[Benzil] Table 11. Kinetic Data for Rearrangement of cis Stilbenediol in 48x Ethanol Solvent, Ionic Strength of 0.13
PH
k , (sec-l) X loa =k std dev
7.5
2.44 f 0 . 3
k, (sec-l) X IO* -2.w
0 Obtained from time dependence of i p fqr cis stilbenediol, 25OC. b Obtained from time dependence of ip for benzoin, 25' C.
The rate constant k , for the rearrangement of the cis isomer can be evaluated by obtaining the time dependence of the voltammetric peak current i, for either the cis isomer or for the benzoin (which is reduced ca. 0.6 V cathodic of the benzil reduction). See Table 11. The relative amounts of benzil, benzoin, and cis stilbenediol under these experimental conditions can be determined by using the respective rate constants and Equation 4, or by measuring the peak current i, for the cis species and relating it to the bulk concentration of the cis isomer. In either case, it is assumed that C* = [Benzil] [Benzoin] [cis]. In the latter case, the proportionality constant relating i, to concentration is assumed to be the same for benzil and the cis stilbenediol. Using either approach, at the end of a 10minute electrolysis, the composition of the sample solution (48 ethanol, pH = 7.5)was found to be
+
+
[Benzoin] [cis]
15% 50%
3575
Spectrophotometric results for the same sample-intermediate mixture after the electrolysis are shown in Figure 3. These results show only a slight increase in absorbance at ca. 255 mp over the period of time (based on electrochemical monitoring of both cis and benzoin) required for the rearrangement to be completed. Because at short times (when the concentration of cis is still appreciable) the absorbance at 255 mp is too great to be attributed to benzil and benzoin alone, it appears that the cis intermediate also has an absorption maximum at that wavelengthLe., A,
(cis) 'v 255 mp
+ 5.
In addition, because the maximum absorbance value increases only slightly over a period of time sufficient to allow a substantial conversion of cis to benzoin, it must be concluded that, at a pH of 7.5, ezsa(cis)
'v
m(Benzoin) 'u 1.2
x l o 4 f 5%
This observation substantiates the assignment of the cis isomeric structure to the less easily oxidized intermediate species, because at the same pH eels is smaller than RECEIVED for review December 4, 1967. Accepted January 18, 1968. Work supported by Public Health Service, Grant No. CA-07773 from the National Cancer Institute.
Solvent Extraction-X-Ray Spectrometric Measurement of Microgram Quantities of Tantalum E. A. Hakkila, R. G. Hurley, and G. R. Waterbury Los Alamos Scientific Laboratory, University of California, LQS Alamos, N . M . 87544 A RAPID ANALYTICAL METHOD was required for determining tantalum when present in concentrations between 0.5 and 10 ppm in pure silver. Several spectrophotometric methods (1-6) for measuring tantalum exist, but these methods suffer from nonspecificity or poor sensitivity. Trace element analysis using x-ray spectrographic techniques has become competitive in recent years with spectrophotometric and emission spectrometric methods. Luke (7) used a curved crystal focussing spectrometer for measuring submicrogram quantities of various elements that were concentrated on ion exchange resin discs '/*-inch in diameter. Limits of detection as low as 0.01 pg were claimed for elements havipg Ka: x-rays in the wavelength region between 1.4 and 1.9 A (zinc and iron). Campbell, Spano, and Green (8) used a flat ( 1 ) C. L. Luke, ANAL.CHEM., 31, 904 (1959). (2) Y . Kakita and H. Goto, fbid.,34,618 (1962). (3) J. 0. Hibbits, H. Oberthin, R. Liu, and S. Kallmann, Talunta, 8, 209 (1961). (4) J. C. Guyon, Anal. Chim. Acta, 30, 395 (1964). (5) S. V. Elinson and A. T. Rezova, Zh. Anal. Khim., 19, 078
(1964). (6) G . R. Waterbury and C. E. Bricker, ANAL. CHEM.,29, 474 (1957). (7) C. L. Luke, fbid.,36,318 (1964). (8) W. J. Campbell, E. F. Spano, and T. E. Green, Ibid. 38, 987 (1966).
