In expanding the rhenium-molybdenum separation to milligram amounts, it was impossible to obtain a good separation with methanol-HC1 or H N 0 3 eluent on a reasonably short column. Using 1-butanol, 0.6M in HC1 and saturated with water as eluent, a separation of 10 mg of rhenium from 10 mg of molybdenum was obtained on a 15 x 1 cm column at a flow rate of 0.4 ml cm- * min-’ (Figure 3).
tungsten remains at the point of origin at all acid concentrations and is identified as a blue spot with the same S n C k KSCN reagent. We have applied this method to the routine identification of rhenium in molybdenum-rhenium alloys containing 2050% Re, and tungsten-rhenium alloys containing 3% Re. Prepared mixtures indicate that all four elements discussed here are readily identifiable in one solution containing as little as 1 Re. An amount of sample sufficient to provide 0.001-0.002 mg per spot of each element is a convenient working range but a factor of ten less material is detectable.
RECEIVED for review May 14, 1970. Accepted November 3, 1970.
Fluorometric Determination of Submicrogram Quantities of Zirconium T. D. Filer Health Services Laboratory, US.Atomic Energy Commission, Idaho Falls, Idaho
FLUOROMETRIC METHODS for the determination of zirconium have been developed using flavonol ( I ) , morin (2), quercetin (3),datiscin ( 4 ) , and calcein blue (5) as reagents. All methods show good sensitivity for the detection of submicrogram quantities of zirconium. All methods also consider interferences due to fluorescent complexes formed by the reagent and foreign ions. However, the flavonol procedure requires a time-consuming electrolysis in a Melaven cell to remove trace quantities of iron and other heavy metals which are serious negative interferences. Morin forms fluorescent complexes with aluminum, gallium, thorium, uranium, tin, and antimony in addition to zirconium. Therefore, zirconium must be determined by difference in the presence of these interferences by repeating the measurement after the addition of a masking agent for zirconium which does not affect the others. The quercetin method requires an extraction with 2-thenoyltrifluoroacetone to avoid milligram quantities of common interferences such as iron, vanadium, and titanium. Microgram quantities of iron, molybdenum, titanium, and thorium are serious interferences in the datiscin procedure. The calcein blue method is more sensitive than any other zirconium procedure but suffers from a lack of selectivity and common elements such as silver, aluminum, bismuth, cadmium, copper, iron, molybdenum, manganese, nickel, lead, thorium, and arsenic interfere seriously. None of these procedures employs a pyrosulfate fusion as a method of sample decomposition and, therefore, cannot ensure that the highly refractory compounds of zirconium are converted to ionic forms suitable for complexation with the flavone. The present method using 3,4’,7-trihydroxyflavoneas the fluorometric reagent is no more sensitive to zirconium than the other compounds, but the fluorescence is measured in a
sulfuric acid solution which permits use of pyrosulfate fusion as the method of sample dissolution. However, the greatest advantage of the present procedure is that many of the common elements associated with zirconium alloys and ores do not interfere in milligram quantities, thus making the direct determination of zirconium possible. EXPERIMENTAL
Apparatus. The instrumentation used was a Beckman D U spectrophotometer with a fluorescence accessory modified as described (6). A combination of Corning Filters with color specification Nos. 0-51 and 7-51 having over 1 % transmittance between 360 and 435 nm with a maximum of 40 % was used for the primary. A combination of Corning Filters with color specification Nos. 3-2 and 5-56 having over 1 % transmittance between 440 and 590 nm with a maximum transmittance of 59% was used for the secondary. A tungsten source was used in the present work, but a medium pressure mercury lamp can be used with the same filter combinations with similar results. Reagents. STANDARD ZIRCONIUMSOLUTIONS,1 mg/ml and 5 pg/ml. Fuse 1.3508 grams of high purity zirconium dioxide with 5 grams of anhydrous sodium sulfate and 3 ml of concentrated sulfuric acid in a 250-ml Erlenmeyer flask. Cool the melt, add 10 ml of concentrated sulfuric acid, 100 ml of water, and dissolve the melt with cooling. Dilute to 1 liter. Dilute 5.00 ml of the stock solution and 10 ml of concentrated sulfuric acid to 1 liter. The solutions contain 1 mg/ml and 5 pg/ml of zirconium, respectively. SODIUMSULFATEIN SULFURIC ACID, 10%. Dissolve 10 grams of anhydrous sodium sulfate in 100 ml of concentrated sulfuric acid with heating as required. Cool and store in a glass-stoppered borosilicate glass bottle. SULFURIC ACID SOLUTION, 0.5714. Dissolve 5.0 grams of sulfamic acid and 14.0 ml of concentrated sulfuric acid in enough water to make 500 ml of solution. Cool and store in a glass-stoppered borosilicate glass bottle. 3,4’,7-TRIHYDROXYFLAVONE SOLUTION, 6.75 x %. The preparation of the flavone has been described (7-9).
