Titanium, and Molybdenum in Tungsten Using Anion Exchange Separations SIR: Because the skace program has significantly increased the importance of trace analysis in ri3fractory metals, an internal research program was initiated to fulfill this demand. Several past investigations (3, 6, 8 ) employing anion exchange resin j have involved wet chemical analyses of macro impurities in ores and refractory metal alloys; however, the literature is essentially devoid of attempts to perform a systematic study for 1 he determination of trace metallic impurities in pure refractory metals with a similar separational technique. l’he separational scheme reported here is similar to that reported by Wilkins (Y). Zirconium is also included in the analysis and is separated from titanium through a liquid-liquid extraction step. The zirconium accompanies titanium quantitatively through the ion exchange column if a sufficient volume of 8M HC1 is employed. ’l’he molybdenum can be eluted with (either 20% HF2570 HCl or 5y0 HF-25Yo HC1 (8). The 5% HF-25% HCl mixture was used in this study becsuse it is less corrosive. However, the molybdenum results might have been improved somewhat by the use of 20% HF-250/, HC1. Essential features of this procedure are presented below as they differ from those of existing methods. EXPERIMEIqTAL
Preparation of Anion Exchange Column. T h e resin (Dowex 2-X8, 200 t o 400 mesh) is cl1:aned thoroughly by decanting t h e fines in t h e usual manner a n d then rinsing the resin contained in the plarjtic ion exchange column (Ledoux & Co.) with three alternate cycles of 4N HCl and 2 N N a O H . Each acid and base rinse (600 to 700 ml.) is followed by a distilled water rinse of equal volume. Orlon wool and woven Saran disks are used to support the anion exchange resin. After the acid and base rinse, the resin is emptied from the column and washed a few times by decantation with 4 N HCl, then it is slurried back into the column t o a height of 9 inches. The flow rate of the column after this treatment is between 5 and 6 ml. per minute. The column is rinsed with about 1000 ml. of 1 : 19 H F , after which it is ready to receive 1,he sample. Dissolution of Sample. Dissolution is accomplished on a steam b a t h by treating a 1- t o $!-gram sample of
tungsten cautiously with 45 ml. of HNOs and 20 ml. of 1 : 1 HF solution in a 100-ml. platinum crucible. After the sample is dissolved, the solution is evaporated t o dryness on a steam bath with a n excess of “Os. The residue is taken u p and dissolved with 50 ml. of 1 : 9 HF on a steam bath. Covering the crucible with a thin polyethylene sheet strapped with a rubber band allows dissolution to occur under slight pressure (6). Following dissolution, the sample is diluted to 100 ml. and transferred to the anion exchange column. Column Separation and Spectrophotometric Determination. After the sample solution is added t o t h e column, the resin is rinsed with about 200 ml. of 1:19 HF. Zirconium and titanium are eluted with 500 ml. of 8M HCl and the first 75 ml. are discarded. Tungsten is immediately eluted with 600 ml. of 10% HF-60% HCl and discarded. Molybdenum is eluted with 1000 ml. of 5% HF-25% HCl, and the first 200 ml. are discarded. T o the 8M HCl fraction containing the zirconium plus titanium, and also to the HF-HCl fraction containing the molybdenum are added 40 ml. of HNOl and 10 ml. of HC104. Each fraction i s evaporated overnight on a steam bath in Teflon evaporating dishes. The perchloric acid solution remaining the following morning from each fraction can be diluted volumetrically with 1 :19 HF and a n aliquot withdrawn or the complete sample taken for analysis. If the 8N HCl fraction is slightly cloudy following the overnight evaporation, it is because of trace amounts of tungsten hydrolyzing out of solution as tungstic acid. It was determined experimentally t h a t as little as 100 fig. can cause this situation. If this occurs, the solution is diluted, cooled, and filtered. The filtrate can then be analyzed for zirconium and titanium. Ctilizing a 2-thenoyltrifluoroacetone extraction (?‘), zirconium is quantitatively separated from titanium by extraction from a 3 M HC104 medium. After the zirconium is reextracted into the aqueous phase, it is evaporated down t o HCIOl fumes. It is then transferred to a 20-ml. beaker with 1 : 1 HNO, and slowly evaporated to dryness, destroying the organic residue. The zirconium is taken up with 1.0 ml. of HCl and determined spectrophotometrically with 2-(2-hydroxy-3, 6-disulfo-1-naphthylazo) benzene arsonic acid or thoron ( 5 ) . The titanium fraction is evaporated down to fuming H2S0, and determined spectrophotometrically with disodium 1,Zdihydroaybenzene-
3,bdisulfonate or tiron (9). I n this procedure, the iron could be reduced more effectively by using a combination of sodium dithionite and thioglycollic acid. Thus, the solution is more stable because the sulfur does not precipitate. The molybdenum fraction is evaporated down to fuming H&Ol and determined with toluene-3, 4-dithiol (4). DISCUSSION A N D RESULTS
Initially the ion exchange separation step proved to be troublesome because of the inability to exclude tungsten from the 8 M HC1 fraction which retains
Table I.
