to stand 2 days, the precipitate was filtered, washed, and dried in a vacuum desiccator. The results, shown in Table VII, suggest that this procedure is adequate to remove (ethylenedinitri1o)tetraacetate from solution but not sufficiently accurate for a quantitative determination. A few experiments with Versene-ol and Versene-diol (obtained from the Dow Chemical Co.) showed that neither compound would precipitate magnesium under the same conditions as did (ethylenedinitri1o)tetraacetate. Khen
Versene-diol was mixed with (ethylenedinitri1o)tetraacetic acid, the rate of precipitation of magnesium EDTA was retarded and only 52y0 of the magnesium was precipitated. A mixture of Versene-ol and (ethylenedinitri1o)tetraacetic acid gave no precipitate a t all with magnesium. LITERATURE CITED
(1) Brintzinger, H., Hesse, G., 2. anorg. u. allgem. Chem. 249, 113 (1942). (2) Brintzinger, H., Munkelt, S., 2. anorg. Chem. 256, 65 (1948).
(3) Brintainger, H., Thiele, H., Muller, U., 2. anorg. u. allgem. Chem. 251, 285 (1943). (4) I. G. Farbenindustrie, Ger. Patent 638,071 (1936). ( 5 ) Ludwig, E. E., Johnson, C. R., IND. ENG.CHEY.. ANAL. ED. 14, 895 (1942). (6) Pecsok, R. L., J . Chem. Educ. 29, 597 (1952). RECEIVEDfor review March 2, 1967. Scce ted May 2, 1957. Presented before the Eighth Pitt.vburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1957.
Separation and Determination of Tantalum GLENN R. WATERBURY and CLARK E. BRICKER' The University of California, 10s Alamos Scientific laboratory, 10s Alamos, N. M. ,Between 0.025 and 3.00 mg. of tantalum are extracted into hexone (4-methyl-2-pentanone) from 6M sulfuric acid-0.4M hydrofluoric acid media and estimated colorimetrically using hydroquinone in concentrated sulfuric acid. Tantalum found averaged 100.2%, with a standard deviation of 1.3%, in 50 analyses of known solutions containing 0.5 to 3.39 mg. of tantalum and various amounts of other elements. No interference in determination of 1 mg. of tantalum was caused by 100 mg. of sodium, potassium, uranium, and plutonium, 20 to 40 mg. of aluminum, chromium(Ill), strontium, vanadium(V), thallium(I), cobalt, and iron(lll), and 2 to 3 mg. of tungsten, neodymium, cerium, zirconium, ruthenium(lV), and gold. Interference of titanium and molybdenum may b e eliminated b y double extraction. Niobium is the only metal that interferes seriously.
B
an analytical procedure was needed for estimating 0.01 t o 2% tantalum in uranium and plutonium alloys, possible methods for the separation and determination of small amounts of tantalum were investigated. Unless large samples are taken, the usual gravimetric method for tantalum is not sufficiently accurate or sensitive. KO specific titrimetric method exists for the determination of tantalum. Of the general separation techniques, extraction methods have been investigated thoroughly and seemed to offer the most promise. Stevenson and Hicks (O),using radioECAUSE
1 Present address, Chemistry Department, Princeton University, Princeton, N. J.
1474
ANALYTICAL CHEMISTRY
tracer methods, found that tantalum could be extracted with diisopropyl ketone from a solution of tantalum and niobium that was 6M in hydrochloric or sulfuric acid and 0.4M in hydrofluoric acid. Other extraction techniques include removal of tantalum from 5M sulfuric acid with 8% tribenzylamine in methylene chloride ( 2 ) , separation of tantalum from 21V sulfuric acid using 5% methyloctylamine in xylene (6), extraction of the tantalum fluoride complex with ethyl acetate (11), and separation of tantalum and niobium from hydrochloric-hydrofluoric acid solution using methyl isobutyl ketone (7, I S ) . For determination of small amounts of tantalum after separation, colorimetric methods seemed to offer the sensitivity required. The pyrogallol (4, 6, IO), perhydrol (8), and chromotropic acid reactions (I) have been adapted for colorimetric procedures. By combining separation of tantalum by extraction with colorimetric determination, a satisfactory procedure has been developed. Although it is written specifically for analysis of samples of uranium or plutonium alloys, the general method, except for the dissolution steps, may be applied to any niobium-free sample that is soluble in 6M sulfuric acid-0.4M hydrofluoric acid. APPARATUS A N D REAGENTS
Platinum dishes, with lip. Hollow-tube type extractor, with 25 x 150 mm. test tubes, as shown in Figure 1. Flasks, 10-ml. volumetric, borosilicate glass. with glass stoppers. Infrared heat lamp with socket, switch, extension cord, and ring stand attachments.