818
ANALYTICAL CHEMISTRY
crystal spectrometer to determine trace elements adsorbed on an ion exchange filter paper. A limit of detectioa of 0.55 pg was claimed for lead using the La1 x-ray at 1.175A. The separation of tantalum from silver using ion exchange papers or discs is not readily accomplished. Extraction of microgram quantities of tantalum from various elements has been reported, however, from either hydrochloric, nitric, or sulfuric acid solutions containing hydrofluoric acid into 4-methyl-2-pentanone (hexone) (6, 9, IO). An investigation of the extraction of tantalum from either a nitric-hydrofluoric or a hydrofluoric-hydrochloric acid aqueous phase into hexone led to the development of a method for the separation and x-ray fluorescence measurement of microgram quantities of tantalum in silver. EXPERIMENTAL
Apparatus and Reagents. A Philips Electronics, inverted, three-position spectrograph, a molybdenum-target x-ray tube, and a scintillation detector were used. Samples were evaporated on optical cover glasses that were 18 mm in
(9) P. C. Stevenson and H. G . Hicks, fbid.,25, 1517 (1953). (IO) G. R. Waterbury, L. E. Thorn, and R. E. Kelly, U. S . At. Energy Comrn. Rept. LA-3465, 1966.
diameter and 0.2-mm thick. Standard laboratory equipment and reagents were used throughout. Silver nitrate of reagent grade purity was used as a source of silver without additional purification. Tantalum was not detected in the silver nitrate using the recommended x-ray fluorescence method. The tantalum standard solution was prepared by dissolving 100 mg of tantalum metal in hydrofluoric acid and diluting an aliquot with hydrofluoric acid and water to obtain a solution 3M in hydrofluoric acid that contained 1 pg/ml of tantalum. The tantalum metal of detected metallic impurities, primarily contained 0.3 niobium and tungsten. The cobalt internal standard, prepared from metal of 99.9x purity, contained 1 pg/ml of cobalt in 3M nitric acid. The hexone was redistilled prior to use. Solvent Extraction Studies. Tantalum is quantitatively extracted from 3.6iM nitric-0.4M hydrofluoric acid into hexone (IO), but significant quantities of silver coextract. The silver is removed by washing with 6 M hydrochloric0.4M hydrofluoric acid. Tantalum also is quantitatively extracted from a 6 M hydrochloric-0.4M hydrofluoric acid solution of the silver. Although a large quantity of solid silver chloride is present, this extraction involves fewer operations and was used in subsequent work. Sample Preconcentration. Concentration of the sample onto the area most intensely excited by the x-ray beam is necessary to obtain good sensitivity. Although the area excited by the FA-60 x-ray tube is 2 by 2.5 cm, the most intense portion of the beam is concentrated over an area approximately 0.5 and 1.8 cm. For maximum sensitivity, the diameter of the area containing the sample should not exceed 1 cm. Optical cover glasses were placed on brass washers having an i.d. of 0.9 cm. and maintained at 325' C, to restrict the sample area as the sulfuric acid solution of the sample was transferred dropwise to the center of the disc. Selection of Internal Standard. The x-ray measurement of trace amounts of tantalum evaporated on glass discs is best performed by using an cdded internal standard. Cobalt, with a Kcr x-ray at 1.790 A was selected as the internal standard. Intensities were approximately 6 counts per second per pg from cobalt evaporated onto glass discs, compared to 3 counts per second per pg from the tantalum Lal x-ray. Background intensities were approximately 10 and 20 counts per second, respectively, for cobalt and tantalum. For tantalum quantities between 0 and 10 pg, 1 pg of cobalt was adequate to yield suitable intensity ratios. Recommended Procedure. Weighed 1-gram samples of silver were dissolved in 5 ml of 70z nitric acid and 5 drops of 4 8 z hydrofluoric acid in Teflon beakers. The solutions were evaporated to dryness, then transferred to 25- x 150mm test tubes using 3 ml of water and 1 mi of 4 M hydrofluoric acid. Five ml of 12M hydrochloric acid were added and each solution was diluted to 10 ml with distilled water. Ten ml of hexone were added to each tube, and mixtures were vigorously agitated with electric stirrers for 5 min. The solutions were allowed to separate at least 5 min, and an 8.00-ml aliquot of the organic phase was pipetted into a 50-ml Teflon beaker. Five drops of 18M sulfuric acid, 1 ml of cobalt solution (1 pg of cobalt), and 5 to 10 ml of water were added, and each solution was evaporated on a steam bath to remove the hexone. The aqueous phase was then fumed to approximately 0.5 ml. A few drops of nitric and perchloric acids were added if charring of the samples occurred. Standards were prepared by transferring 0, 5 , and 10 pg of tantalum, 1 pg of cobalt, and 5 drops of sulfuric acid into Teflon beakers and performing the evaporating and fuming operations as described for samples. The nitric-perchloric-sulfuric solutions were evaporated dropwise onto the glass discs heated on brass washers. A total of 6400 counts were accumulated at the tantalum Lal and cobalt Kcr x-rays while the sample was rotated under the x-ray beam. The background intensity, measured for a
Table I. Precision of Hexone Extraction-X-Ray Fluorescence Spectrometric Method for the Determination of Ta in Ag
Ta added, pg 0 1.0 3.0 5.0 8.0 10.0
z
pg
Standard deviation Ta Rel.
