(7) or by using the Thoron spectrophotometric method (4) or other available methods. If traces of sulfates or phosphates are present, conductometric titration of the distillate with lanthanum acetate (6) is recommended. The fluoride content of solid complex fluorides, such as those of thorium, zirconium, aluminum, boron, and uranium, which is not easily obtained by the fluosilicic acid distillation method of Willard and Winter (14), is readily determined by pyrohydrolysis, Pyrohydrolysis can also be used to determine the fluoride content of aqueous solutions of such complexes. For samples that are aqueous solutions of these fluoride complexes, it is reconiniended that the fluoride in solution be converted to sodium fluoride, evaporated to dryness, mixed with the accelerator, and then p yroh ydrolyzed. h n o t her technique is precipitation by means of a slurry of magnesium oxide; any coprecipitation of cations present in the sample solution tends to minimize errors that would otherwise result from incomplete p ecipitation of the fluoride. This precipitate is centrifuged, transferred to a filter paper, and dried carefully a t l l O ° C . An aliquot of the dried precipitate
is intimately niised with the pyrohydrolytic accelerator and transferred quantitatively to the nickel boat. The subsequent steps of the pyrohydrolysis are conducted according to the procedure described b y Warf (9. 13). Certain metals, such as molybdenum and niobium. form volatile metal fluorides or oiyfluorides. Their presence in a n y sample may preclude tlie use of this method.
LITERATURE CITED
mination of Fluoride in ZirconiuniUranium Fuel Processing Solutions,” U. S. Atomic Energy Comm., Rept. 100-14405 (1957). (3) Hibbits, J. O., ASAL. CHEU.29, 1760 (1957). (4) Horton, A. D., Thomason, P. F., Miller, F. J., Ibid., 24, 548 (1952). (5) ICubota, H., Sur&, J. G., Ibid., 31. 283 11959). (6)-Lee, J. E:, Jr., Edgerton, J. H., Kelley, T., Ibid.7 28,1441 (1956). ( 7 ) Nicholas, M. L., Kindt, B. H., Ibid., 22,785 (1950). ( 8 ) Powell. R. H.. Menis, O., Ibid., 30, 1546 (1958). (9) Rodden, C. J., ed.-in-chief, “-halytical Chemistry of the Manhattan Project,” Sational Nuclear Energy Series, 1st ed., Div. 17111, Val. I, p. 728ff, McGram-Hill, New York, 1950. (10) Shank, R. C., Rein, J. E., Huff, G. A., Dykes, F. W.,ANAL. CHEM.29, 1730 (1957). (11) Simons, J. H., “Fluorine Chemistry,” Vol. 2, pp. 83-9, Academic Press, New York, 1954. (12) Susano, C. D , White, J C., Lee, J. E., Jr., AKAL.CHEY. 27, 453 (1935). (13) Warf, J. C., Cline, IT. D., Tevebaugh, R. D., Ibid., 2 6 , 342 (1954). (14) Willard, H. H., Winter, 0. B., IKD.ENG. CHEJI., AXAL. ED. 5 , 7 (1933).
(1) Adams, P. B., Killiams, J. P., Ciiernist Analyst 48,48(1939). (2) Dykes, F. W.,Boonian, G. L., Elliott, RI. C., Rein, J. E., “Remote Deter-
RECEIVEDfor review February 6, 1958. Resubmitted October 12,1959. Accepted October 12, 1959.
.
ACKNOWLEDGMENT
The authors are indebted to L. C. Henley for suggesting the all-Teflon construction of the closure to the sample port; to lf. T. Kelley for design suggestions; t o P. F. T h o ~ ~ a s ofor n suggestions made during the course of the work; and to the Special Analyses Laboratory and the Pilot Plant Control Enit, supervised b y TI-. R. Laing and C. E. Lamb, respectively. for the precision data reported herein,
I
Deter mination Of Niobium in the Parts per . Million Range in Rocks F. S. GRlMALDl U. S. Geological Survey, Washington 25, D. C. F A modified niobium thiocyanate spectrophotometric procedure relatively insensitive to titanium interference is presented. Elements such as tungsten, molybdenum, vanadium, and rhenium, which seriously interfere in the spectrophotometric determination of niobium, are separated by simple sodium hydroxide fusion and leach; iron and magnesium are used as carriers for the niobium. Tolerance limits are given for 28 elements in the spectrophotometric method. Specific application is made to the determination of niobium in the parts per million range in rocks. The granite G-1 contains 0.0022% niobium and the diabase W-1 0.00096y0 niobium.
