magnesium and phosphate. Several hundred fold amounts of other ions may be tolerated, provided : m o u n t is taken of the electrolyte effect thus introduced when the calibration clirves are drawn up. The reagent is sta.sle and the color formation is rapid and is also stable for 24 hours! when protected against atmospheric carbon dioxide. The calcium complex of the reagent is apparently not a very strong one, because (a) the electrolyte effect is appreciable, (b) it can readily be broken down by EDTA, and (e) a la:*geexcess of reagent is necessary to ensure maximum color formation. I n the recommended procedure when 10 ml. of 10-3LTfreagent is used ejlOmlr=4500, with 15 ml. of reagent t5lOmU = 5200, and with 20 ml. E ~ =~ 5900. ~ , Against ~ the increased sensitivity, however, m m t be set the high resultant, background absorption of such a large m c e s of reagmt. The range of the method may be extended considerahly beyond 70 pg. by such methods and caii be taken up to 130 pg., but the recommended procedur: is felt to be a reasonable compromise between the two considerations involved. I n applying the pimocedure to the determination of calcium in various substances, the electroj yte level should be kept to a minimum. Thus, for example, calcium in an organic matrix would beat be brought into solution by
a n oxygen flask combustion, as has been demonstrated previously ( I ) , rather than by a wet digestion process with subsequent neutralization, etc. The reagent is applicable in the presence of large amounts of masking agents such as cyanide and fluoride, and consequently should be applicable to much larger amounts of interfering ions than were examined in this study, though large amounts of metals such as iron (111),which are difficult to maintain in solution a t p H 12 even with masking agents, would probably have to be separated. When calcium is being determined in the presence of Sr+2 and Ba+z, difficulty may arise in obtaining a calcium-free salt for incorporation in the blank and calibration solutions. Under these circumstances, and indeed generally, the method of standard addition of pure calcium to the test solution should prove a successful and convenient alternative, provided that the total calcium still lies within the Beer’s law range. Both murexide (emsmr = 10,000) and glyoxal bis-(2-hydroxyanil) (e5ajrn,, = 15,000 to 16,000) are more sensitive than Calcichrome (e61jm, = 7600), but both these reagents and their calcium complexes are very unstable. Calcichrome also has a very much higher toleration for strontium and, probably, most other metals.
ACKNOWLEDGMENT
The sample of Calcichrome used in these studies was synthesized by R. A. Close, formerly of this department. We thank members of the staff of H.M. Government Chemists’ Laboratory, London, for checking and passing constructive criticism on the proposed method, and J. Bernal, University of Zaragoza, Spain, and R. Belcher, University of Birmingham, for their interest. LITERATURE CITED
(1) Belcher, R., Macdonald, A. M. G., West. T. S.. Talanta 1. 408 11958). (2) Close, R.‘ A,, West,‘T. S., Ibid., 5 ,
221 (1960).
(3) Diggins, F. IT7.,Analyst 80,401 (1955).
(4) Goldstein, D., Stark-Mayer, C., Anal. Chim. Acta 19, 437 (1958). ( 5 ) Gorsuch. T. T . Posner. A M..Nature 176, 268 (1955). ’ (6) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 3rd ed., p. 83, Interscience, Kew York, 1959. ( 7 ) Ibid., pp. 368-80. (8) West, T. S., Anal. Chim. Acta 2 5 , 301 (1961). (9) Williams, K. T., Wilson, J. R., ANAL. CHEW33, 244 (1961). 1-1
RECEIVEDfor review May 20, 1963. Accepted September 13, 1963. Work done with financial support from the Spanish Comisaria de Protecci6n Escolar y Asistencia Social and the British Welding Research Association, which provided a scholarship for one of us (M.H.L.).
