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. T h e thiocyanatestannous chloride ether extraction (11 ) of the molybdenum was considered b u t 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, b u t 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, b u t 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 the mqlybdenum is essential. The nitric acid also o ~ i d i ~ ethe i: 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 a n H-type polarographir cell with a saturated calomel electrode ( 7 , 8)were used.
ANALYTICAL CHEMISTRY
1442
The cell was kept in :I constant temperature water bath at 25.0" k 0.1" C', Dissolved oxygen was displaced by bubbling nitrogen through the solution for 10 minutes prior to the polarographic determination. The nitrogen was purified by passing it through vanadous sulfate solution (9). The capillary used has m and t values of 1.99 mg. of mercury per second, and 3.95 seconds, respectively, a t -0.235 volt in the supporting electrolyte used in the analysis, giving an m2j 3 t1 / l e constant of 1.989 mg. Z / 3 sec.-1/2. Standard Molybdenum Solution 1 (1 ml. = 0.0010 gram of molybdenum). Dissolve 1.500 grams of pui e molybdenum trioxide in a minimum amount of l5", sodium hydroxide solution and dilute to 1 liter in a volumetric flask. Standard Molybdenum Solution 2 (1 ml. = 0.00010 gram of molybdenum). Pipet 50 nil. of standard molybdenum solution 1 into a 500-ml. volumetric flask 0.0010 0.0030 0.0050 0.0070 0.0090 0.0110 and dilute to the mark with water. 1:thyI ether, reagent grade, absolute. GRAMS OF MOLYBDENUM PER 50 ML. Sulfuric acid, 95 to 9 6 7 , specific gravity, 1.84. Figure 1. Calibration Curve for 0.010 to 0.00110 Gram of IIydrochloric acid, specific gravity, 1.19. Molybdenum Sitric acid, specific gravity, 1.42. Peichloric acid, 70%. Ammonium hydroxide, specific gravity, 0.90. Ge1:ttin Solution, 0.10";. Dissolve 0.10 gram of gelatin in Table I. Molybdenum in Synthetic Samples of Titanium about 90 ml. of hot water. Cool and dilute t o 100 ml. Prepare Alloys Containing Less than 1% Iron frcsh daily. Standard Dilute Hydrochloric Acid, 5s. Mix 5 parts of hydrochloric N o Found, % Deviation, S o . of acid with 95 parts of water. ,\Io Added, (Average) 5% Detns. Dilute Ammonium Hydroxide, 2 0 5 . Mix 1 part of ammo0,0023 0.00043 4 nium hydroxide with 4 parts of water. 0.0030 0.020 0,0023 5 0,020 Dilute Ammonium Hgdroxide, lo;. Mix 1 part of ammonium 0.102 0.0041 6 0.100 hydroxide with 99 parts of water. 0.490 0.0179 6 0.500 1.00 5.01
1.00 5.00
6
0.003 0.091
PROCEDURE
For samples containing up to 1% molybdenum proceed as folIons: Dissolve 1 gram of the sample contained in a 250-ml. beaker with 30 ml. of hydrochloric acid by warming on the hot plate. Add 1 ml. of nitric acid and 5 ml. of sulfuric acid and evaporate t o fumes of sulfuric acid. -4dd about 10 ml. of water and swirl t o dissolve the salts. Cool to room temperature, add 20 ml. of hydrochloric acid, and dilute to 50 ml. with water. The 50-ml. level should be indicated by a mark on the side of the beaker. Transfer the solution to a 250-ml. separatory funnel. Use only about 2 ml. of water to rinse the beaker. If silicon or tungsten is present, leave most of the precipitate in the beaker. Add 40 ml. of ethyl ether to the separatory funnel and shake for about 15 seconds. Allow the layers to separate for a minute or more. Drain off the bottom aqueous layer into the original 250-ml. beaker. Pour off the ether layer through the top of the separatory funnel into a clean 250-ml. beaker. Transfer the aqueous layer back into the separatorv funnel. Do not rinse the beaker. Make four additional extractions using 25-ml. portions of ether. Evaporate the combined ether extracts to dryness by heating on the steam bath. Add 5 nil. of nitric acid and 10.0 ml. of perchloric acid and wash down the sides of the beaker mith a little water. Evaporate to fumes of perchloric acid by heating on the hot plate, and fume lightly for about 10 seconds. ,idd about 15 ml. of water and swirl to dissolve the salts. Transfer the solution to a 50-ml. volumetric flask, add 4.0 ml. of 0.10% gelatin solution, and dilute to the 50-ml. mark with water. Make a polarogram of the solution over the range of +0.3 to -1.0 volt us. the saturated calomel electrode. Convert the wave height from millimeters to microamperes by multiplying by the sensitivity. Find the per cent molybdenum in the sample by consulting a calibration curve in which grams of molybdenum are plotted against microamperes. Tu-o calibration curves should be used, one for 0.0010 to 0.0110 gram of molybdenum, and another for 0,0000 to 0,0010 gram of molybdenum. The calibration curves are ohtained by adding standard molybdenum solutions 1 and 2 to pure titanium sponge and carrying the samples through all steps of the procedure. The calibration curves obtained by the authors are shown in Figures 1 and 2. For samples containing 1 to 5% molybdenum proceed as follows: Dissolve a 1-gram sample in hydrochloric acid and oxidize with nitric acid as in the regular method. Dilute to 100 ml. in
Table 11. Molybdenum in Synthetic Samples of Titanium Alloys Containing More than 1% Iron (I-gram sample) ,\Io Added, "0
Fe Added, 7%
1\10 Found,
5 0 5.0 10.0 10.0 10.0 5 0 10.0 10.0
1.00 1.00 1.00
%
0 47 0.46 0.49 0 47 0.52 0.52 0 99 0 98 0.96
5.0
0.50 0.50 0.50 0.50 0.50 0.50
Table 111. IIolybdenum in Actual Titanium Alloys Sample
.4verage M o Found. 7%
WA 4 WA 7
3.34 1.94
Standard Deviation,
%
No. of Detns.
