amount of radioactivity on the sample surface, register the minimum quantity of beta emissions. The same reasoning applied to BP-907 epoxy adhesive, which is thought to be structurally similar to E-705,and to AF-30 phenolic-nitrile adhesive (Figures 2 and 3). A further investigation is planned to obtain evaporative rate curves on overcured samples. It is surmised here that, because chain cleavage is being initiated, the curves will begin to rise because of an increase in attractive sites and, thus, a greater retention of radiochemical. Thus, through use of very little time (approximately 2 min per analysis) and comparatively simple electronics, the rate of cure of polymeric materials may be followed as a function of pressure, temperature, and time. The most attractive features are the economical aspect and the simplicity of the method in-
volved. Although the sophisticated computer analysis through complex chemical instrumentation will remain the number one analytical tool in the evaluation of polymeric structure in relation to cure, a straightforward method such as evaporative rate analysis may initiate a trend toward simplicity in the investigation of long chain molecules. ACKNOWLEDGMENT
The author is grateful to Gary Greene of Ametek Technical Products for his assistance in setting up the experimental procedures. RECEIVED for review July 24, 1968. Accepted September 13, 1968.
Determination of Tantalum and Its Separation from Niobium and Other Closely Associated Elements Using Tetra-N-Pentylammonium Bromide R. G . Dosch Sandia Laboratory, Albuquerque, N.M . NUMEROUS TECHNIQUES and methods have been proposed for separating tantalum and niobium from closely associated elements, as well as from one another; the latter has proved to be the more difficult task. A long list of gravimetric procedures for the separation of tantalum from niobium can be found in the literature. The more popular of these appear to be the tannin procedure as described by Schoeller ( I ) , the N-benzoyl-N-phenylhydroxylamineprocedure (2, 3), and the selenous acid procedure (4). These gravimetric methods have proven unattractive for reasons such as being extremely time consuming and tedious; giving incomplete separation or incomplete recovery of the precipitated element; and having precipitation conditions involving very careful pH control in a relatively narrow range, high complexing ion concentration, and rigid limitations as to amounts and ratios of elements present in the sample to be analyzed. For these reasons, most of the current analytical work involving tantalum-niobium systems is being done using ion exchange techniques to effect the necessary separations. Ion exchange techniques for separating niobium from tantalum, which are valid for any ratio of the two metals, are outlined in a general review of the analytical chemistry of niobium and tantalum by Kallmann ( 5 ) . These techniques have the added advantage of separating niobium and tantalum from closely associated elements in Groups IV and V as well as from each other. The main disadvantage results when these analyses are done on a nonrepetitive basis. In this situation, preparation of an ion exchange system becomes time consuming, while the separation time may only take 4-6 hours. (1) A. R. Powell and W. R. Schoeller, Analyst, 50, 485 (1925). (2) A. K. Majumdar and A. K. Mulherjee, Nufurwissenschuffen, 44, 491 (1957). (3) R. W. Moshier and J. E. Schwarberg, ANAL.CHEM., 29, 947 (1957). (4) F. S. Grimaldi and M. M. Schnepfe, ibid., 30, 2046 (1958). (5) S. Kallmann in “Treatise of Analytical Chemistry,” Part 11,
Vol. 6, Section A, I. M. Kolthoff and P. J. Elving, Eds., Interscience, New York, N. Y.1962, pp 177-406.
