of this reaction mixture mere delivered into 4.0 ml. of the dithizone reagent in an ice bath, and the usual procedure was followed. Two blank tubes, one in which the mercurial was replaced by an equal volume of mater and the other in which the protein was replaced by an equal volume of water, were required and the method was calibrated by comparison with a standard cysteine solution. RESULTS
Figure 1 compares the results obtained with this method and the Boyer assay in following the reaction of p mercuribenzoate with partially purified milk xanthine oxidase. The initial rate given by the dithizone method was somewhat more rapid than that given by the Boyer procedure, but the end points were in excellent agreement. Because aliquots of the reaction mixture
are shaken with dithizone in carbon tetrachloride, it is possible that the greater initial rate observed with the dithizone method reflects the “unco~-ering” of protein sulfhydryl groups by the denaturation attendant upon shaking with the organic phase. Figure 2 shows the excellent agreement between the present procedure and that of Boyer ( 1 ) when applied to crystalline bovine serum albumin and to a commercial, trricecrystallized preparation of muscle triosephosphate dehydrogenaee which had been incubated with 0.002M cysteine at p H 7.0 for 5 hours and then dialyzed against 0.01% Versene (pH 7.0) to remove the cysteine. A possible source of variability in the dithizone method is incomplete separation of the organic from the aqueous phase. mith care, however, good replication of results mag be obtained. Thus, the data shown in Figure 1 repre-
sent the means of triplicate deterniinations p;hich agreed to within 3%. LITERATURE CITED
(1) Boyer, P. D., J . Am. Chem. SOC.76,
4331 (1954). (2) lIalmstrom, Bo G., “Methods of Biochemical Analysis,” D. Click, ed., Vol. 3, p. 327, Interscience, Sew York, 1956. (3) Sandell, E. B., “Colorimetric Determination of Trace Metals,” p. 321, Interscience, Sew York, 1944. ( 4 ) Whitmore, F. C., Woodward, G. E., “Organic Syntheses,” Coll. Vol. I, p. 159, Wiley, Xem York, 1941.
RECEIVED for review September 4, 1966. Accepted February 2, 1957. rlssisted in part b y Contract AT-(40-1)-289 between Duke University and the U. S. Atomic Energy Commission and Grant RG-91 from the National Institutes of Health. Work done during the tenure of a U. S. Public Health Service Postdoctoral Fellowship of the National Heart Institute to Irlvin Fridovich.
Separation of Rhodium from Platinum, Palladium, and Iridium by Ion Exchange WILLIAM
M. MacNEVlN
and EDWARD S. McKAYI
McPherson Chemical laboratory, The Ohio State University, Columbus, Ohio
b The occurtence of rhodium as a yellow cation form and its conversion to a pink anion form are not well understood and cause erratic results during separations. Study of the preparation of these two forms shows that the cation form can b e prepared exclusively by oxidizing the rhodium to the quadrivalent state, reducing it to trivalent with hydroquinone, precipitating the yellow hydroxide, and dissolving it in hydrochloric acid. Platinum, palladium, and iridium treated in the same way behave as anions, if palladium is allowed to age at the hydroxide stage. This principle of cation-anion differentiation was used in the separation by ion exchange of rhodium from mixtures with platinum, palladium, and iridium. Rhodium chloride of high purity was obtained.
S
of milligram quantities of rhodium(III), platinum(IV), palladium(II), and iridium(1V) by ion exchange was reported in 1953 by MacNevin and Crummett (7), who used the principle of cation-anion differentiaEPARATION