Recovery of Zirconium from Tungsten”
Zir-
conium conium readded, covered, rg. MI3. Zir-
Tungsten, grams 1 00-2 00 1 00-2 00 1 00-2 00 1 00-2 00 1 0 0 - 2 00 1 00-2 00 100-200 1 0 0 - 2 00 1.00-2.00 1 00-2 00 1 00-2 00 1 00-2 00 1 00-2 00 a
= 6%
Table 11.
Tungsten, * grams 00-2.00 00-2 00 00-2 00 00-2 00 00-2 00 00-2 00 00-2 00 00-2 00 00-2 00
H3.
12 1 12 7 11 1 22 7 23 3 24 8 261 25 6 111
-0 4 +o 2 -1 4 - 2- 3_ -1 7 -0 2 1 1 1 +O 6 +11
97 8 102 96 2
-2 2 $2 -3 8
100
Using the expression
std. dev. level.
1 1 1 1 1 1 1 1 1
12 5 12 5 12 5 25 0 25 0 25 0 250 25 0 100 100 100 100 100
Difference,
0
p--;T, -___
absolute i t the 1OOpg.
Recovery of Titanium from Tungsten”
TitaTitanium nium readded, covered, rg. rg. 14 5 14 5 14 5 145 145 145 145 145 145
13 7 13 9 14 2 151 154 146 142 145 153
Difference, -0 8 -0 6 -0 3 +6 +9 +1 -3 0 +8
d. w ,
Using the expression .. std. dev. = 5Yc absolute at the 145 p g . Tungsten metal and also tungstlc oxide were used for the matrix material. a
VOL. 36,
NO. 7, JUNE 1964
e
1373
Table 111.
Recovery of Molybdenum from Tungsten"
MolybMolyb- denum denum readded, covered, rg. Mg. 12.5 9.7 12.5 11.9 12.5 9.3 250 235 250 238 250 240 250 238 250 242
Tungstic oxide, grams 1 .m-2 00 1 00-2 00 1 00-2.00 1 00-2 00
i
00-2 00 1 00-2 00 1 00-2 00 1 , oo-2.00
Difference, PG
-2.8 -0.6 -3.2 - 15 - 12 - 10 - 12 -8
Using the expression
std. dev. level.
=
3% absolute at the 250
pg.
zirconium and titanium. This problem was eliminated by washing the resin thoroughly with dilute sodium hydroxide and dilute hydrochloric acid. Also, the resin columns were covered with
brown wrapping paper to try to stabilize the oxidation state of tungsten as it passed through the column. Results were not studied. Repeated analyses of the 8 M HCI fraction indicated the maximum tungsten concentration to be about 100 pg. If 100 fig. of tungsten are present, they will hydrolyze out of solution and consequently, can be filtered off without interfering in the procedure. If no tungsten hydrolyzes out of solution, there is not enough present to cause any interference in the titanium procedure. Various elements form fluoride complexes as noted by Faris ( I ) . Each of the elements, aluminum, arsenic, beryllium, chromium, iron, lead, magnesium, mercury, thorium, tin, uranium, and vanadium at the 100-fig. level were tested for interference in the spectrophotometric procedures for zirconium, titanium, and molybdenum. The procedure used for zirconium included the 2-thenoyltrifluoroacetone extraction step. Results indicated that only vanadium interfered to any significant
extent In the disodium-1, 2-dihydroxybenzene-3, 5-disulfonate procedure for titanium (Tables I, 11, and 111). Further studies are being conducted relative to other important impurities and matrices. LITERATURE CITED
Faris, J. P., ANAL. CHEM.32, 520 119601. (2) Hague, J. L., Brown, E. D., Bright, H. A., A'atl. Bur. Std. 53, 261 (1954). (3) Hague, J. L., Machlan, L. A,, Ibid., 62, 11 (1959). (4) Hobart, E. W.,Hurley, E. P., Anal. Chim. Acta 27. 144 11962). (5) Horton, A. D., AIUAL.CHEM.25, 1331 (lM31. ( 6 j Kallmann, S., Oberthin, H., Liu, R., Ibid., 34, 609 (1962). (7) Marsh, S. F., Maeck, W.J., Booman, G. L., Rein, J. E., Ibzd., 33, 870 (1961). ( 8 ) Wilkins. D. H., Talanta 2. 355 11959). (9) Yoe, J.'H., Armstrong, A. R:, IND. EXQ.CHEY., ANAL. ED.19, 100 (1947). (1)
KES F. SUGAWARA Air Force Materials Laboratory (MAYA) Wright-Patterson Air Force Base, Ohio.
