Spectrophotometric Determination of Molybdenum as the Quercetin Complex in an Alpha-BenzoinoximeChloroform-Ethyl Alcohol Medium GERALD GOLDSTEIN, D. L. MANNING, and OSCAR MENIS
Oak Ridge National laboratory, Oak Ridge, Tenn.
b A spectrophotometric method i s presented for the r a p i d determination of molybdenum in slurries of thorium oxide, solutions of uranyl sulfate, and samples of steel. Molybdenum i s extracted into a solution of a-benzoinoxime in chloroform, after which the complex of molybdenum and quercetin i s formed in a portion of the organic extract b y the addition of quercetin in ethyl alcohol. The absorbance of the yellow complex is measured a t a wave length of 420 m l * . The optimum concentration range was found, by the method of Ringbom, to b e 0.12 to 1.8 y of molybdenum per ml. Of the extraneous ions which were tested, only tungstate and vanadate interfere. The coefficient of variation of the method is 3%.
A
and precise method was needed for the determination of molybdenum in aqueous slurries of thorium oxide that contain uranium and corrosion products, such as iron, nickel, and chromium. Moreover, as molybdenum interferes seriously with the determinatiqn of uranium by the spectrophotometric thiocyanate method or by the volumetric methods that involve the reduction of uranium and subsequent titration with a standard oxidizing reagent, a procedure was also necessary for the separation of molybdenum and uranium. To achieve the required sensitivity and rapidity, colorimetric methods of determination combined with extraction procedures were reviewed. Most of the extraction procedures which are described by Sandell (12) and revientd by Waterbury and Bricker ( I S ) are not sufficiently specific for molybdenum; therefore, further separations are necessary. Acetylacetone (2,4pentanedione) which has been used in a recently dereloped extraction method (Y), is highly selective formolybdenum; however, a time-consuming wet oxidation step is required to destroy the acetylacetone prior to the determination of molybdenum. Probably the most selective reagent for molybdenum is a-benzoinoxime which is commonly RAPID
utilized in gravimetric determinations ( 3 ) . I n strong acid solution, this reagent can be used as a selective precipitant for microgram quantities of molybdenum (9). Preliminary tests revealed that molybdenum can be rapidly and efficiently extracted into a chloroform solution of a-benzoinoxime. To avoid further separation, several chromogenic reagents known to react with molybdenum (12) were tested to determine if a color reaction takes place in the chloroform-a-benzoinoxime medium. An extremely sensitive color reaction was found to take place with quercetin, a reagent which has been utilized in the determination of many elements ('J 5, 6> 'O). This investigation, therefore, was concerned with the development of a method for the extraction of molybdenum with a solution of a-benzoinoxime in chloroform, and the subsequent determination of molybdenum in the organic phase as the quercetin complex.
and evaporate the solution to the heavy fumes of perchloric acid. Allow the solution t o cool and transfer to a 25-ml. volumetric flask which contains approximately 10 ml. of water. Adjust the volume of test solution to the mark with water. URANIUM.Uranyl sulfate, supplied in an aqueous solution was used in these tests. STEEL. The procedure described in the ASTlI manual was applied in dissolving the alloy. Extraction and Color Development.