1 Present address, Department of Chemistry, University of Tulsa, Tulsa, Okla.
1220
0
ANALYTICAL CHEMISTRY
tion to form the cationic Pd(NH3)ZtT complex and separate palladium(I1) from the stable chloride anions of the other three metals. Separation of rhodium and iridium by elution as anions was only 957, complete and iridium could not be removed from the resin. I n the s u n e year, Stevenson and associates (1 I ) reported successful separation of the four metals by elution of the perchlorates from a n ion exchange resin. Repeated attempts in this laboratory to bring mixtures of milligram amounts of these four metals into solution by fuming with perchloric, and nitric acid preparatory to separation have not been successful. Palladium remains insoluble, apparently as perchlorate. Berg and Senn ( 1 ) also used cationanion differenti‘ition between rhodium and iridium bv converting rhodium to a cation by heating with thiourea. The rhodium is retained by a cation resin and the anionic iridium appears in the eluate. The nature of the thiourea complex was not reported. Large amounts of organic material must be destroyed before the separated metals are used further. Cluett, Berman, and McBryde (S) have reported the separation of rhodium and iridium by elution in chloride solu-
tion. Very large volumes of eluate containing a high salt concentration i r e eluted. Several years’ experience in this laboratory have shown that the behavior of rhodium is uncertain and erratic. This suggested that the reactions of rhodium should be more fully understood, if its behavior was to be controlled. lleyer and Kawczyk (IO) and Tuthill (12) had observed the change in color of freshly prepared rhodium chloridesolutions from pink to yellow. Kraus and Umbacli (6) reported a yellow ionic form and pink un-ionized form of rhodium sulfate. When the yellow form of the chloride is treated with silver nitrate, silver chloride precipitates; no precipitate is obtained with the pink form. The change from pink to yellow form in 3 months) a t room temis slox (l5yO perature, In preliminary experiments the yellow rhodium chloride Lvas prepared by redissolving the freshly precipitated hydroxide in hydrochloric acid. The yellow form behaved as a cation and the pink form as a n anion toward ion exchange resins. These and other reactions have been used in developing a series of separations of rhodium from several mixtures with platinum, palladium, and iridium.
EXPERIMENTAL
Stock Solutions. Solutions of each of the metals in their common oxidation states were prepared from their chlorides and standardized by analysis according to the Gilchrist-Richers (5) methods. Ion Exchange Resins. Dowex 50 cation exchange resin, 20- t o 50-mesh, in the hydrogen form was used. The cross linkage n as indicated by the designation X-8. The columns were 50 em. X 14 mm., contained about 20 grams of resin, and were surrounded by water jackets for temperature control. -4copper coil inserted in the inlet lead to the condenser could be heated for work a t higher temperatures. An automatic shutoff device-a thin layer of fine sea sand-prevented liquid from dropping below the top level of resin. Khen the liquid level dropped to the top of the resin bed, the sand packed tightly and stopped the flow completely. METHODS OF DETECTION
In order to study separation processes, quick methods of detection are needed. Platinum was detected by its reduction to the free metal on the addition of formic acid. Dimethylglyoxime was used to detect larger amounts of palladium, and the stannous chloride brown color reaction of Burstall ( 2 ) was used for smaller amounts. Rhodium was also detected b j the red color produced with Burstnll’s reagent. Iridium could be detected by its reaction with cwicentrated nitric acid, in which a brown color appears. Spectrographic analj.sis was also used to detect traces.
Rhodium from Platinum. Known mixtures containing 5 to 20 mg. of platinum and 7.06 to 14.12 mg. of rhodium as chlorides in 20 ml. were used. About 100 mg. of sodium chloride mas added, and the solution was evaporated to 5 ml. and then cooled. Strong sodium hydroxide (30 grams per 100 ml.) was added until yellow rhodium hydroxide appeared and then 1drop extra of base was added. After 10 minutes, 3iv hydrochloric acid was added until the p H was 3.5. The resulting clear solution was yellow. The solution was passed over a 25-cm. length of cation resin, Dowex 50, X-8, 20- to 50-mesh. Fifty milliliters of water was sufficient to elute the platinum completely. However, some rhodium was not retained. Six molar hydrochloric acid was used to remove rhodium from the column. The temperature of the column was raised t o 60” C. Approximately 200 ml. of 6M acid was needed to remove the rhodium quantitatively. The eluted rhodium solution was evaporated to dryness and the residue found to be spectroscopically free from platinum (Table I). The unsorbed rhodium is believed due to the presence of some pink anionic form in the yellow cationic rhodium.
