obtained by using the cupferron method described earlier for total oxides. The proper volume was then calculated to transfer exactly 23.58 mg. of mixed oxides to a 100-ml. volumetric flask. These volumes were measured with a buret. A 10.00-ml. aliquot was taken and the color developed in a 25-ml. volumetric flask, exactly as described under the standardization procedure. Typical data for such a series are tabulated in Table V. The average deviation for the zirconium oxide results given is =t0.5%, which compares favorably with gravimetric results. DISCUSSION
The methods of differential spectrophotometery can be applied successfully
to the determination of zirconiumhafnium ratios. Mixtures in which hafnium predominates can be analyzed spectrophotometrically by comparing fixed total weights of oxides against a highly absorbing null. The method described above has given good results for the range and conditions for which it was designed. Naturally occurring zirconium minerals and ores contain only a small per cent of hafnium. With the ever expanding utilization of zirconium, the search for new sources intensifies, and the determination of low level hafnium becomes increasingly important. By operating a t the highest sensitivity of the photomultiplier, the
slit width could be decreased with the consequent use of a more strongly absorbing null. The lengthening of the light intensity scale could then make possible improved accuracy in the low hafnium range. LITERATURE CITED
Bastian, Robert, ANAL.CHEM.23, 580 (1951).
Manning, D. L., White, J. C., Ibid., 27, 1389 (1955).
Reilley, C. N., Crawford, C. M., Zbid., 27, 716 (1955).
RECEIVED for review November 17 1956. Accepted August 1, 1957. Divisfion of Analytical Chemistry, 132nd Meeting, ACS, New York, N. Y., September 1957.
Determination of Tantalum in Niobium MARY LOUISE THEODORE Materials Engineering Department, Westinghouse Elecfric Corp., East Pittsburgh, Pa.
Small amounts of tantalum in niobium can b e separated by an extraction procedure in which tantalum is preferentially extracted from a mixedacid solution. The niobium sample is dissolved in a mixture of acids, and the tantalum is extracted with methyl isobutyl ketone and precipitated with ammonium hydroxide. Re-extraction of the tantalum from this precipitate reduces the coextraction of niobium to a negligible amount. The tantalum is determined photometrically b y the pyrogallol method a t 355 mp. The proposed procedure has been successfully applied to the analysis of commercially available niobium samples.
T
ability of niobium-base alloys to withstand extremely high temperatures makes them potentially important for use in aircraft. Knowledge of the impurities in niobium used in such an application is essential. The most common impurity in commercial-grade niobium is tantalum, yet there are few published methods for determining small amounts of tantalum in niobium. To satisfy this need, the present method was developed. HE
PHOTOMETRIC DETERMINATION OF TANTALUM
Although methods have been published in which Rhodamine B (3) and hydrogen peroxide (11) have been used for the photometric determination of tantalum, pyrogallol is the most widely
used color-developing reagent for this element. Previous methods in which the tantalum-pyrogallol color was measured in sulfuric or phosphoric acid solution (6, 6, 10) were subject to strong interference from niobium, titanium, and tungsten, and some corrections had to be applied. By measuring the tantalum color in hydrochloric acid solution a t 325 mp, Dinnin (8) found that the interference caused by niobium and titanium could be greatly diminished. Marzys (8) extended this work to include a simultaneous determination of tantalum and niobium in one sample solution. Tantalum and niobium calibration curves were prepared with reagent solutions as described by Marzys. To keep the niobium absorbance to a negligible amount, the concentration of niobium in the color-developed solution must be kept below 0.75 mg. in 100 ml. of solution. Tantalum must, therefore, be separated from the bulk of the niobium sample before a photometric determination can be made. SEPARATION OF TANTALUM FROM NIOBIUM
