ANALYTICAL CHEMISTRY
1438 chromium is present, add hydrochloric acid dropwise to the fuming perchloric acid solution to volatilize the chromium as chromyl chloride. With the cover lid ajar evaporate to dryness but do not bake. Bllow to cool, and wash down the sides of the beaker and the cover lid. Remove the cover lid. Evaporate to dryness once more but do not bake. Allow to cool. Wash down the sides of the beaker with about 20 ml. of water. Boil down to a volume of 5 f 1 ml. Allow ta cool somewhat, and add 15.0 ml. of composite aluminon reagent. Cover with the watch glass and heat on the steam bath (high steam) for 30 minutes. .411ow to cool to room temperature, transfer to a 100-ml. volumetric flask, and dilute to the mark. Compare colorimetrically with the reagent blank a t 540 mp. Convert the readings to per cent aluminum by consulting the calibration curve. PRECAUTION. Do not use beakers that have been used for the analysis of steels, as the iron becomes absorbed in the glass and cannot be removed even with cleaning solution. RESULTS
The results obtained for aluminum in synthetic samples prepared by adding standard aluminum solution to I-gram samples of sponge titanium are shown in Table I. The results obtained for aluminum in the presence of possible interferences are shown in Table 11. None of the metals found in commercial titanium alloys interferes with the method. The
presence of more than 0.10% phosphorus causes low results. Fortunately, phosphorus is rarely present in commercial titanium alloys except in traces. LITERATURE CITED
(1) Craft, C. H., and iMakepeace, G. R., IXD. ENG.CHEM.,A 3 . 4 ~ . ED.,1 7 , 2 0 6 (1945). (2) Hammett, L. P., and Sottery. C. T., J . Am. Chem. SOC.,47, 142 (1925). (3) Luke, C. L., Ax.4~.CHEM.,24, 1122 (1952). (4) Luke, C . L., and Braun, K. C., Ibid., 24, 1120 (1952). (5) Lundell, G. E. F., and Hoffman, J. I., “Outlines of Methods of Chemical Analysis,” p. 117, Kew York, John Wiley B: Sons, 1938. (6) Olsen, A. L., Gee, E. A., and McLendon, V., IND.ENG.CHEM., ANAL.ED.,16, 169 (1944). (7) Roller, P. S.,J . Am. Chem. SOC.,55, 2437 (1933). (8) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” p. 116, Kew T o r k , Interscience Publishers, 1944. (9) Smith Chemical Co., G. F., Columbus, Ohio, “Cupferron and Neo-Cupferron,” p. 14, 1938. (10) Strafford, N., and Wyatt, P. F., A n a l y s t , 68, 319 (1943). (11) Ibid., 72, 54 (1947). (12) Titanium Metals Corp. of ;Imerica, S e w York, “Handbook on Titanium Metal,” 5th ed., p. 56, 1952. (13) Yoe, J. H., and Hill, W. L., J . Am. Chem. SOC.,49, 2395 (1927).
RECEIVED for reriea February
10, :%3.
Accepted August 18, 1953.
(Methods for Analysis of Titanium Alloys)
Determination of Molybdenum in Molybdenum-Titanium Alloys by Precipitation as the Sulfide GEORGE NORWITZ AND &MAURICECODELL Pitman-Dunn Laboratories, Frankford Arsenal, Philadelphia, Pa.
