a n estimation unless only water is added before the pyridine-water reagent. The triphenylmethyl acetate suffers a rapid acid-catalyzed hydrolysis in comparison to the other esters present. Triphenylsilanol also reacts with the ethyl acetate reagent, but the acetate formed is immediately hydrolyzed by the pyridine-mater reagent. This was verified by isolating triphenylsilanol quantitatively after the pyridine-water was added, and by obtaining essentially 1 0 0 ~ oacetylation by the N-methylaniline-perchloric acid finish, where no mater is present to hydrolyze the triphenylsilyl acetate. INTERFERENCES
I n ethyl alcohol-water mixtures, perchloric acid catalyzes the acetylation of ethyl alcohol even though more water is present than the amount of anhydride in the acetylating reagent. However, for quantitative reaction of the ethyl alcohol it is neccssary to have a definite excess of anhydride over water. Table IX shows the results with 11 mmoles of acetic anhydride added. By increasing the concentration of acetic anhydride in the acetylating mixture it should be possible to determine more dilute solutions of ethyl alcohol in water. I n Table X the effect of compounds containing various functional groups on the acetylation of alcohols is studied. The interference of simple ketones, such as acetone, is eliminated by chilling the sample and acetylating the reagent to 0” C., or by using the pyridine reagent
at room temperature. Cyclic ketones, which are somewhat enolic, interfere at 0” C. in ethyl acetate, but do not interfere when the acetylation is carried out in pyridine. Aldehydes interfere seriously in both solvents, but this interference has also been noted in existing acetylation methods (9, 19). The carbonyl group in benzoin does not interfere with the acetylation of the ahydroxyl group. The following compounds do not interfere with the acetylation of alcohols: indene (in pyridine) , thiourea, urea, and triphenylmethane. Enols are acetylated to varying degrees, as are imides, hydrazides, and oximes. I n ethyl acetate, double bonds and furan rings interfere, but triple bonds do not. Amines, phenols, mercaptans, and some oximes are quantitatively acetylated. ACKNOWLEDGMENT
The authors express their appreciation to J. P. LaPlante who performed some of the preliminary experiments on this problem. They also thank the following companies for the alcohol samples they supplied: The Doiv Chemical, Eastman Chemical Products, General Aniline and Film, Pennsalt Chemicals, and Quaker Oats, The Lucidol Division of Wallace and Tiernan generously supplied the two samples of hydroperoxides used. LITERATURE CITED
(1) Burton, H., Praill, P. F. G., J . Chem. Soc. 1950, 1203. (2) Zhid., 1951,522. (3) Christensen, 13. E., Clarke, R. A.,
ENG.CHEM., AXAL. ED. 17, 265 (1945). (4) Conant, J. B.,Bramann, G. h l . , J. Am. Chem. SOC.50,2305 (1928). (5) Delahy, R., Sabetay, S., Bull. S O C . chzm. France 1935, 1716. (6) Erdos, J. B., Bogati, A. G., Reu. SOC. qutm. Mez. 1,223 (1957). (7) Fritz, J. S., “Acid-Base Titrations i n Nonaqueous Solvents,” p. 13, G. F. Smith Chemical Co., Columbus, Ohio, 19.52. ( 8 ) Fritz, J. S., Fulda, M. O., . \ s . ~ L . (8) CHEM.25, 1837 (1953). (9). Fritz, J. S., Hammond, G. S., “Quan(9) titative Organic Analysis,” p. 261, Wiley, New York, 1057. IO) Ibid., p. 278. 11) Fritz, J. S., Yaniamura, S . S., Bradford, E. C., -4ri.4~.CHEM.31, 260 (1959). (12) Goering, H. L., Reeves, R. L., Espy, H. H., J.Am. Chem. SOC.78,4926 (1956). (13) Gold, V., Jefferson, E. G., J . Cheni. Soc. SOC.1953. 1953, 1409 1409. (14) Hudson, C. S., Brauns, D. H., (14)Hudson, J . Am. Chem. SOC.37,2736 (1915). (15) Mehlenbacher, V. C., “Orymic. Analysis,” Vol. I, pp. 1-38, Interscience, New York, 1953. (16) Mesnard. Mesnard, P.. P., Bertucat. Bertucat, M.. M., Bull. SOC. soc. chim. France 1959.307. (17) Morgan, K. J., Anal. C h i n . ..lcta 19,27 (1958). (18) Pesez, M., Bull. SOC. chim. France 1954, 1237. (19) Siggia, S., “Quantitative Organic Analysis via Functional Groups,” p. 9, Wiley, New York, 1054. (20) Smith, G. F., “Analytical Applications of Periodic Acid and Iodic Acid,” D.61. G . F. Smith Chemical Co.. Columbus, Ohio, 1950. (21) Toennies, G., Kolb, J. J., Sakanii, W., J. Biol. Chem. 144, 193-227 (1942). IND.