A critical review of the analytical chemistry of tantalum and niobium published in 1952 (1) described the existing methods for separating tantalum and niobium as cumbersome, tedious, and unreliable. With the advent of solvent extraction procedures for these two elements (4, 7 , 9, 16-16), the way was opened for a new approach
to the separation of tantalum and niobium. The extraction procedure described by Werning and associates (16) using methyl isobutyl ketone seemed most promising for the separation of tantalum from large amounts of niobium. Extraction conditions were chosen so that the least possible amount of niobium was coextracted. APPARATUS
Platinum dishes, 100-ml. capacity Polyethylene bottles, 4-ounce and 32ounce, with screw caps Polyethylene funnel, 60' Polyethylene dropper, with rubber bulb Polystyrene graduate, 10-ml. capacity with 0.2-ml. subdivisions Ultramax separatory funnels, 125-ml. and 250-ml. capacity. These funnels, supplied by the Fischer & Porter CO., have a Teflon valve which requires no lubricant. Silica crucibles and lids, opaque, 50ml. capacity Spectrophotometer, Beckman Model R u
Absorption cells, silica, 5-cm. light path Infrared lamp, Fisher Infra-Radiator supplied by Fisher Scientific Co. REAGENTS
Methyl isobutyl ketone, 4-methyl-2pentanone, Eastman Kodak Co. Hydrofluoric acid, hydrochloric acid, VOL. 30, NO. 4, APRIL 1958
465
ammonium hydroxide, Mallinckrodt TransistAR reagent Kitric acid, Baker and Adamson reagent Potassium bisulfate, fused, powder, Mallinckrodt reagent Standard tantalum solution, 1 ml. = 0.401 mg. of tantalum. Dissolve 0.2004 gram of spectrographically pure tantalum powder in a platinum dish with 10 ml. of hydrofluoric acid and about 10 drops of nitric acid. Add 10 ml. of hydrochloric acid. Transfer to a 500ml. volumetric flask, and dilute to the mark with water. Mix well. Store in a polyet hylene bottle. Tartaric acid, 15%. Dissolve 300 grams of tartaric acid in water, transfer to a 2-liter volumetric flask, and dilute to the mark with water. Mix well. Store in a polyethylene bottle. Ammonium oxalate-hydrochloric acid solution. Dissolve 30 grams of ammonium ovalate in 300 ml. of water, and add 1520 ml. of hydrochloric acid. Transfer to a 2-liter volumetric flask, and dilute to the mark with water. Mix well. Store in a polyethylene bottle. Pyrogallol solution. Add 150 ml. of water to 50.00 grams of fresh pyrogallol in a 250-ml. volumetric flask. Shake frequently until dissolved. Stir 4.52 grams of stannous chloride dihydrate in 35 nil. of hydrochloric acid until dissolved, and add to the pyrogallol solution. Dilute to the mark with water. Mix well.
PREPARATION OF CALIBRATION CURVE
Transfer 0, 5.0, 10.0, 15.0, 20.0, and 25.0 ml. of standard tantalum solution to six 4-ounce polyethylene bottles. Taking into account the amount of hydrofluoric and hydrochloric acids present in the tantalum solution, add acids so that the final volume of solution is 100 ml. and consists of 7.4 ml. of hydrofluoric acid, 8.3 ml. of hydrochloric acid, 2.7 ml. of nitric acid, and 81.6 ml. of water. Carry each standard through the complete extraction procedure individually. Transfer the standard to be extracted to a 32-ounce polyethylene bottle with a tight-fitting screw cap. Add 50 ml. of methyl isobutyl ketone, tighten the cap, and shake for 2 minutes. Using a polyethylene 60' funnel, pour the solution into a 250-ml. Ultramax separatory funnel. Discard the lower aqueous layer and, with the aid of a polyethylene funnel, transfer the ketone layer to a 125-ml. Ultramax separatory funnel. Add 5 nil. of ammonium hydroxide to the ketone layer and shake for 30 seconds. Allow the stoppered separatory funnel to stand for 2 minutes. Transfer the lower ammoniacal layer to a 100-ml. platinum dish. Rash the ketone layer three times, using 5 ml. of water for each washing, and place washings in the platinum dish. Add 1.5 ml. of hydrofluoric acid to the platinum dish and swirl the dish gently until all the precipitate dissolves; then add 12.0 ml. of hydrochloric acid. Transfer the solution to the 32-ounce
466
0
ANALYTICAL CHEMISTRY
