2
Similar separation was obtained lvith five times as much of each metal. The concentration of (ethylenedinitri1o)tetraacetic acid was increased to 0.2;11 and the potential FTas reduced to 300 volts because of the higher conductivity of the solution. TKO- and three-membered combinations of these four metals were separated in a similar m y . Rhodium(III), palladium(II), and iridium(1V) were separated a t any pH between 4 and 9. The presence of platinum(IV) required a pH of 9, to decrease the platinum zones to one. Procedure for Separation of Platinum(IV), Palladium(II), Rhodium(111), and Iridium(1V). Evaporate t o dryness 1 ml. of the chloride solutions of the metals. Take up the residue in 0.5 ml. of 0.1N hydrochloric acid. The total concentration of the metals should not exceed 0.05M. Convert the rhodium to the yellow cationic form by making the solution sufficiently alkaline with dilute sodium hydroxide to redissolve any precipitate that forms. Immediately adjust the solution to pH 2 to 3 with dilute hydrochloric acid and add 2 to 3 moles of solid (ethylenedinitri1o)tetraacetic acid for each mole of metal present. Adjust the solution to pH 9 with dilute sodium hydroxide and dilute to 1.0 nil. with water. Apply 0.1 ml. of this solution to a 1 X 13 inch Eaton-Dikeman paper No. 652, which has been washed with 0.l.M (ethylenedinitri1o)tetraacetic acid solution adjusted to pH 9, drained, and blotted. Apply the solution about 3 inches from the cathode end. Place the strip in the horizontal cell for paper electrochromatography, and apply 500 volts for 45 minutes. Remove the strip and test for each metal as in descending paper chromatography. CONTINUOUS PAPER ELECTROCHROMATOGRAPHY
Apparatus. The apparatus is similar to t h a t described by Durrum (3). The cell was made from Lucite plastic
and could accommodate an 11 X 13 inch paper supported vertically. The power source was a 1000-volt 1-ampere direct current generator driven by a 3-hp. motor. The output of this generator was varied from 100 to 1000 volts by a 2750-ohm slide-wire resistance in the 125-volt direct current line which energized the generator. The output ripple was reduced to 1.5% by passing the current through six 2-pfd. condensers. Eaton-Dikeman No. 652 paper, 11 X 13 inches in size. was provided (Figure 3), nith 12 drip points of equal size along the bottom of the sheet, starting 2 inches from either side. A similar set of notches was cut along the top edge of the paper. The speed of flow of solvent could be controlled by the length of these points dipping into the solvent in the solvent trough. A iTick of Khatman KO.2 filter paper, inch wide, was attached below notch 3. Its free end dipped into the sample beaker. It was purposely made of thinner paper than that used for the separation medium, so that the rate of flow of sample would be less than that of the solvent.
Figure 3. Paper pattern for continuous electrochromatography
A series of slits was cut along the sides of the paper '/4 inch from the edges. Lengths of platinum wire were woven through the slits to serve as electrodes for application of the voltage. An extra thickness of paper was added a t A and A', to increase the rate of flow of solvent over the platinum wire electrodes. The extra solvent served t o m s h out electrode reaction products and to keep the paper and solvent cool. Sloping slots located just inside the platinum wire electrodes conducted electrode reaction products and excess wash liquid away from the center of the paper. An attempt was made to prevent overheating of the paper by pressing water-cooled glass plates against the paper. Erratic and unusual migration occurred and it was concluded that the paper could not be cooled by direct contact with a cold surface. The only satisfactory arrangement &-as to allow the paper to hang freely and avoid excessive voltage and amperage. Decreased migration could be counteracted
by longer paper, or the rate of solvent flow could be decreased to allow more time for separation. The size and design of paper and the concentration of solvent used are a practical comproniise among these factors. Continuous separations of some mixtures of the four metals, -sere successful. The actual paths and locations of the separated metals on the paper were observed by stopping the process and spraying the paper with stannous chloride-potassium iodide reagent. Platinum was indicated by yello\\- t o brownish color, palladium by pink to dark purple, and rhodium by orange to maroon. Detection of iridium requires application of concentrated nitric acid, which produces a b r o m color. By this means, the sharpness of separation can be shown. Continuous separations were first attempted with mixtures containing 10 mg. of the metals. Later, 100 mg. of each metal were used with equal success. Continuous paper electrochromatography is possible only if the rate of movement in descending chromatography for each metal is different from the rate of horizontal migration. If not, the metals reach the drip point a t different times. The wide differences betrveen separations by horizontal electroniigration and descending chromatography indicate that a continuous process can be developed. HolTever, best separation of the four metals with horizontal electromigration occurred a t pH 9, a t which rhodium precipitated and remained a t the point of application. A continuous process would hare to avoid precipitation of rhodium. The purity of metal obtained a t separated drip points was studied by x-ray fluorescence analysis as in horizontal paper electrochromatography. I n each example described below, the impurity metal did not exceed 1 part per thousand of the metal collected a t any drip point. Platinum and Palladium. Platinum(1V) and palladium(I1) in chloride solution were evaporated just to dryness and the residue was diluted n i t h O . 1 M hydrochloric acid until the total concentration of metal was 0.05M. A twofold excess of solid (ethylenedinitri1o)tetraacetic acid v a s added and the p H of the solution adjusted to 9 with dilute sodium hydroxide. The solvent was 0.1X (ethylenedinitri1o)tetraacetic acid adjusted to p H 9. The paper was placed in the apparatus ivith half the length of the feed points immersed in solvent. Solvent f l o ~began and was allom-ed to \\et the paper before the sample was added. The sample was applied by dipping the wick, attached to feed point 3, into the sample solution. A potential of 130 volts n-as applied