Table 1. Separation of Rhodium from Platinum on Dowex 50 Metal Unsorbed, Mg Metal Eluted, Rlg. Metal Taken, M g . Pt Rh Pt Rh Pt Rh 5 20
5.20
5 20
14.12
0 00
0 10
6 95
5.20
Table II. Separation of Rhodium from Palladium on Dowex 50 Metal Taken, M g . Metal Unsorbed, Mg. Metal Eluted, hlg. Pd Rh Pd Rh Pd Rh
Table 111.
Separation of Rhodium from Palladium on Dowex 50 in Presence of Versene Metal Taken, M g . Metal Unsorbed, Mg. Metal Eluted, M g . Pd Rh Pd Rh Pd Rh 8.32 7.06 8.30 0.10 Trace 6.90 8.32 14.12 8.20 0.15 Trace 14.0
Table IV.
Separation of Rhodium from Palladium on Dowex 50 with Rhodium(lV) Intermediate
Metal Taken, M g . Pd 8 32 8 32
Rh 7 06 14 12
Metal Unsorbed, hlg. Pd 8 32 8 32
Metal Eluted. Mg.
Rh 0 00 0 00
Pd 0.09 0 LJO
Rh 7 10 14 00
Rhodium from Palladium. Three methods of preparing the sample were tried. METHODI. The sample containing rhodium(II1) and palladium(I1) as chlorides was treated in the way described above for the rhodium-platinuni mixture (Table 11). The column and elution treatments were also similar. The rhodium obtained in the rvaporated eluate was spectroscopicnlly free from palladium in two out of four experiments. The eluted palladium contains a small amount of rhodium not retained by the resin, which is again attributed to the presence of some anionic form. METHOD11. hlach-evin and Kriege (8, 9) have shown that pnlladium(I1) forms a stable complex with (ethylenedinitri1o)tetraacetic acid (EDTA). The separation described in Method I was repeated, except that 5 nil. of O.1M EDTA n as added before precipitation of the rhodium hydroxide with base. This retained the palladiuni in solution but had no evident effect on the rhodium and did not improve the retention of rhodium bi the resin (Table 111). The eluted rhodium proved to be less free from palladium than in the previous separation without EDTA. METHOD111. In the Gilchrist-Wichers scheme, rhodium and palladium are precipitated hydrolytically along with iridium. Rhodium is, however, in the quadrivalent form and forms an olive green hydroxide. When redissolved in
hydrochlui~cacid, a green solution rcsults, which acts as an anion and is not adsorbed by a cation exchange resin. Upon reduction with hydroquinone, yeliom trivalent rhodium chloride is produced. This method of preparing the yellow cationic form of rhodium n as therefore attempted, with the hope that a 100% retention of rhodium by the column might be achieved (Table IV). The palladium unsorbed by the resin mas spectroscopically free of rhodium and the rhodium removed from the column was free from palladium. Thus this method of preparing the yellow cationic form of rhodium chloride produces a 100% cationic form. Rhodium from Iridium. I n prcliminary work with separate solutions of yellow rhodium(II1) chloride and iridium(1V) chloride, it was observed that the rhodium behaved largely as a cation and the iridium as an anion. However, nhen a mixture of the two was passed over a cation resin, a large fraction of the iridium was retained bj the resin along with all the rhodium and upon subsequent elution, the separation was far from complete. This apparent “induced” behavior of iridium by rhodium was also observed in the ion exchange work of Crummett (4) and in the electrolytic deposition of the two metals by Tuthill (12). If quadrivalent iridium was added to the yellow cationic form of rhodium after it had been treated to get it into this form, the iridium did not deposit on the VOL. 29, NO. 8, AUGUST 1957
1222