(
- - - - I
I
,
~
RECEIVEDfor review June 3, 1950. Accepted August 24, 1959. Division of Analytical Chemistry, 136th Meeting, ACS, Atlantic City, N. J., September 1959.
Spectrophotometric Determination of Niobium in Tantalum KARL S. BERGSTRESSER Universify of California, Los Alamos Scientific laboratory, Los Alamos, An improved spectrophotometric method for 1 to 100 p.p.m. of niobium in high-purity tantalum metal or oxide is based on a separation of niobium from tantalum with an anion exchange resin. An average error of 0.05 y of niobium was observed in samples which contained from 4 to 9 7 of niobium. Other metals in amounts expected in high-purity tantalum metal do not interfere.
S
of niobium from tantalum by ether extraction and subsequent determination by photometric measurements of a niobium thiocyanate complex are subject to error because EPARATION
18 12
a
ANALYTICAL CHEMISTRY
N. M.
tantalum tends to hydrolyze. Hume and coworkers (1, 7) minimized the effect of tantalum by adding tartaric acid and changing the amount of reagents and the manner in which they were added. Their method was applicable at tantalum to niobium ratios as high as. 100 to 1. Hastings and McClarity (6) controlled the effect of tantalum by strict attention to details of concentration, manipulation, and timing in the addition of reagents which included hydrofluoric acid. For samples of pure tantalum metal they reported a range of 0.01 to 0.30% of niobium with good reproducibility.
The determination of niobium by means of its thiocyanate complex is a useful sensitive method. Crouthamel and coworkers (3) reported a molar absorbance index of 38,OOO for the niobium complex in an aqueous-acetone medium. However, the disturbing effect of tantalum appeared t o be too great for successful analysis of tantalum metal which contained as little as 10 t o 100 p.p.m. of niobium. Other methods for separating 1 to 10 y of niobium from 100 mg. of tantalum were investigated. Again the influence of tantalum was observed; extraction of niobium oxinate into chloroform (6) gave inadequate separa-
tions whenever a niobium solution also contained more than 1 mg. of tantalum. By using an anion exchange resin and the procedure described by Cabell and Milner (8) in place of extraction, separation of microgram quantities of niobium from tantalum was accomplished and determination of niobium with its colored thiocyanate complex was possible. This combination of methods was useful with tantalum to niobium ratios which were as large as lo6 to 1. APPARATUS AND REAGENTS
Spectrophotometer, Beckman Model DU, with 1-cm. Corex cells. Ion Exchange Columns. These were made from methyl methacrylate tubing 0.5 inch in outside diameter, of l/16inch wall, and a length of 8 or 9 inches. The diameter at one end of the tubing \vas reduced to approximately 5/32 inch by careful heating and pulling. Because the tubing was more brittle after this treatment, it mas cut with a thin file. The reduced-diameter end was fitted with a short length of polyethylene tubing which was previously softened by heating and pulled to a capillary tip. A small amount of fine turnings, cut from a rod of Kel-F (The 11.W,Kellogg Co.) was forced into the reduced-diameter end of the tubing to serve as a support for the resin. A reservoir which was an inverted, 2 ounce, narrowmouth polyethylene bottle from which the bottom had been cut was press-fitted to the opposite end of the tubing. Sufficient resin was transferred to the nsscmbled column with water to give a rmin height of 12 cm. The resin in the column was washed with 100 ml. of acid wash solution before use. Evaporating dish, Teflon, lOO-ml., Chemware (Fisher Scientific Co.). Anion Exchange Resin, D o w x 1-X8, 100 to 200 mesh, chloride form (Bio-Rad Laboratories). Stirrer, hollow-tube type, as described by Waterbury and Bricke? (8). Acid Wash Solution, 3M hydrochloric acid-0.1M hydrofluoric