r i d of 11/'2 hours nt slightly h o v e rooill tcmpcrrrturc give more consistent rcsults. 'L'lie firilure of the method with nitroaliphtics probably lies in the f:ict that they react msily with sulfuric. ncitl nt relativdv low tcrnpcr:tturc, :is comp a r d with riitrotiromntics which nrc appnrcntly iinrcwtivc lit reduction temperat u ru . 'i'hc p r i tn:~ry nitro p:rra ffins, whcn trc:itcrl with :I strong niincrd acid, relict to forin n cnrbosylic acid (possibly B sulfonritrtl product with sulfuric acid) crnd hydrosyltminc. Also secondary IIit ro pnr:tffins-e,g,, 2-ni troprop:incitre ciitircly decomposed by heating helow 100' C. with hydrochloric :rcid (4). Thc prob:ilic course of such n reaction iri cotiretitr:ited sulfuric acid during the reduction period cithcr :Lt rooin tvnipcraturc or slightly abovp, hns not bccn dctcrinirietl. 'Froin tlic results, lion.v.i.cr, it is cviclriit tlint coinplcte dccoinposi-
tion docs riot take phcc. \Vhen actual digestion waa started, it WLLSinvariably noticed that brown fumes were present in the Bask, and also n strong odor of nitrogen tetroxide present. This mxne condition exists with compounds containing a nitro group attached to R tcrtinry carbon. The failure of the method with pyrnzolonc and trinzole rings may be due to thcir rcsistnncc to eithcr reduction or oxidation. No sntisfactory espLinntion can be given for the failure of 2,4dichlornnitrobenzene to respond to the met hod , and repeated dcterminatione, while showing only slight vnrhtions, still fall below the calculated value. I n spite of these shortcomings it is felt that the outlined procedures offcr a method for nitroaromatics, nonrefmctor3. conipounds, and possibly azo compounds,
and also n mctliod for hetcrocyclics containing n pyridine nucleus. ACKNOWLEDGMENT
The suthor is indcbtcd to the Eastern Chemical Corp. for supplying the orgnnic chemicals used in this invcstigntiou. LITERATURE CITED
(1) hsboth, Am. Chem. J . 7, 108 (1885). (2)Bradstreet, R. B., ANAL. CHEX.26, 235 (1954). (3) Ibid., 29, 944 (1957 (4) Dunstan, W. R., kouldin E. J . Chem. SOC. 77, 1262 (1900$); Chetn. Zenlt. 2, 184 (1901).
(5) hIcCutchan, Philip, Roth, W. F., ANAL.CAEM.24, 369 (1952). (6) Stebbins, Chem. Zenlr, (3) 17, 161 (1885).
RECEIVEDfor review October 1, 1958. Accepted Piovembcr 3, 1939.
JOHN G. SURAK,' DALE 1. FISHER, C, L. BURROS, and L. C. BATE Analytical Chernisfry Qivision, Ook Ridge National Laboratory, Oak Ridge, fenit.
B An efficient, inexpensive pyrohydrolysir apparatus i s described for dstcrmination of fluorides in materials from which fluorides are difficult to separate by fluosilicic acid distillation. The reaction chamber is constructed of nickel. An unmodifled, standard tube furnace functions as a superheater for the steam and as the heater for the reaction chamber. In the absence of interferences due to anions of volatile acids, the hydrofluoric acid evolved from about 30 mg. of fluoride i s determined by direct alkalimetric titration. h r o n may be used for colorimetric determinationof the hydrofluoric acid evolved from samples containing microgram amounts of fluoride. For samples that contain about 30 mg. of fluoride, the precision i s within 1%.
methods for the determination of halides, and of fluorides in particular, have proved effective and reliable (P,8, 6, 8-10,18,18) especially in the presence of interferences (11) which form fluoride coniplexes that are difficult to decompose by the fluosilicic acid distillation method of Willard nnd Winter (14) using perchloric YROHYDROLYTIC
1 ORINS Rewaroh Participantl Marquette University, Milwaukee 3 , W ie.
acid. The pyrohydrolytic method is especially useful tor solid samples, bec m m the fluorides can be separated directly without a preliminary fusion step or dissolution. Apparatus of several typical designs have been described (1, 6, 8, 12, IS). An efficient apparatus has been designed for the pyrohydrolysis of about 30 mg. of fluoride in complex salts, vhich is siniple in construction and inexpensive to build and operate. It is rugged, and particularly suited for continuous operation. Several of these apparatus have been built since 1955 and have given very satisfactory service. Dykes et al. (2) m d Shank et al. (10) have adapted this apparatus for the remotely controlled determination of fluoride in reactor fuels. They titrate the evolved hydro5uoric acid to a conductometric end point by means of 3 standard solution of thorium nitrate. For test portions that contained from 0.25 to 1 meq. of fluoride, Dykes el al. (a) report their "precision is 0.056 meq. standard deviation for a single determination."