polyethylene extraction bottle, using 16.5 ml. of water to rinse the platinum dish. Add this rinse water to the bottle. Add 25 ml. of methyl isobutyl ketone, tighten the cap, and shake for 2 minutes. Using a polyethylene 60' funnel, pour the solution into a 125-ml. Ultramax separatory funnel. Discard the lower aqueous layer and, with the aid of a polyethylene funnel, transfer the ketone layer to another 125-ml. Ultramax separatory funnel. Add 5 ml. of ammonium hydroxide to the ketone layer, and shake for 30 seconds. Allow the stoppered separatory funnel to stand for 2 minutes. Transfer the lower ammoniacal layer to a 50-ml. silica crucible. Wash the ketone layer twice, using 5 ml. of water for each washing, and add the washings t o the silica crucible. Place the silica crucible under an infrared lamp nith the bottom of the lamp bulb 6 inches from the top of the crucible, and allow the solution to evaporate to dryness. (It is most convenient to allow the evaporation to take place overnight, so that the photometric procedure can be started the next morning.) ,4dd 12.5 grams of potassium bisulfate t o the dried residue. cover with a silica lid, and fuse. Place 125 ml. of tartaric acid solutian in a 250-ml. beaker and heat to boiling. Fill the crucible three fourths full n-ith hot, tartaric acid solution, and stir with a glass rod until most of the melt has dissolved. Using distilled water to rinse the crucible and lid, transfer the contents to the beaker containing the remainder of the tartaric acid solution and stir, with heating, until all the melt has dissolved. Filter the hot solution through No. 40 filter paper into a 250-ml. volumetric flask. Cool the solution to room temperature in a n-ater bath, dilute to the mark with water, and mix well. Immediately transfer a 25.0-ml. aliquot to a 100-nil. volumetric flask to await color development. Before color development, any crystals formed in the aliquots should be dissolved by warming the solutions. Pipet 50.0 ml. of ammonium oxalatehydrochloric acid solution into each 100-ml. volumetric flask and swirl to mix. Then pipet 20.0 ml. of pyrogallol solution into each flask, dilute to the mark with water, and mix well. Let stand for 30 minutes. Measure the absorbance a t 355 mp in 5-cm. silica cells using a Beckman Model B spectrophotometer, with the standard containing no tantalum being set a t zero absorbance. Plot a calibration curve of absorbance us. milligrams of tantalum per 100 ml. Check several points on the calibration curve each time a 2-liter quantity of ammonium oxalate-hydrochloric acid solution is prepared.
DETERMINATION OF TANTALUM IN NIOBIUM
Transfer 2.000 grams of the niobium sample to a 100-ml. platinum dish, and add 3.0 ml. of hydrofluoric acid. Using
a previously calibrated polyethylene dropper, add nitric acid dropwise, several drops a t a time, to the niobium. After the first few additions, tilt the platinum dish when the nitric acid is added so that it will drop directly onto the undissolved niobium. Record the amount of nitric acid that is used. After about 1.0 ml. of nitric acid has been used, add 3.0 ml. of hydrofluoric acid. Continue adding the nitric acid in the same way until the sample has completely dissolved. Filter the solution through No. 40 paper in a 60' polyethylene funnel into a 4-ounce polyethylene bottle, and add acids so that the final volume of solution is 100 ml. and consists of 7.4 ml. of hydrofluoric acid, 8.3 ml. of hydrochloric acid, 2.7 ml. of nitric acid, and 81.6 ml. of water. Prepare a blank containing the same amounts of acids and water. Extract both sample and blank by the double extraction procedure described above, and continue mith the precipitation, evaporation, and color-development steps. With the aid of the calibration curve, determine the milligrams of tantalum present in the aliquot used for color development. If the tantalum content of the niobium is greater than 0.5%, a onetenth aliquot will give an absorbance too high t o be read on the calibration curve. In this case, a smaller aliquot is used, and sufficient blank solution is added to make a total of 25.0 ml. For niobium samples containing less than 0.05% tantalum, fuse the dried residue with 5.0 grams of potassium bisulfate, dissolve the melt in 50 ml. of hot tartaric acid solution, and dilute the solution to 100 ml. The 25.0 ml. used for color-development is then a onefourth aliquot of the sample.