acid. Concentrated hydrochloric acid (625 ml.) and concentrated hydrofluoric acid (12.5 ml.) were added to distilled water and diluted with water to 2.5 liters. This solution was stored in a polyethylene container. Aluminum Chloride Solution. Aluminum chloride hexahydrate (4.5 grams) was dissolved in 50 ml. of 6N hydrochloric acid and added t o 200 ml. of distilled water. Stannous Chloride Solution. Stannous chloride dihydrate (16 grams) \vas dissolved in 120 ml. of concentrated hydrochloric acid and added to 80 ml. of distilled water. Tantalum Oxide. One-gram portions of tantalum metal, containing approximately 65 p.p.m. of niobium, were dissolved in concentrated hydrofluoric acid. Each portion of dissolved metal was transferred t o resin columns in a manner described below for analytical samples, except that the niobium was removed by eluting with 500 instead of
200 mf. of acid wash solution. After displacing the acid wash solution from the column with distilled water, the wct resin was transferred to a platinum dish, allowed to settle, and freed from as much water as possible by decantation. The rcsin in the platinum dish was dried on a steam bath, then carefully ignited over a Meker burner and, finally, heated in an electric mufflc furnace a t 700" C. until a pure white oxide residue was obtained. Tantalum purified in this manner contained less than 0.5 p.p.ni, of niobium. Niobium AIetal Ponder. The average assay for this material was 09.24y0 of niobium. Reagent grade chemicals and doubledistilled water were used in making other required solutions. PROCEDURE
Transfer a sample of tantaluni mct:il or oxide, containing no more than 10 y of niobium, to a 10-ml. platinum dish or crucible. For a 100-mg. sample add 1 to 2 ml. of concrntratcd hydrofluoric acid, covcr the platinum container, and nlloiv the sample to dissolve compktely. \\'arm the platinum Container on a steam bath to increase the rate of dissolving or add nitric acid dropwisr t o metal samples, but remove this acid by evaporation after dissolution of samplc. Remove the platinum cover and rinse it and the inside wall of the platinum vessel with a few drops of hydrofluoric acid. Carefully evaporate the sample solution to a minimum volume (about 0.25 ml.) by heating on a stvnm Ixth to c,liminatc excess hydrofluoric aritl. Strictly avoid any formation of solid tantalum compoiind; if this should occur, redissolve in concentratrd h!-drofluoric acid and again evapor:Lte to a minimum volume of clcar solution. Dilute the dissolved samplc to approximately 5 ml. with acid wash solution (3M hydrochloric acid-0.1 .If hydrofluoric acid). Place a 250-ml. polyethylene beaker under a properly preparctl ion exchange column to permit collrction of all effluent. AM to the column 5 ml. of acid wash solution and then directly afterwards, the dissolved sample. Use about 15 ml. of acid \\.ash solution in srveral portions to ensure complcte transfer of the sample solution to the column. Allow solution to drain through the column until liquid level is just above the resin and then rinse the iippcr part of the column Kith several sm::ll portions of acid n.ash solution. \\'hen this rinse has drained through the column and thc liquid level is slightly above the resin, continue addition of acid wash solution to the reservoir of the column until the total volume of effluent is 200 ml. This normally requirrs about 2 hours. Transfer the effluent in suitable increments to a 100-ml. Teflon evaporating dish, placed on a hot plate with a surface tcmperature between 225' and 250" C. Continue evaporation until the volume is reduced from 200 to approximately 2 ml., but avoid evaporation to dryness. Add 5 ml. of aluminum chloride solution
to thc cv&porotingdish and t h m trLmhfcr the aluminum-niohium solution