resin, but behaved as an anion and was eluted quantitatively. Trivalent iridium hydroxide is soluble, although a precipiate appears after long boiling with sodium hydroxide, but this is considered the result of air oxidation to quadrivalent iridium hydroxide. The failure of iridium and rhodium to separate from a mixture is the result of their simultaneous precipitation as hydroxides. If this could be avoided, the separation should be possible. (This suggests formation of one ionic species containing both metals.) These ideas have been incorporated into a separalion. The mixture or rhodium(J.11 and IV) and iridium(1V) chlorides was treated with 1%hydroquinone until the color change was complete and the iridium was all in the trivalent state. Strong sodium hydroxide was then added until precipitation of rhodium hydroxide was complete. Trivalent iridium hydroxide did not precipitate. Hydrochloric acid ( 3 N ) was added until the pH was 2.8. After 10 minutes, chlorine gas was bubbled through the solution to restore the iridium to the quadrivalent state. The solution was then applied to Dowex 50 resin and the column washed with 50 ml. of 10% chlorine water. The iridium was washed through quantitatively and the rhodium was retained (Table V). Rhodium mas removed by elution with 6N hydrochloric acid. The eluted rhodium was spectrographically free from iridium. However, rhodium was not quantitatively retained by the resin, and hence the iridium is contaminated to this extent. A second pass through fresh resin was not attempted, as the failure of all of the rhodium to adsorb was considered due to its partial anionic character. Rhodium from Platinum and Palladium. Separation of rhodium from platinum and palladium is affected by the pretreatment of the sample. When a solution containing chlorides of these three was heated with excess base and redissolved in acid in order to produce cationic rhodium, much of the palladium was in the cationic form. If the combined hydroxides were allowed to age a t room temperature before they mere redissolved in acid, palladium behaved as an anion and was not retained by the cation resin. Platinum also behaved as an anion. The unsorbed mixture contained a small amount of rhodium. The rhodium removed from the column was spectrographically free from platinum but contained traces of palladium (Table VI). Rhodium from Palladium and Iridium. The separation was carried out exactly as with the rhodiumiridium mixture. Palladium introduces no difficulties (Table VII). When (ethylenedinitri1o)tetraacetic acid was added in the preparation of the 1222
ANALYTICAL CHEMISTRY
Table V. Separation of Rhodium from Iridium on Dowex 50 Metal Eluted, Mg. Metal Taken, Mg. Metal Unsorbed, Mg. Rh Ir Rh Ir Rh Ir 7.06 7.14 0.10 0.00 7.14 6.90 0.00 7.14 0.10 7.14 7.06 6.95 14.12 7.14 0.30 7.14 13.8 0.00 14.12 7.14 0.28 0.00 7.14 13.9 14.12 7.14 14.0 0.00 7.14 0.10 0.00 14.28 0.15 14.28 14.12 13.9 35.2 14.28 1.5 0.00 14,28 33.7 0.00 33.5 35.70 1.8 35.70 31.6 Table VI.
Separation of Rhodium from Platinum and Palladium on Dowex 50
-~ hietel Taken, Mg. Rh
Pd
Pt
14.12 7.06
8.32 8.32
5.20 5.20
Metal Unsorbed, Mg. Rh Pd Pt 0.12 0.10
8.25 8.15
5.20 5.20
Metal Eluted, Mg. Rh Pd Pt 14.0 Trace 0.00 Trace 0.00 6.9
be due to a small amount of pink anionic form of rhodium chloride in equilibrium with the yellow cationic form. The pink form is converted to the yellow cationic form by precipitation with strong sodium hydroxide and redissolving in acid. Upon standing, the yellow species develops noticeable pink color. This indicates the existence of an equilibrium between the yellow and pink forms. The yellow cationic form is obtained exclusively by oxidizing the rhodium to the quadrivalent state and then reducing it with hydroquinone. Precipitation of the yellow hydroxide, followed by immediate solution in hydrochloric acid, produces a completely cationic form of rhodium This work also indicates that iridium(1V) produced by oxidation with chlorine is an anion, while iridium(II1) behaves as a cation.