DISCUSSION
Dissolution of Niobium. Niobium cannot be dissolved in a mixture of hydrofluoric and hydrochloric acids but can be dissolved in hydrofluoric acid by the dropwise addition of nitric acid. The amount of niobium extracted increases with increasing amounts of nitric acid; therefore, the niobium must be dissolved with a minimum amount of nitric acid. Upon dissolution of certain commercially available niobium samples with hydrofluoric and nitric acids, a white, insoluble residue was obtained. This residue, which was analyzed by x-ray diffraction methods and found to be zirconium dioxide, can be removed by filtration before the extraction step. Removal of Tantalum from Solvent. Tantalum is removed from the methyl isobutyl ketone by precipitation with ammonium hydroxide (15). This precipitate was difficult to filter; however, by separating the ammoniacal layer from the ketone, placing it in a polyethylene beaker, and adding acetone, a precipitate was formed which could be
filtered very easily. The precipitate and paper were dried, ignited, heated a t 1000" C., cooled, and weighed. The mixed hydroxide precipitate was filtered, ignited, and n-eighed only when the coextraction of niobium had t o be calculated. All other times, the ammoniacal layer containing the precipitate was added to a silica crucible in n-hich it n a s evaporated to dryness under a n infrared lamp. This dried residue fused much more easily than the oxides which nere heated a t 1000" C. Use of Silica Crucibles. Fused silica crucibles instead of platinum had to be used for the fusions because stannous chloride, which is present i n the pyrogallol solution, giyes a yellow color n i t h tiaces of platinum. Extraction of Niobium. The amount of niobium extracted varies n i t h the concentrations of acids, t h e weight of sample. and the volume of solution t o be extracted. The percentage of niobium extracted decreases with increasing sample weight. I n one extraction, l.lY0 of the niobium was extracted n i t h the tantalum from a 2-gram sample. This amount of niobium stilbinterfered in the photometric procedure for tantalum. -1 re-extraction of the mixed hydroxide precipitate decreased the amount of niobium extracted by a factor of 20. The amount of niobium extracted n.ith the tantalum in a double extraction of a 2-gram sample is 0.06%. After a double extraction, the amount of niobium present in an aliquot taken for photometric analysis shows negligible absorbance. Extraction Conditions. I n t h e extraction procedure of Kerning (15), the ratio of extracting solvent to aqueous solution was 1 to 1. An organic to aqueous phase ratio of 1 to 2 can be used without affecting the extraction of tantalum. The acid concentrations used were chosen so that maximum tantalum extraction and minimum niobium extraction were obtained. The acid concentrations chosen for the first extraction were 2 . 2 S hydrofluoric acid and 1.ON hydrochloric acid. I n the second extraction, dissolution of the niobium sample did not have to be considered, and the concentrations chosen were 0.9S hydrofluoric acid and 2.9N hydrochloric acid. Stability of Color. A standing time of 30 minutes is necessary for complete development of the tantalam color, but after this time the color is stable for a t least 4 hours. Tantalum Calibration Curve. Tantalum which n a s extracted from acidic solution, precipitated by ammonium hydrouide, evaporated. fused, and
determined photometrically did not show t h e same absorbance as t h e same amount of tantalum determined directly by the photometric procedure. This loss of tantalum in the extraction procedure was compensated for by preparing the calibration curve from standard tantalum solutions which were also1 extracted, precipitated, evaporated, fused, and determined photometrically. The calibration curve obtained by the double extraction procedure conforms to Beer's lan- over the range from 0 to 1.Ol nig. of tantalum per 100 ml.