quantitatively to a 40-rnl. borosilicate glass centrifuge tubc with 5 ml. of distilled water. Add a drop of nivth!.I red indicator solution atid sufficic~nt concentrated aminoiiiuiii Iiydroxi~li: to make the solution just slightly alk:ilinc. Centrifugc the tube for 5 minutes :tntl discard all supernatmt liquid bl. tlecantation. (The possibility of completing thc procedure in tho centrifuge tulic~ was considered, but tuhc tli:tnictc,r w:is too large to give \vork:thlr height t o liquid mixturc during vxtrac.tion.) Dissolve the :~luriiinuiii hydroxiil~~ precipitatc in 5 ml. ( J f stannous c l i l o r i t l ~ ~ solution. Transfer this solution, \\ itli 5 ml. of 4% (w./v.) aqucous Iioric wid solution, to a 1)orosilicate gl:iss tvst tri1,r (25 X I50 mm.)\vliich cont:tins c,s:tr,tly 5 ml. of 20y0 (w./v.) :tqucfioiis:tmnioniu~n thiocyanntv solution. i i t l t l from :t pi1,i.t exactly 5 ml. of rcqgrnt gmdc ilic~tli,~.l ethrr to tlic trst t u t r ~ n r ir;inirtli:itt~ly l vxtract thv ! ~ l l o \ v niol)iuni t l i i ~ ~ v , ~ ~ : t i i : ~ t ~ ~ complcx iiito t l i c h c,tlic,r Iiy :igit:ttiirK tlic. mixturc i n tlic. tu1)c witli :I nicitor-tli.ivtbn. hollon.-tulx typc stirrtsr for 2 niinuti~s. ltcxmovc thc stirrcr, : t I h tlic. I:i>.c,rsto sqm-atc for 1 minute, and thrri \\.itlioiit tlt*l:ty trarisfrr :L portion of thc ct1ic.r layer to a I-c*ni.Corcx cell ivitli :t 3 - t i 1 1 . pip& Covcsr the crll and irnnic~li:itc~I\~ measure thv ahsorhnce of thr c,tlic,i, solution, \vith reagcmt gr:dc tlic,tli>.l ether as a rvfrrence liquid in :inotlic,r cwverctl cvll, :it 386 m p and 0.1 -nini. slit. Dctc~rmiric: a re:tgrnt h l m k l)y hcginning R dc8tcwnin:ttion tvitli 200 ml. of :wid \\ash strliition. t;iil)tr::c,t the rcbagcmt Mink :tiid any (*(,I1 corrtbc.tioti from the o l ) s ; m d al)sorhnnce. 1)cmtl.rmine thcs c:qiiiv:tlcnt amount of n i ~ i i ) i u n i in microgr:trns for the corrected 111)sorbance by rrfcrcnce to a calilmtioii curve. RESULTS AND DISCUSSION
Calibration Curve. D a t a for a calibration curve mere obtained aft(ir preparing a standard niobium solution by dissolving a wcighed portion of niobium metal p o w d u in hytlrofluoric acid and diluting with ariil wash solution. A weighed aliquot of the niobium solution was adtletl to 122 mg. of purified tantalum ositlc which were previously tlissolvrtl i n hydrofluoric acid. Then the tantaluniniobium solution was analyzed accortling to the recommended proccdurcb. From 12 determinations, made during a %day period with several differrnt prcparations of reagent solutions, the slope of a calibration curve was talculated as 0.1046 absorbance unit per microgram of niobium. An cstimate of the standard deviation calculated from these data was equal to 0.003 absorbance unit. After corrwtion for reagent blank the intercept was 0.004 absorbance unit; this was assumed to be caused by rpsidual niobium in the purified tantalum oxide. When other individual samples of VOL. 31, NO. 1 1 , NOVEMBER 1959
1813
the oxide (122 mg.) were analyzed without addition of niobium, the results were 0.004, 0.003, and 0.003 :Lbsorbance unit. Tantalum M e t a l Analysis. Some typical results for metal analysis are shown in Table I. These represent single determinations for niobium in high-purity tantalum metal, obtained with the recommended procedure. For comparison, data for x-ray fluorescence analysis are also shown in Table I. The anion exchange resin used in these determinntions was discarded after a single tantalum-niobium separation. blthough tantalum may be quantitnti\.csl!. recovcrrd by elution with 4.1.1 aminoniuni Table I .
Determination of Niobium in Tantalum Metal
,ZIetal NO. A
B C
I) E F G H I J K L
Niobium, P.P.I\l SpectroX-ray photofluoresmetric cence 309 97 420 60 66 41 57 67 123 78 76 78
285 95 395 65 55 . . .
50 80 120 45 90 60
Table II. .4.
13.
Repetitive Determinations of Niobium in Metal H Individually weighed 'portions of met,al H Weighed aliquot8 of solution which contained 0.1422 gram of metal H per gram of solution
Niohium, P.P.M. B-
Average Std. dev.