0.05 0.02
3 3
a volumetric flask. Pipet out an aliquot of solution that will contain about 0.0050 t o 0.0100 gram of molybdenum. A4dd5 ml. of sulfuric acid, evaporate to fumes of sulfuric acid, add 10 ml. of water and 20 ml. of hydrochloric acid, dilute to 50 ml., and proceed with the extraction as described above. For samples containing more than 0.01 gram of iron modify the above method as follows: Proceed to the point where the ether has been evaporated. Then dissolve the precipitate in a minimum amount of dilute hydrochloric acid (5%). Add 5 drops of nitric acid and heat the solution to boiling. Cool, wash down the sides of the beaker, and dilute to about 50 ml. with water. Make the solution ammoniacal and add 10-ml. excess ammonium hydroxide. Heat to boiling and filter through a coarse filter paper (Whatman S o . 41). Wash with hot dilute ammonium hydroxide solution (1%). Save the filtrate. Dissolve the precipitate off the filter paper into the original beaker with hot dilute hydrochloric acid (5y0)and wash the filter paper with hot water. Dilute to about 50 ml. and make a second ammoniacal separation. Dissolve the precipitate with hot dilute hydrochloric acid (5%) and make a third ammoniacal separation. Combine the ammoniacal filtrates.
V O L U M E 25, NO. 10, O C T O B E R 1 9 5 3
1443
Boil until the ammonia has been expelled. Add 10 ml. of nitric acid and eva orate to dryness to remove the ammonium salts. Add a secon8lO-ml. portion of nitric acid and repeat the evaporation to dryness. Add 5 ml. of dilute ammonium hydroxide (20%) and warm to dissolve the precipitate. Add 5 ml. of nitric acid and 10.0 ml. of perchloric acid. Evaporate to fumes of perchloric acid, fume lightly for about 10 seconds, add 15ml. of water, transfer to a 50-ml. volumetric flask, and polarograph the solution as described above. The results obtained by the authors on several synthetic samples prepared by adding standard molybdenum solution to pure titanium sponge are shown in Table I. The results obtained on synthetic samples containing iron are shown in Table 11. The results obtained for molybdenum in two actual alloys are shown in Table 111. EXPERIMENTAL
To investigate the effect of hydrochloric acid concentration on the extraction of molybdenum by ether, aliquots of standard molybdenum solution containing the equivalent of 0,0050 gram of molybdenum were added to several 1-gram portions of sponge titanium. The samples were dissolved in hydrochloric acid, oxidized with nitric acid, and evaporated to fumes of sulfuric acid, as described in the method. Various amounts of hydrochloric acid were then added and the solutions diluted to 50 ml. with water. The molybdenum was then extracted with ether, using three or five extractions. The wave heights obtained were compared with the wave height obtained using a perchloric acid solution containing 0.0050 gram of molybdenum.
0.M)OlO GRAMS
Figure 2.
1 0.00050 MOLYBDENUM PER 50 ML.
I 0.0010
OF Calibration Curve for 0.0000 to 0.0010 Gram of Molybdenum
The results for the extractions are shown in Table IV. The optimum amount of hydrochloric acid is seen to be 20 ml. The solution is approximately 4.7 -1:in hydrochloric acid and 3.6 A\r in sulfuric acid. Five extractions are necessary. T o show the necessity of adding gelatin, several polarograms were run with and without gelatin. Without gelatin a maximum was obtained; 0.008% gelatin completely suppressed this maximum I n order to show the effect of perchloric acid concentration on the wave height, aliquots of standard molybdenum solution containing the equivalent of 0.0020 gram of molybdenum were pipetted into a series of 50-ml. volumetric flasks Various amounts of perchloric acid xere then added, 4 ml of 0.1% gelatin solution were added, and the solutions were diluted to 50 ml and
Table IV.
