tration of 1 gram of sample per 10 ml. of acid. A series of nine determinations on NBS sample 130 indicat,e the precision which inay be espected from this procedure. The follon-ing results mere obtained from determinations made on separate films taken on 3 different days.
ACKNOWLEDGMENT
Table 11. Comparison of Spectrographic and Chemical Determinations of Lead in Leaded Steels
Lead, % Diff ., SpectroSpectrographic graphic Chemical Chemical 0.00
LITERATURE CITED
0.00 0.00 0.00
(1) Gillette, J. &I., Boyd, B. R., ShurBus, A. A. A p p l . Spectroscopv 8 , 162-8 (1954). (2) Pagliassotti, J. P., E d . , 9, 153-8 f1955). (3) R;Zi, J.’T.,Iron Age 157, KO.3, 42-6 (1946). (4) Woodruff, J. I?., J . Opt. SOC.iiiner. 4 0 , 192-6 (1950).
+o. 02 yo Lead, certified
value % Lead found
957, confidence
interval
0.204 0.203, 0.199, 0.211, 0.212, 0.215, 0.210, 0.205, 0.198, 0.202 Av. 0.20G zk5.9yo
A comparison of determinations on leaded steels by this procedure and the lead molybdate precipitation procedure is shown in Table 11.
-0.02 0.34 0 28 0.33
0.26 0.24 0.24 0.28 0.23 0.23 0.29
0.31 0.30 0.30
$0.03
0 23
+O.Ol
0.28
0.22 0.26 0.23
0.24 0.27
The author wishes to thank the management of Jones & Laughlin Steel Corp. for permission to publish this report.
-0.02
+O. 03
-0.02
$0.02
+o. 02
0.00 -0.01
+o. 02
RECEIVED for review October 12, 1956. Accepted December 20, 195G. Presented in part, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February-March l95G.
Spectrophotometric Determination of Rhenium with Alpha-Furildioxime VlLLlERS W. MELOCHE, RONALD L. MARTIN, and WILLIAM H. WEBB1 Deparfmenf of Chemistry, Universify o f Wisconsin, Madison, Wis.
A simple, sensitive, and accurate spectrophotometric determination of rhenium i s based upon complexation with a-furildioxime. The rheniumdioxime complex i s formed upon reduction of perrhenate b y tin(l1) chloride in the presence o f a large excess o f afurildioxime. The absorbance of the complex i s measured a t 532 mp. Beer’s law i s followed within 0,270for 20 to 3007 o f rhenium in 50 mi. o f solution. This method i s free from interference b y most ions, molybdenum being the chief interference. Prior extraction o f the molybdenum-ethyl xanthate complex by chloroform overcomes this source o f interference.
W
a-furildioxime and tin(I1) chloride are added to a dilute hydrochloric acid solution containing perrhenate, an intensely colored dienium-furildioxiine complex results. A spectrophotometric method for rhenium based on this reaction is described in this paper. The color reaction of dimethylglyoxime with rhenium in the presence of tin(I1) chloride in strong acid solution HEN
1 Present address, Missouri School of Mines and Metallurgy, Rolla, &Io.
was reported by Tougarinoff (12) in 1934. Tribalat (16) found that CYbenzildioxime was a more sensitive reagent than dimethylglyoxime for the detection of rhenium. Investigations here have shown that a-furildioxime is even more sensitive. Peskova and Gromova (10) reduced perrhenate with tin(I1) chloride in 9 to 12N sulfuric acid in the presence of 0-furildioxime. Under these conditions, apparently a number of rhenium-furildiosime coniplexes are formed, with colors varying froni yellowish green to orange to red. As only the red coniplex mas extracted by chloroform, rhenium was determined by measuring the absorbance of the chloroform extract. Work in this laboratory (5, 16) indicated that a more satisfactory method results from forming the furildioxime complex in about O.SN hydrochloric acid solution. A 26% acetone solution is used, so that the rheniumdioxime complex is kept in solution. Under these conditions, only a reddish purple rhenium-furildioxime complex is observed. The absorbance is measured directly on the original solution without extraction. A molar absorbancy index of about 41,300 is obtained, compared to 24,000 reported by Peskova and Gromova.