Simultaneous Determination of Niobium and Tantalum by Neutron Activation Using Niobium-94m and Tantalum-‘I 8 2 m and Rapid Radiochemical Separations C H O N G K. KIM’ and ‘W. WAYNE
MEINKE2
Department of Chemist,ry, University of Michigan, Ann Arbor,
b The niobium and tantalum content of rocks, graphite, and stainless steel samples has been determined by thermal neutron activation analysis with a 10-minute irra’diation of samples in a neutron flux of 10’2 n second-’. Niobium and tantalum were separated simultaneou:jly, followed b y an extraction of niobium-free tantalum with 1 -pentanol In-amyl alcohol), The entire radiochemical separation was completed in less than 10 minutes. The radioactivity and half life of the chemically separated niobium-94m and tantalum-1 82m were determined by gamma spectrometry. In practice, microgram amounts c f niobium and tantalum can be determined simultaneously in 30 minuter. b y this method.
T
Mich.
HE ELEMENTS tantalum and niobium occur together in nature and have close chemical properties. Thus i t is a difficult problem to separate tantalum from niobium quantitatively, and to determine the exact amount of tantalum and niobium in a sample. Among the many methods currently used for the analysis of these elements, the colorimetric procedure appears to be simple, rapid, and fairly sensitive (3, 16). However, this method requires effective means of separating niobium and tantalum from interfering iron, titanium, molybdenum, tungsten, and other elements. The clean separation of niobium from tantalum is also difficult and lengthy. Moreover, corrections for reagent blanks become appreciable at low concentrations.
Determination by activation analysis of tantalum in the G-1 and W-1 “standard” rock samples of the U. S. Geological Survey was carried out by Morris and Olya in 1960 ( 1 4 ) , and Atkins and Smales in 1960 ( 1 ) using the long-lived tantalum-182. No work, however, has been reported to date on the niobium in this sample or of the use of short-lived tantalum-182m in neutron activation analysis. The simultaneous determination of niobium and tantalum by thermal
On leave from Atomic Energy Research Institute, Seoul, Korea. Present address, Department of Nuclear Engineering, University of Maryland. * Present address, Analytical Chemistry Division, National Bureau of Standards, Washington, D. C. VOL. 35,
NO. 13,
DECEMBER 1963
2135
400 channel pulse height analyzer. This equipment has been described in
neutron activation of short-lived niobium-94m and tantalum-182m should give not only a rapid analysis but also a more sensitive detection of these two elements than the colorimetric technique.
detail elsewhere (11, l a ) . Burrell mechanical shaker. Filter paper-glass fiber, Type A, Gelman Instrument Go., Chelsea, Mich. Reagents. Tantalum foil, 0.00025 inch thick, Ethicon Suture Laboratories, New Brunswick, N. J. Niobium oxide powder, high purity, Fansteel Metallurgical Corp., North Chicago, Ill. n-Amyl alcohol, analytical reagent grade, Baker and Adamson. Potassium bifluoride powder, Merck and Co. Sodium peroxide, granular, reagent grade, Baker and Adamson. -411 other chemicals were of analytical reagent grade. The niobium-95 radioactive tracer was purchased from Oak Ridge National Laboratory. Standard solutions of niobium and tantalum were prepared by dissolving
EXPERIMENTAL
Apparatus. Samples were irradiated in polyethylene snap-type “rabbits” in the pneumatic tube system of the Ford Nuclear Reactor of t h e University of Michigan. This system permits irradiations a t thermal neutron fluxes of about 1OI2 n 0111.12 second-1 (when the reactor is operating at full power of 1 Mw.) and delivery to a hood in the neighboring Michigan Memorial Phoenix Laboratory within 3 seconds after irradiation. Samples were then processed chemically and were measured by a 3-inch x 3-inch SaI(T1) crystal coupled 11-ith a R I D L
Table 1.
Activation Analysis of G - 1 Rock Sample for Niobium and Tantalum“
Sample G-1 U. S. Geol. Survey 1
2 3 4 5 6
m7t. of sample, gram 0 2286 0.3238 0 3549 0 5259 0 3121 0 5189
Yield of radiochemical separation, % Nb Ta 21 4 s3 40 7 61 34 62 50 67 15 54 30 60
Level of Nb and Ta in sample, p.p.m. Sb Ta 31 undetectableb 32 undetectableb 30 undetectableb 35 undetectable* 30 undetectableb 36 undetectableb
Av.
c
=
32 f 3“
Value reported in literature is 5 to 8p.p.m. niobium and 1.33 to 5 p.p.m. tantalum (6). Below 10 p.p.m. Standard deviation.