T
HERE is little information in the literature concerning the
determination of molybdenum in titanium alloys, although molybdenum-titanium alloys are important. The “Handbook of Titanium Metal,” 1952 edition (25), states that no satisfactory method has been proposed for the determination of molybdenum in titanium alloys. Various procedures that are used for the determination of molybdenum in other types of materials are not very satisfactory when applied to titanium alloys. The thiocyanate-stannous chloride extraction method (3,18) leaves much to be drsired on account of the instability of the color. There are no volumetric methods for molj bdrnum that can be used in the presence of large amounts of titanium. The m-benzoinoxime precipitation method (1,9) is not very reliable for the determination of molybdenum in titanium alloys. I t was decided to investigate the possible application of the sulfide method to the determination of molybdenum in titanium alloys. Quantitative precipitation of molybdenum as the sulfide has always bren considered a difficult matter (6, 10). In the usual procedure the molybdenum sulfide is precipitated from a hot solution, digested for about 2 hours, filtered, and ignited to the oxide ( 2 , 7 ) . Two precipitations u i t h intervening oxidation of the molybdenum are usually made ( 7 ) . Even with two precipitations only about 2 to 10 mg. of moljbdenum can be precipitated (8). Some investigators have recommended the use of a pressure flask for the precipitation of molybdenum sulfide. However, according to Hillebrand and Lundell (6) this does not help to any appreciable extent. Another disadvantage of the methods that have been proposed for the precipitation of molybdenum as the sulfide is the incomplete separation of the molybdenum in the 3resence of such elements as vanadium (6) and the contamination i f the precipitate by such elements as titanium which hydrolyze, especially when the solutions are heated. From the above it would seem that hefore a wccevful method for the determination of molybdenuni in molybdenum-titanium alloys could be devel-
oped, the proper conditions for the precipitation of molybdenum sulfide would have to be studied. Such a study was undertaken. REAGENTS
Standard Molybdenum Solution (1 ml. = 0.0050 gram of molybdenum). Dissolve 7.500 grams of pure molybdenum trioxide in a minimum amount of 15y0 sodium hydroxide and dilute to I liter in a volumetric flask. Hydrochloric acid, specific gravity 1.19. Sitric acid, specific gravity 1.42. Sulfuric acid, specific gravity 1.84. Perchloric acid, 70%. Phosphoric acid, 85%. Hydrofluoric acid, 48%. Tartaric Bcid Solution, 2070. Dissolve 200 grams of C.P. tartaric acid in water and dilute to 1 liter Kith water. Hydrogen peroxide, C.P. 30%. Sulfuric Acid-Hydrogen Sulfide R a s h Solution. Add 15 ml. of sulfuric acid to 1 liter of water and saturate with hydrogen sulfide. Sulfuric Acid-Tartaric Acid-Hydrogen Sulfide Wash Solution. Add 15 ml. of sulfuric acid and 20 ml. of 20% tartaric acid solution to 1 liter of water and saturate with hydrogen sulfide. F4CTORS INVOLVED IN IIOLYBDENUM SULFIDE PRECIPITATION
Necessity of Allowing Sulfide Solutions to Stand. Overnight. Aliquots of standard molybdrnum solution were pipetted into 250-ml. beakers and 5 ml. of sulfuric acid were added. The solutions were diluted with water to 175 nil., and hydrogen sulfide was passed through the solutions for 15 minutes a t room temperature. One set of solutions was allowed to stand 2 hours and then filtered; a second set was allowed to stand overnight before filtering. The precipitates werr washed with sulfuric acidhydrogen sulfide wash solution and ignited a t 500” C. The results obtained, shown in Table I, indicate that the solutions must stand overnight. S o experiments were conducted with hot solutions because heating a dilute sulfuric acid solution containing 1 gram of titanium leads to hydrolysis of the titanium.
V O L U M E 2 5 , NO. 10, O C T O B E R 1 9 5 3 Table I.
Effect of Standing Mo Found after Standing Overnight, Gram 0.0049 0,0098 0.0248 0.0496
110 Found after 2-Hour
N o .4dded, Gram
Standing, Grain 0,0041 0.0078 0.0175 0.0330
0.0050 0.0100 0.0250
0.0500
Table 11. Effect of Different Acids M1./175 hll.