Transfer a portion of the sample solution which will contain approximately 0.05 t o 5 mg. of molybdenum t o a 125-ml. separatory funnel. Add 50 ml. of a 5% hydrochloric acid solution. Extract the solution three times with 15-ml. portions of the a-benzoinoxime solution, and finally extract with 20 ml. of chloroform. Combine the three 15ml. portions of the organic extract which contain the molybdenum with the 20ml. portion of chloroform. K a s h this organic solution with 25 ml. of a 5% solution of hydrochloric acid, and then combine this acid phase with the original aqueous solution. If no further deterAPPARATUS AND REAGENTS minations are to be made for elements Recording spectrophotometer, Warwhich are retained in the aqueous phase, ren Spectracord. this solution may be discarded. Transfer Spectrophotometer, Beckman Model the organic phase to a 100-ml. vo!uDU. metric flask, and dilute to volume wlth Distilled, deionized water and reachloroform. gent grade chemicals were used in the Transfer a 1-ml. aliquot of the organic preparation of all solutions. phase ( 2 ml. if the original solution conSTAXDARD MOLYBDEXUM SOLUTIOX, tains less than 0.5 mg. of molybdenum) 1.0 mg. per ml. Dissolve 0.92 gram of to a 25-ml. volumetric flask; then add ammonium molybdate in 500 ml. of 10.00 ml. of ethyl alcohol. Add 3.00 water. Less concentrated solutions can ml. of the quercetin reagent; then dibe prepared by diluting this standard to lute the sample t o volume with chlorodesired volumes. form. Wait until any turbidity disap~-BENZOIKOXIME (No. 1877 Eastman, pears; then measure the absorbance of Rochester, K. Y.), 0.1% in chloroform. the solution at a wave length of 420 mp Dissolve 1 gram of a-benzoinoxime in 1 us. a reagent blank in a 1-cm. cell. If liter of chloroform. the original sample contains less than QUERCETIN(S. B. Penick and Co., 0.5 mg. of molybdenum, use 5-cm. cells 50 Church St., New York, 5 . Y.), in measuring the absorbance. 0.1% in ethyl alcohol. Dissolve 1 gram of quercetin in 1 liter of ethyl alcohol.
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EXPERIMENTAL PROCEDURE
Dissolution of Sample.
THORIUM OXIDE. Weigh out a 0.5-gram sample of thorium oxide into a 250-ml. beaker, and add 20 ml. of a 1 t o 1 so!ution of water and nitric acid containing 1 to 2 drops of hydrofluoric acid, then boil gently until the sample dissolves. Add 3 ml. of perchloric acid
Extraction of Molybdenum w i t h Preliminary tests a-Benzoinoxime.
revealed t h a t molybdenum can be extracted with a O.lyc solution of abenzoinoxime in chloroform. The data presented in Table I were obtained by extracting 1 t o 5 mg. of molybdenum from 50 ml. of a 5YC VOL. 30, NO. 4, APRIL 1958
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Table 1. Degree of Extraction of Molybdenum with a-Benzoinoxime in Chloroform Extraction with three 15-ml. portions of 0.1% a-benzoinoxime in chloroform
Molybdenum in Aqueous Phase.a , ME. Before After extraction extraction
No.
Extracted, %
0.011 0.007 0.012 4 0,018 5 0.051 Determined by thiocyanate 1 2 3
98.9 99.6 99 6 99.5 99.0
method.
solution of hydrochloric acid in accordance with the recommended procedure. T h e quantity of molybdenum t h a t remained in t h e aqueous phase was determined by t h e colorimetric thiocyanate method (11) after t h e chloroform that dissolved in the aqueous medium had been removed by boiling the aqueous solution. Table I shows that over 99% of the molybdenum present is extracted by following the recommended procedure. Essentially the same results are achieved when a 5% solution of hydrochloric, perchloric, or sulfuric acid is utilized; however, extractions from phosphoric or hydrofluoric acid media are incomplete and nonreproducible. The concentration of acid is not critical, in that the same results are obtained when the acid concentration is varied from 2 to 20%. K h e n more than 0.5 mg. of molybdenum is present in the initial solution, a turbidity is visible in the organic extracts, which is probably a result of the limited solubility of the molybdenum-0-benzoinoxime complex in the organic solvent; hoiyevcr, this turbidity does not cause any difficulty in the extraction of the molybdenum. A t the time this study was completed the work of Jeffery ( 4 ) ) who used a