Table
11.
Activation Analysis of NBS-446 Stainless Steel and Graphite Sample for Niobium and Tantalum
Sample
Spectrographic Stainless Steel Std., KBS446 1 2 3
Yield of radiochemical separation, % ’ Yb Ta 16 40 13 40 4.4 17
jTt. of sample, gram 0.0670 0.0761 0.05925
Level of Nb and Ta in sample, % Nb Ta 0.52 0.015 0.60 0.018 0.61 0.012 Av. 0.58 f 0.015 0.05 0.003a
Value reported by others, 70 Nb Ta 0.6 -0.03
Ta 0.010 0.008 0.012 __ 0.01 f 0.0024
Ta 0.01
~
Graphite
Std1002 Synthetic Sample, Spex Ind-Co. 1
2 3
Ta
0.0647 0.1121 0.0948
2s
32 25 Av.
Standard deviation. ~
2136
~~
ANALYTICAL CHEMISTRY
=
a known weight of niobium oxide powder and tantalum foil in 48% reagent grade hydrofluoric acid and diluting with 1% by weight oxalic acid solution to give comparative standard solutions of 70 pg. of niobium per ml. and 98.2 pg. of tantalum per ml. in a final concentration of 1M hydrofluoric acid. Sample Irradiations. U. S. Geological Survey rock sample, G-1 (powdered); National Bureau of Standards, sample No. 446; Spectrographic stainless steel; and Spex Industries Corp., Scotchplains, X‘. J., spectroscopic graphite powder S o . 1002 in which 43 different elements are contained, were used. The powder samples u-ere n-eighed and transferred into polyethylene capsules. These capsules were then positioned inside the rabbit and irradiated along n i t h standard gold foil flux monitors. Procedure. Place a known amount of niobium-95 tracer, 10 mg. of niobium carrier, and a n e-iact amount of tantalum carrier (4.0 mg. in this experiment) in a platinum crucible a n d evaporate to dryness. Drop the irradiated sample into t h e crucible and fuse i t with 2 t o 3 grams of potassium bifluoride for two minutes. Cool the outside of the crucible by dipping into water Lvhile manipulating to solidify the melt in a thin layer. Dissolve the melt by adding 3 to 4 ml. of concentrated sulfuric acid and 1 ml. of 48% hydrofluoric acid, and dilute with water to make a total volume of about 20 ml. Transfer to a 50-ml. Boston round bottle containing 10 ml. of n-amyl alcohol and shake in a mechanical shaker for 1 minute. Transfer the solution to a 100-ml. separatory funnel, draw off the aqueous layer ( A ) and set aside. Wash the remaining organic layer twice with 10-ml. portions of a solution 1 . 2 s in hydrofluoric acid and 2X in sulfuric acid and discard the washing. Transfer the organic portion to a 125-ml. wide mouth Erlenmeyer flask and count Talg2”activity in the scintillation spectrometer. Pour the aqueouj solution ( A ) into a 400-ml. beaker containing a boiling solution of 20-ml. of concentrated nitric acid and maintain boiling for two minutes. .Idd 2 grams of boric acid powder to the boiling mixture. Collect the precipitate on glaii fiber filter paper in filter chimney and mount on a counting card for activity measurement. Determine chemical yield of niobium by area under the 0.76-1n.e.v. KbQ6 photopeak. Determine tantalum recovery by reirradiation of the organic tantalum extracts and coniparison of resulting tantalum activity n ith that from a 1-ml. sample of tantalum standard. For the stainless qteel and graphite samples, use a sodium peroxide fusion (9) in a zirconium crucible. Dissolve the melt in 15 ml. of water and acidify with 30 ml. of concentrated nitric acid. Heat the solution to boiling and add 0.5 gram of potassium perchlorate powder in small portions, boiling for a few seconds after each addition. Cen-
-5OC
cn W
k I)
z 5 400 m \
E
30C
3
0 0
v
I-> -
20c
I-
100
f
C
20
40
60
IO0
80
ENERGY ( K e v ) Figure 1.