Electric hot plate. Polystyrene pipet, 2-ml., with 0.5-ml. graduations. Beckman Model DU spectrophotometer, with Corex cells of 1-cm. light path. Hydrofluoric acid, 48%, reagent grade. Hydrofluoric acid, 4M. Dilute 16.26 grams of 48y0hydrofluoric acid to 100 ml. with water. Store in a polyethylene bottle. Nitric acid, concentrated, reagent grade. Sulfuric acid, concentrated, reagent grade. Dispense from a 25-ml. reservoir buret equipped with silica gel drying tubes on the air inlet and top vent. Ammonium persulfate crystals. Hexone (4-methyl-2-pentanone), Eastman white label, or methyl isobutyl ketone, Matheson, Coleman, and Bell. Hydroquinone solution, 55 mg. per ml. Dissolve 5.50 grams of Eastman white label hydroquinone in concentrated sulfuric acid and dilute to 100 ml. with this acid. Dispense from a 5-ml. Koch microburet equipped with silica gel drying tubes at the top of the buret and a t the reservoir vent. Sodium hydroxide solution, 5 N . Dissolve 20 grams of reagent grade sodium hydroxide pellets in water and dilute to 100 ml. Store in a polyethylene bottle. Standard tantalum solution, 0.6 mg. of tantalum per gram. Accurately weigh about 0.1 gram of pure tantalum metal into a 50-ml. platinum dish, and add 4 ml. of water and 2 ml. of nitric acid. Add hydrofluoric acid dropwise to maintain a slow dissolution rate until the tantalum is dissolved. Cautiously add 35 ml. of concentrated sulfuric acid and evaporate the solution to strong fumes of sulfur trioxide. Transfer the solution to a weighed 125-ml. glassstoppered flask and dilute to about 100 ml., using concentrated sulfuric acid to wash the dish and for the dilution. Weigh the flask and contents and calcu-
late the amount of tantalum per gram of solution. (The niobium content of the tantalum metal was less than O.lyo by spectrographic analysis and caused no interference.) DISSOLUTION OF SAMPLE
Uranium alloys are dissolved by treating a known weight of the sample in a platinum dish with 3 ml. of 6 M sulfuric acid and 5 ml. of concentrated nitric acid for each gram of alloy. The nitric acid is added in small increments after the dish has been covered with a watch glass. When the initial reaction subsides, the mixture is warmed under a heat lamp until effervescence ceases. After the watchglasshasbeenwashedand removed, 0.5 ml. r: concentrated hydrofluoric a c J is added for each gram of sample, and the reaction mixture is warmed again. The hydrofluoric acid treatment is repeated until the sample is dissolved, and then the solution is diluted to known volume. If a green precipitate of uranium(1V) fluoride persists, a few milliliters of nitric acid are added and the solution is evaporated to fumes of sulfur trioxide before final dilution. For plutonium alloys, a known weight of the alloy is placed in a centrifuge tube and 2 to 3 ml. of water are added. Concentrated hydrochloric acid is added dropwise a t a slow rate to prevent rapid evolution of hydrogen or loss of sample by frothing or spraying. After the evolution of hydrogen ceases upon further addition of hydrochloric acid, the centrifuge tube and its contents are warmed to ensure the complete reaction of any metal. The reaction mixture is centrifuged, the supernatant liquid is transferred to a volumetric flask, and any residue is placed in a platinum dish. The residue is treated with hydrofluoric, nitric, and sulfuric acids and evaporated until strong fumes of sulfur trioxide appear. The resulting mixture, if com-
ORGANIC
-1 im.