(1) W. C. Alford, L. Shapiro, and C . E. White, ANAL.CHEM.,23, 1149(1951). (2) R. A. Geiger and E. B. Sandell, Anal. Chim. Acta., 16, 346 (1957). (3) D. M. Hercules, Tulunta, 8,485 (1961).
(4) A. P. Golovina, I. P. Alimarin, E. A. Bozhevol’nov, and L. B. Agasyan, Zh. Anal. Khim., 17,591 (1962). ( 5 ) R. V. Hems, G. F. Kirkbright, and T. S. West, ANAL.CHEM.,
42,784 (1970).
(6) C. W. Sill and C . P. Willis, ANAL.CHEM., 31,598 (1959). (7) D. G. Roux and G. C . deBruyn, Biochem. J.,87,439 (1963). (8) Katsuzo Yamaguchi, Nippon Kagaku Zasshi, 1963 148. (9) Z. I. Jerzmanowska and M. Michalska, Rocz. Chem., 35, 353 (1961).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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. I
I
, . 0 0 0 e 0 0 -
,0111
I
I
I
’“t
80
U I-
v)
z
3,4’, 7
- T R I HY D R o x Y FLAVO NE c o Nc E NT R AT Io N ( x IO4v0
Figure 1. Effect of 3,4’,74rihydroxyflavone concentration 1. Blank 2. 5 - p g Zr standard 3. 5-pg Zr standard corrected for blank
40
30t.--.-.-.-.-.-.-.-
2ol
1
10
I N HCIO4,rnl
I N NaOH,rnl
Figure 2. Effect of acidity
Transfer 6.75 mg to a 100-ml volumetric flask and dilute to volume with 95 %ethanol. Procedure. The procedure given below for preparation and measurement of the fluorescence is that used in the development of the procedure using pure zirconium solutions. It is also to be followed when zirconium has been separated and can be obtained in concentrated sulfuric acid free of interfering elements. However, many applications of this procedure can be made without separations, provided the sample size is chosen so that the heavy metal content does not exceed the permissible levels described below. Place the zirconium standard or other zirconium solution into a 100-ml beaker. Add 2 drops of 7 2 z perchloric acid, 1 ml of 10% sodium sulfate in sulfuric acid, and evaporate the solution carefully to dryness on a n asbestos-covered hot plate. Heat until all the sulfuric acid including that condensed on the beaker walls has been volatilized and fuming has ceased. Cool the sodium acid sulfate residue, add 2 ml of water, and 3 drops of 25% hydroxylammonium sulfate. Cover the beaker with a watch glass and boil the solution until the volume has been reduced to about 0.5 ml. Remove the cover glass and rinse with a few drops of water. Add 15.00 ml of the sulfuric acid solution and transfer the solution quantitatively to a 25-ml volumetric flask. Add 1.00 ml of 3,4’,7trihydroxyflavone solution, mix, and dilute to volume. Mix thoroughly and place in a constant-temperature bath a t 25 “C for 20 minutes. Measure the fluorescence using the technique described previously (6, 10). Permanent glass standards (6) can be used to reproduce the same instrumental sensitivity from day to day. Place 1 ml of water for a blank and 1 ml of the 5-pg/ml standard zirconium solution in separate 100-ml beakers, add 1 drop of 72% perchloric acid and 1 ml of the sulfuric acidsodium sulfate solution. Evaporate carefully to dryness on a n asbestos-covered hot plate until evolution of sulfuric acid fumes has ceased, and treat as described above. Substract the blank from the standard and express the sensitivity as microgram zirconium per net scale division. Correct the (10) C. W. Sill, C. P. Willis, and J. K. Flygare, Jr., ANAL.CHEM., 33,1671 (1961). 470
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
1. Blank 2. 5-pg Zr standard 3. 5-pg Zr standard corrected for blank