T
thiocyanate method is probably the most widely used for the determination of small amounts of niobium. The niobium thiocyanate reaction was first developed into a quantitative colorimetric procedure by Alimarin HE
and Podval’naya ( I ) . Additional studies, modifications, and applications of the method have been made ( 2 , 4-11). Two procedures are uspd a t present. I n one, the color is de\-eloped in homogeneous water-acetone medium as originally proposed by Freund and Levitt (4) and in the other the yellow niobium thiocyanate coniplex is estracted into a n organic solvent such as ether, as originally proposed by hlimarin and Podral’naj-a ( 1 , and studied in detail by Lauw-Zecha, Lord, and Hume ( 5 ) . llolybdenum, tungsten, vanadium, and rhenium seriously interfere in the thiocyanate method; titanium interferes to a much smaller estent It is necessary in geochemical studies on abundance and distribution of niobiuni in rocks and minerals to handle such elements adequately, preferably n ith a simple procedure. llodifications in the thiocyanate method broaden its applicability. Specific a;iplication is made to
the determination of niobium in the parts per million range in rocks. Ethyl acetate is substituted for ether in the estraction of the niobium thiocyanate only for convenience. REAGENTS AND APPARATUS
All reagents were of analytical grade. The tantalum and niobium pentoxides ryere supplied by the Fairmount Chemical Co., Inc. The purity of each dried product ivas better than 99.97, by spectrographic tests. The titanium dioxide was the Johnson, Matthey &. Co., Inc., Specpure grade. STAKDARD NIOBIUMSTOCK SOLUTIOS, 1 ml. equals 1mg. of niobium pentoside. Fuse 0.2 gram of niobium pentoxide with 3 grams of potassium pyrosulfate in a 50-ml. Vycor crucible. Dissolve the melt in the crucible, by heating with 30 ml. of concentrated sulfuric acid. Cool and Carefully transfer the solution into 150 ml. of (1 2) sulfuric acid. Dilute with water to 200 ml. in a volumetric flask and store in a desiccator. STANDARDKIOBIUMDILUTESOLU-
+
VOL. 32, NO. 1, JANUARY 1960
119
Table I. Niobium Content of G-1 and W-1 (Per cent) \ V i t h w t Addetf It', 3f3, V, rind Ru With Added W,510; V, an'tl Re Ci- 1 \v- 1 ti-1 \V-1 Averngc SM. dev. No. of san1ples Rfrnge
0,00217 (J . 1 m
10 0,00'10 to 0.0023
O10QO96
O.OOOoO 10
0 0000 1 to 0.0011 I
1 nil. equals 0.01 nig. of niobium pentoside. Trike a suitable aliquot of thc stock Polution and dilute to volume with 40y0 IV.,'~. sulfuric acid. Store flask i n a desiccator. This solutioii is s t a l k for a t lcnst a month. Aminonium thiocgannte, 25% iv./v. in n.:iter. Thc solution is stable for at lcnst a month. Stannous chloridc dihydmtc, 40% n./v, in concrntratcti hydrochloric acid. Tartaric acid. 25% u'./v. in water. Bcckman DU sp&tropliotonictcr. TIOS,
PROCEDURE
' h n s f e r 0.25 grnrii of fincly po\\ticrctl silicate rock samplc to a gold crucible and ignitc a t GOOo to 700" C. to reniovc organic ninttcr and oxidize sulfides. Fuse with 3 grams of sodium hydroxide (30 pellets) a t 350" to 375" C. for 30 to 45 rninutcs, uncoverctl. Aiis the melt a t 10-minute intervals by swirling the crucibIe, held with Blair tongs. Cool. Lmch the mclt with 90 nil. of -rater. Rcniove crucible, police, and wash nith about 10 nil. of wtter. Digest the samplc on a stcwn h t h for about 30 minutes. Add a