Figure 1 . Hollow stirrer extractor with test tube
pletely soluble in water, is added to the solution in the volumetric flask. If a white insoluble residue remains, 2 drops of concentrated hydrofluoric acid and 1 t o 2 ml. of water are added and the mixture is warmed. This solution is added to the solution in the volumetric flask. If the sample is not completely soluble by this procedure, a sodium nitrate-sodium hydroxide fusion is performed (12). An aliquot from the solution of the dissolved sample is taken for analysis. Because of the somewhat limited solubility of plutonium in 6-44 sulfuric acid, aliquots taken for analysis should not contain over 300 mg. of plutonium, unless larger volumes are used in the extraction. Aliquots containing several grams of uranium have been analyzed by the recommended procedure. RECOMMENDED PROCEDURE
Transfer an aliquot of the solution of the sample, containing 0.025 to 3.0 mg. of tantalum, to a platinum dish. Add sufficient sulfuric acid to make a total of 3.3 ml. and evaporate the solution under a heat lamp and on a hot plate until strong fumes of sulfur trioxide are evolved. Transfer the solution to a 25 X 150 mm. test tube, using a total of 5 ml. of water to wash the dish. Cool the solution and finally wash the dish with 1 ml. of 4M hydrofluoric acid and 1 ml. of water. Cool the solution again, add 10.0 ml. of hexone, and stir the mixture for 5 minutes with the hollow stirrer. After the layers have separated completely, pipet 8 ml. of the organic layer into a 30-ml. platinum dish containing 3 drops of 5N sodium hydroxide. For samples containing over 0.5 mg. of tantalum, take 2-ml. aliquots and obtain duplicate results. Slowly evaporate the aliquot to dryness under a heat lamp with frequent mixing by swirling. After the dish has cooled, add about 10 drops of water, and then cautiously add 5 drops of concentrated hydrofluoric acid. For samples containing milligram amounts of titanium or molybdenum, add 3.5 ml. of concentrated sulfuric acid, evaporate the solution to fumes of sulfur trioxide, and perform a second extraction as described above. For other samples, add 2 to 3 ml. of concentrated sulfuric acid to the residue and evaporate the solution under a heat lamp until fumes of sulfur trioxide are evolved. Cool the solution, add 5 to 10 mg. of ammonium persulfate crystals, and warm on a hot plate until evolution of gas ceases. Repeat the ammonium persulfate treatment until the sulfuric acid solution is colorless. Then heat the solution on a hot plate and under a heat lamp until very strong fumes of sulfur trioxide are evolved for 3 to 5 minutes. Allow the dish to cool, and as soon as it may be handled conveniently, transfer the solution to a dry 10-ml. volumetric flask, using small portions of concentrated sulfuric acid t o wash the dish. The total volume of the solution and washings should be less than 7 . 0 ml. When the solution has cooled to room temperature, add from a buret
3.0 ml. of hydroquinone solution (55 mg. of hydroquinone per ml. of sulfuric acid), and dilute to 10 ml. with concentrated sulfuric acid. Xeasure the absorbance (ATII) of the solution a t 375 mp relative to a reference containing 3.00 ml. of hydroquinone solution and sulfuric acid t o make 10 ml. Repeat the procedure, using concentrated sulfuric acid in place of the sample aliquot to obtain the absorbance (&lank) for the reagent blank. and determine the absorbance (&ta&xd) for a standard, using a known weight of the standard tantalum solution. Take aliquots of the same size from the organic layers for the sample, blank, and standard. Use the following equation to calculate the milligrams of tantalum in the sample aliquot: Ta in sample aliquot, mg. = (Ta in std. mg.) (AT*Astsndard
-
Ablank)
Ablank
EXPERIMENTAL RESULTS AND DISCUSSION
Extraction of Tantalum. Stevenson and Hicks (9) reported that tantalum n-as 97% extracted into diisopropyl ketone from 6 M sulfuric or hydrochloric acid containing 0.4M hydrofluoric acid. A similar extraction efficiency for tantalum was obtained when the analyses of the solvent layers were based upon the tantalum-hydroquinone color. Because of the coextraction of molybdenum, iron, and other interfering substances from hydrochloric acid solution, only the use of the sulfuric-hydrofluoric acid media was considered for the determination of tantalum. Factors affecting tantalum extraction from sulfuric acid solutions were investigated to determine optimum experimental conditions. The effects of acid concentrations, tantalum concentration, temperature, extraction time, and extraction solvent were studied. Extractions of 0.90 mg. of tantalum from solutions 6M in sulfuric acid and O.OM, 0.2MJ and 0.4M in hydrofluoric acid were 2.6, 76, and 98y0 efficient, respectively. Higher