Effect of Hydrochloric Acid Concentration on Extraction of Molybdenuni (5 ml. H S O , present per 50-ml. volume)
I-ICI, B11./50 hll. 0 5 8 9 10 15 15 20 20 25 25 30 35
No. of Extractions 3
3 3 3 3 3 5 3 5 3 5 3
hfo Added, Gram 0.00500 0.00500
hlo Found, Gram 0.00000 0.00000 0.00500 0.00116 0.00500 0.00250 0.00500 0.00359 0.00500 0.00458 0.00500 0.00454 0,00500 0,00447 0.00500 0 00493 0.00500 0 00430 0.00500 0.00490 0.00~00 0,00406 Solutions mut,ually soluble Solutions mut ually soluble
%lo Recovered. % 0
0 23.2 50.0 71 8 91 5 90 8 89 4 98 6 85.9 97 9 81 2
polarographed. The wave heights obtained (microamperes) are shown in Table V. It is readily seen that the wave height increases with increasing amounts of perchloric acid. Ten milliliters of perchloric acid per 50 ml. was chosen as most suitable for the method. T o establish the half-wave potential polarogram were run on varying amounts of molybdenum. Ten milliliters of perchloric acid and 4 ml. of 0.1% gelatin were present in all these determinations. I n calculating the half-wave potentials corrections were made for the I R drop across the resistors (IO). The halfwave potential was determined to be -0.235 volt us. S.C.E. A study of interfering elements that might be found in titanium alloys was made. Up to 0.05 gram (equivalent to 5%) of the following caused no interference: nickel (added as nickel chloride), cobalt (added as cobalt chloride), copper (added as copper sulfate), magnesium (added as magnesium sulfate), and aluminum (added as aluminum sulfate). Up to 0.13 gram of manganese (added as manganous sulfate), and 0.13 gram of chromium (added as chromic sulfate) caused no difficulty. Up to 0.02 gram of vanadium (added as vanadyl chloride) caused no significant interference; 0.01 gram of boron (added as boric acid) did not interfere. Tin interfered because it was extracted by the ether and on fuming with perchloric acid was precipitated as metastannic acid. The precipitate of metastannic acid was found to occlude significant amounts of molybdenum. This occlusion was not totally unexpected, as nietastannic acid will occlude many oxides such as antimony oxide, phosphorus oxide, and arsenic oxide. Fortunately, tin is rarely present in commercial titanium alloys. Up to 0.01 gram (1%) of iron caused no interference in the simple extraction procedure. When more than this amount of iron is present, the ammoniacal separation method must be used. I t was found that only about one quarter of the iron present was extracted by the ether. This incomplete extraction of the iron could be due to the acidity or the presence of the sulfuric acid. Up to 0.02 gram of tungsten (added as sodium tungstate) or 0.02 gram of silicon (added as sodium silicate) caused no difficulty.
Table V.
Effect of Amount of Perchloric Acid on Wave Height (0.0020gram of Mo) Wave Height, 19.6 24.0 42.8 48.4 59.2 71.2
FIC101.Ml./50 M I . 1 3 5 7 10
12 15
pa.
80.4
LITERATURE CITED
(1) Haight, G. P., ANAL.CHEM., 23, 1505 (1951). (2) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 107, New York, John Wiley & Sons, 1929. (3) Hokhshtein, Ya. P., J . Gen. Chem., U.S.S.R., 10, 1725 (1940). (4) Holtje, R., and Geyer, R., 2. anorg. Chem., 246, 265 (1941). (5) Johnson, M. G., and Robinson, R. J., ANAL.CHEM.,24, 366 (1952). (6) Ranerskii, E. .i.,and Shvartsburd, L. A,, Zauodskaya Lab., 9, 283 (1940). (7) Kolthoff, I. M., and Lingane, J. J., “Polarography,” p. 242, New York, Interscience Publishers, 1941. (8) Lingane, J. J., and Laitinen, H. .I., IND. ENG.CHEX.,4h-AL. ED., 11.504 (1939). (9) Meites. L., and Meites, T., ANAL.CHEM.,20, 984 (1948). (10) Sargent and Co., E. H., Chicago, Ill., “Manual of Instructions for Model XXI Visible Recording Polarograph,” p. 23. (11) Scott, TV. W.,“Standard Methods of Chemicrtl Analysis,” p. 606, Vol. I, New York, D. Van Nostrand Co., 1939. (12) Stackelburg, hZ. von, Klinger, P., Koch, W., and Krath, E., Tech. Mitt. K ~ u p p Forsch. , Be?.. 2 , 59 (1939). (13) Uhl, F. A . , 2. anal. Chem., 110, 102 (1937). RECEIVED for review January 17, 19,5X. 4ccepted August 18, 1953
.