Unfortunately, in this determination, as n-ith most other methods for the determination of rhenium, molybdenuni interferes. Molybdenum does not form a colored complex with a-furildioxinie, but low results are obtained for rhenium when even small amounts of molybdenum are present. A large increase in the amount of furildioxime added increases the rhenium yield, but the results are not satisfactory when larger amounts of molybdenum are present. As the most common source of rhenium contains molybdenum, a means for eliminating the molybdenum interference was necessary. The ethyl xanthate-molybdenum extraction method (4, 6) was mccessfully adapted for use in the procedure described. REAGENTS AND APPARATUS
Spectrophotometric measurements were made on a Cary Model 1431 recording spectrophotometer with matched 1cm. silica cells. The perrhenate stock solutions were prepared from potassium perrhenate obtained from the University of Tennessee. The purity of the potassium perrhenate mas checked by determination with nitron ( 2 ) . a-Burildioxime Stock Solution, Dissolve 0.70 gram of the diosime in 200 nil. of acetone, Prepare this solution fresh VOL. 29,, NO. 4, APRIL 1957
527
daily, as on prolonged standing it turns yellow. a-Furildioxime obtained from the Eastman Kodak Co. was used without purification. Tin(I1) Chloride Stock Solution. Dissolve 10 grams of analytical reagent tin(I1) chloride dihydrate in 10 ml. of concentrated hvdrochloric acid and sufficient water to make the final volume 100 ml. Potassium Ethyl Xanthate Solution. Dissolve 27 grams of the salt in 35 ml. of mater, filter, and dilute t o 50 ml. Prepare this solution fresh daily. Potassium ethyl xanthate obtained from the Eastman Kodak Go. was used without purification, PROCEDURE
Molybdenum Absent. Add an aliquot containing less than 300 y of rhenium as perrhenate t o a 50-ml. volumetric flask, and sufficient hydrochloric acid to make the final amount equal t o 35 mmoles. Dilute the contents of the flask t o about 30 ml. with mater, add 13 ml. of ~~-furildioxime solution and then 5 ml. of tin(I1) chloride solution, and dilute to exactly 50 ml. Allow the solution to stand for 45 minutes, and then read the absorbance a t 532 mp. Make the absorbance reading against a blank containing all the reagents except rhenium. The absorbance of the blank mill vary only slightly from that of water. Convert the absorbance reading obtained to concentration of rhenium by using Beer’s lam or a standard curve. Large Amounts of Molybdenum Present. If necessary, put the rhenium-containing material in solution with a sodium peroxide fusion (6). Make the proper dilutions, so that a 25ml. aliquot mill contain no more than 200 mg. of molybdenum and 400 y of rhenium. Pipet a 25-ml. aliquot into a 250-ml. beaker. Neutralize the sample with concentrated sulfuric acid and add 2 drops in excess. Add 10 drops of liquid bromine. Place the sample on a hot plate a t lorn heat and heat without boiling, until the excess bromine is driven off. The bromine oxidation can be omitted only if it is certain that all the molybdenum exists in its highest valence state. Add 5N sodium hydroxide solution until the sample is neutralized, and 2 or 3 drops in excess until a pH of 9 to 11 is reached. Transfer to a glass-stoppered separatory funnel, cool to room temperature in running water, add 10 ml. of xanthate solution, and mix. Then add 6 ml. of concentrated hydrochloric acid, shake for 5 seconds, and finally add 50 ml. of chloroform. Shake vigorously for 30 seconds and let stand until the two layers separate completely. The water layer should be colorless or a very pale pink. The chloroform layer mill be an intense red-violet color. Dram. off the chloroform layer and discard it. Wash the aqueous layer twice with 25-ml. portions of chloroform. Quantitatively transfer the sample solution in the separatory funnel to a 100-ml. volumetric flask, add 0.3N potassium permanganate dropwise until the per528 *
ANALYTICAL CHEMISTRY
manganate is no longer decolorized (about 10 drops), and then add a drop or two in excess. Dzstroy the remaining color of the permanganate with a drop of the tin(I1) chloride solution, and then add 26 ml. of furilcioxime solution and 10 ml. of tin(I1) chloride solution. Dilute the contents of the flask to exactly 100 ml. , After 60 minutes, read the absorbance a t 532 mp. To convert absorbance to concentration of rhenium, compare with standard: carried through the entire procedure, ANALYTICdiL RESULTS