hydrofluoric acid concentrations were avoided because of etching of the glassware and the satisfactory recOveries obtained with the 0.4M concentration. For solutions 0.4M in hydrofluoric acid and 2.0MJ 4.Oi$f, and 6.OM in mineral acid, extraction efficiencies of 73, 92, and 98%, respectively, were found. Solutions more concentrated in sulfuric acid were inconvenient to extract with hexone because of the slow separation of the phases and the partial miscibility of the layers. Whereas the solubility of hexone in 6M sulfuric acid solutions containing 0.4M hydrofluoric acid is about 1%, hexone is miscible with the aqueous phase when the sulfuric acid concentration is increased to about 9M. This study indicated that the acid conVOL. 2 9 , NO. 10, OCTOBER 1957
* 1475
centration of 6 X in sulfuric acid and 0.4M in hydrofluoric acid is optimum. The efficiency of extraction into hexone at the optimum acidities was independent of the tantalum concentration from 0.0 to 3.0 mg. per 10 ml. and independent of the temperature from 0" to 35' C. No significant change in efficiency was observed when the time of extraction was varied from 2 to 10 minutes for 10-ml. volumes of the organic and aqueous phases. For larger volumes or solutions containing precipitates, extraction times have been increased to 10 to 15 minutes t o ensure complete extraction. Diisopropyl ketone, hexone, n-amyl acetate, and bis-2-chloroethpl ether mere used as solvents for the extraction of tantalum. Hexone and diisopropyl ketone extracted about 98% of the tantalum present in a single 5-minute extraction; n-amyl acetate and bis-2chloroethyl ether were 74 and 84y0 efficient, respectively. Although hexone and diisopropyl ketone had the same extraction coefficient for tantalum, hexone was used because it was readily available in sufficient purity to prevent excessive charring when evaporated. Preparation of Extracts for Color Development. When the diisopropyl ketone or hexone layers were evaporated to dryness under a heat lamp without first making the extract alkaline with sodium hydroxide, the organic solvent charred. Ignition or wet oxidation of these charred residues, followed by dissolution of the tantalum in hydrofluoric and sulfuric acids or a potassium bisulfate fusion, gave low and erratic results because of volatilization of some tantalum fluoride and mechanical loss during oxidation. Purification of the organic solvent by distillation prior to extraction produced no significant improvement. Experiments in which hexone containing small amounts of sulfuric and hydrofluoric acids was evaporated, showed that the acids in the extract were causing the charring. By making the extract alkaline with sodium hydroxide and mixing the hexone and sodium hydroxide frequently by, swirling while slowly evaporating the organic layers, the charring was practically eliminated. TVhen the residue from the evaporation was dissolved in hydrofluoric and sulfuric acids, the slight discoloration which appeared could be removed by treating the solution with 5 t o 10 mg. of ammonium persulfate and gently warming the mixture on a hot plate. Large amounts of ammonium persulfate caused a loss of tantalum because of the evolved gas and spray. Approximately 11% of the tantalum was lost from a solution to which 800 mg. of ammonium persulfate were added. Up to 225 mg. of ammonium persulfate caused no 1476
ANALYTICAL CHEMISTRY
340
350
360
370
380
390
400
MILLIMICRONS
Figure 2. Absorption curves for tantalum-hydroquinone system
significant tantalum loss when formation of spray was minimized by carefully warming the reaction mixture. Colorimetric Determination of Tantalum. Of the colorimetric methods for determining tantalum ( I , 4, 5, 8, IO), it was decided to investigate the use of chromotropic acid. This method consisted of adding 0.1M chromotropic acid in concentrated sulfuric acid to a solution of tantalum in concentrated sulfuric acid and measuring the absorbance at 375 mp. Because the absorbance of the reaction product between tantalum and chromotropic acid never reached a constant value a t room temperature but continuoqsly increased for several hours, an attempt was made to stabilize the color and increase the sensitivity of the reaction by heating the reaction mixture. An optimum heating time was not found because the absorbance of the solutions increased steadily with heating time a t 90" C.; heating a t 130' C. charred the organic reagent and destroyed the color. However, the color of the tantalum-chromotropic acid prepared a t room temperature obeys Beer's law on dilution with concentrated sulfuric acid and with various tantalum concentrations under stringently controlled experimental conditions. In an effort to find a more suitable
Table I.
