Separation and Spectrophotometric Determination of Microgram Amounts of Niobium GLENN R. WATERBURY and CLARK E. BRICKER' University of California, 10s Alamos Scientific laboratory, los Alamos, N. M. b Niobium in amounts between 25 and 1000 y can b e extracted into hexone (4-methyl-2-pentanone) from a 6.3M sulfuric-1.6M hydrofluoric acid medium and estimated colorimetrically using hydroquinone in concentrated sulfuric acid. An average niobium recovery of 99.9%, with a standard deviation of 2.0%, was obtained for 38 analyses of solutions containing known amounts of niobium and various other metals. O f 31 metallic ions tested only tantalum and large amounts of molybdenum interfered seriously with the method.
D
THE DEVELOPMENT of an analytical method for tantalum (S), niobium was found to extract partially from 6.0111 sulfuric-0.4M hydrofluoric acid solutions and to interfere with the colorimetric determination of tantalum n ith hydroquinone. When metallurgical investigations in this laboratory created the need for an analytical method for the determination of 0.05 to 2.0% of niobium in plutonium and uranium alloys, the tantalum method was modified and used successfully for the determination of niobium. The recommended procedure is specifically for the analysis of uranium or plutonium alloys, but the general method may be applied to any sample that is soluble in 6.3111 sulfuric-l.6M hydrofluoric acid and does not contain tantalum or large amounts of molybdenum.
URIXG
EXPERIMENTAL
A majority of the reagents nere prepared as for the tantalum procedure ( 3 ) . T o prepare a standard niobium solution, weigh accurately about 0.3 gram of pure niobium metal and place in a 50-nil. platinum dish containing a fen- niilliliters of dilute (1 to 1) nitric acid. Add 10 nil. of sulfuric acid and then add hydrofluoric acid dropwise t o maintain a slow reaction rate. When solution is complete, evaporate to strong fumes of sulfur trioxide and transfer the cooled solution to a 100-ml. volumetric flask, using 10 ml. of 4 M hydrofluoric acid to dissolve any residue. Use water to wash the dish and dilute the solution t o volume. Mix the solution and then transfer it to a polyethylPresent address, Chemistry Department, Princeton University, Princeton, N. J.
ene bottle. Dilute 10 ml. of this stock solution t o 100 ml. with water t o prepare a solution containing 0.3 mg. per ml. of niobium. The spectroscopically pure niobium metal used for this work had a tantalum content of less than 0.170, n-hich was too lovi to interfere. The dissolution of the samples, the apparatus, and the techniques of extraction and colorimetric determination are essentially those described in the tantalum method. Extraction of Niobium. I n previous work niobium mas incompletely extracted from 6 M sulfuric-0.4M hydrofluoric acid solution. Therefore, an investigation of the effect of certain variables on the extraction efficiency was made to determine the best conditions. Because of the coextraction of molybdenum, iron, and other metals from hydrochloric acid solutions, only sulfuric-hydrofluoric acid media were considered for the extraction of niobium. Extractions of 0.6 mg. of niobium into 10 ml. of hexone (4-methyl-2-pentanone) mere made from an equal volume of solution that was 0.1 to 2.0-V in hydrofluoric acid and 3.6 to 7 . 2 M in sulfuric acid. The results, given in Table I, show that the extraction efficiency is highly dependent upon the total acidity and the fluoride concentration, especially for low acidities. For constant sulfuric acid concentration the amount of niobium extracted increases by 72% when the hydrofluoric acid concentration is changed from 0.1 t o 1.2M and by only 6.670 when the hydrofluoric acid concentration is increased further to 2.0M. Thp efficiency of the extraction also increases with the sulfuric acid concentration, and this effect is greater for solutions with low hydrofluoric acid concentrations. Therefore, to obtain efficient extraction of niobium, the sulfuric acid concentration should be as high as practicable and the hydrofluoric acid concentration should be greater than 1.251. Solutions more concentrated in sulfuric acid than 6 to 7 X are inconvenient to extract because the phases separate slowly. Hydrofluoric acid concentrations above 2.0X would increase the efficiency of the niobium extraction, but the handling problems would be greater; metals, such as plutonium, vould also be less soluble in the solutions with high fluoride content. When all factors were considered, acid concentrations of 6.3M sulfuric and 1.6M
Table 1.