similar extraction procedure for the separation of microgram quantities of molybdenum, was not known to the authors. He utilized relatively larger concentrations of the reagent for this purpose. I n this study the a-benzoinoxime concentration was kept a t a minimum so as not to interfere in the quercetin complex formation as described below, The substitution of 0.01% solution for the 0.1% concentration, however, was only suitable for solutions relatively free from major components. I n solutions containing a high concentration of thorium and other ions, the extractions with 0.01% a-benzoinoxime led to low and erratic results; consequently, the use of a 0.1% solution of the reagent was necessary to assure quantitative extraction of molybdenum from solutions of this type. 540
ANALYTICAL CHEMISTRY
sorption of the complex. The absorbance of the solutions varied with ethyl alcohol concentration and the maximum absorbance is achieved when the solutions contain approximately 50% ethyl alcohol by volume. A 10.00ml. portion of ethyl alcohol which, along with the alcohol in 3.00 ml. of the quercetin reagent, will produce a suitable concentration of alcohol, can be added to the aliquot taken for absorbance measurement, COXCENTRATION OF QCERCETIN REAGENT. The amount of quercetin which is essential to the production of maximum absorbance n-as established by measuring the absorbance of solutions Figure 1. Absorption spectra of that contained fiyed quantities of molybdenum-quercetin complex molybdenum and ethyl alcohol and Molybdenum, 1.25 X 1 OJM various amounts of a 0.1% solution of A. Quercetin vs. ethyl alcohol quercetin. The relationship between E. Molybdenum-quercetin complex vs. ethyl absorbance and the quercetin concenalcohol C. Molybdenum-quercetin complex vs. quercetration is presented graphically in tin Figure 2. Maximum absorbance in solutions containing 10 y of molybdenum can be attained by adding 3.00 Molybdenum-Quercetin Complex. ml. of quercetin reagent, which is The yellow color of t h e complex of sufficient to obtain a Eeer’s law remolybdenum and quercetin, which exlationship in solution3 that contain hibits a maximum absorbance a t a up to 50 y of molybdenuni. wave length of 420 mp, is developed by COSCEKTRATIOS OF a-BESZOISOXIJIE. adding a n alcoholic solution of quercetin The effect of the concentration of 01to a n aliquot of the extractant of chlorobenzoinoxinie on the development of form and a-benzoinoxime which conthe complex of molybdenum and quertains the molybdenum. cetin was examined. The oxime conI n Figure 1 the partial absorption centration is considered a variable, spectra, obtained by a Warren Specas it is dependent on the number of tracord, are g k e n of the quercetin extractions which are made and the reagent blank us. ethyl alcohol, the volume of the aliquot of the organic quercetin-molybdnum complex us. ethyl extract taken in measuring the abalcohol, and the quercetin-molybdenum sorbance. The absorbance of the conicomplex us. a quercetin reagent blank. plex decreases in proportion to increasTo establish the optimum conditions for ing amounts of this reagcnt. Yariathe color reaction, the effects of such tions in the concentration of a-benzovariables as the concentration of alcohol, inoxime between the limits of 0.002 amount of quercetin, and quantity of oto 0.004y0 of the final-volume have no benzoinoxime were studied. CONCESTRATION OF ETHYL ALCOHOL. effect on the formation and subsequent absorbance of the molybdenuni-querceThe addition of ethyl alcohol to the tin complex. Greater concentrations extract was necessary to effect the reof the a-benzoinoxime decrease the moval of any turbidity, and t o lead to absorbance measurements beyond the a considerable enhancement in the ablimit of error of the method; consequently, a new calibration curve, based * -on the quantity of oxime present, is required. For maximum sensitivity, the oxime concentration in the final volume should be kept to a minimum. Adherence to Beer’s Law. Calibration curves for the determination of molybdenum were prepared after extracting 0.05 t o 5 mg. of molybdenum as described in the procedure. The absorbance was measured in either 1- or 5-cm. cells according to the concentration of molybdenum in the initial test portion. A linear relationship was found to exist between the Figure 2. Effect of excess quercetin absorbance and the molybdenum conon absorbance of molybdenum-quercecentration over the range investigated. tin complex The coefficient of variation of the measVolume, 25.0 ml. urements taken for the calibration line Ethyl alcohol, 13.0 ml. is less than 2%. The molar absorbance Molybdenum, 10.0 y 032