Tantalum Taken, Mg.
0.249 0.258 0.259 0.278 0.356 0.292
reagent for the colorimetric determination of tantalum, compounds related t o chromotropic acid were tried: 2hydroxynaphthalene - 8 - sulfonic acid, which was less sensitive than chromotropic acid for tantalum; 1,5-naphthalenediol, which showed no characteristic absorption with tantalum; 2,7-naphthalenediol, m hich appeared to have strong fluorescence that was quenched by tantalum; resorcinol, JThich mas very insensitive to tantalum; catechol, which showed an absorption with tantalum a t 355 mp but a lower sensitivity than chromotropic acid; and hydroquinone, which shoTved the most immediate promise. Early in the investigation of the tantalum-hydroqhinone reaction, it was found that water and heating had a very detrimental effect on sensitivity. Therefore, all reactions were carried out a t room temperature in concentrated sulfuric acid protected from atmospheric moisture. It was difficult to obtain reproducible values when absorbances were measured at the wave length of maximum absorbance (355 mp) because of the extremely strong absorption of hydroquinone a t this wave length. As shown in Figure 2, the absorption of hydroquinone decreases rapidly to a low value at 375 mp, whereas the absorption of the tantalum - hydroquinone color decreases only 357, from its maximum value. This disadvantage of having a slightly reduced sensitivity at the higher wave length is more than offset by eliminating the necessity for exactly reproducing the hydroquinone concentration from sample to sample. Thus, by measuring the absorbance of the tantalum-hydroquinone color a t 375 mp, good linearity with tantalum concentration as well as excellent reproducibility from day to day was found. The effect of hydroquinone concentration on color mas investigated by adding various amounts of 0.5M hydroquinone solution in sulfuric acid to known amounts of tantalum in fumed sulfuric acid, diluting the solution to 10 ml. with the acid, and measuring absorbance against a reagent blank containing the same amount of hydroquinone. The results (Table I) show that the tantalum-hydroquinone color
Effect of Hydroquinone on Tantalum-Hydroquinone Color Hydroquinone hdded , Moles Hydroquinone . Absorbance, Absorbance, Mg. Moles Tantalum 375 hlfi Mg. Tantalum 0.833 0.207 72.8 11.0 1.32 0.340 176 27.5 1.67 0.434 349 55.0 2.04 0,567 652 110.0 2.27 0.807 763 165.0 2.44 0.713 1239 220.0
~~~
Table II.
Tantalum Taken,
~~
~
~~
Formation and Stability of Tantalum-Hydroquinone Color
Age of
Absorbance After After 60 min. 120 min. 0.497 0.810 1.352
0,2219 0.3503 0.6052
Hydroquinone Soln., Hr. 2 2 2
Immediately 0.491 0.803 1.343
After 30min. 0.492 0.802 1.348
0,2160 0.2820 0.4462
20 20 20
0.494 0.638 0.992
0.498 0.640 0.998
0.501 0.642 1.004
0.502 0.643 1.006
0.507 0.647 1.016
0,2230 0.2918 0.3728
44 44 44
0.507 0.650 0.822
0.502 0.648 0.819
0.505 0.652 0.822
0.505 0.650 0.820
0.506 0.654 0.824
0.499 0.652 0.818
0.2778 0.2808 0.3921
116 116 116
0.608 0.613 0.840
0.609 0.613 0.841
0.609 0.613 0.841
0.610 0.613 0.841
0.610 0.612 0.839
0.607 0.612 0.835
Mg.