Acid Added
iMo Found, Gram 0.0249 0.0251 0,0248 0.0247 0.0237 0.0113 0.0246
HzSOa HCI HF HC10, HsPOa HNOt HzSO4 HF Ha904 HNOr HF HNOi HF. Tartano acid HtYO, Hh03
0.0221 0,0247 0.0248 0.0249
Effect of Difierent Acids. Aliquots of standard molybdenum solutions containing 0.0250 gram molybdenum were pipetted into 250 ml. beakers and different acids and mixtures of acids were added. The molybdenum sulfide was precipitated a t room temperature and the solutions were allowed to stand overnight. The results obtained are shown in Table 11. Good recoveries are obtained from sulfuric, hydrochloric, hydrofluoric, and perchloric acids. Nitric acid alone badly interferes with the precipitation, but when some nonoxidizing acid is present, 2 ml. of nitric acid causes no difficulty. I t Tvas decided to use hydrofluoric acid for the determination of molybdenum in titanium alloys, as hydrofluoric acid has the advantage of rapidly dissolving titanium alloys and holding titanium, tungsten, silicon, and vanadium in solution by the formation of stable complexes I t was also decided to add 2 ml. of sulfuric acid to maintain the acidity, since some hydrofluoric acid may volatilize in dissolving the sample. Effect of Acidity. Detailed studies of acidity were made only with sulfuric, hydrochloric, and hydrofluoric acids; 0.0250 gram of molybdenum was added in every case. The hydrogen sulfide was passed through the solutions a t room temperature and the solutions allowed to stand overnight. The results are khonn in Table 111.
Table 111. Effect of Acidity H ~ S O IM1./175 , MI. 0 1 2 5
7 8.5
10
20 30
HCl, ML/175 MI. 1 2 5 10 13 20 30
0.0002 0.0235 0.0243 0.0248 0.0247 0.0247 0,0229 0.0216 0.0171 0 0 0 0 0 0 0
0247 0262 0250 0249 0248 0231 0205
HF, 311./175 hll. 1
2
6 8
10
1
.\Io Found, Grain
0.0243 0.0280 0 0249 0 0253 0.0267
1439 Good results were obtained for molybdenum in solutions cont,aining 5 to 8.5 ml. of sulfuric acid per 178 ml. (1.0 to 1.7 N , or 2.9 to 4.90/0),1 to 15 ml. of hydrochloric acid per 175 ml. (0.07 ta 1.0 or 0.6 to 8.6%), and 2 to 8 ml. of hydrofluoric acid per 175 ml. (0.3to 1.3Nor 1.1 to 4.6%). High results are obtained for larger amounts of hydrofluoric acid, owing to attack of the glassware by the hydrofluoric acid. For the amount of hydrofluoric acid used in the proposed method ( 5 ml. per 175 ml.) high results were never obtained. ,411 precipitations involving hydrofluoric acid must be carried out a t room temperahre. If the solutions are heated, the glassware will be attacked. The titanium samples must be dissolved in platinum dishes. The results show that the acidity must be regulated. When 1 gram of titanium is present, the acidity is best adjusted by wing the proper amount of acid in dissolving the sample. Should an attempt be made to adjust the acidity after neutralizing with ammonium or sodium hydroxide, difficulty will be encountered in redissolving the mass of titanium hydroxide. Choice of Wash Solution. For washing molybdenum sulfide precipitates a dilute acid solution saturated with hydrogen sulfide should be used. h halide acid (hydrochloric, hydrofluoric) cannot be used because molybdenum halides are volatile, and some molybdenum can be lost during the ignition to molybdenum trioxide. The use of sulfuric acid seemed best. The wash solution must not he too weakly acid; otherwise some molybdenum sulfide will dissolve. rl solution containing 15 ml. of sulfuric acid and 1000 ml. of water was found to be satisfactory. The presence of tartaric acid in the wash solution caused no difficulty. Need for a Rapid Stream of Hydrogen Sulfide. It has been stated that a rapid stream of hydrogen sulfide must be used for precipitating molybdenum; otherwise the hydrogen sulfide causes reduct,ion of the molybdenum and incomplete precipitation ( 7 ) . This point was checked by passing a slow stream of hydrogen sulfide for 30 minutes through two solut.ions containing 0.0250 gram of molybdenum and 5 ml. of sulfuric acid (per 175 ml.). The results obtained, 0.0223 and 0.0236 gram of molybdenum, indicate that too slow a stream of hydrogen sulfide cannot be used. In all subsequent work the hydrogen sulfide was passed through the solution rapidly (about 1400 cc. per minute) for 