samples for a n appropriate blank carried through the entire procedure including separations, if any, and calculate their zirconium content from the sensitivity value obtained from the standard. Sample Preparation. Because of the refractory nature of the compounds of zirconium and associated elements and the inability to dissolve them completely even in boiling concentrated acids, pyrosulfate fusion is always employed to ensure their complete dissolution and conversion to a soluble ionic form. A procedure for the decomposition of refractory silicates which involves a potassium fluoride fusion and a transposition to a mixed alkali pyrosulfate fusion has been described (11). In cases where silicates are known to be absent, the potassium fluoride fusion can be omitted and the sample treated initially with the appropriate mineral acid followed by pyrosulfate fusion. The solution obtained can be diluted to any desired volume and a n appropriate aliquot taken for analysis and treated as described previously. Many samples can be analyzed directly following dissolution, but others will require separation of zirconium from other elements. This will be true when only trace amounts of zirconium are present in a large excess of some interfering element. Zirconium can be extracted quantitatively from beryllium, aluminum, lanthanum, thorium, cobalt, nickel, manganese, lead, zinc, bismuth, gallium, scandium, indium, thallium, and yttrium by chloroform in the presence of 0.005M cupferron at pH values less than zero (12). Hindman (13) has also shown that zirconium can be extracted from large amounts of iron(I1) by using the same system in the presence of ascorbic acid and potassium thiocyanate. (11) C. W. Sill, ANAL.CHEM.,33, 1684 (1961). (12) Jiri Stary, “The Solvent Extraction of Metal Chelates,” The Macrnillan Co., New York, N.Y., 1964, pp 115-21. (13) F. D. Hindman, Health Services Laboratory, U. S. Atomic Energy Comm., Idaho Falls, Idaho, work in progress, 1970.
RESULTS AND DISCUSSION
Effect of 3,4’,7-Trihydroxyflavone Concentration. The results obtained when various concentrations of 3,4’,7-trihydroxyflavone were used are given in Figure 1. Curve 2 shows that the intensity of fluorescence of 5 pg of zirconium increases with increasing flavone concentration to a maximum at about 6 x lO-4%. If the highest precision is desired, the higher concentration of flavone should be used because the maximum fluorescence signal is produced at this level and the instrument can be operated in the range that has maximum stability. Also, at the higher concentration of flavone, small changes in concentration will not produce significant variations in fluorescence readings. However, the concentration of flavone can be adequately controlled so that it will not be a significant factor in precision even on the steeper portion of curve 2. On the other hand, the intensity of the zirconium fluorescence per unit blank fluorescence is greater at lower concentrations of the flavone. If the instrumental sensitivity can be increased so that the relatively weak fluorescence obtained at lower flavone concentrations can be spread over the full range of the instrument without significant loss of precision of measurement through instrument instability, smaller quantities of zirconium can be detected. The minimum detectable quantity of zirconium and the proper concentration of flavone to be used are dependent on the value of the blank, and the stability and sensitivity of the instrument. The arrows show that the recommended flavone concentration occurs at 3 X Lower concentrations result in smaller blanks, but the instrumental instability at these levels causes a significant decrease in