Tlie results reilorted in Table I indicate the reproducibility and accuracy to be expected from this method. These data were olltained by analyzing aliquots of a perrhsnate stock solution by the procedure f x samples not containing molybdenuin. Tlie data for the concentration range illustrated in Taole I show strict adherence to Beer’s law, as the average deviation of the molar absorbancy indexes from the wierage is only 0.2%. Higher concentraticns of rhenium do not follow Beer’s law exactly, but somewhat higher concentrations can be made to follow Beer’s Ian- by increasing the concentration of both acetone and furildioxime. The results of analyses of synthetic mixtures of inolybllenum and rhenium are given in Table 11. The lorn results for rhenium may kave been caused by a small amount of molybdenum which was not removed. Rhenium was determined in these solutions by compaiison with standards which did not contxin molybdenum and had not been run through the ethyl xanthate extraction procedure. More reliable results are obtained by extrapolation from a standard curve prepared by running the entiie procedure on solutions containing molybdenum as well as known amounts of rhenium. DlSClJSSlON
Effect of Reagent Concentrations. By forming the rhl?nium-dioxime complex in various acid concentrations, it mas determined ;;hat the maximum color intensity is developed with a hydrochloric acic. concentration of 0.65 t o 0.9iV. [ntensity decreases somewhat for higher and lower concentrations of acid. Forty-five milligrams of furildioxime (13 ml. of the stock solution) are capable of converting up tcs 300 y of rhenium in 50 ml. of solution quantitatively to the rhenium-furildioxirie complex; 15 mg. are sufficient to convert 100 y of rhenium completely ;o the complex, but larger amounts of rhenium are incompletely complexed. A large excess of furildioxinie is always necessary for formation of the dleniurn complex,
Table 1. Analytical Data RheAbsorb- Molar Abmum, ance sorbancy %. y/50 Measure- Coefficient DeviaM1. ment x 10-1 tion 23.28 0.103 4121 -0.3 0.104 4161 $0.7 46.56 0.206 4121 -0.3 0.207 4141 +0.2 93.12 0.413 4131 -0.05 0.415 4151 i-0.4: 139.68 0,620 4135 $0.05 0.619 4128 -0.1 186.24 0.826 4131 -0.05 0.828 4141 $0.2 232.80 1.030 4121 -0.3 1.033 4133 0.0 279.36 1.238 4128 -0.1 1.234 4115 -0.4 Av. 4133 0.2 Table II. Determination of Known Amounts of Rhenium in Presence of Molybdenum (125.0 mg. of molybdenum present) Rhenium, blg. E ~ Saniple Present Found % 0.1941 -2.6 1 0 2000 0.1952 -2.4 2 0.2000 0.1940 -3.0 3 0.2000 0.0977 -2.3 4 0,1000 0.0970 -3.0 5 0,1000 0,0972 -2.8 6 0.1000
L
For 50 y of rhenium in 50 nil. of solution, a dioxime-rhenium ratio of about 15 is necessary before the purple complex can be observed by the eye. The dioxime solution also supplies sufficient acetone to keep 6he rhenium-dioxime complex in solution. Larson (6) found that the rate of formation of the complex is dependent on the concentration of acetone as well as the other reagents; the reaction rate is highest for acetone concentratiolls OE about 20 volume yo. A concentration of 26% mas chosen for the procedure, so that all the rhenium-dioxime complex mould remain in solution for the higher rhenium concentrations. Higher acetone concentrations are avoided, because the rate of complex formation becomes smaller. A large excess of tin(I1) cl?loride is required to increase the rate of formation of the rhenium-dioxime complex. Experiments have indicated that 5 ml. of a 10% solution are satisfactory for 300 y of rhenium in a 50-ml. volume, but larger amounts can be added with no damaging effects. Order of Addition of Reagents. The furildioxime is added t o the solution before the tin(I1) chloride. The time interval between the addition of the tm-o reagents is not important, unless tin(I1) chloride is added be-
~
~
~
,
fore the dioxime, when the interval must be less than 10 minutes. When the time interval is too large, the color development is incomplete, probably because a stable compound of rhenium(IV) is formed. Stability of Rhenium-Dioxime Complex. If the proportions of reagents suggested are used, color formation is about 977* complete after 10 minutes and mavinium color developnient occurs in about 45 minutes. When maximum color intensity is reached, i t undergoes no appreciable change for 24 hours when kept a t room temperature in stoppered Aasks. Ethyl. Xanthate-Molybdenum Extraction. After investigation of methods for the separation of large amounts of molybdenum from small amounts of rhenium (1,5,4,6,8,9),the ethyl xanthate-molybdenum extraction method of Hurd (4) and Malouf and JTThite (6) was chosen for use. It combines a simple separation with a minimum of interfeience to the dioxime procedure. Changes in the proportions of reagents used by AIalouf and White mere made, so that the procedure could be more effectively applied to this deterniination of rhenium. Chloroform mas used in place of the benzene-carbon tetrachloiide mixture, because a more satisfactory separation of phases was obtained. The ethyl xanthate-molybdenum extraction caused a change in the aqueous