Techniques such as cellulose chromatography and solvent extraction, which have been successfully used in separating Nb from Ta, present somewhat the same problem when applied to a nonroutine or nonrepetitive analytical situation. In the work reported here, tetra-n-pentylammonium bromide is proposed as an analytical reagent for the isolation and determination of tantalum by gravimetric techniques. Tantalum is quantitatively precipitated as white, easily filterable, tetra-n-pentylammonium tantalum hexafluoride (TPATF), which is subsequently fired, and the tantalum weighed as Tanor. Interfering substances are limited to anions such as chromate, permanganate, perrhenate, tungstate, vanadate, and molybdate which form nonvolatile tetra-n-pentylammonium compounds (6). Elements which form insoluble fluorides also interfere; however, these can be easily eliminated by filtration prior to precipitation of the tantalum. Elements closely associated with Ta, those in Groups IV, V, and VI, were studied relative to their quantitative effect upon the precipitation of the TPATF. Quantitative separations were obtained in mixtures containing Ta to impurity ratios ranging from 10 :1 to 1 :10 with amounts of tantalum ranging from approximately 10-150 mg. Larger ratios of impurity to tantalum or larger amounts of tantalum have not been attempted at this time. Total time required for the separation and determination of Ta is approximately 2 1 / 2hours. This allows 1 / 2 hour for dissolution of the sample, precipitation of TPATF, and the filtering process, and 2 hours for firing the precipitate to Ta205. EXPERIMENTAL
Reagents. Tetra-n-pentylammonium bromide (TPABr) was used as received from Eastman Organic Chemical Co. without further purification. The metals used in this work (6) R. G . Dosch, ANAL.CHEM., 40,829 (1968). VOL. 41, NO. 1, JANUARY 1969
193
Table I. Effect of Acid Concentration on Precipitation of TPATF
zHF
z"01
(V/V)
2.5 2.5 2.5 2.5 2.5 5 10 15 pH = 2a pH = 5
( 4 8 Z HF,
V/V) 5 2.5 1.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5
Tantalum, mg Taken Found 102.20 100.47 99.58 101.97 106.25 101.83 99.86 104.86 101.00
102,03 100.14 99.54 101.96 106.25 102.0 99.96 102.75 101.12
No precipitate formed Samples were dissolved in 5 ml of HN03-l ml 4 8 z HF and diluted to 200 ml. The pH was then adjusted with NHaOH.
(Ta, Nb, W, Mo, Ti, Zr, Hf) were of AR grade or higher purity and were obtained from a wide variety of sources. All other chemicals were AR grade and were used without further purification. Stock solutions of 0.13M TPABr were prepared by dissolving 10 grams of TPABr in water, filtering through a medium porosity porcelain crucible, and diluting to 200 ml. The solution used in washing the precipitated TPATF consisted of 0.5 gram TPABr, 5 ml of concentrated "Os, and 1 ml of 4 8 z HF, diluted to 200 ml with distilled water. TPATF was prepared by adding TPABr to a solution containing an excess of tantalum. The compound was washed with distilled water until the filtrate gave a negative Brtest and had a pH equal to that of the distilled water. During this procedure, some of the precipitate dissolved. However, washing was necessary to remove excess tantalum and bromide. The remaining material was left in the filter paper and dried in vacuo at 75 "C for 15 hours. The dry compound had a melting point in the range 155-65 "C. No loss of weight was detected upon further drying under the above conditions or at 105 "C in a drying oven. Attempts to
Ta (mg taken) 4.72 50.44 101.27 204.44 401.94 5.12 11.08 30.10 72.35 105.85 10.98 53.57 101.51 5.59 10.69 17.92 50.63 100.76 10.09 57.87 103.80 11.13 54.61 100.61 10.80 103.24
194
... ...
...
100. 5 103.8 70.2 29.9 11.1
... ... ... ...
...
...
110.6 47.1 12.3
... ... ...
... ... ... ... ... *.. .*. ... ... ... ... ... ...
100.6
103.6 136.6 53.3 12.5
...
...
... ... ...
...
,..
...
...
...
...
.,.