5 minut,es. Then a moderately rapid stream of hydrogen sulfide (about, 800 cc. per minutr) was passed through the solution for another 10 minutes. Manner of Ignition. There has been considerable disagreement concerning the proper temperature for ignition of molybdenum trioxide (MoOa). Recently, however, Duval(5) and Dupuis (4),using the Chevenard thermobalance, have conclusively settled the problem. They found that the pyrolysis curve for molybdenum trioxide showed a horizontal between 343" and 782" C. A temperature of 500" C. was chosen for the proposed method, because this is a convenient temperature approximately midway between 343" and 782" C. To check on the effect of the length of the ignit,ion, a precipitate containing 0.0250 gram of molybdenum \vas ignited for t,he usual 1-hour period, and then ignited again for 3 more hours. The loss in weight for the second ignition period was 0.2 mg. Obviously, the t,ime of ignition is not critical. I t is known that carbon and carbon monoxide can reduce molybdenum trioxide a t red heat (12); therefore, i t was decided to check on n-hether burning off the filter paper was critical. Two samples containing 0.0250 gram of molybdenum and 5 ml. of SUIfuric a(-id ( per 175 ml.) were precipitated, papers and precipit,ates were placed directly in the muffle at 500" C., and the doors were closed. The results obtained after a 75-minute ignition period were 0.0251 and 0.0282 gram. These results would seem to indicate that the manner of burning off the filter paper was not critical. However, because of the danger that uncharred filter paper might remain after the ignition and because of the danger of loes of material caused by evolution of sulfur dioxide and steam, i t is recommended that the filter papers be charred before the ignition by heating the crucibles on the hot plate. Porcelain crucibles
ANALYTICAL CHEMISTRY
1440
must be used in igniting sulfides. If platinum cruvibles itre used, they will be attacked by the sulfide. Amount of Molybdenum Precipitated. Test5 showed that as little as 0.1 mg. of molybdenum was quant,itativelyprecipitated from solutions containing 5 nil. of eulfuric acid, 5 ml. of hydrochloric arid, or 5 ml. of hydrofluoric acid (per 175 ml.). However, because of the errors involved in weighing small amounts of precipitates, the method is not recommended for the determination of less than 2.5 mg. of molybdenum. To determine whether as much as 1 gram of molybdenum could lie precipitated as the sulfide, 1.500 grams of pure molybdenum trioxide (equivalent to 1.000 gram of molybdenum) was dissolved in the minimum amount of 15% sodium hydroxide solution, the solution was neutralized to methyl red with sulfuric acid (1 to l), and 10-ml. excess sulfuric acid (1 to 1) was added. The solut'ion was diluted to 175 ml., treated with hydrogen sulfide, and allowed to stand overnight. The result obtained for molyb(Ienuni was 0.9992 gram. Effect of Valence of Molybdenum. Solutions containing 0.0250 and 0.050 gram of molybdenum and 5 ml. of sulfuric acid w i ' e treated with titanous sulfate solution until the solutions Tvei'e blue. By this treatment, the molybdenum (VI) n-as reduced to molybdenum(V) (16, 24). The solutions n-ere treated with hydmgen sulfide for 15 minut,es, allowed to stand overnight, and filtered. The solutions on being treated with the hydrogen sulfide became a purple color (possibly molybdenum blue) and a very small amount of brownish precipitate appeared. The results obt:iiiied for molybdenum were 0.0006 and 0.0010 gram, respectively. Solutions containing 0.0250 and 0.0500 gram of molybdenum and 5 ml. of sulfuric acid were treated with granulated zinc for a few minutes in a covered beaker. This reduced the molybdenun1 from the t 6 to the f3 st,ate ( 1 7 ) . After the zinc had dissolved, the solutions were immediately diluted to l i 5 ml. withwater and treated with hydrogen sulfide. A brownish-black precipitat,e appeared and the solution remained colorless. The results obt:ained for molybdenum were 0.0097 and 0.0160 gram, respectively. The composition of the precipitates obtained by precipitating molybdenum sulfide from the lower valence states is uncertain. is precipi31:iwrow and Nikolow (11, 