Analyst, January, 1967, Vol. 92, j!$. 4 3 4 6 43 The Determination of Niobium in Metals and Alloys BY A. I. WILLIAMS* (Metallurgy Division, National Physical Laboratoyy, Teddington, Middlesex) A method is described for the determination of niobium in metals and alloys. Niobium is separated from the matrix elements by adsorption on to Dowex 1 anion-exchange resin from a solution of hydrofluoric acid and nitric acid. The niobium is eluted in a mixture of hydrofluoric and nitric acids and determined by the spectrophotometric measurement of the blue complex formed between niobium (V) and bromopyrogallol red. The method has been applied to the determination of niobium in iron, steel, tantalum and zirconium. CURRENT interest in the mechanical properties of low carbon steel containing small amounts of niobium has resulted in the need for a simple method of analysis for this element.The problem posed requires for its solution, first, a simple quantitative method for separating the niobium from the matrix, and secondly, an accurate and precise method for determining the niobium. The separation of niobium from steel has been a difficult problem, especially when tantalum is present. Methods in which tannin is used for the fractional precipitation of niobium and tantalum in complexes with oxalic acid and tartaric acid,1,2 and the behaviour of the ammonium fluoro complexes of niobium and tantalum in a cellulose column with ketone solutions3 are complicated by the presence of large amounts of other ions. For some time past in this laboratory, work has been carried out with anion-exchange resin and eluents of mixtures of hydrofluoric and nitric acids at various strengths.This has led to the development of simple techniques for the separation of elements such as niobium and tantalum from each other, and from iron, iron alloys, cobalt, chromium, nickel and zirconium. The advantage of the procedure evolved is that the niobium is separated “cleanly” in a solution which can be evaporated to leave a negligible residue, apart from the niobium. Also, no errors caused by hydrolytic precipitation occur in the hydrofluoric acid - nitric acid mixtures used. The only alloying elements found in steel that are retained on the anion-exchange resin, together with niobium, are tantalum and boron. Tantalum remains adsorbed when the niobium and boron are eluted.Boron is volatilised and completely removed during subsequent evaporation of the hydrofluoric acid - nitric acid eluate. The only other elements known to be adsorbed under the conditions described, are rhenium, iridium and tell~rium.~ The separa- tion procedure can be used to concentrate small amounts of niobium from large amounts of sample, thus enabling amounts of niobium as small as 1 p.p.m. to be determined. The technique in which reactions that form colours are used, followed by spectrophoto- metric measurement, is accepted as the most suitable for determining small amounts of n i ~ b i u m . ~ Until recently only one colorimetric method was known that gave accurate results, namely that with hydrogen peroxide, followed by measurement of the absorption in the ultraviolet range.This method is not very sensitive, and the presence of titanium causes interference. Determination by measurement of the optical density of the yellow thiocyano complexes formed between niobium(V) and thiocyanate is inaccurate because the decomposition products of thiocyanic acid absorb under the same conditions, and vanadium, tungsten, molybdenum and titanium cause interference.* The spectrophotometric determination of niobium(V) with bromopyrogallol red has recently been de~cribed.