precision, Effect of Acidity. The effect of changes in acidity on the fluorescence of the zirconium-3,4’,7-trihydroxyflavonecomplex was studied. The excellent efficiency and high buffering capacity of the system is shown in Figure 2 . The arrows mark the points on the buffer curves that result under the recommended conditions. Spectral Characteristics. Figure 3 shows the excitation and emission spectra for the reagent and its zirconium complex in 1M perchloric acid. The fluorescence of 3,4’,7-trihydroxyflavone exhibits its excitation maximum at 377 nm and a fluorescence emission maximum at 485 nm. The zirconium complex shows its excitation maximum at 417 nm and fluorescence emission maximum at 475 nm. All values are uncorrected for emission characteristics of the light source or the response of the detector. Detection Limit and Precision. The detection limit and determination limit of this procedure were determined as defined by Currie (14). T o determine these values and the precision obtained with larger quantities of zirconium, 10 blanks and ten 5-pg zirconium standards were analyzed under the recommended conditions, including the evaporation of zirconium solutions to dryness in the presence of sulfuric acid and the transfer from beaker to volumetric flask. The mean for the 5-pg zirconium standards was 95.8 scale division with a standard deviation of k 0 . 5 scale division. The mean for the blanks was 25.1 scale divisions with a standard deviation of 1 0 . 2 scale division. The results indicate a detection limit of 0.05 pg of zirconium. The minimum quantity of zirconium that can be determined with a precision of 10% is 0.14pg. Linearity. The effect of zirconium concentration on the fluorescence was investigated at a 3,4 ’,7-trihydroxyflavone
concentration of 2.50 X 10-7 mole per 25 ml to determine the linearity under analytical conditions. Nonlinearity is not detectable up to 10 pg of zirconium since it is not greater than the precision of the procedure--i.e., about 1%. As the quantity of zirconium is increased beyond 10 pg, deviation from linearity becomes very pronounced. Effects of Other Substances. In anticipation of applying the present procedure to a wide variety of sample types without separations, a detailed investigation was made of the effect of many other substances on both blanks and 5-pg zirconium standards. The element or compound investigated was added before fuming with sulfuric and perchloric acids to determine its effect under the recommended conditions. No error could be detected on blanks and less than 1 % on 5-pg zirconium standards in the presence of 5 mg of lithium, potassium, beryllium, cerium, magnesium, zinc, gadolinium, lutetium, and iron or 1 mg of mercury, arsenic, calcium, manganese, cadmium, platinum, rubidium, and cesium. Errors produced by other substances are shown in Table I. HAFNIUM, ANTIMONY, AND TIN. Hafnium, antimony, and tin produce a fluorescence with the reagent of sensitivity such that not more than about 0.02 pg of any of these can be present without producing detectable error. Fluorescence due to hafnium in a sample will be reported as zirconium. Both antimony and tin can be removed easily as the bromides from sulfuric acid by volatilization with hydrogen bromide (IS). This can best be accomplished by adding three 0.5gram increments of solid sodium bromide to the fuming sulfuric acid solution and sweeping the bromidesfromthe solution
(14) L. A. Currie, ANAL.CHEM., 40,586-93 (1968).
(15) J. E.Hoffman and G. E. F. Lundell, J . Res. Nut. Bur. Stand., 22, 22 (1939).