62) state that llo2S6.5H~0 tated from molybdenum solutions that have been reduced with zinc. However, these investigators worked with strong sulfuric acid solutions (greater than 20'%) and obtained by reduction n.ith zinc a red colored solution, whose valency was uncertain. In the reduction procedure used by the authors a dilute sulfuric acid solution was employed, and a light yellow solution obtained. The valency of the reduced solution would seem to be 1 - 3 ( 1 7 ) . Probably the composition of the small amount of precipitate obtained by passing hydrogen sulfide through the solution that was reduced with titanium(II1) m s Mo&, and the composit,ion of the precipitate obtained by passing hydrogen sulfide through the solution that had been reduced with zinc was probably A'fO&$. All sulfides of molybdenum on ignition give AI003 (19). The above experiments show that it is necessary for the molybdenum to be in the +6 state, The conversion to this state after dissolving t,he sample is most effectively accomplished by adding hydrogen peroxide (14). The hydrogen peroxide first forms perosyniolybdic acid, H&foO,. The valence of molybdenum in peroxymolybdic acid is still +6, since it consists of molybdenum conihined with peroxide -0-0- groups ($1). The excess peroxide is destroyed by heating in a platinum dish prior to the hydrogen sulfide precipitation, as otherwise the peroxide will oxidize hydrogen sulfide t o sulfur (15). Ordinarily, if 1 gram of titanium is present, the titanium-peroxide complex formed when hydrogen peroxide is added is very stable. However, the titanium-peroxide complex is destroyed readily when hydrofluoric acid is present ( I S ) . I n destroying the hydrogen peroxide the peroxymolybdic acid is converted to molybdic acid. Possibility of Weighing Molybdenum as Sulfide. Two solutions containing 0.0500 gram of molybdenum and 5 ml. of sulfuric
acid Mere treated with hydrogen sulfide as usual. The solutions were filtered through sintered-glass crucibles (medium). One precipitate was washed with sulfuric acid-hydrogen sulfide wash solution. The other precipitate was washed with sulfuric acidhydrogen sulfide wash solution, then three times with carbon disulfide and three times with ether (to remove sulfur). The results obtained for molybdenum (assuming that the precipitate was MO&) were 0.0783 gram for the precipitate washed only with the sulfuric acid-hydrogen sulfide wash solution, and 0.0615 gram for the precipitate washed with the carbon disulfide and ether. Obviously, the method is not satisfactory, owing to the presence of water of hydration, sulfur, or sulfides other than MoSa. Sidgwick (20)has indicated that some MoSl may be formed. Interferences. Only interferences that might be present in titanium alloys were investigated, and only the sulfuric acidhydrofluoric acid medium (2 ml. of sulfuric acid and 5 ml. of hydrofluoric acid per 176 ml.) used in the proposed method for the determination of molybdenum in molybdenum-titanium alloys was considered. It was found that 0.1 gram of the following caused no interference: chromium (added as chromic sulfate), aluminum (added as aluminum sulfate), manganese (added as manganese sulfate), and iron (added as metallic iron); 0.05 gram of the following caused no interference: nickel (added as nickel chloride), cobalt (added as cobalt chloride), magnesium (added as magnesium sulfate), vanadium (added as vanadyl chloride), and silicon (added as sodium silicate); 0.01 gram of boron (added as boric acid) caused no difficulty. Tin was found to precipitatepartially withmolybdenum,and copper was found toprecip itate quantitatively. Fortunately, these two elements are rarely found in commercial titanium alloys. More than 0.0020 gram of tungsten will give high results for molybdenum. However, if 5 grams of tartaric acid are added ( 7 ) to the sulfuric-hydrofluoric acid medium a clean-cut separation of molybdenum from 0.05 gram of tungsten is obtained. When molybdenum is separated from tungsten, the precipitate must be washed with a solution containing sulfuric acid, tartaric acid, and hydrogen sulfide.
Table IV. Molybdenum in Synthetic Alloys M O
Present,
Average 110 Found,
%
Standard Deviation,
%
.
s o . of
Detns.