~ This colour system is stable and repro- ducible, and the reaction seems to be the most sensitive spectrophotometric method in the visible range so far proposed for niobium. Aylesbury, Bucks. A method in which PAR is used has also been publi~hed.~,~ * Present address : Ministry of Technology, Forest Products Research Laboratory, Princes Risborough,44 [Analyst, Vol.92 COLOUR FORMATION- Belcher, Ramakrishna and Westg gave the following experimental conditions for the formation of the blue niobium(V) - bromopyrogallol red complex. The tartaric acid solution containing niobium(V) ions was nearly neutralised with sodium hydroxide solution until it was within the pH range of 5.7 to 6.6. I t is within this pH range that the reaction between niobium(“) and bromopyrogallol red takes place. The colour-forming reaction was specific for niobium in a solution containing an acetate buffer that was made 0.3 per cent. w/v with respect to EDTA. It was found that colour development was complete after 90 minutes in the presence of EDTA.On measuring the optical density 34 hours later, no change was observed. Gelatin was added to act as a protective colloid to prevent the colour complex settling out of solution as a blue precipitate. Belcher et aL9 showed that appreciable amounts of many elements could be tolerated by the reagent in the presence of EDTA. They also state that for good reproducibility, the reagent, which is subject to oxidation, should be prepared afresh every 7 days. WILLIAMS: DETERMINATION OF NIOBIUM IN METALS AND ALLOYS EXPERIMEKTAL A suitable amount of sample was dissolved in a solution that was 7 M with respect to nitric acid, and 5~ with respect to hydrofluoric acid. The residue, if any, was filtered off on a Whatman No. 542 filter-paper and discarded.The niobium was separated from the matrix by adsorption on to Dowex 1 x 8 anion- exchange resin, 200 to 400 mesh, from a solution that was about 7 M with respect to nitric acid, and 5 M with respect to hydrofluoric acid. After eluting the matrix with nitric acid (7 M) - hydrofluoric acid (5 M) solution, the final runnings were checked for iron content with thio- glycollic acid, when iron or steel was the matrix, and the niobium was eluted with a solution that was 5 M with respect to nitric acid, and 0.2 M with respect to hydrofluoric acid. Tantalum, when present, was adsorbed with the niobium, but it had formed a strong complex and did not elute with the niobium. Tantalum was removed from the anion-exchange resin by eluting with a solution that was 12 M with respect to nitric acid and 5 M with respect to hydrofluoric acid.Slight degradation of the resin occurs in the acid mixture at this strength. This procedure gave a simple separation of niobium from tantalum, which was useful when analysing steels containing both niobium and tantalum. The eluate containing the niobium was evaporated to remove nitric and hydrofluoric acids. The residue contained a small amount of anion-exchange resin that was removed by ignition which left niobium pentoxide. Fusion with potassium hydrogen sulphate enabled the niobium to be dissolved in a tartaric acid solution. COLOUR MEASUREMENTS- Measurements of optical densities were made at wavelength 610mp with a Unicam SP500 spectrophotometer with 4-cm cells, or by using a “Spekker” absorptiometer fitted with a tungsten lamp, Ilford 607 filters and 4-cm cells.The spectrophotometer calibration graph was constructed in the range 0 to 40 pg of niobium, and a straight-line relationship was obtained. The absorptiometer calibration graph was a straight line in the range 0 to 40 pg of niobium, but became curved from 40 to 60 pg of niobium. NIOBIUM IN TANTALUM- The determination of niobium in tantalum required a different procedure at the initial separation stage. The sample was dissolved in a nitric acid and