0
WAVELENGTH, nm
Figure 3. Excitation and emission spectra of reagent and zirconium complex in 1Mperchloric acid
+ +
A . Zr reagent. Excitation at 417 nm B. Zr reagent. Emission at 475 nm C. Reagent alone. Excitation at 377 nm
D. Reagent alone. Emission at 485 nm
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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Element Hf Sb Sn A1 Ga In
Quantity, mg
Table I. Effects of Other Substances Error, scale division. Remarks Blanks 5 pg Zr Fluorescent; 0.125 pg/sc. div. +40.0 > +10 Fluorescent; 0.12 pg/sc. div. +8.6 +4.9 Fluorescent; 0.11 pg/sc. div. $9.0 $10.2 Fluorescent; 39 pg/sc. div. +25.4 >+lo Fluorescent; 1.8 pg/sc. div. +5.6 +3.3 Fluorescent; 450 pg/sc. div. $2.2 +1.6 Fluorescent; 270 pg/sc. div. +3.7 $2.0 Fluorescent; 560 pg/sc. div. +1.8 +3.3 Fluorescent; 530 pg/sc. div. +1.9 0.0 Yellow color when flavone is added -2.2 -11.0 Yellow color when flavone is added -1.1 -2.1 Yellow color when flavone is added -12.1 -62.3 Yellow color when flavone is added -0.3 -5.6 Yellow color when flavone is added -4.1 -15.2 Yellow color when flavone is added +8.9 -50.8 Faint pink color 0.0 -3.9 Faint green color -1.3 -0.5 Faint blue color -1.9 -2.0 Faint yellow color +3.9 -4.4 Very turbid because of anhydrous Crn(SO& f3.0 -35.9 Few flocs of dehydrated SOz present f3.1 -2.1 Turbid f2.1 -11.0 Slight turbidity +3.0 -3.7 Elemental gold precipitates +32.5 -6.0
0.005 0.001 0.001 1 .o 0.01 1 .o Tl 1.0 B 1 .o Th 1.o W 0.01 0.1 Mo 0.01 Nb Ta 0.01 Bi 0.1 Ge 1 .o co 1 .o Ni 1.0 1 .o cu U 1 .o Cr 0.1 Si 1.0 Ba 1.0 Sr 1 .o Au 1 .o +6.2 +3.6 Ag 1 .o Ti 0.1 +0.7 -2.6 +4.8 -8.2 sc 1 .o La 1 .o -1.9 -0.1 Y 1 .o 0.0 -4.3 0.0 -6.2 V(+5) 1 .o Se 1.o +1.2 -2.2 P 1 .o +0.5 -81.4 F +2.2 -0.8 1 .o a Blank, 25.1 sc. div.; 5 pg Zr standard, 95.8 sc. div.; sensitivity, 0.0708 pg/sc. div. f1.0 sc. div. on standards probably indicate significant effect of added substance.
by careful addition of 5 ml of concentrated hydrochloric acid. ALUMINUM AND GALLIUM.Aluminum and gallium produce a fluorescence with the reagent of sensitivity such that not more than 100 pg of aluminum or 0.5 pg of gallium can be present without producing detectable error. Both interferences can be eliminated by an extraction with chloroform in the presence of cupferron as described previously. However, gallium is not abundant enough to cause much concern, and aluminum can be accommodated without loss of reliability by proper choice of sample size. For example, if the total sample taken for analysis is limited to 1 mg, zirconium can be determined in concentrations greater than 5 X loF3 without detectable interference from as much as 10% aluminum. Because the average soil or air dust sample would not be expected to have more than 10 % aluminum, direct determination of zirconium on such samples is possible. OTHERFLUORESCENT COMPLEXES. Indium, thallium, boron, and thorium show some interference at the 1-mg level due to their fluorescence with the reagent. Part of the fluorescence observed might be due to traces of zirconium. CHROMIUM, NIOBIUM,AND TANTALUM.Chromium, niobium, and tantalum are particularly serious interferences because of their strong absorption of both the emitted zirconium-3,4 ',7-trihydroxyflavone fluorescence and the exciting radiation, and because of the insolubility of anhydrous chromic sulfate, niobic acid, and tantalic acid. The presence of chromium can be detected quite sensitively by evaporating the solution to fumes of sulfuric acid in the presence of perchloric acid. A bright yellow color will develop with less than 0.1 mg of chromium, but will disappear quite rapidly because of thermal decomposition of the dichromate in the 472
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
Yellow color when flavone is added Bright yellow in concentrated HzSOa Added as NaHzPOabefore fuming Added before fuming Differences larger than 10.4 sc. div. on blanks or