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I
index of this complex is about 36,000. The color is stable for a t least 24 hours. To establish the optimum range where highest precision can be achieved in the determination of molybdenum, the method of Ringbom (11) was applied. B y plotting the per cent transmittance us. the logarithm of the concentration of molybdenum, a n Sshaped curve is obtained. The optimum range of concentration is established from the portion of the curve which is nearly linear, and has the maximum dope. The optimum range, in 1-em. cells, \vas from 0.6 to 1.8 y of molybdenum per milliliter of final volume. The calculated value of the coefficient of rariation derived from the maximum slope, for a variation of 1% in the photometric error. is about 3'3, over this range. For loner concentrations, measured in 5-cni. cells, the optimum range is from 0.12 to 0.36 y of molybdenum per milliliter of fine1 volume. Spectrophotometric Determination of Empirical Formula and Instability Constant. T h e slope-ratio method (2) was applied to the determination of the empirical formula of the molybdenumquercetin complex. To utilize this method, solutions were prepared that contained a n excess of quercetin and varying quantities of molybdrnum. A straight-line curve for thwe solutions could be constructed when the absorbance measurements were plotted us. the concentration of molybdenum. A secbnd series of solutions was prepared that contained a n excess of the molybdenum equal to the excess of quercetin present in the first series of solutions. Various increments of quercetin were added to this solution, after which the absorbance was measured and plotted us. the concentration of quercetin. From these results also a straight-line plot was obtained. The ratio of the slopes of these two straight lines represents the molecular ratio of the components of the complex. The two straight-line curves were parallel when plotted graphically, showing t h a t the ratio of their slopes is 1 to 1 ; consequently, the molecular ratio of quercetin to molybdenum is 1 t o 1. The dpgree of dissociation, CY, was established from the relationship: -a=- E m
- E,
is no evidence of absorption due to the presence of reagent, to avoid the necessity of correcting the absorbance measurements for unreacted reagent. For a solution that was 8.32 X 10-6M in molybdenum, i t \vas found that E , is equal to 0.295 and E, is equal to 0,100. The degree of dissociation was calculated t o be 0.661. From this value of cy, the instability constant was calculated from the relationship : K = - LY2C
that 0.5 gram of either thorium or uranium does not interfere in the determination of molybdenum. I n general, the extraction procedure has the same degree of specificity for the determination of molybdenum as that which is ordinarily attributed to the corresponding gravimetric method ( 3 ) . The aqueous phase was found to be suitable for the determination of the major components, thorium or uranium, by a n y of the methods ordinarily used for these purposes.
1--a
n-here C = concentration of the complex in moles per liter. The value for the instability constant, K , under these particular conditions, was found to be 1.1 x 10-6: Effect of Diverse Ions. Separate tests n-ere made to determine the degree of interference of various cations and anions. Table I1 indicates that the proposed procedure is practically specific for molybdenum. but there are some exceptions. Of the substances tested, tungstate is the most serious interference, n-hile vanadate interferes to a lesser degree. As tungstm and vanadium are not commonly associated with samples which are tested in this laboratory, methods to eliminate these interferences Tvere not developed. Rioderate amounts of other elements, including phosphate and fluoride, do not interferc. It is of special interest
Table II.