phase, such that the subsequent development of the rhenium-dioxime complex mas incomplete. To eliminate this interference, potassium permanganate was added dropwise until a faint color persisted. After this treatment, dioxime complex developed without difficulty upon addition of the colorforming reagents. Rhenium-Furildioxime Complex. The purple compound used for spectrophotometric determination is the furildioxime complex of a reduced valence state of rhenium. I n dilute hydrochloric acid solution an excess of tin(I1) chloride normally reduces perrhenate to rhenium(1V) (7, 13, 14). The addition of furildioxime to rhenium(1V) solutions did not produce the purple rhenium-dioxime complex, but after addition of excess tin(I1) chloride the rhenium-dioxime complex formed quantitatively. Apparently, tin(I1) chloride takes the rhenium below the quadrivalent state in the presence of the complexing agent. Divalent rhenium may be involved in the rheniumfurildioxime complex. All attempts to form the rheniumdioxime complex by electrolytic reduction have been unsuccessful. Most other reducing agents also are not capable of forming the complex. The complex forms after reduction with chromium(I1) chloride or vanadium(I1) chloride, but neither of these reducing agents is Satisfactory for quantitative
formation of the rhenium-furildioxime complex. Larson (6) concluded from spectrophotometric data that the rheniumdioxime complex probably represents a ratio of 1 rhenium to 2 furildioximes. EFFECT OF FOREIGN IONS
The dekrmination of rhenium with furildioxime is free from interference from moet anions, exceptions being thiocyanak, nitrate, and large amounts of fluoride. A thousandfold excess of most metal ions does not interfere, except where the interfering ion absorbs in the 530-inp range. Several metal ions, such as copper(I1) and palladium(II), form dioxime complexes in acid solution, and thus interfere with the rhenium test. Interference from oxidizing agents can be eliminated by prior reduction with sulfite or tin(I1) chloride. Small amounts of most acids do not interfere. Larger amounts should be neutralized with sodium hydroxide before the test for rhenium is made. Nitric acid and its salts interfere. Basic solutions should be neutralized with sulfui-ic acid before the rhenium determination. Table I11 illustrates the effect of a number of ions and compounds on the furildioxime method for rhenium. Some of the data in this table were taken from Preuss (11). LITERATURE CITED
Table 111. Effect of Foreign Ions (100 y of rhenium present) Re Found, Y Comments
Foreign Compound Added HClO,
Mg.
100
500 500
100
200
200
1000 200 100 100 y 100 x g 100 Ba 100 Cd 100 ZI1 100 Ni
100 Fe 100 Fe 100 As
COC12 CUCll
uozso, Se02
PdCI,
NaJloOa
100 Bi 100 A1 1c r 10 Cr
1c o 10 c o 1c u
10 c u 100 u 1 Se 10 Se 10 Pd
50 y hlo
100 y &Io
99 96 100 100 98 100
100 99
..
.. 100
Complete interference Complete interference
100
100 100 100 100 100 100
100 100
100 110 100 103 95 92 102 96
}:
Cr(II1) color interferes Cu forms dioxime complex Insoluble Se dispersed throughout solution Complete interference, dioxime complex formed Dioxime concentration increased five times
Fisher, S. A., Meloche, V. LV., ANAL. CHEW24, 1100 (1952). Geilmttnn, RT.,Voight, A., Z. anorg. allgem. Chenz. 193, 311 (1930). Hillebrand, W,F., Lundell, E. F., Bright, H. A., Hoffman, J. I., ('Applied Inorganic Analysis," 2nd ed., p. 320, Wiley, New York, 1953. Hurd. L. C.. IND. ENG. CHEII..ANAL. ED.' 8, ll'(1936). Larson, W.J., Ph.D. thesis, Universitv of Wisconsin. 1950. iCIaldml, E. E., Whke, M. G., ANAL. CHEII.23, 497 (1951). Maun, E. K., Davidson, N., J . Am. Chenz. SOC.72, 2254 (1950). Meloche, V. W.,Preuss, A. I?., ANAL. CHE\I. 26, 1911 (1954). Meyer. R. J., Rulfs, C. L., Ibid., 27, 1387 (1955). Peskova, V. M., Gromova, M. I., Vestlzik Moskov, Univ. 7 , No. 10, Ser. Fiz.-Mat. , i Estcstven Nauk, No. 7, 85 (1952). Preuss, A. F., Rohm & Haas Co., Philridelphia, Pa., private communication. (12) Tougarinoff, M. E., Bull. SOC. chim. Belgt! 43, 111 (1934). (13) Tribalat, S., Ann. chinz. 4,289 (1949). (14) Tribalat, S., Compt. rend. 222, 1388 16) Webb, W.H., Ph.D. thesis, University of Wisconsin, 1949. RECEIVED for review September 29, 1056. Accepted November 27, 1956. Work supported in part by the Research Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation. VOL. 29, NO. 4, APRIL 1957
529