ANALYTICAL CHEMISTRY
RESULTS AND DISCUSSION The tetra-n-pentylammonium salt used in this work was the bromide, for reasons described in previous work (6). The precipitate formed when this reagent is added to an aqueous solution containing Ta-F complex ions appears to be [(CSH11)4Nl[TaF6]based on the following: Elemental chemical analysis; and the infrared spectra of the compound
Table 11. Analytical Results W (mg) Hf (mg)
Nb (ms)
...
detect the presence of water using Karl Fischer reagent and infrared spectra were negative. In the elemental analysis of the material, the Ta was determined by weighing as Ta206 after ignition at 800 "C, the N and F were obtained by activation analysis, and the C and H by microcombustion techniques. The following results were obtained, the values in parentheses being the theoretical values based on the formula [(C,Hll)aNl[TaF6]: Ta 30.4 + 0.1 (30.49); C 39.4 + 0.5 (40.47); H 7.11 i: 0.3 (7.47); F 19.7 =t 1 (19.21); and N 2.2 + 0.1 (2.36). Procedure. The tantalum and other metals were dissolved in polyethylene beakers using approximately 5 ml of concd H N 0 3 and 1 ml of 4 8 z H F to effect dissolution. The solutions were transferred to borosilicate beakers containing approximately 100 ml of distilled water. The tantalum was precipitated by addition of TPABr, and the solution volume was adjusted to 200 ml. The precipitate, TPATF, was allowed to stand for a minimum of 5 minutes and was filtered through Whatman No. 42 filter paper using vacuum to decrease filtration time from 15 minutes to 5 minutes. The use of vacuum is not essential as the TPATF is fairly coarse and easily filtered. The precipitate was transferred from the beaker to the filter and washed, when necessary, with the solution described previously. Slight adherence of the TPATF to glass occurs at the solution-air interface, but is easily removed by standard techniques; and the washing procedure may be described as typical for a gravimetric procedure. The filter paper containing the TPATF was placed in a Pt crucible and put in a muffle furnace at room temperature. The temperature was slowly increased to 800 "C (30-40 minutes) and the precipitate was ignited for 1 hour at this temperature, cooled in a desiccator, and weighed as Ta205.
...
...
... ... ... ... ...
.,.
... ... ... ... ... ... ... ... ...
... ... ...
Ta (mg found)
... ... ... ...
... ... *.. ... ... ... ... ... ... ... ... ...
... ... ... . * .
... ... ...
...
... . I .
...
,..
...
103.9 55.8 10.9
...
...
...
102:6 47.3 11.5
...
...
...
* . .
102.7 11.3
4.99 50.38 101.20 204.40 399.83 5.17 11.37 30.11 72.42 105.63 11.09 53.96 101.43 5.67 11.01 17.84 50.48 100.77 10.17 51.78 103.55 11.20 54.22 100. 57 10.91 103.10
over the range 400 to 700 cm-’ compares very well with the spectra of TaF6- species as reported by Fordyce and Baum (7). The difficulty in washing and drying the compound and the deviation from theoretical stoichiometry make the determination of tantalum as [(C5H11)4r\TI[TaF6] impractical. By igniting the compound to form Ta205,excellent analytical results can be obtained. The conditions necessary for quantitative precipitation of tantalum are easily obtained, as can be seen from the data listed in Table I. In solutions containing H F concentrations higher than approximately 1 tetra-n-pentylammonium fluoride will precipitate slowly upon standing. This has no effect on the analysis as the compound completely volatilizes at temperatures considerably lower than 800 OC,but it can be very disconcerting to find an unexplained precipitate forming in a filtrate solution. While a fairly wide range of H F concentrations appears to have little effect on the method, it was found that TPATF was appreciably soluble in HNOITPABr solutions which contained no HF. Sufficient TPABr to provide a 3 : 1 mole ratio of TPABr to Ta is needed for quantitative work. An excess of TPABr greater than threefold does not interfere in the determination. A precipitation time of approximately 15 minutes was used for all data reported in this work. The reaction appears to be complete within 5 minutes, and quantitative results were achieved with precipitation times ranging between 5 minutes
z,
(7) J. S. Fordyce and R. L. Baum, J . Chem. Phys., 44, 1159 (1966).