...
0.26 0.25 0.5 1.1 0.8 1.2
25 17 23 10 13
Table 11. Accuracy of Solvent Extraction-X-Ray Measurement of Ta
Ta added, 0 1 2 5 10
No Ag 0.02, -0.02, -0.02 0.8, 1 . 0 , 0 . 9 , 0 . 9 1 . 9 , 2.0, 1 . 6 5.0,5.1,4.6,6.0 10.3,9.2, 10.1, 10.6
1 gram Ag -0.02, 0.06 1.1,0.5,0.8 1.6, 1.6, 1.7 5.0, 5.2, 5 . 6 9.3, 9.2, 9 . 7
blank cover glass, was subtracted and the amount of tantalum in the samples was calculated by comparing the tantalum to cobalt intensity ratio from samples to the ratio measured from the standards. RESULTS AND DISCUSSION Precision. The precision of the method was determined by repeated measurements of known quantities of tantalum extracted from 1-gram portions of tantalum-free silver nitrate processed as described in the recommended procedure. Fourteen samples were analyzed at each concentration, The data (Table I) show that the relative standard deviation of the separation and measurement of 1 pg to 10 pg of tantalum is 2 5 z or less. The standard deviation of the concentration-x-ray measurement (without the extraction separation) also was determined; generally this deviation was about half of the value obtained when the extraction step was included. The standard deviation for measuring a blank was 0.13 pg, indicating that the lower limit of reliable measurement was approximately 0.4 pg of tantalum (3u). Accuracy. The accuracy of the solvent extraction-x-ray method was determined by comparing the tantalum found in a series of standards without silver to the tantalum found in equivalent standards that contained 1 gram of silver. The results (Table 11) show that silver does not cause a bias. Interferences. Interference in x-ray fluorescence trace analysis may be of two types: absorption or enhancement by elements which may be coextracted from the sample in large amounts, or x-ray line overlap by extracted trace constituents. Iron, gallium, antimony, arsenic, molybdenum, selenium, and tellurium, in addition to tantalum and niobium, are among the elements extracted from 6 M hydrochloric-0.4M hydrofluoric acid (9). Of the elements which may interfere by x-ray line overlap with either the Lal line for tantalum or the Kcr line for cobalt, only those elements having x-rays with a relative intensity of 10 or greater within a 20 angle of 1 degree (two line half widths) of the lines measured were considered in this study. Twenty-three elements were tested by extracting 1 or 10 pg of tantalum from 1 gram of silver containing up t o 100 mg of the added element. The results showed that greater VOL. 40, NO. 4, APRIL 1968
819
than 1 mg of coextracted elements such as niobium result in smearing of the sample on the glass disc, and erratic count rates. As much as 100 mg of elements such as cerium, neodymium, lithium, sodium, potassium, calcium, strontium, magnesium, manganese, or nickel, which are not extracted and do not have overlapping x-rays, do not interfere. The effect of larger amounts was not studied. Interference was not observed in the presence of 50 mg of zirconium; 30 mg of titanium; 5 mg of rhodium or platinum; 2 mg of tungsten; 1 mg of uranium, vanadium, or chromium; 250 pg of molybdenum; 200 pg of copper, 50 pg of iron, or 10 pg of cobalt. High recoveries observed when extractions were performed from greater than 50 mg of zirconium, 30 mg of titanium, 2 mg of tungsten, or 1 mg of niobium probably were caused by residual tantalum in the metals. Although the method applies specifically to the determination of microgram amounts of tantalum in silver, it can be
readily applied to other materials using the hydrochlorichydrofluoric acid system, or adapted to other types of samples by using nitric-hydrofluoric acid or sulfuric-hydrofluoric acid solvents. One analyst can extract and measure approximately 30 samples per day. ACKNOWLEDGMENTS
The authors gratefully acknowledge the helpful suggestions and assistance of Dr. C. F. Metz, under whose supervision this work was performed, and the assistance of Miss H. L. Barker and W. G . Baughman for performing some of the X-ray measurements and extractions. The Los Alamos Scientific Laboratory is operated under the auspices of the Atomic Energy Commission. RECEIVED for review December 11, 1967. Accepted February 2, 1968.