hydrofluoric acid mixture. The strength of the acid was adjusted so that it was 5 M with respect to nitric acid and 0.2 M with respect to hydrofluoric acid. The tantalum was adsorbed on to Dowex 1 anion-exchange resin. Finally, the niobium was eluted with an eluent that was 5~ with respect to nitric acid and 0.2 M with respect to hydrofluoric acid, and the tantalum then eluted with an eluant that was 12 M with respect to nitric acid and 5 M with respect to hydrofluoric acid.RE s ULTS The procedure described was applied to steels of the British Chemical Standards series that contained niobium. It was also applied to a sample of tantalum, and to two zirconium - niobium binary alloys. The results are given in Table I. These results show that the procedure developed for determining niobium after anion- exchange separation is quantitative, and possesses the required precision.January, 19671 WILLIAMS : DETERMINATION OF NIOBIUM IN METALS AND ALLOYS TABLE I RESULTS OBTAINED FOR NIOBIUM BY THE PROPOSED METHOD 45 Sample B.C.S.No. 273 Mild steel B.C.S. No. 275 Mild steel B.C.S. No. 277 Mild steel B.C.S. No. 312 P.M.A. B.C.S. No. 261 18/12 Stainless steel ALA 13 (2 per cent. manganese steel) ALA 15 (2 per cent. manganese steel) Tantalum 1 Zirconium 1 Zirconium 2 Niobium content, per cent. 0.0003 0.035 0.02 Niobium + Tantalum = 1-33 Tantalum = 0-13 Niobium + Tantalum = 0.71 Tantalum = 0.04 0.10 0.10 0.0012 0.5 1.0 Sample weight, g 2*0000 0.5000 0*5000 0*5000 0*5000 0.5000 0.5000 0.5000 0.5000 0-5000 0~6000 0.5000 0.5000 0.5000 0*5000 0.5000 0.2500 0.2500 0.2500 0-2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.5000 0.5000 0-5000 0.5000 0*5000 0~5000 0*5000 0*5000 0.5000 0.2500 0.2500 1~0000 1~0000 1~0000 0.1 140 0.0885 Volume of test solution, ml 50 50 50 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 250 250 250 250 250 250 100 100 100 100 100 100 100 100 100 100 100 50 50 50 200 250 METHOD APPARATUS AND MATERIALS- is used.A Unicam SP500 spectrophotometer or Spekker Aliquot taken, nil 50 50 50 10 15 20 10 15 20 10 15 20 20 20 20 20 0.5 1.0 1-5 1.0 1.5 2.5 2.5 5.0 5.0 7.5 7.5 5 5 5 5 5 10 10 5 5 10 10 10 10 25 10 7.5 Niobium in aliquot, CLg 9.0 2.5 2.5 18.0 27.0 35.5 17.5 27.5 35.5 17.3 26.8 34.5 21-6 21.6 21.5 21.6 14.5 30.5 45.2 29.2 44.7 16.7 15.9 33.0 33.0 50.8 50.8 26.3 26.0 26-3 26.3 26.0 51.0 51.7 26.5 26.0 26.5 263 2.5 2.5 5.5 31.8 27.0 Niobium found, per cent. 0.0004, 0.0005 0.0005 0.036 0,036 0.035, 0.035 0.035, 0.035, 0.035, 0.034, 0.0346 0.021, 0.021, 0.021, 0.021, 1.16 1.22 1.21 1.17 1.19 0.67 0.64 0.66 0.66 0.65 0-65 0.105 0.104 0.105 0.105 0.104 0.102 0.103 0.106 0.104 0.106 0.105 0.0012, 0.0012, 0.001 1 0.56 1-02 absorptiometer with tungsten lamp Ion-exchange column-This consisted of a polythene tube, 30 cm long by 1 cm internal diameter, narrowed at the outlet end and fitted with a polythene funnel at the inlet.The column was charged with l o g of resin. Resin-Dowex 1 x 8, 200 to 400 mesh, chloride form. Before use, wash with nitric acid (12 M) - hydrofluoric acid (5 M) solution. Polythene, PTFE and platinum apparatus are required for handling hydrofluoric acid solutions. REAGENTS- purity ethanol and dilute to 1 litre with water. Use fresh solution every 7 days. 6 ml of glacial acetic acid and dilute to 1 litre. Bromopyrogallol red solution-Dissolve 0.14 g of bromopyrogallol red in 500 ml of high Bufer solution, pH 6.0-Dissolve 80 g of high purity ammonium acetate in water, add46 WILLIAMS EDTA solution-Dissolve 50g of EDTA (disodium salt dihydrate) in water and dilute GeEatin solution-Dissolve 1 g of gelatin in warm water, cool and dilute to 100 ml.Use high purity grade for all other reagents. PROCEDURE- Dissolve the sample in 20 ml of a solution that is 7 M with