little paper pul and filtw through a 7- to 9-em. medium-fast filtcr such as S EL' S 589 mhitc ribbon. Wash the prccipitate with (1 19) ammoniuni hydroside solution. Discard filtrate. On washing, a whitc precipitate, possibly calcium carbonate, may form in the filtrate. Disregard this. Flush the residue into the original beaker. Dissolve any rcmaining residue off the papcr with 10 ml. of hot (1 1) hydrochloric acid, collecting the filtratc in tlic original beaker. Wash the iinpcr thoroughly wi$i ivntcr and rcscrve for the filtration of the anirnonium hydroside precipitate that follovs. Heat the solution, and precipitate with ammonium hydroxide at the methyl red end point. Digest tIic precipitate 5 to 10 rninutcs and filter on tho reserved paper. Wash the prccipitate with hot 2% w./v. ammonium chloride solution. Burn the precipitate a t about 000" C. in the gold crucible used for sodium hydroside fusion. .4dd 3 mg, of powdered magnesium oside and mis with a rod, Rcpcat thc sodium hydroxide fusion, leach, filtrntion, and washing with dilute ammonium hydroxide, 8s before. Ignite the precipitate in a silica crucible. Add 0.5 to 1 gram of otnssium pyrosuIfate (prefembly in t t e direction of the smaller amount) and fuse until a clear melt is obtained. Coo!. Add 0.67 ml. of concentrated sulfuric acid with a 1-mL graduated pipet, Gently heat
+
+
0.00210
0.00100
O.oUoI1 7
4
0.0020 to 0.00'13
0.oooo3 0.00008 to 0.00104
(use Blair tongs to hold cruciblc) o w :In oprn flamc as briefly as poseihle until the mclt is di~intcgrated:ml a Inilky solution results. Cool. Pipet 20 ml. of leach solution (medc by mixing 20 ml. of 25% tartaric acid, 65 nil. of hydrochloric acid, and cnou h water to givc a total volume of 120 m f ) into a 50-ml. beaker. Transfrr some of thc solution in the beaker to the silica crucible coiitnining the sample. Stir and trmsfer to tlic beaker. Rcpcat until a complete trnnsfer is m:idc. Scrub and wish the crucible \vitli two I-ml. portions of water. Heat the solutinn to incipicnt boil, A calcnr solution should. be obtainctl. Cool. If a little silica IS prcscnt, atid n iittle pulp nntl filter on a small medium-fast paper, collccting the filtrate in B 25411. volumetric fltisk. W:uh with water, make to volume, and mis. The solution thus prepnrcd for rocks is stable for a t lrnst 3 days. Trnnsfcr a 15.ml. aliquot t o a 60-ml. sepamtory funnel: This size of aliquot wiil contain the aniounts of tartaric acid, sulfriric acid, and hydrochloric : i d nccded in the extraction. If n mailer aliquot is taken, first add compensating amounts of thcsc rengcnts to the separatory funnel before addition of the sample. Add tvatcr to give a total volume of 19.5 ml. At the same time prrpare a blank in another separ:ttory funnel containing 2 mi. of tartnric acid, 0.8 nil. of (1 1) sulfuric acid, 13 ml. of (1 1) hydrochloric acid, and enough wntcr to givc 19.5-ml. total volume. From this point, proccss the sample and blank in the samc manner. Add 5 nil. of ammonium thiocyanate by pipet and mi\-. Add 0.6 ml,. of stmnous chloridc by pipet and mix. Add 20 ml. of ethyl acetnte and shake vi orously for 1 minute. Withdraw neiirb all of the aqueous phasc. A t this point prepare a stripping solution by mising 26 ml. (1 1) hydrochloric acid, 13 ml. of water, 10 ml. of animoniuni thiocyanate, and l ml, of stannous chloride. Wash