sample, the eluted rhodium was spectrographically free from palladium. Rhodium from Platinum and Iridium. This separation followed the plan for rhodium and iridium (Table VIII) * Rhodium from Platinum, Palladium, and Iridium. Rhodium was separated from the other three members of the group as follows: One per cent hydroquinone was added until no further color change was observed, 5 ml. of 1%EDTA was added, and the solution was brought to boiling and made alkaline with strong sodium hydroxide. The suspension was cooled to room temperature, and adjusted to pH 2.8 with hydrochloric acid. Chlorine gas was bubbled through the solution for 10 minutes to reoxidize the iridium. The solution was then applied to the column and the column was washed with 50 ml. of 10% chlorine water. This removed platinum, palladium, and iridium. Rhodium was removed from the resin with 3N hydrochloric acid and was found to be spectrographically free from platinum, palladium, and iridium.
LITERATURE CITED
( I ) Berg, E., Senn, W., Jr., ANSL. CHEM. 2 7 , 1255 (1955). (2) Burstall, F. H., J . Chem. SOC.1936, 1 v.7
l(J.
(3) Cluett, M. L., Berman, S. S., McBryde, W. A. E., Analyst 80, 204 (1956). (4) Crummett, W. B., Ph. D. dissertation, Ohio State University, 1951. (5) Gilchrist, R., Wichers, E., J . Am. Chenz. SOC.5 7 , 2565 (1935). (6) Kraus, F., Umbach, H., 2. anorg. Chem. 180, 42 (1929). (7) MacKevin, W. M., Crummett, W. B., .4NAL. CHEM.2 5 , 1628 (1953).
DISCUSSION
The separations described produce rhodium chloride of high purity. Rhodium is not quantitatively recovered, and a small amount of rhodium usually contaminates the separated metals. The failure of rhodium to be retained completely by the resin is considered to
Table VII.
Separation of Rhodium from Palladium and Iridium on Dowex 50 Metal Cnsorbed, Rlg. hletal Eluted, Mg. Rh Pd Ir Rh Pd Ir Trace 0 00 12.8 7.14 1.3 8.25 7.14 8.32 Trace 0 00 12.6 7.14 8.22 7.14 1.5 8.32
hIetal Taken, Mg. Rh Pd Ir 14.12 14.12 Table VIII.
Separation of Rhodium from Platinum and Iridium on Dowex 50
Metal Taken, Mg. Rh Pt Ir 14.12 13 40
520 5 20
7.14 7.14
Metal Unsorbed, hlg. Rh Pt Ir 1.5 ... .,. 1.4
..
,
..
Metal Eluted, Mg. Rh Pt Ir 0.24 Trace 12.4 0.31 Trace 12.0
(8) MacSevin, W. hl., Kriege, 0. H., Zbid., 28, 16 (1956). (9) hlacSevin, W . M., Kriege, 0. H., J . Am. Chem. SOC. 77, 6149 (1955).
(10) Meyer, J., Kawczyk, $1. Z . anorg. u. allgem. Chem. 226,295 (1936). (11) Stevenson, P. C . , Franke, A. A., Borg, R., Nervik, W.,J . Am. Chem. SOC.75,4876 (1953).
(12) Tuthill, S., Ph.D. dissertation, Ohio State University, 1958.
RECEIVED for review December 10, 1956. Accepted February 11, 1957.
Determination of Calcium or Zinc Additives in Lubricating Oils and concentrates By an (Ethylenedinitri1o)tetraacetic Acid Titration Method P.
B. GERHARDT and E. R. HARTMANN
Products Research Division, ESSO Research and Engineering Co., linden, N. 1.
b A method has been developed for determining calcium or zinc directly in oils without prior ashing of the sample. The sample is mixed in acetone and after the metal has been chelated with EDTA, excess EDTA is titrated with standard magnesium chloride.