RESULTS A N D DISCUSSION
To evaluate the method further on different sample types, the described procedure was applied to the estiniation of molybdenum in a steel sample from the K'ational Bureau of Standards as well as on synthetic slurries of thorium oxide and solutions of uranyl sulfate (Table 111). The molybdenum was determined by dissolving 0.5 gram of the steel sample in aqua regia, after which the solution was evaporated t o fumes with about 3 nil. of perchloric acid. The solution was diluted t o 50 ml. and treated in accordance with the recommended procedure. The results reported for the KBS No. l l l A steel, the major constituent of which is iron, indicate that the method is applicable to the determination of molybdenum in ferrous alloys. As larger quantities
Effect of Various Ions on Determination of Molybdenum
Quantity, M g .
Ion
Present
3.01 3.52 3.20 3.10 3.02 2.96 3.07 3.07 2.99 3.11 3.07
5
Table 111.
Sample Solutions of uranyl sulfate (110 mg. of U )
Thorium oxide slurry (500 mg. of Th) NBS Ni-Mo Steel No. 111 A
-0.06 -0.1 -0.1 $0.08 -0.1 +o. 01 $0.52 $0.20 $0.10 $ 0 . 02 -0.04 $0.07 +O. 07 -0.01 $0.11 $0.07
Determination of Molybdenum in Various Samples
Em
where E,,, = maximum absorbance of a given amount of molybdenum in the presence of a large excess of quercetin (used to ensure the complete complexation of molybdenum), and E , = absorbance of the same amount of molybdenum mixed with a stoichiometric amount of quercetin (1 mole of molybdenum to 1 mole of quercetin). Measurements of E m and E, were made at a wave length of 440 mp, where there
Molybdenum, hIg. Found Differ en ce
Present
Molybdenum, Mg. Found 0.01 1.04 3.07 3.07 5.09
0.00 1.oo
3.00 3. 004 5.00
Error +0.01 +0.04
+0.07 +O. 07 $0.09
3.00"
2.97
-0.03
2.22
2.19
-0.03
Contained 1mg. each of Fe+*,Cr*, NifB,and Ti+'. VOL. 30, NO. 4, APRIL 1958
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of the substances listed in Table I1 are not likely to be encountered in samples of uranium and thorium, no further tests were made; however, it was demonstrated that large concentrations of uranium and thorium could be tolerated. Based on experience in gravimetric procedures ( 3 ) , one can conclude that large quantities of other materials can be tolerated if a n extraction step is used. B y the recommended procedure, molybdenum can be separated from and determined in a variety of materials. The coefficient of variation of the method is less than 3yG. The use of a n organic medium for the color development and subsequent absorbance measurements eliminates the necessity of a rigid control of pH, a condition which is commonly encountered in aqueous systems. hloreover, the pro-
cedure is rapid; one determination can be completed in about 30 minutes. ACKNOWLEDGMENT
The authors acknowledge the assistance of AI. A. Marler and C. D. Susano in the preparation of this manuscript. LITERATURE CITED
(1) Grimaldi, F. S.,White, C. E., Aw.4~. CHEM.25, 1886 (1953). (2) Harvey, A. E., Manning, D. L., J. Am. Chem. SOC.72, 4488 (1950). (3) Hillebrand, W, F., Lundell, G. E. F. Bright, H. A,, Hoffman, J. J., “Applied Inorganic Analysis,” 2nd ed., p. 310, \%ley, Xew York, 1953. (4) Jeffery, P. G., Analyst 81, 104 (1956). (5) Komenda, J., Chem. listy 47, 531 (1953).
(6) Liska, K., Ibid. 49, 1656 (1958). (7) hIcKaveney, J. P., Freiser, H., AXAL. CHEY. 29, 291 (1957). (8) hlenis, O., Manning, D. L., Goldstein, G., Ibid., 29, 1426 (1957). (9) Nichols, N. L., Rodgers, L. H., IND. ESG. CHEM.,ASAL. ED. 16, 137 (1944). (10) Oka, Y., Matsuo, S., J. Chem. SOC. J a p a n , Pure Chem. Sect. 74, 931 (1953). (11) Ringbom, A., 2. anal. Chem 115, 322 (1939). (12) Sandell, E. B., ‘‘Colorimetric Determination of Traces of Metals,” 2nd ed., Chap. XXIX, Interscience, ?;em? York, 1950. (13) Waterbur G. R., Bricker, C. E., ANAL. &EM. 29, 129 (1957). RECEIVEDfor review June 13, 1957. Accepted December 6, 1957. Work carried out under Contract KO. W-7405-eng26 a t Oak Ridge Kational Laboratory, operated by Union Carbide Nuclear Co., division of Union Carbide Corp., for the Atomic Energy Commission.