A 70.9 67.3 68.4 68.2 68.7 1.5
66 8 67.6 67.1 66.9 67.1 0.4
chloride-1M ammonium fluoride (2), the small quantity of resin required for each determination did not justify the time required for this operation. One sample of metal I) was analyzed with one change in the procedure. The 200 ml. of effluent were collected in four separate 50-ml. portions and each portion was analyzed individually. The first 50 ml. of emuent contained 98.8% of the total niobium recovered and the second portion contained 1.2%. S o niobium was found in the third or fourth portions in this analysis. The same value for reagent blank (0.018 absorbance unit) was obtained by following the recommended procedure or by beginning with a few milliliters of 6N hydrochloric acid, adding 5 ml. of aluminum chloride solution, and completing the determination with a hydroxide precipitation and ether extraction. Apparently the separation with anion exchange resin and evaporation in a Teflon dish do not affect the reagent. blank. Ether extractions were always made with minimum delay to avoid variable losses by evaporation. No standard samples for tantalum metal were available; reliability of niobium determinations was estimated by other means. In Table I1 the results for repetitive determinations for metal H are shown. To detect nonhomogeneity for this metal, the tantalum was analyzed with individually weighed samples and with a single large sample which, after dissolution, was carefully divided into appropriate portions by taking weighed aliquots. I n other sets of repetitive determinations, with large samples of metal dissolved and divided into four aliquots, estimates of the standard deviation were in the rangc of 0.1 to 0.7 p.p.m. when the metal contained between 40 and 70 p.p.m. of niobium. The accuracy with which niobium can be recovered from tantalum was estimated with the data in Table 111. I n this set of eight determinations
Table .II. Analysis of Tantalum Samples Containing Added Niobium Wt. of Tan Calcd. Nbb S b Added Total Nb Nb Found, Sample, Mg. in Ta, y t,o Ta, y in Ta, y Y Error, y -0.02 5.07 5.05 0.88 04.30 4.19 6.07 6.15 +0.08 1.91 93.67 4.16 -0.05 7.22 7.17 4.44 2.78 99.91 +O .04 8.25 8.29 3.87 98.64 4.38 6.70 6.76 f0.06 1.65 5.05 37.24 0.00 4.97 4.97 2.66 51.95 2.31 4 08 4.12 +O .04 1.88 2.20 42.27 -0.12 7.34 7 22 5.60 39.25 1 .74 a To obtain homogeneous samples, 1.3123 grams of metal F (Table I ) were dissolved
and accurately measured aliquots were used for individual samples. Weight of niobium in each tantalum sample was calculated by using average of 8 determinations for metal F, 44.4 p.p.m., with a standard deviation of 0.7 p.p.m.
1814
b
ANALYTICAL CHEMISTRY
th(1 average error is 0.05 y of niubium which is equivalent to 0.5 p.p.m. of niobium in a 100-mg. tantalum metal sample. Interfering Elements. Some of the metals which form colored complexes with thiocyanate were added either to solutions which were used to determine the reagent blank or to known amounts of niobium. When 20 to 40 y of molybdenum, tungsten, titanium, or rhenium were added in this manner, they caused positivr vrrors in the blank, or in the rwults for niobium, which were equivalrnt to 0.1 to 0.3 y of niobium. Thrsv errors were irregular, least for molybdenum, and insignificant when the amount of interfering element was less than 10 y . High-purity tantalum metal is unlikely to contain interfering amounts of these metals; for example, metal D was analyzed with a modification of Greenberg's procetIurP 14) and contained only 2 p.p.m. of niolybdenum and 5 p.p.m. of tungsten. The following elements were added separately, in 5-mg. amounts as soluble salts, to niobium samplos prior to analysis: zirconium, thorium, yttrium, and berylliuni. Only slight positive errors were observed, in no casc tquivaient to inore than 0.04 y of niobium. Platinum is an interfering metal because it forms a yellow complex with stannous chloride which is to some extent extracted by ether. This iiiterference required a change from platinum to Teflon dish for evaporation of the effluent from the ion exchange column. ACKNOWLEDGMENT
The author expresses his appreciation to E. Arnold Hakkila for his efforts in analyzing the tantalum metal samples by x-ray fluorescence. LITERATURE CITED
( 1 ) Bukhsh, M. N., Hume, ANAL.CHEM.27, 116 (1955).
D. F.,
( 2 ) Cabell, M. J., Milner, I., .inal. Chim. Acta 13, 258 (1955). ( 3 ) Crouthamel, C. E., Hjelte, B. E.,
Johnson, C. E., ANAL. CHEM. 27,
507 (1955). (4) Greenberg, P., Ibid., 2 9 , 896 (1957). ( 5 ) Hastinns, J., McClarity, T. A . , Ibid.,
Grove, E.'L., Zb ( 7 ) Lauw-Zecl '
. Zbid.,
29, 1475 (1957
RECEIVEDfor review May 4, 1959. .4ccepted July 20, 1959. Work performed under auspices of U. S. htomic Energy Commission.