Gamma-ray spectrum of 6.6-minute Nbg4mseparated from NBS-446 sample Spectra taken a t 5.5-minute intervals
trifuge the niobium and tantalum pentoxide formed, decant the supernate, and dissolve the precipi;ates in a solution of 1 ml. of 48% hydrofluoric acid and 3 ml. of concentrated sulfuric acid diluted to 20 ml. with water. Extract tantalum with n-amyl alcohol. Precipitate the niobium re naining in the aqueous layer by adding concentrated ammonium hydroxide d-op by drop t o avoid the precipitation of any remaining tungsten ( 7 ) . Collect the precipitates on glass fiber filter paper in filter chimney by rapid auction and count as above RESULTS AND DISCUSSION
Results obtained from the activation analysis of the G-1 rock sample are summarized in Table I , the graphite sample and stainless steel sample in Table 11, respectively. In the first three experiments with the G-1 rock sample, th: niobium content was four to six times higher than the value reported by the spectrographic method ( 5 ) . The data mere reexamined by making three more :Lctivation runs but there w m little change. Thus it would appear that there is much more niobium in the G-1 standard than the 8 p.p.m. previously reported although some deviation might bc attributed to inhomogeneity of the G-1 sample (6). It was not possible to aetect tantalum in the G-1 sample although chemical recovery of the tantalum was 60 to 80%. Thus the amount of tantalum in the sample must be less 1,han 10 p.p.m.,
the lower limit of this method for tantalum analysis. The chemical recovery of niobium throughout the experiment fluctuates greatly since acid hydrolysis of niobium is seldom quantitative. Niobium recovery is particularly poor in the S.B.S. and graphite samples since the acid hydrolysis must be done prior t o the tantalum extraction to eliminate exces peroxide from the sodium peroxide fusion. Spectra obtained from the niobium and tantalum fractions are shown in Figures 1 and 2 . The characteristic x-rays a t 16.6 and 57.8 k.e.v, for niobium (6,10) and tantalum (19), respectively, were measured for the quantitative determination. The half life of the niobium x-ray peak mas found to be 6.6 minutes. The peak corresponding to the 67-k.e.v. gammaray from 111-day tantalum-182 formed during the 10-minute irradiation was subtracted from the combined tantalum-182m1 tantalum-182 peak. The nuclide Ta'82m is easily distinguished from Tals2 and other nuclides (6). Calibration curves using a known amount of N b and Ta were made under several conditions of measurement of the Nbg4m and Ta1R2m x-rays. The amount of Xb9*m and Ta152m was determined from the area under the 16.6k.e.v. and 57.8-k.e.v. photopeak, respectively. The curves are straight lines which were then used to evaluate the niobium and tantalum content in the samples. Background and inter-
fering Conipton radiations from other activities remaining in the samples were eliminated by extrapolation of the base line from both sides of the photopeak. Tantalum extraction by n-amyl alcohol u-as adopted to separate niobium, tungsten, and other elements from it. It was found that the extraction of tantalum by n-amyl alcohol is not closely dependent on acidity of the solution, nor on the temperature of the solution. Furthermore, the extraction leaves little contamination of other elements, and gives more than 85% extraction of tantalum in a single shaking with 10 ml. of n-amyl alcohol I t also permits a fast and clear-cut separation of two immiscible layers immediately after the shaking. Iron (+3), aluminum. beryllium, gallium, indium. and selenium might be possible contaminants (15),but no radiochemical interference is found since isotopes of some of these are very short-lived while others have comparatively low cross sections for neutrons. Tannin precipitation (4), 2,6-dimethyl 4-heptanone, and methyl isobutyl ketone (17) extractions were tried, but these are all seriously contaminated when applied to low concentration tantalum and niobium samples. The di-isopropyl ketone method (17, 18) was not used because of difficulty in obtaining the solvent and its cost. Tungsten and titanium are the main contaminants in the niobium hydrolysis VOL. 35,
NO. 13, DECEMBER 1963
2137
300
200
IO0
0
40
120
80
160
200
ENERGY ( K e v ) Figure 2.