Further Studies with 2,4,7-Trinitrofluorenone as a Reagent for Microscopic Fusion Analysis DONALD
E.
LASKOWSKI and WALTER C. McCRONE
Department of Chemistry and Chemical Engineering, Armour Research Foundation, and Department of Chemistry, Illinois lnsfitute o f Technology, Chicago, 111.
b This work extends previous information on the use of 2,4,7-trinitrofluorenone as a fusion reagent for the identification of polynuclear aromatics and investigates the applicability of this reagent to the identification of benzene derivatives. Mixed fusion data are presented for 1 4 naphthalene derivatives and 1 1 benzene derivatives. The values of the significant melting points are presented and in most cases they are sufficient for identification of the compound under test. 2,4,7-Trinitrofluorenone may be used as a fusion reagent for the identification of benzene derivatives, although volatility of the starting compounds is an important limiting factor. The data reported provide additional information for the characterization of unknown organic compounds which may b e used as an adjunct to the classical organic characterization schemes.
T
use of 2,4,7-trinitrofluorenone as a reagent for microscopic fusion analysis and a general outline of the technique have been described (4). This paper gives additional data on naphthalene derivatives and extends HE
542
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ANALYTICAL CHEMISTRY
the studies of benzene derivatives t o include melting point data ( 5 ) . Previous studies (4)have indicated that the general technique is applicable to polynuclear aromatics. I n view of the findings (6) that niolecular addition-compound formation bet ween 2,4,7trinitrofluorenone and benzene deriratives is common, it was especially desired to determine limitations when the method is applied to benzene deriratives. EQUIPMENT A N D REAGENTS
ii polarizing microscope with a 16-mm. objective and a 1OX eyepiece was used. Melting points were taken on a Kofler hot stage (Arthur H. Thomas Co., Philadelphia, Pa., hot stage No. 274) fitted with a voltmeter (0 to 50 volts, alternating current) in parallel. A Varitran variable transformer was used to adjust the heating rates. Mixed fusions were prepared on a Kofler hot bar (W. J. Hacker and Co., Inc., Ken- York, N. Y.). The 2,4,7-trinitrofluorenone (obtained from Dajac Laboratories, Division of Monomer Polymer, Inc., Leominster, Nass.) was purified b17 recrystallization from an acetic acid-water solution, followed by trituration with 957, ethyl alcohol and vacuum drying. The melting point found was 175.8-176.8’ C.,
and the literature value was 175.5176.5” C. ( 2 ) . Solid compounds were purified by recrystallization; liquid compounds were purified by distillation, usually under vacuum. Most of these compounds were purchased from the Eastman Kodak Co. EXPERIMENTAL
Procedure. T h e follon-ing method identifies aromatic compounds by microscopic mixed fusion analysis with 2,4,7-trinitrofluorenone. A small amount of t h e reagent is melted between a cover glass and microscope slide. A small amount of t h e unknotvn compound is melted and allowed t o flow into contact with the reagent. Finally, the preparation is melted back, so that a zone of mixing exists between the reagent and unknown. Because there is a concentration gradient on the microscope slide, ranging from pure reagent to pure unknown, there exists on the slide a n area of composition corresponding to each composition on the binary phase diagram of the two components. If the unknown forms a molecular addition compound with the reagent, the addition compound and the two eutectics may be clearly seen on microscopic examination during and after solidification. For identification purposes, four significant melting points are measured