Table I.
Precision of Tantalum Determination
No.
% Tantalum
Kennametal Niobium 0.070 0.070 0.076 0.066
1 2 3
4 Average
0 .070
Standard deviation
=
lo-' = 0.004% tantalum
d"'."'
Fansteel Niobium 1 2 3
4 5 6
7 Average Standard deviation =
0.327 0,329 0.325 0.326 0.331 0.317 0.335 0.327
= 0.006% tantalum
Table 11. Recovery of Tantalum Added to 2-Cram Niobium Samples Tantalum, Mg. Difference, Added Found Difference %
-
T a Added to Kennametal Xb (0.070% Ta) .n ,5202 n- 810 +I.O _ ~+o.008 ,. +0.008 +0.006 +0.104
'0,802 0.810 1 , 6 0 4 1.610 2.406 2.510
+1.0 +0.4
+4.1
T a Added to Fansteel S b (0.327% Ta) 2.005 2.005 4.010 4.010
1.980 2.090 3.915 3.993
-0,025 $0.085 -0 095 -0.017
-1.2 +4.2 -2.4 -0.004
RESULTS
Precision. The precision of the' method was checked by analyzing commercially available niobium samples (Table I). Accuracy. As no tantalum-free niobium was available, t h e accuracy of the method was checked by adding standard tantalum solution t o the niobium samples which were analyzed for precision data (Table 11). The average difference between milligrams of tantalum added and found is 1.8%. LITERATURE CITED
Atkinson, R. H., Steigman, J., Hiskey, C. F., ANAL.CHEXI.24, 477 (1952).
Dinnin, J. I.,Zbid., 25,1803 (1953). Gillis, J., Mikrochemie 31,273 (1944). Hicks, H. G., Gilbert, R. S., ASAL. CHEM.26, 1205 (1954). Hunt, E. C., Wells, R. A., Analyst 79, 345 (1954).
Ikenberry, L., Martin, J. L., Boyer, W. J.. ANAL. CHEX 25. 1340
The calibration curve shifts slightly when a new batch of ammonium oxalate-hydrochloric acid solution is used. Several points on the calibration curve should be checked each time a 2-liter batch of this solution is prepared. Interference Check. T o determine if any interfering elements xi-ere extracted along with t h e tantalum, tantalum was extracted from a solution containing 1 mg. of each of t h e following elements: iron, nickel, chromium, molybdenum, tin, bismuth, titanium, tungsten, wnadium, thorium, and uranium. The precipitated tantalum was filtered and ignited, and the resultant tantalum pentoxide was analyzed spectrographically. The concentrations of these elements found in the tantalum precipitate were negligible.
(1953): (7) Leddicotte, G., hloore, F. L., J . Am. Chem. Soc. 74, 1618 (1952). (8) Marzvs. A. E. O., Analust 80, 194 (1955). Milner, G. IT. C., Barnett, G. A., Smales, A. A., Zbid., 80,380 (1955). Norwitz, G., Codell, AI., blikula, J., Anal. Chim. Acta 11, 173 (1954).
Palilla, F. C., Adler, X.,Hiskey, c. F., ANAL.CHEhf. 25,926 (1953). Scadden, E. XI., Ballou, N. E., Zbid., 25, 1602 (1953).
Stevenson, P. C., Hicks, H. G., Zbid., 25, 1517 (1953).
Werning, J. R., Higbie, K. B., Znd. Eng.Chem. 46, 2491 (1954). Werning, J. R., Higbie, K. B., Grace J. T., Speece, B. F., Gilbert, H. L., Ibid., 46, 644 (1954). RECEIVED for review February 20, 1957. Accepted December 27,1957. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, hlarch, 1957. Work carried out under U. S. Air Force Contract No. AF 33(616)-3316.
VOL. 30,
NO. 4, APRIL 195%
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