Effect of Acid Concentration on Extraction Efficiency (0.6 mg. of niobium taken)
H2SOa, -11 6
6 6 6
7.2 3.6 6.3
HF, JI 0.1 0.2 0.3
Nb Extracted, % 17.2 36.7 47.6
04
67 7
0.8 1.6 1.6
91.8 71.8 96.6
hydrofluoric seemed optimum. At these acid concentrations hexone estracts 95.6% of the niobium present. Apparently, hexoiie is a more efficient extractant for niobium than diisopropyl ketone, which extracts only 90% of the niobium from 6J1 sulfuric-9M hydrofluoric acid and is less efficient a t lower acidities (2). With the optinium acidities the efficiency of the niobium extraction is independent of the niobium concentration up to 1.0 mg. in 10 ml., of the teniperature from 0' t o 35' C., and of the time of evtraction from 2 to 10 minutes. Extraction times of 15 minutes are used if a prccipitate forms in the aqueous phase. Colorimetric Determination of Niobium. Ikenberry (1) reported a broad absorbance maximum at about 400 mfi for t h e niobium-hydroquinone color in sulfuric acid solutions rontaining some phosphoric acid. A similar absorption curve was obtuined in the absence of phosphoric acid, 10 a wave length of 410 nip was selectcd for measuring the absorbances of thv wmples. The niobium-hydroquinonc color obeys Beer's law at this wave length for niobium concentrations up to 1.0 mg. in 10 nil. The effect of hydroquinone concentration on the color was investigated by adding various amounts of hydroquinone solution (55 mg. per ml.) to 0.167 mg. of niobium in fumed sulfuric acid and diluting t o 10 ml. with the acid. Reagent blank solutions containing the same amount of hydroquinone in sulfuric acid were used as references in measuring the absorbances of these VOL. 30, NO. 5, MAY 1958
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Table II. Effect of Hydroquinone on Niobium-Hydroquinone Color
Hydroquinone Added, Mg. 27.5 55.0 82.5 110.0 137.5 165.0 220.0 275.0
Absorbance, 410 M p 0.115 0.280 0.440 0.600 0.760 0.920 1.180 1.319
solutions. The results (Table 11) show the high dependence of the absorbance on the hydroquinone concentration. Although 3.0 ml. of the hydroquinone solution (165 mg. of hydroquinone) in 10 ml. does not produce a limiting value for the absorbance, larger amounts of hydroquinone are not practical because of the limited solubility of the reagent in sulfuric acid and the final volume of 10 ml. Evaporation of the hexone fractions after extraction caused charring of the solvent unless the extract was made alkaline. Any slight discoloration that formed during the evaporation was removed by oxidation with small amounts of ammonium persulfate, as described in the recommended procedure. To demonstrate the stability of the niobium-hydroquinone color, the absorbances of several solutions containing various amounts of niobium and 3.0 ml. of hydroquinone solution in 10 nil. were measured immediately upon mixing and then after standing for definite periods, The absorbances decreased by 2.1% in the first 20 minutes, 3.0% in the next 70 minutes, and only 0.5% in a final 45-minute period. Although the color fades slowly, measurements of the absorbance should be made within a few minutes of the time of mixing the reagents for the most accurate results.
dense fumes of sulfur trioxide are evolved. Transfer the solution to a test tube (25 X 150 mm.), using 2.0 ml. of water to wash the dish. Chill the solution in an ice bath, wash the dish with 4 ml. of 4M hydrofluoric acid, and finally with 0.5 ml. of water; slowly add the washings to the chilled solution. Add 10.0 ml. of hexone, and stir the mixture for 5 minutes with a hollow stirrer. After the layers have separated completely, pipet two 3-ml. aliquots of the organic layer into a 30-ml. platinum dish containing 2 or 3 drops of 5N sodium hydroxide. For samples containing milligram amounts of gold, iridium, manganese, platinum, titanium, vanadium, or molybdenum, pipet 8 ml. of the organic layer into a platinum dish containing 4 t o 5 drops of 5N sodium hydroxide. Slowly evaporate the aliquots to dryness under a heat lamp with frequent mixing, cool the dish, and cautiously add about 10 drops of water and 5 drops of concentrated hydrofluoric acid. If an 8-ml. aliquot was taken, add 3.5 ml. of concentrated sulfuric acid, evaporate
Table 111. Analytical Results for Niobium in Known Solutions with Diverse Ions Added
Foreign Element Added, M g .
.....