twice, first with 15 mi. and then with 10 ml. of stripping solution, shaking each timc vigorously for 1 minute. Kithdrrtn the final ethyl acctatc layer into a 25-ml. volumetric flask. Adjust to mark with ethyl acetate and mix, Determine the absorbance of the sample against the blank in 5-cm. cells at 385 mp, With a Beckman spectrophotomctcr use a slit width between 0.15 and 0.20 mm. For the best results process only one wmple and one blank a t one time. Determine thc concentration of niobium from n calibration CUNC prepared with solutions containing the amount of added pyrosulfate present
+
+
+
in thu sntnple aliquot. l'roccss tho standards in exactly thc s m e ' tnanticr tis the snmple aliquot. Magnetite in rocks mny only partially decompose on sodium hydroxide fusion but eventually yields to the pyrosulfatc. Because the hydroxide fusion rcmoves interfering elements, rocks Kith apprccinblc m o u n t s of iiiagnctite could fail to enter complctclg into the purification step and could cause erroneous results. Of the possiblc intwfcring clcnicnts in inngnctit(*,wn:itliuni is important. In a rock contiiiiiiiig 2% niagnetite with 0.25% ~:~nntliiiin pcntosidc in ttic niagnetitc (an usudly lnrge amount), PI'C'I~if nonc of the vanadium w r c Icachcd out, the crror would not escccd 0.7 ii.p.m. of niobium. Thc actual error ~ o u l db u snin1lcr, bccnusc magnetite yields to sonic cxtcnt in nearly cvwy stcp of tlic proccdure. In situations where the mngnctite may bc a problem, it is suggrsted that 0.1 gram of sodium nitruto be includcd with the sodium 1iyrlr.osidc in the first fusion. The niiwd flus is vcry cffectivc in dccornposing fiilcly divided magnetite. EXPERIMENTAL
Operating Conditions. Tlic objcctiw \vas to deterininc conditions that would minimizc the c s t r n t of estrwtioii of titiinium, tungsten, rheniuiii, van II tl i u ni, and mol ybd en u 111. X i n noniuni thiocyanate and hydrochloric acid concentrations n ere varied simultaneously : t h e hydrochloric acid io steps of 1.5, 3, 5, 7 , and S.5 nil., t h e amnioniiiin thiocyanatc iii steps of 3, 4.5, 0. i . 5 , and 0 nil. ,of 257, w./v. solution for each concentration of hydrochloric acid. A total volume of 25 nil. nns cstractcd, the conccntration of stannous chloride, tartaric acid, and sulfuric acid being kept fiscd a t the samc lrvels as in t h e proccdurc. The final conditions adopted for the extraction v u e : total volume 25 ml., containing 2 ml. of tartaric acid, 0.4 ml. of sulfuric acid, B total of 7 ml. of hydrochloric acid, 0.5 ml. of stannous chloride, and 5 ml. of ammonium thiocynnntc. These conditions arc necessarily a compromise madc primarily on thc basis of sensitivity and stability of the niobium rcaction, and high tolerancc for titnnium. Tlic stripping tcchnique wits found necessary to prevcnt interierenco from iron and minimize interfcrencci from titanium, The niobium thiocynnate coniplcs in ethyl acetatc is reasonably stable. LESS than a 2% decrease in nbsorbanco is obtnined after standing for 1 hour Then the absorbancc is inensured against o. blank standing for thc same timc, The pcr cent extracted is temperaturedependent, Tho apparent distribution ratio decreases with incream in tern-