fuming sulfuric acid. Niobium and tantalum can also be detected quite sensitively by the appearance of a yellow color when the flavone is added. Chromium can be removed easily from sulfuric-perchloric acids by volatilization with hydrogen chloride gas as chromyl chloride. Niobium and tantalum, unfortunately, are not readily separated from zirconium, and the presence of trace amounts will interfere seriously with this procedure. OTHER COLOREDCOMPLEXES.Tungsten, molybdenum, bismuth, germanium, and scandium form yellow complexes with the reagent. Bismuth and scandium interferences can be eliminated by the cupferron extraction. Tungsten and molybdenum are incompletely extracted by chloroform in the presence of cupferron and, therefore, are still serious interferences. TITANIUM.Titanium produces very little interference a t the 0.1-mg level. If more titanium is present, however, turbidity will be encountered because of its extreme hydrolytic tendencies. However, if the sample size is limited to 1 mg, zirconium can be determined in concentrations greater than 5 X loLa%without detectable interference from as much as 10% titanium. The presence of fluoride or FLUORIDE AND PHOSPHATE, phosphate in the final solution used for fluorometric determination produces serious negative interference, 1 mg of either being sufficient to complex the zirconium and eliminate the fluorescence completely. Fortunately, the fluoride will be eliminated from the original sample by the pyrosulfate fusion. Pyrosulfate fusion should be carried out in platinum ware rather than in glass when fluoride is present to prevent dissolution of zirconium from glassware.
IRON. In the absence of sulfamic acid and hydroxylammonium sulfate, iron produces serious negative interference, 1 mg of iron being sufficient to eliminate the fluorescence completely. This is due to the strong absorption of both the emitted zirconium-3,4’,7-trihydroxyflavonefluorescence and the exciting radiation by the ferric ion. However, the reduction to ferrous sulfamate virtually eliminates this interference and 5 mg of iron can be tolerated without separation.
ACKNOWLEDGMENT
The author acknowledges the assistance of his associates during many helpful discussions. Special thanks are due to E. G. Paul for preparation of the flavone used in the present investigation. RECEIVED for review July 31, 1970. Accepted November 4, 1970.
Liquid-Liquid Extraction of Rhenium(Vl1) with Mesityl Oxide V. M. Shinde and S. M. Khopkar Department of Chemistry, Indian Institute of Technology, Bombay-76, India SOLVENTEXTRACTION STUDIES of transition elements with mesityl oxide showed that rhenium(VI1) can be quantitatively extracted with 75 mesityl oxide-methyl isobutyl ketone from 1M hydrochloric acid containing 1M potassium chloride. Rhenium from the organic phase is stripped with dilute ammonia and is subsequently determined photometrically at 430 mp as its thiocyanate complex ( I ) . Various extracting agents were used for the solvent extraction of rhenium. Among alcohols, butyl alcohol (2), amylalcohol (3),isopentyl alcohols ( 4 ) were used for its extraction and separation from few ions, while in ketones, ethyl methyl ketone ( 5 ) or methyl isobutyl ketone ( 6 ) were used. Pyridine (7) or quinoline (8,9) were used for its extraction from alkaline solutions. The ion association complexes of rhenium with tetraphenyl arsonium chloride (10) or with tributyl ammonium salt (11) were extracted in chloroform or dichloromethane. Tributyl phosphate (12, 13) was used for its extraction from mineral acids. However, most of these methods suffered from several disadvantages. Thus, backstripping of rhenium from the organic phase was cumbersome and needed special reagents (13); it was essential to use high concentration of the extractant to effect quantitative extraction