Niobium Taken. Av. Niobium Found, ?’& Mg. 0.600
U, 150 0.450 0.300
Pu, 51.2 Pu, 102.4 Pu, 153.6
0.293 0.293
An absorptivity of about 5.5 per mg. of niobium in 10 ml. of solution containing 165 mg. of hydroquinone is obtained. For determinations to within 5%, a t least 0.010 mg. of niobium should be present. For absorbances greater than 0.100, the precision of the niobium determination is within 2%. The absorbance of the niobium-hydroquinone color is linear with niobium concentration to absorbances of a t least 1.7. The absorbance per milligram of niobium decreases slowly with the aging of the hydroquinone solution and a sample of known niobium content should be determined each day for calibration purposes. PROCEDURE
Transfer an aliquot of sample solution containing 0.025 to 1.0 mg. of niobium to a platinum dish. Add sufficient sulfuric acid to make a total of 3.5 ml. and evaporatc the solution under a heat lamp and on a hot plate until 1008
ANALYTICAL CHEMISTRY
98.6, 103.3, 98.5, 99.3, 100.8 98.6
103.5, 100.5 99.1 96.1,
99.6, 100.0 99.8, 101.2 98.8, 98.2, 102.2 100.2,97.7 98.8 100.0 100.0 96.5 100.8 98.6 103.7 101.4
0.293
0.293
97.7 99.6 99.8 97.2 97.7 102.9 99.4 103.5 101.2 101.0 101.9
Av. 99.9 Std. dev., % ’ 2.0 a Double extraction performed. * Double extraction performed 98.8% value only.
for
the solution to fumes of sulfur trioxide, and perform a second extraction as described above. For other samples add 2 to 3 ml. of concentrated sulfuric acid and evaporate the solution under a heat lamp until fumes of sulfur trioxide are evolved. Cool the solutions, add 5 to 10 mg. of ammonium persulfate crystals, and warm the solution on a hot plate until the evolution of gas ceases. Repeat the ammonium persulfate treatment until the sulfuric acid solution is colorless. Then heat the solution on a hot plate and under a heat lamp until very strong fumes of sulfur trioxide are evolved for 3 to 5 minutes. Allow the dish to cool, and as soon as it may be handled conveniently, transfer the solution to a dry 10-ml. volumetric flask, using small portions of concentrated sulfuric acid to wash the dish. The total volume of the solution and washings should be less than 7.0 ml. When the solution has cooled to room temperature, add from a buret 3.0 ml. of hydroquinone solution (55 mg. hydroquinone per ml. of sulfuric acid), and dilute to 10 ml. with concentrated sulfuric acid. Measure the absorbance (ANC)of this solution a t 410 mp relative to a reference containing 3.00 ml. of hydroquinone solution and sulfuric acid t o make 10 ml. Repeat the procedure using concentrated sulfuric acid in place of the sample aliquot to obtain the absorbance (&lank) for the reagent blank, and defor a termine the absorbance (&) standard using a known volume of the standard niobium solution. Aliquots of the same size should be taken from the organic layers for the sample, blank, and standard. To calculate the niobium in the sample aliquot, divide the milligrams of niobium in the known volume of standard solution by the net absorbance (Astd.- Ab]&) of the standard and multiply the dividend by net absorbance ( ~ 4 N b- A b l s n k ) of the sample. EFFECT OF DIVERSE IONS
When 0.293 to 0.600 mg. of niobium was extracted from solutions that contained 170 to 500 times as much uranium, plutonium, sodium, or potassium, 30 times as much aluminum, bismuth, cadmium, cerium, chromium(111), copper, iron, lead, magnesium, mercury, neodymium, nickel, ruthenium, thallium, silver, strontium, tin, zinc, or zirconium, and seven times as much titanium, no significant interference was observed (Table 111). Interference caused by 10 mg. of gold, iridium, manganese, platinum, titanium, or vanadium and 2 mg. of molybdenum may be eliminated by performing a double extraction as described above. Chromium(V1) interferes unless it is reduced by adding hydrochloric acid before the sample is evaporated to fumes with sulfuric acid. Tantalum and large amounts of molybdenum cannot be separated from niobium by a double extraction and therefore interfere seriously with the method.
I n addition to the interference caused by the metallic ions, water causes about 0.1% decrease in the niobiuni-hydroquinone color for every 1 mg. present in 10 ml. of solution; phosphate causes about 0.07% decrease per mg.; but fluoride, added as potassium fluoride, has no observable effect. It is obvious that reagents used in this procedure should be free of phosphate and protected from atmospheric moisture.
Table IV. Results for Niobium in Uranium-Titanium-Niobium Alloys
I
0.487 0.470 0.490 0 .46ga 0 . 466n 0 . 500a 0.480 f 0.014 1.02 1.00
0.469 0.465 0.469O 0.4744 0,477a
RELIABIL\TY
KO standard samples of plutonium or uranium alloys containing niobium were available. Therefore, the reliability of the method is based upon the analytical results for solutions containing known amounts of niobium and upon the reproducibility of results for uraniunititanium-niobium alloys. The solutions were fumed with sulfuric acid and treated as solutions of samples; the results should indicate the reliability for actual samples. Any precipitate that formed during fuming was transferred quantitatively to the test tube and the entire mixture was extracted. Data for 38 representative determinations, as giren in Table 111, show an
0.5
I1
Av. 0.471 & 0.005b 1.0 1.01 1.02
1.02 1.00
1.01
1.01
Av. 1.01 f 0.01
I11
0.5
0.258 0.2.58 .~
.