and the resulting current was 120 ma, Under these conditions the two metals reached the lower edge of the paper in 2.5 hours. Palladium appeared exclusively a t drip point 5, while platinum appeared only a t points 6 and 7. Palladium and Iridium. Palladium(I1) and iridium(1V) in chloride solution were mixed with twofold excess of solid (ethylenedinitri1o)tetraacetic acid and the p H tvas adjusted t o 9 with sodium hydroxide. A potential of 120 volts was applied, and a current of 200 ma. passed. Palladium appeared a t drip point 5 and iridium a t points 6 and 7. Rhodium and Iridium. Rhodium(111) and iridium(1V) in chloride solution were mixed with a twofold excess of solid (ethylenedinitri1o)tetraacetic acid. The solution was then made strongly basic with sodium hydroxide to convert the rhodium to the yellow cationic form. The p H of the solution wis then adjusted to 4 with hydrochloric acid. The solvent was O . 1 M (ethylenedinitri1o)tetraacetic acid adjusted to pH 4. A potential of 150 volts was applied, a current of 180 ma. passed; rhodium appeared entirely a t drip points 4 and 5 and iridium a t drip points 8 and 9. Rhodium and Platinum. Rhodium(111) and palladium(I1) in chloride solution were made strongly alkaline with sodium hydroxide, to convert rhodium to the yellon, cationic form. A twofold excess of solid (ethylenedinitri1o)tetraacetic acid was added and the pH adjusted to 4. The solvent was 0.1M (ethylenedinitrilo) tetraacetic acid adjusted to p H 4. At 140 volts the current was 180 ma. Rhodium appeared entirely a t drip points 4 and 5, while platinum appeared a t points G, 7, and 8. Rhodium and Palladium. Palladium(I1) and rhodium(II1) in chloride solution were made strongly basic with sodium hydroxide, a tIvofold excess of (ethylenedinitri1o)tetraacetic acid added, and the p H adjusted to 4. At 150 rolts, rhodium appeared only a t drip points 4 and 5 , m-hile palladium appeared exclusively a t points 6 and 7. Rhodium, Palladium, and Iridium. Rhodium(III), palladium(II), and iridium(1V) in chloride solution were made strongly alkaline with sodium hydroxide, a twofold excess of solid (ethylenedinitri1o)tetraacetic acid added, and the p H adjusted to 4. At a potential of 150 volts, rhodium appeared entirely a t drip points 4 and 5 , palladium a t points 6 and 7, and iridium a t points 8 and 9.
separated a t pH 4, although the platinum is more diffusely located. It may be concluded: The use of (ethylenedinitri1o)tetraacetic acid to complex palladium and iridium makes it possible to work in alkaline solutions a t p H 9, a t which platinum behaves as a single species. Platinum(IV), palladium(II), rhodium(III), and iridium(1V) can be separated in limited amounts by horizontal paper electrochromatography with 0.1M (ethylenedinitri1o)tetraacetic acid as solvent a t pH 9. These metals cannot be separated in the same solvent by simple descending chromatography. The difference in rates of movement of the four metals in descending chromatography and in horizontal electrochromatography makes possible the establishment of a continuous process separation for certain combinations of the four metals. Continuous separations of mixtures of platinuin(1T’) and palladium(II), palladium(I1) and iridium(IV), rhodium(II1) and iridium(rV) ,rhodium(II1) and platinum(IV), and rhodium(II1) and palladium(I1) and iridium(1V) have been accomplished. Rfixtures of all four metals have not been successfully separated because of the diffuse behavior of platinum a t pH 4.
REFERENCES
(1) Anderson, J. R. A., Lederer, XI.,
Anal. Chim. Acta 5 , 321 (1951). (2) Burstall, F. H., Davies, G. R., Wells, R. A., Tinstead, R. P., J. Chem. SOC.1950, 516. (3) Durrum. E. L.. J . Am. Chem. Soe. 73, 4875 (1951).’ Kriege, 0. H., Ph.D. dissertation, Ohio State University, 1934. MacXevin, W.&I.,Hakkila, E. A., ASAL. CHEW.,2 9 , 1019 (1957). MacNevin, W. M., Kriege, 0. H., Ibid., 26, 1768 (1954). I b i d . , 27, 535 (1955). I b i d . , 28, 16 (1956). MacNevin, W. M., Kriege, 0. H., J . Am. Chem. SOC.77,6149 (1955). MacNevin, TV. X, McKay, E. S., ;IXAL. CHEX. 29, 1220 (1957). Ward. F. L.. Lederer. M., Anal. Chim. Acta’6, 355 (1952): RECEITEDfor review March 14, 1957. Accepted August 15, 1937.
DISCUSSION
A mixture of all four metals could not be continuously separated. The presence of rhodium requires a slightly acid solution (pH 4), to prevent precipitation of rhodium hydroxide. At pH less than 9, platinum distributes in several diffuse zones involving palladium and iridium. However, a mixture of rhodium(II1) and platinum(1V) can be
Argentimetric Method for Epoxides-Correction I n the article on “Argentimetric Method for Epoxides” [Stenmark, G. A,, ANAL.CHERI. 29, 1367 (1957)], the fifth line from the bottom of the second column should read 2,6-di-tert-butyl-4methylphenol instead of 2,4-di-tertbutyl-6-methylphenol. VOL. 29, NO. 12, DECEMBER 1957
1809