at lower acidity (12); the extractions were quantitative only if larger volumes of extractants were used and multiple extraction was needed ( 3 , 4 ) ; at other times it was necessary to preequilibrate the extractant with alkali hydroxide before metal extraction was carried out ( 7 , 9 ) ; and, finally, it was possible to accomplish good separation if temperature was critically controlled ( 3 , 4 ) . The solvent extraction of rhenium(VI1) with mesityl oxide eliminates all such limitations. It is possible to quantitatively extract rhenium at low acidity with 75 mesityl oxide in only (1) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, N. Y., 1959, p 758. (2) H. Skiba and M. Wojtwosicz, Chem. Anal. (Warsaw), 10, 183
(1965). (3) V. Yatirajam and R. Prosad, Z . Anal. Chem., 220, 343 (1966). (4) Ibid., p 340. ( 5 ) T. M. Cotton and A. A. Woolf, ANAL.CHEM.,36, 248 (1964). (6) V. Yatirajam, 2. Anal. Chem., 219, 128 (1966). (7) D. T. Meshri and B. C . Haldar,J. Sci. Znd. Res., 20B, 551 (1961). (8) U. B. Talwar and B. C. Haldar, Znd. J. Chem., 3, 452 (1965). (9) S. J. Rimshaw and G. F. Malling, ANAL.CHEM., 33, 751 (1961). (10) S . Tribalat, I. Pamm, and M. L. Jungfleisch, Anal. Chim. Acta., 6 , 142 (1952). (11) M. Ziegler and H. Schroeder, Z . Anal. Chem., 212, 395 (1965). (12) V. I. Plotnikov, L. I. Zelenskaya, and L. P. Ustova, Sb. Znst., Tscet. Metall., 112 (1965). (13) N. Idrdanov and S . Mareva, C. R . Acad. Bulg. Sci., 19, 913
(1966).
30 seconds, while rhenium from the organic phase can be stripped just by shaking with dilute ammonia (0.3N “,OH). It is necessary neither to use large volumes of organic phase nor employ multiple extractions. Thus, rhenium is quantitatively removed with only 10 ml of mesityl oxide in a single extraction. It is not necessary to preequilibrate mesityl oxide before actual extraction. Mesityl oxide is less volatile as compared to other extractants like ether; and it can be reused after distillation. The extraction can be carried out without any critical control of temperature. The method suggested by us is thus very rapid, extremely simple, reproducible, and affords clean cut separation of micrograms of rhenium in the presence of several cations as well as anions. EXPERIMENTAL Apparatus and Reagents. Type 3 K H 57 filter photometer was used. The stock solution of rhenium(VI1) was prepared by dissolving 0.1550 gram of potassium perrhenate (Spec. pure of Johnson Matthey and Co., London) in 1 liter of water containing 5 ml of 6N sulfuric acid. The solution was standardized gravimetrically by the Nitron method (14). It contained 100 pg of Re(VI1) per ml. The test solutions [25 pg of the Re(VI1) per ml] were prepared by fourfold dilution. Mesityl oxide (B.D.H., bp 128.7 “C) was used after double distillation. General Procedure. An aliquot of solution containing 25 pg of rhenium(VI1) was mixed with hydrochloric acid and potassium chloride to a volume of 10 ml so that the concentration of acid and salting-out agent was 1M. It was then introduced into a separatory funnel and shaken for 30 seconds with 10 ml of 75z mesityl oxide in methyl isobutyl ketone. The two layers were allowed to settle and separate. The aqueous layer was removed first and then rhenium was removed from the organic layer by back extraction with 10 ml of 0.3N ammonium hydroxide and then with 10 ml of water. Rhenium(VI1) from the aqueous phase was determined colorimetrically as its thiocyanate complex at 432 mp ( I ) . RESULTS AND DISCUSSION Effect of Acidity and Mesityl Oxide Concentration. The concentration of hydrochloric acid was varied from 0.1251.OM and that of mesityl oxide from 19-100% (1.62-8.7OM)
(14) W. W. Scott, “Standard Methods of Chemical Analysis,” Vol. I, D. Van Nostrand, New York, N. Y., 1939. ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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