respect to nitric acid and 5 M with respect to hydrofluoric acid. Warm the solution to expel oxides of nitrogen, cool, filter if necessary through a Whatman No. 540 filter-paper, and add it to the anion-exchange column. Elute the matrix by adding in small increments 90 ml of a solution that is 7 M with respect to nitric acid and 5~ with respect to hydrofluoric acid.Discard the eluate containing the matrix. Elute the adsorbed niobium with 200 ml of a solution that is 5 M with respect to nitric acid and 0.2 M with respect to hydrofluoric acid. Fuse the residue with 0.3 g of potassium hydrogen sulphate, cool and extract the melt with 10 ml of 20 per cent. w/v tartaric acid solution. Dilute the solution to 80ml with water and adjust the pH to 6-0 with 20 per cent. w/v sodium hydroxide solution. Transfer the solution to a 100-ml calibrated flask, dilute to the mark and mix. Place a suitable aliquot of the test solution in a 100-ml calibrated flask. Add 6 ml of EDTA solution, 10 ml of bromopyrogallol red solution, 10 ml of buffer solution and 2 ml of gelatin solution. Mix, allow the solution to stand for 90 minutes, dilute it to 100 ml, and mix.Measure the optical density of the blue complex against a reagent blank prepared in a similar way, in 4-cm cells at 610 mp on a Unicam SP500 spectrophotometer or on a Spekker absorptiometer with Ilford 607 filters. CALIBRATION- Preparation of standard solution A-Fuse 0.1431 g of niobium pentoxide with 3.5 g of potassium hydrogen sulphate. Extract the melt with 20ml of 20 per cent. w/v tartaric acid solution and add another 80ml of tartaric acid solution. Dilute to 1 litre with water. 1.0ml of standard solution A = 1OOpg of niobium. Prejbaration of standard solution B-Take 20 ml of standard solution A, add 38 ml of Adjust the pH to 1 litre. Evaporate the eluate containing the niobium to dryness and ignite the residue. 20 per cent.w/v tartaric acid solution and dilute to 180ml with water. of the solution to 6.0 and dilute to 200ml. 1.0 ml of standard solution B = 10 pg of niobium. Transfer aliquots of 1,2,3,4 and 5 ml to 100-ml calibrated flasks and continue as described in the last paragraph of the procedure. Plot optical density against micrograms of niobium. NIOBIUM IN TANTALUM- Dissolve the sample in a mixture of 5 ml of nitric acid and 5 ml of hydrofluoric acid, and evaporate the solution until recrystallisation just starts. Dissolve the residue in 25 ml of a solution that is 5 M with respect to nitric acid and 0.2 M with respect to hydrofluoric acid. Add it to the anion-exchange column and elute the niobium with 200 ml of 5 M nitric acid - 0.2 M hydrofluoric acid solution. Continue as described in the second paragraph of the procedure. REGENERATION OF THE ANION-EXCHANGE RESIN- Tantalum absent-Pass 25 ml of a solution that is 7 M with respect to nitric acid and 5 M with respect to hydrofluoric acid through the resin bed. Tantalum present-Pass 200 ml of a solution that is 12 M with respect to nitric acid and 5 M with respect to hydrofluoric acid through the resin bed. Then pass 50 ml of a solution that is 5 M with respect to nitric acid and 0.2 M with respect to hydrofluoric acid. 1. 2. 3. 4. 5. 6. 7 . 8. 9. REFERENCES Powell, A. R., and Schoeller, W. R., Analyst, 1925, 50, 486. Schoeller, W. R., IBid., 1932, 57, 750. Burstall, F. H., and Williams, A. F., Ibid., 1952, 77, 983. Huff, E. A., Analyt. Chein., 1964, 36, 1921. White, G., and Scholes, P. H., Metallurgia, 1964, 70, 197. Belcher, R., Ramakrishna, T. V., and West, T. S., Talanta, 1962, 9, 943. Bacon, A, and Milner, G. W. C., Annlytica Chim. Ada, 1956, 15, 129. Belcher, R., Ramakrishna, T. V., and West, T. S., Talanta, 1065, 12, 681. I , Ibid., 1963, 10, 1031. _ _ _ _ - Received Mny 2nd, 10G6