I
ORDER to control certain processing or blending operations, petroleum testing laboratories are frequently required to make rapid determinations of calcium or zinc in petroleum products. Although emission spectrographic or flame photometric methods are available, this costly equipment cannot be justified in some operations. Hence, a rapid method of chemical determination of calcium and zinc is often required. A method has been developed which permits determination of calcium or zinc directly in oils without prior ashing of the sample. Essentially, it consists of mixing the sample in acetone and after chelating the metal with (ethylene dinitri1o)tetraacetic acid (EDTA), titrating the excess EDTA with standard magnesium chloride. Although solution is not always complete, the analysis is quantitative. The reactions involved are based essentially on the chelating properties of EDTA (1-3). Because the method requires neither ashing nor precipitation steps, it is extremely rapid. An analysis can be completed in about 20 minutes.
x
REAGENTS
(Ethylene dinitri1o)tetraacetic acid (EDTA), approximately 0.W. This solution does not require standardization. Magnesium chloride, 0.1N. Stand-
ardize by titrating a weighed portion of reagent grade barium chloride in water as described in the procedure, First dry the barium chloride for 2 hours a t 125" t o 130" C. Indicator, Eriochrome Black T. Triturate 0.2 gram of the dye with 100 grams of ammonium chloride and store the mixture in a tightly stoppered bottle. (This mixture is not stable and should be discarded after about 3 weeks.) Buffer solution. Dissolve 67.5 grams of ammonium chloride in about 200 ml. of distilled water, add 570 ml. of concentrated ammonium hydroxide, and dilute t o 1liter with distilled water. PROCEDURE
Weigh an amount of the sample sufficient to contain 20 mg. of metal into a 300-ml. Erlenmeyer flask, and add about 130 ml. of acetone. Swirl the flask until the sample is completely dissolved or well dispersed, When the product under test is an oil blend, the size of sample should be about 10 grams weighed to the nearest 0.1 gram. When it is an additive concentrate, the size should be 0.2 to 0.4 gram, weighed to the nearest milligram. I n some cases a slight cloudiness may persist. This has no effect upon the final result. Into a second flask put the same amount of acetone, but no sample. By means of pipet, add 25 ml. of the 0.1N EDTA solution to each flask. To ensure complete mixing, swirl and warm both flasks at steam bath temDerature for 5 minutes. Cool the flasks slightly and add to each, 2 ml. of concentrated ammonium hydroxide, 10 ml.of the buffer solution, and about 0.3gram of the indicator mixture. Titrate both solutions with the standard magnesium chloride solution to a wine-red end point, Calculate the metal content of the sample by the equation: M = (B
- S)
X N X e X 100
W-
where B = volume in milliliters of magnesium chloride solution needed for titration of blank S = volume in milliliters of magnesium chloride solution needed for titration of sample N = exact normality of magnesium chloride solution e = milliequivalent weight for metal insam le W = weight ofsample, in grams M = weight per cent of metal in sample REMARKS
The estimated method is:
precision of
yo Level Calcium Zinc
0.5-1.5 0.5 9.0
the
Precision
0.020 0.012 0.11
The method has the following limitations: Samples must contain only one bivalent metal; and, it cannot be used for the analysis of used lubricating oils because of interference from iron, lead, copper, etc. When the reagents are added, certain einc products cause the mixture to assume a bluish green color. This does not affect the final result. LITERATURE CITED
(1) Banewics, J. J., Kenner, C. T., ANAL. CHEM. 24 1186 1952). (2) Martell, A. E., alvln, M., "Chemistry of the Metal Chelate Com-
d
pounds," 5th ed., Vol. 1, Van Nostrand, New York, 1952. (3) Schwarzenbach, G., Biedermann, W., Helv. Chim. Acta 31, 456 (1948).
REC~IVED for review June 11, 1956. Accepted March 5, 1957. VOL. 29, NO. 8, AUGUST 1957
1223