and 3 hours. However, if solutions or the precipitate in solution were allowed to stand overnight, low results were obtained. For 100 mg of tantalum, these recoveries ranged from 99-99Sz in polyethylene or containers made of Teflon (DuPont) to 96-98z in borosilicate containers. In the general procedure described, borosilicate beakers were more satisfactory because of the adherence of TPATF to teflon or polyethylene which necessitated wiping the inside of the beakers with filter paper to quantitatively remove the precipitate. The reason for low recoveries upon long standing has not been satisfactorily explained at this time. The data in Table I1 illustrate the recoveries one can achieve in the determination of tantalum in solutions containing no other metallic elements. These are comparable to data listed, also in Table 11, which were obtained for Ta analyses in the presence of elements which are generally associated with tantalum, and whose presence has been shown to present difficultiesin current analytical methods for tantalum. Emission spectrographic analyses were done on the Ta205 resulting from precipitation as TPATF and subsequent ignition of 50 mg of Ta from solutions containing Ta:Nb ratios from 2:l to 1 :2. Results show Nbz05 contamination of less than 0.1 to 0.25 by weight over the above range.
z
z
RECEIVED for review March 4, 1968. Accepted September 12, 1968. Work supported by the United States Atomic Energy Commission. The contents of this paper are taken in part from a doctoral thesis to be submitted at the University of New Mexico.
Direct Ultraviolet Spectrophotometric Determination of Benzene in Aqueous Solutions Dennis G . Marketos’ Nuclear Research Center “Democritos,” Radiation Chemistry Laboratory, Aghia Paraskevi-Attikis, Athens, Greece
IN OUR studies on the determination of relative rate constants of reactions of the OH radicals with various substances in irradiated aqueous solutions by competition techniques, benzene was used as a reference solute. The absolute reacis known ( I , 2) with high accuracy. tion rate constant koH+CaHs Thus, according to the principle of the method, it is necessary that the concentration of benzene in each particular experiment be known with high accuracy. It is reported (2) that a difference of about 25% was observed between an earlier (I) and a more recent ( 2 ) value of koH+csHbdetermined by the same workers in pulse radiolysis studies of aqueous benzene solutions, the latter being considered the more accurate because of greater certainty in the benzene concentration. The adjustment of the desired concentrations by appropriate dilutions of saturated aqueous solutions of benzene results in (a) errors due to loss of benzene because of its high volatility, 1
On leave from the Greek State Chemical Laboratories.
(1) L. M. Dorfman, I. A. Taub, and R. E. Buhler, J . Chem. Phys.; 36,3051 (1962). (2) L. M. Dorfman, I. A. Taub, and D. A. Harter, ibid., 41,2954 (1964).
and (b) uncertainties due to the rather poor agreement between the values of its solubility in water, reported in the literature (3-10). The difference between the extreme values amounts to approximately 50 of the lower value. Furthermore, values cited in Handbooks ( I I , 1 2 ) are too low. The present investigation provides a highly accurate technique for the spectrophotometric determination of benzene in dilute aqueous solutions. (3) L. J . Andrews and R. M. Keefer, J . Amer. Chem. SOC.,71, 3644 (1949). (4) A. P. Brady and H. Huff, J . Phys. Chem., 62,644 (1958). (5) D. S. Arnold, C. A. Plank, E. E. Erickson, and F. P. Pike, Ind. Eng. Chem., Chem. Eng. Data Ser., 3,253 (1958). (6) D. M. Alexander, J. Phys. Chem., 63, 1021 (1959). (7) F. Franks, M. Gent, and H. H. Johnson, J . Chern. SOC.,1963, 2716. (8) C . McAuliffe, J . Phys. Chem., 70, 1267 (1966). (9) A. A. Taha, R. D. Grigsby, J. R. Johnson, S.D. Christian, and H. E. Affsprung, J . Chem. Educ., 43, 432 (1966). (10) J. D. Worley, Can. J . Chem., 45, 2465 (1967). (11) “Handbook of Chemistry,” N. A. Lange, Ed., 10th ed., McGraw-Hill, New York, 1961, p 421. (12) “Handbook of Chemistry and Physics,” R. C. Weast, Ed. in
Chief, 45th ed., Chemical Rubber Publishing Co., Cleveland, Ohio, 1964-65, p C-146. VOL. 41, NO. 1, JANUARY 1969
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