PROCEDURE FOR DETERMINATION OF MOLYBDENUM IN MOLYBDENUM-TITANIUM ALLOYS
Transfer 1 gram of the sample to a large platinum dish and add 10 ml. of water and 2 ml. of sulfuric acid. Add in small portions 5 ml. of 48% hydrofluoric acid measured with a Bakelite graduate. After the reaction has ceased add 1.5 ml. of 30% hydrogen peroxide, and dilute to about 60 ml. with water. The solution should be yellow, owing to the titanium-peroxide complex. If i t is not yellow, add more hydrogen peroxide. Heat on the hot plate until the yellow titanium-peroxide complex is destroyed. If the volume is reduced below 50 ml., add more water. Cool to room temperature and dilute to about 80 ml. with water. Wash the contents of the dish into a 250-ml. beaker containing about 50 ml. of water. Add some paper pulp and dilute to 175 ml. with water. Pass a rapid stream of hydrogen sulfide through the solution for 5 minutes and then a moderately rapid stream for an additional 10 minutes. Allow to stand overnight. Filter through a medium filter paper (Whatman No. 40) and waRh with sulfuric acid-hydrogen sulfide wash solution. Place the filter paper and precipitate in a tared porcelain crucible and heat the crucible on the hot plate until the filter paper is charred. Ignite a t 500" C. for 1 hour. Cool and weigh as MoO3. The factor for converting Moo3 to Mo is 0.6667. For samples containing over 0.2V0 of tungsten proceed as above but add 25 ml. of 2Op, tartaric acid solution to the beaker after transferring from the platinum dish. Wash the sulfide precipitate with sulfuric acid-tartaric acid-hydrogen sulfide wash solution.
V O L U M E 2 5 , NO. 10, O C T O B E R 1 9 5 3 Table V.
1441
I b i d . , p. 247. Ibid., p. 255.
Molybdenum in ~Iol~-hdenum-Titanium illoys
Sample WA 4 WA 6 w.4 7
Average JIo Found,
Standard Ileviation,
%
Detns.
3.34 9.20 1.87
0 02 0.03 0 02
5 5 5
%
Knowles, H. B., J . Rewarch Natl. BUT.StandaTds, 9, 1 (1982). Lundell, G. E. F., and Hoffman, J. I., “Outlines of Methods of Chemical Analysis,” p. 52, New York, John Wiley & dons, 1938. Mawrow, F., and Nikolow, M., 2.anorg. Chem., 95, 185 (1916). IMellor, J. W,, “Comprehensive Treatise on Inorganic mid Theoretical Chemistry,” Vol. XI, p. 541, London, Longmans, Green & Co., 1931. Prescott, A . B., and Johnson, 0. C., “Qualitative Chemic-nl Analysis,” p, 385, New York, D. Van Nostrand Co., 1933. Ibid., p. 450. Ibid., p. 505. Sehaefer, E., Arch. Eisenhiiteuw, 2, 297 (1937). Scott, W.W., “Standard Methods of Chemical Analysis,” 1-01. I, p. 596, New York, D. Van Nostrand Co., 1939.
KO.of
RESULTS
The results obtained for molybdenum in synthetic niolybdenum-titanium alloys prepared by adding standard molybdenum solution to I-gram samples of titanium are shown in Table IV. The results obtained for molybdenum in three representative molybdenum-titanium alloys are shown in Table V.
I b i d . , p. 606.
Sidgwick, K. T., “The Chemical Elements and Their C:oii~pounds,” Yol. 11. p. 1037, London, Oxford University Press, 1950. Ibirl., p. 1038. I b i d . , p. 1045. Ibid., p. 1049. Titanium 3Ietols Corp. of .inierica, New York, “Handbook on Titanium Metal,” 5th ed., p. 46, 1952. Trawrs, -4.. Compt. rend.. 165,362 (1917).
LITERATURE CITED
American Society for Testing Materials, Philadelphia, “AST11 Methods of Chemical Analysis of hietals,” p. 112, 1950. Ibid., p. 114. Ibid., p. 116. Dupuis, T., Compt. rend., 228, 541 (1949). Duval, C., ANAL.CHEM.,23, 1271 (1951). Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 246, New York, John Wiley & Sons, 1929.
RECEIVEDf o r review innuary 1 7 , IR.i,R.