*
APPARATUS
The design, materials, and constructional detaiie of the reaction chamber nnd condenser are indicated in Figure 1. The complete assembly of the pyro-
hydrolysis apparatus is shown in Figure 2. The reaction chamber is constructed from nickel (12). The condenser jacket is fabricated from copper and is silversoldered to the nickel. The nickel entryway for the steam and the nickel exit tubes for the hydrofluoric acid and steam are made integral parts of the reaction chamber by means of fusion welding. The closure to the sample port is made of Teflon. A Hevi-Duty, Type 50, 'iM-watt, 115-volt-23~volt, 12-inch tube furnace is used y t h o u t modification both for superheating of the steam and for pyrohydmlysis. The furnace mechanically supports the reaction chamber. With this arrangement no steam trap is necessary. The siniple steam generator is shown. in Figure 2.
If samples t o be pyrohydrolyaed contain only microgram m o u n t s of fluoride (of the order of 50 p.p.m.) the amount of liquid condensed with apparatus of this size is excessive for good results. A reactor of smaller dimensions would be more suitable. The volume of distillate can be reduced by introducing steam at a slower rate or by passing air saturated with water vapor rather than steam into the apparatus, However, for samples containing larger amounts of fluoride (of the order of 30 mg,) the use of steam is recommended. The temperature of VOL 32, NO, 1, JANUARY 1960
1 17
6
Moich w i t h a
'EACTION CHAMBER END PLUG
i S EZMBLY: 1. Position 5 in I and then iio1 weld the two together ,UT Id the chamfer. 2 . Position items 3 , 4 , a n d 6 sh own ond fosien together with silver solder. . Dnri+i"" c in 4 With 5 . , " .,.. ",, ,/ . ...... tubes on 4 So' with 1 u n d fusion weld :he junction of 6
-
-
7
iiqned by
J.C.Surok ",J,
-LE. ,= :,-,,=,
I
. c Bote
. L .Burros ) bY
.L Burros
SAMPLE PORT CLOSURE
i
/ ond Couniersink
/Drill
Item
PARTS LIST - Pyrohydrolysis Apporo1us Moteriol Remarks 1 Nickel 0.125 W a l l I Teflon 2 Brass 4+"
No.Reqd.
I
z 3
s
4
1
5
I
Cooper Nickel
N -
;
CONDENSER JACKET
Figure
,.
-
I
,
.
.. .
me lenon port closure must ne kept below 300" C. to prevent appreciable thermal decomposition. PROCEDURE
The pyrohydrolysis procedure has been described by Warf, Cline, and Tevebaugh (13) and is readily adapted to macro-, semimicro-, and microtechniques. The sample boat, made of nickel, is placed in the hottest section of the reaction chamber, the temperature of which should he about 1000" C. (9, 13). A pyrohydrolytic accelerator, such as UaOs or V,OS, is used t o increase the speed of the pyrohydrolytic reaction. Powell and Menis (8) have reported the use of UsOsor WOs as accelerators. If WO1 is used, the ten]. perature must be kept below the volatilization temperature of WO,. How-ever, a t temperatures of the order of 800" C. in using WOa as the accelerator, it is difficult to decompose sodium fluoride completely in a reasonable period of time. For salts of fluoride content in the 1 18
ANALYTICAL CHEMISTRY
reaction c h a m b e r
and cond
range from 1to 50%, the evolved hydrofluoric acid in the distillate is titrated directly with a standard solution of base, using phenolphthalein as the indicator. The spectrophotometric method for the determination of evolved hydrofluoric acid with 1-(0-arsonophenylazo) - 3 - naphthol - 3,6 - disulionic acid (Thoron) is used (4)for samples of fluoride content in the part-permillion region or for samples containing nitrate or halide n-hich will interfere. If about 30 nig. of fluoride is pyrohydrolyzed, the relative standard deviation is about 1%. Bhont 100 to 500 y of evolved hydrofluoric acid can be determined by the spectrophotometric method, but the relative standard deviat,ion becomes about 9% for equipment of bhis size. The dist,illation rate is so regulated that about 30 t.o 50 ml. of liquid will be condensed in about 30 minutes. I n this period of time, slightly more than 99% of the total fluoride content of the sample will have been evolved as hydrogen fluoride. The presence d anions of strong acids,
Figure 2. installed
Pyrohydrolysir apparatus
such as hydrobromic, hydrochloric, hydriodic, nitric, phosphoric, and sulfuric, which are volatile and distill over during the pyrohydrolysis, must be taken into account. These anions will cause a positive error in alkalimetric titrations. Interferences .from nitrates and halides other than fluoride can be circumvented by using a thorium nitrate titration a i t h Alizarin Red S indicator
(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 by 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 oforn suggestions made during the course of the work; and to the Special Analyses Laboratory and the Pilot Plant Control Enit, supervised by 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
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