p!~iiture, probably bcoausr of grratcr 1. Do not interfere in the amourits sliown dissociation of thc complex at highcr (these nmounts are the maximum ttmpcm.turcs. For cxamplc, a t t l ~ r twted for cach oxide). 10-y levcl of niobium pentoside the 50 mg. CnO, hIgO, F e a r , Alto3 ticcrease in thc amount extractcd is 1% Y~OS, CelOI 25 mg. COO, CrlOS, ZnO, XiO, Sc& iwr degree. When the tempcrature SbtO,, PbO, BigOa, Tho, 10 rng. differs by more than 2' from thc tcm2. Do not interfere in moderate amounts. pcmiture when the caIibration curve wns Ti02 4 mg. S o intcrfcrence. m:tde, a t h s t one stnndard should bc Gave 1.3 and 10.3 y run at ncnriy the sRme time as thc blob when 1 and 10 s:i m plcss. y of NbgOa were taken. Thc calibration curvc is prncticnlly a %rO?, 3.5 mg. h'o interference. 10 mg. No intprfercnce at straight linc, the npparrnt cstinctioii the l - y IiiblOs Icvcl. c-opfficicnt being a few per cciit siiinllrr 3 mg. Gnvc 0.8 y instead of :it the 10-7 level of niobium pcntosid(8 10 or mZo5. 5 mg. Gave 9.4 -( iiistcad ivhcn coniliared to the I-? Icvcl. This of 10 y of yllos. s ~irobnhly bccausc of thc grcatcl T ~ Q O2.0 ~ ,mg. KO interference if c omplcsing effect of sulfate a t higher the operations prior to and niobium concriitration. An nbsor1,including the estrnction arc made within 20 : m e diffcrcncc from the blank of 0.5 minutes. is obtained nith n &em. light path for 3.0 mg. KO intcrfcrericc nt 10 y of niobium pcntosidc in 25 nil. of the 1-y level of h'h20s hut so111t ion, gave 9.6 y of SI,:Os a t the IO-? level. Behavior of other Elements. Each Ii&O,. Sulfate diminishes t h o inelcrncnt n u s tested in t h r absence of tenPity of the niobium niobium and in tlic prcsencc of 1 thiocyiinntc color. Just and 10 y of niobium pcntosidc. Stovk undcr a 2% error is obtained in the range of 1 to solutions of salts wire used and cstrac10 y of ZiblOs when 0.3 tion and stripping wcre carried out as i n grnm of potassium pjrothe procrdurc. Practically no diffvtsulfate is present in the encc in behavior was observrd. Tiic sample and the calibration curve is made without various elemcntP cnn be grouprd in thrcc pvrosulfatc. An error of c l a w s (see table a t right). 7yo results with 0.6 grnni Pintinuin utrnsils cnnnot bc iiscd i n of potnssium pyrosulhte, the decomposition of thc snmplc. E Y ~ This interference is eliminated by prc aring calibrav i t h mild treatment of the sanipl(~, tion curves Kom solutions such n s Ii~tlrofluoi~ic-sulfuricacid drcontnining the same composition, sufficicnt platinunn is disamounts of added pyrosolvrci to c:iuse srrious error w f ~ onl!. i sulf:1tC. a few micrograms of niobium arc to bc 3. Interfere n t all levels. dt,tvmined. hlthougl. gold is nn iritwEquiv. to 1 -, Element K bc05 fcring clcment, insignificant amounts of gold R X ci~ssolved n.hcn 6 grams of Re(VI1) 4 0 W(V1) 9.5 sodium hydroside I! ith or without 0.2 Pt(1V) 11 grain of sodium nit:ate are fused for F V(V\ 2R -_ hours at 375" C. hiO(ii1) GO SEPARATIOX OF MICROGRAM Avov~~s CU(I1) 880 Au(II1) 1,100 OF YIOBIUM FROM MILLIGRAM AMOCNTS U(VI ) 11,700 OF TUKGFTEN, &IOLPBDENU?vf, ]'ANADIUM,ASD Ramrmr. This separation is accomplished by sodium hydroside fusion. Two to 20 y of NblOs are defiltrates from the sodium hydroxide contaminated in one fusion Kith 5n fusions of G-1 were tested for the efficiency of at least 99% from as presence of niobium and none was much as 15 mg. each of the oxides of found. molybdenum, tungsten, vanadium, and The mineral labuntsa\