Combined gamma-ray spectrum of 16.5-minute T a i S 2 m plus Tals2separated f r o m graphite sample
Top curve taken immediately after separation; middle curve, 15 minutes later; and bottom curve, 145 minuter after middle curve
step. However, these elements give little interference to the 16.6-k.e.v. Sbg4m activity measurement unless present in about tenfold excess. The reirradiation technique of tantalum extracts for the tantalum yield determination was carefully studied and found accurate, simple, and fast. The neutron flux mas monitored for each irradiation by sending a known weight of gold foil along with each sample and standard since it has been observed that, in the same pneumatic tube during 1-Mw. operation, the flux varies between 1.42 and 1.12 X lo’* n. cm,-2 second-’ (IS). In this may, all measured activities were normalized to the same specific activity of gold. Seutron selfshielding effects in general have been well evaluated by Gilat and Gurfinkel(6), and Hqjgdahl (8) and amount to no more than 1 to 2% in these samples. The advantage of this type of analysis for niobium and tantalum is that one sample irradiation gives a single analysis of two elements simultaneously in less
2138
ANALYTICAL CHEMISTRY
than 30 minutes and correction for reagent blank is unnecessary. ACKNOWLEDGMENT
Thanks are due to the staff members the of Ford Nuclear Reactor for their help in making the irradiations. LITERATURE CITED
(1) -4tkins, D. H. F., Smales, A. A., Anal. Chim. Acta 22, 463 (1960).
(2) Ciuffolotti, L., Giori, C., Mandelli, hI., Energia Nucl. JIilan 8, 422-4 (1961). (3) Cockbill, M. H., Analyst 87, 611629 (August 1962). (4) Das, A I . S., Venkateswarlu, C., Athavale, V. T., Analyst 81, No. 961, 239-243 (1956). (5) Fleischer, M., Stevens, R.E., Geochim. Cosmochim.Acta 26,525-543 (1962). (6) Gilat, L., Gurfinkel, Y., Israel Atomic Energy Comm. Rept. IA-756 (December 1962). ( 7 ) Hillebrand, E.F., Lundell, G. E. F., Bright, H. -4., Hoffman, J. I., “Spplied Inorganic Analysis,” p. 588, Wiley, YPW Ynrk 19.53 ---( 8 j Hggdahi, 0. T., Michigan Memorial Phoenix Project X e p t . MMPP-226-1 (December 1962). I
(9) Kim, C. K., Meinke, 17.JT., Talanta 10,83-89 (1963). (10) RIaddock, R. S., Neinke, K. W., U. S. Atomic Energy Comm. Rept. AECU-4438, 63-67 (November 1959). (11) Ibid., Rept. TID-17272, 30-44 (November 1962). (12) Meinke, W.W., 12litcleonics 17, Yo. 9, 86 11959). (13) keinkk, W.W., U.S. Atomic Energy Comm., Rept. AECU-3887 (1958). (14) Morrie, D. F. C.. O l p , -\ , 7’alanta 4, 194 (1960). (15) Morrison, G H Freiser, H., “Solvent Extraction in .Inalytical Chemistry,” JViley, Philadelphia, Pa., 1957. 1161 Sandell. E. B.. “Colorimetric Deter‘ mination of Traie of Metals,” p. 682, Interscience, Kew York, 1999. (17) Steinberg, E. P., Suclear Science Series Rept. NAENS-3039, p. 1753, National Research Council, Washington 25, D. C. (August 1961). (18) Stevenson, P. C., Hicks. H. G.. ANAL. CIIEM.25. 1517 (19;i3 1 (19) Sunya;, A. T$.> Axel, P., Phys. Rev. 121, No. 4, 1158-1168 (February 15, 1961). ~
RECEIT-ED for review August 9, 1963. Accepted September 16, 1963. Work Fa8 supported in part by the U.S. Atomic Energy Commission.