Determination of Carbon in Sodium by Photon Activation Analysis G . J. Lutz and D. A. De Soete National Bureau of Standards, Washington, D. C. 20234 LIQUIDSODIUM has a number of thermodynamic and nuclear properties which make it very useful as a heat transfer medium for nuclear power plants. These properties include thermal stability, high boiling temperature, a high heat transfer coefficient and a fairly low thermal neutron cross section ( I ) . The main disadvantage of sodium is its ability to carburize or decarburize metals with which it comes in contact, causing subsequent deleterious effects (2). These reactions are generally interpreted in terms of carbon going to a state of lower thermodynamic activity. Thus, it is necessary to have an accurate knowledge of the carbon content in the sodium. Stoffer and Phillips (3) have described a procedure for the determination of carbon in sodium-potassium alloy. This technique involves heating the sample in a combustion tube to a temperature of 950" C and absorbing the liberated COZin an ascarite-drierite absorption bulb. Modifications using manometric (4) or gas chromatographic (5) measurements have been described. Pepkowitz and Porter (6) have published a method for the determination of carbon in which the sodium sample is dissolved in water, the solution neutralized, evaporated to dryness, and the carbon oxidized with the Van Slyke reagent. Kallman and Liu (7) have described a method whereby the total carbon content of sodium metal is determined by lowtemperature ignition of the sample in an oxygen-argon mixture followed by treatment with dilute sulfuric acid and con-
ductimetric determination of the liberated COz. The carbonate content is measured by steam decomposition of the sodium sample and subsequent acid liberation of the COZ. In sodium combustion analysis, apparatus and reagent blanks may be quite significant, thus limiting the sensitivity of the method. In addition, the possibility of surface contamination between the time of sampling and of measurement exists. In activation analysis, however, the possibly contaminated surface can be removed after irradiation, and an analysis, representative of only the bulk of the sample, is obtained. This technique is free from a blank and the sensitivity is only limited by the radioactivity induced in the sample and the detector background. NUCLEAR CONSIDERATIONS
(1) Marshall Sitting, "Sodium: Its Manufacture, Properties and Uses," Reinhold Publishing Company, New York, 1956. ( 2 ) R. W. Lockhart and G. Billuris, IMD Special Report Series NO. 12, Nuclear Metallurgy AIME IX, 1963. (3) K. G. Stoffer and J. H. Phillips, ANAL.CHEM., 27,773 (1955). (4) J. Herrington, U. K. Atomic Energy Authority, AWRE Report
Natural carbon has two isotopes, 12C and I C , with isotopic abundances of 98.89z and 1.11 respectively. There are several nuclear reactions which can be used in an activation analysis. The reaction 13C(n,y)14C can be rejected because of the low abundance and low capture cross section of l3C and the very long half-life of 14C. The fast neutron reactions, 12C(n, p)12B, lC(n,p)l3B and lC(n,cr)lOBe, have half lives either too short or too long for activation analysis. The reaction lC(n,2n)11C is difficult to induce because its threshold is 18.7 MeV. Albert and coworkers (8) have used the reaction '2C(d,n) 1aN for the determination of carbon in iron. This would not appear suitable for the present problem, as charged particles do not penetrate deeply into the sample, and it is intended to remove the sample surface after irradiation but before separation and counting. The nuclear reaction chosen for this work was '2C(y,n)l1C.
( 5 ) T. G. Mungall, J. H. Mitchen, and D. G. Johnson, ANAL.CHEM., 36, 70 (1964) (6) L. P. Pepkowitz and J. T. Porter, Zbid., 28, 1606 (1956). (7) S . Kallmann and R. L u . Zbid., 36, 590 (1964).
(8) Ph. Albert, G. Chaudron, and P. Sue, Bull. SOC.Chim. France, 1953, 97.
z
0-62162
820
ANALYTICAL CHEMISTRY