0.250 0.2E16~
0.243~ Av. 0.253 5
0.006
_
1.01 =I= 0.01 0.240 0.2615 0 .256a 0.253 0.252 0.252 =I= 0.008
Double extraction performed. Standard deviation is precision index used for all results. a b
average of 99.9% for the niobium found, with a standard deviation of 2.0y0. The niobium in the alloy samples, which contained 0.25 to 1.00% of niobium and titanium, was determined by two analysts. Although the titanium concentrations were low, double extractions were performed with some of the samples to test the method. As shown in Table IV, the results for the three representative samples obtained by the two analysts agree within 2% and have a precision of 1 to 3%. The time required for analysis depends upon the ease of solution of the sample. After the samples are dissolved, 15 to 20 determinations may be made in 1 day. LITERATURE CITED
(1) Ikenberry, L., Martin, J. L., Boyer, W.J., ANAL.CHEM.2 5 , 1340 (1953). ( 2 ) Stevenson, P. C., Hicks, H. G., Ibid., 25,1517 (1953). (3) Waterbury, G. R., Bricker, C. E., Ibid., 29, 129 (1957). RECEIVED for review September 16, 1957. Accepted January 8, 1958. Work done under the auspices of the U.S. Atomic Energy Commission.
Determination of Microgram Quantities of Fluoride H. M. NIELSEN Utah Stafe University, logan, Utah The determination of trace amounts of fluoride in the quantities found in some animal tissues is difficult, especially when the sample size must b e restricted. By employing an ion exchange technique for concentrating the fluoride and freeing the solution from interfering ions, 1 - to 10-7 quantities can b e estimated with a precision within 5% of the fluoride present.
S
papers have reported methods for determining fluoride in small amounts. Usually the sample is isolated from interfering substances by the Willard-Winter (IO) distillation or by some modification of this procedure. The isolated fluoride has been estimated, apparently with satisfactory results, by titration (8-4, 8), fluorimetry (6, 9). and spectrophotometry (2, 3, 7 ) , if the fluoride concentration is not too low. In the isolation of fluoride by the distillation procedure, rather large amounts of distillate must be collected to ensure quantitative recovery; the concentration may thus be too low for satisfactory estimation when certain substances are analyzed. Concentration of the distillate by EVERAL
evaporation usually is unsatisfactory because interfering substances present in small amounts may be concentrated to a point where they exert a significant effect. Making the solution basic to prevent loss of fluoride and subsequently adjusting to the proper p H for measurement add to the total salt concentration of the sample, a factor which must be controlled when very small amounts of fluoride are being det ermined. The method reported here employs an ion exchange resin for concentrating the fluoride. Estimation is by a modification of the spectrophotometric method of Megregian ( 3 ) . Attebury and Boyd ( 1 ) first showed that the halides could be absorbed on an anionic exchange resin, from which they could then be separately eluted. They used the strongly basic resin, Dowex 2, in the nitrate form. Nielsen and Dangerfield (6) applied this principle to the determination of atmospheric fluoride, using Duolite A41 in the hydroxyl form. The hydroxyl forms of both Duolite and Dowex 1 and 2 failed to give results of sufficient precision for the author’s purposes, so a more suitable
form was sought. Dowex 1 in the acetate form has been used in the laboratory for the past year and has given satisfactory results in determining fluorides in 0- to 10--y amounts in volumes of as much as 250 ml. APPARATUS A N D REAGENTS
Beckman Model DU or B spectrophotometer or equivalent instrument. 10-mm. matched cuvettes. Anion exchange resin, Dowex 1-X8, 200 to 400 mesh. Sodium acetate solution (1M) in distilled water. Sodium acetate solutions ( O . l M , 0.2 144, and 0.3M) made by diluting the 1 M acetate solution. Standard fluoride solution, 10 y per ml. Reagent A, 1.000 gram of Eriochrome Cyanine R (Geigy) dissolved in distilled water and diluted to 1 liter. Reagent B, 0.175 gram of zirconyl nitrate dihydrate, dissolved in 500 ml. of distilled water, t o which are added 500 ml. of concentrated hydrochloric acid (reagent grade, specific gravity 1.19). Reference solution (for setting the zero point on the spectrophotometer). To 105 ml. of distilled water are added 10 ml. of Reagent A and 5 ml. of concentrated hydrochloric acid. VOL. 30. NO. 5, M A Y 1958
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