.\ccepted AugiLit 18, 19S3.
(Methods for Analysis of Titanium Alloys)
Polarographic Determination of Molybdenum in Titanium Alloys MAURICE CODELL, JAMES J. MIKULA, AND GEORGE NORWITZ Pitman-Dunn Laboratories, Frankford Arsenal, Philadelpia, P a .
G”“”
METRIC methods such as the sulfide method are not applicable t o the accurate determination of small amounts of molybdenum in titanium alloys. Therefore, an investigation was undertaken t o develop a polarographic procedure for the determination of molybdenum in titanium alloys, as the polarograph is especially adaptable to the determination of small amounts of metals. A few experiments indicated that molybdenum could not be satisfactorily determined polarographically in the presence of large amounts of titanium. A separation of the molybdenum from the titanium was therefore deemed necessary. An extraction separation of the molybdenum seemed desirable. The thiocyanatestannous chloride ether extraction (11 ) of the molybdenum was considered but rejected because it was found that considerable tin was extracted by the ether, a fact that led to complications. An extraction procedure that seemed to offer promise of success was the ether extraction of molybdenum from a hydrochloric acid solution ( 2 ) . Such a procedure has been used for a long time for the separation of iron, but has been little used for the quantitative separation of molybdenum. Hillebrand and Lundell ( 2 ) state that the molybdenum is 80 to 90% extracted under conditions used for the extraction of iron. I n the application of the extraction procedure t o titanium alloys it was not feasible to use a hydrochloric acid solution alone. Titanium alloys dissolve readily enough in hydrochloric acid, but it is not possible to calculate accurately the hydrochloric acid that would be lost by volatilization in dissolving the sample. Regulation of the acidity is important. It was decided t o dissolve the sample in hydrochloric acid, oxidize with nitric acid, and then evaporate to fumes of sulfuric acid. The optimum amount of hydrochloric acid could then be added, and the solution diluted to a definite volume. I n order to extract the molybdenum it is necessary that the molybdenum be in the f 6 state. Therefore, the use of the nitric acid to oxidize i: the mqlybdenum is essential. The nitric acid also o ~ i d i ~ ethe titanium from the f 3 to the +1state
Many p;ipers have been published on the polarographj- ot’ niolybdenum. Uhl ( I S ) showed that a double wave was obtained from a dilute nit,ric acid solution containing lactic and os:ilic acids. Stackelburg, Klinger, Koch, and Krath (12) found th:itm molybdenum does not produce a wave from neutral or alkdine solutions but does produce a well-defined wave from a sulfuik acid medium. These authors also found that molybdate in hydrochloric acid produces an ill-defined wave starting from z e v ) applied e.m.f. KanevskiI and Shvartsburd (6) obtained a doul~le wave from a phosphoric acid solution. Hokhshtein (3)investigated the polarographic det>erminationof molybdenum from iiitric acid solutions. Johnson and Robinson (5) investigated t>hepolarography of molybdenum in sulfuric and nitric acid solutioris, and pointed out that the presence of nitric acid leads to a large increase in the reduction current attributable to catalytic riitr:itc~ reduction. Holtje and Geyer (4)worked out procedures using sulfuric and perchloric acid media. Haight ( 1 ) investigated the use of a perchloric-sulfuric acid medium for the determination of molybdenum and applied the procedure to the direct deternination of molybdenum in st,eels. The waves obtained for molybdenum in a perchloric acid medium are largely due to a catalytic reduction of the perchlorate ion in the presence of the molybdenum. The use of a perchloric acid medium in preference to other media was chosen by the authors not only because of the sensitivity of the method but because the procedure could be readily adapted t o the determination of molybdenum after evapornt,ion of the ether from the ether extraction. The method described in this paper is applicable to samples containing 0.003 to 5% molybdenum. For samples containing more than 0.01 gram of iron an ammoniacal separation of the iron from the molybdenum must be made after the ether extraction to eeparate the iron from the molybdenum. APPARATUS AND REAGENTS
A Sargeiit polnrograph, Model XXI, and an H-type polarographir cell with a saturated calomel electrode ( 7 , 8)were used.