Polarographic Determination of Para ba nic Acid SIR: In the course of an investigation of the electrochemical oxidation and reduction of purines and pyrimidines, it became desirable to determine parabanic acid (I) in the reaction product mixtures obtained on controlled potential electrolysis. Polarographic examination of the electrolysis solutions suggested itself as a likely method. H-N-C=O
~l
o=c c-0
Y H
On investigating the polarographic behavior of parabanic acid at the dropping mercury electrode over the normal p H range with commonly used buffers, polarograms of parabanic acid in solutions above ca. pH 4.6 containing phosphate give evidence of interaction with the phosphate. -4s the p H is increased, the diffusion current and the slope of the polarographic wave decrease, resulting finally in the appearance of a post-wave. These phenomena are clearly evident in the curves published by Hladik ( I ) , but are attributed by the latter to the increased concentration of singly and doubly charged anions of parabanic acid. Thus, a t p H 5.6 (Figure I ) , it is possible to assume the 1374
e
ANALYTICAL CHEMISTRY
existence of an appreciable concentration of the singly charged anion since pK, for parabanic acid has been estimated from electrometric data to be 6.10 at an ionic strength of 0.1 (3) and Hladik ( I ) estimated pK1 to be 6.2 and pK2 to be 10.8. It is unlikely, however, that the post-wave is due to the presence of the anion. It is much more likely that these phenomena are due to complexation with phosphate since polarograms run in acetate buffers covering the same p H range show neither the post-Vvave nor the change in slope (Figure 1). The half-wave potentials and wave slopes [given in parentheses and estimated by the relation, slope = 0.056 / - Ea,r)] in acetate buffer a t 25" C. are -0.75 volt (1.08) a t p H 4.0, -0.78 (1.02) a t p H 4.8, and -0.84 (0.97) a t pH 5.6. I n alkaline solution, parabanic acid hydrolyzes to oxaluric acid. For the specific purpose of the experiments involved, it was helpful to use a mixed acetate-phosphate buffer, viz., an acetate-McIlvaine buffer of p H 5.1. I n this medium, the diffusion current of parabanic acid is proportional to concentration; the diffusion current constant, I (equals idj/Cm2/3 PI6), for the normal polarographic range of 0.1 to I m M is 3.60. Hladik (1) postulated that the parabanic acid is reduced in a two-electron, two-hydrogen ion reaction to 5-hydroxy-
hydantoin; the value of 3.6 for I and the pH-dependence of Ellz are consistent with such a reaction. The well-defined wave and the relatively low half-wave potential for an organic compound indicate the selectivity that may be expected in the polarographic determination of parabanic acid. In the electrolytic oxidation of uric acid in aqueous 1M acetic acid solution, 1.00 ml. of the supernatant solution obtained on centrifuging a portion of the electrolyzed reaction mixture was diluted to exactly 10.0 ml. with 0.5M pH 6.6 McIvaine buffer. The p H of the resulting solution was 5.1, since the acetic acid in the 1-ml. aliquot from the reaction mixture exceeded the capacity of the buffer. This solution was immediately transferred to the polarographic cell and deaerated with nitrogen. Polarograms were run from -0.5 to - 1.8 volts us. S.C.E.d typical polarogram is shown in Figure 2. The concentration of parabanic acid was estimated by comparison of the limiting current values of the first wave with a limiting current-concentration curve of parabanic acid prepared in the same buffer mixture a t the same pH. EXPERIMENTAL
Polarograms were recorded with a Leeds & Xorthrup Type E ElectroChemograph using a water-jacketed H-