After
180 min.
After
Absorbance,
1 day
Mg. Tantalum
0.511 0.880 1.400
2.213 2.292 2.219 2.241 2,287 2.262 2.223 2.257 2.274 2.228 2.205 2.236 2.189 2.183 2.142 2.171
Av.
Av.
Av.
Av.
is very dependent on hydroquinone concentration. Although 3.0 ml. of the hydroquinone solution in 10 ml. does not produce a limiting value for the absorbance of the tantalum-hydroquinone color, the effect of the hydroquinone concentration is less if 3.0 ml. of the reagent are used rather than smaller amounts. Larger amounts are not practical, because the limited solubility of hydroquinone in sulfuric acid does not permit solutions much more concentrated than 0.5M and larger volumes cannot be used if the final volume is to be 10 ml. The nature of the tantalum-hydroquinone complex was investigated by the slope ratio method ( 3 ) . The absorbances of two series of solutions were measured. One series contained a constant concentration of hydroquinone (8.0 X 10-5;M) and concentrations of tantalum varying from 3.0 X 10+ to 11.5 X 10-4M. In the other series, the tantalum concentration was constant a t 2.7 X l O - 4 M and the hydroquinone concentration varied from 1.17 X to 18.75 X 10-3M. The slope of the line for constant tantalum concentration is 0.0463 and that for constant hydroquinone concentration is 0.0223. The ratio of these two slopes, 2.08 to 1, suggests that the formula of the tantalum-hydroquinone complex must be tantalum (hydroquinone)2. To demonstrate the stability of the tantalum-hydroquinone color, the absorbances of several solutions containing various amounts of tantalum and 3.0 ml. of hydroquinone solution were measured as soon as possible after mixing and then after standing for definite periods. The results of this study (Table 11) clearly show that the absorbance of the tantalum-hydroquinone color immediately reaches a virtually constant value. I n some cases, absorbance increases slowly with time, but in others it is constant for a day
or more. Generally, for the most accurate measurement of the tantalumhydroquinone color, absorbance should be measured within an hour after reagents are mixed. Table I1 also indicates that the absorbance per milligram of tantalum is essentially constant for a hydroquinone solution that is a t least 2 days old. As the hydroquinone solution stands, the absorbance per milligram of tantalum decreases slowly, but hydroquinone solutions that have stood 10 days are satisfactory, if a known tantalum sample is run each day. The tantalum - hydroquinone color shows an absorbance of about 2.2 per mg. of tantalum in 10 ml. of solution (Table 11). Thus, the limit of detection of tantalum is about 0.01 mg. in 10 ml. of solution. For determinations of tantalum to within 5%, a t least 0.025 mg. should be present. Because the absorbance of the tantalum-hydroquinone color is linear with tantalum concentration to absorbances of a t least 1.5, the maximum amount of tantalum that can be determined in 10 ml. of solution is about 0.7 mg. For absorbances greater than 0.100, the precision of the tantalum determination is well within 2%. Interferences. When 1 mg. of tantalum was extracted from solutions t h a t contained a t least 100 times as much sodium, potassium, uranium (VI), plutonium(III), or plutonium(IV), 20 t o 40 times as much aluminum, chromium(III), strontium, vanadium(V), thallium(I), cobalt, or iron(III), and 2 to 3 times as much tungsten, neodymium, cerium(III), cerium(IV), zirconium, ruthenium(IV), or gold, no interference was observed in either the extraction or subsequent colorimetric determination. No evidence has ever been obtained that any of these metallic ions is extracted with the tantalum, and recovery of tantalum
in all these studies was complete within the reproducibility of the method. On the other hand, about 0.5% of any titanium present is extracted by the hexone. Molybdenum alone is not extracted appreciably by hexone, but if tantalum is present, significant amounts of molybdenum are coextracted with tantalum. Niobium is largely removed from the aqueous phase by the hexone. Because titanium, molybdenum, and niobium all react with hydroquinone and produce colored solutions which show considerable absorption a t 375 mp, these elements interfere in varying degrees with the determination of tantalum. The interference of equivalent weights of titanium and molybdenum on the tantalum determination is not detectable, but ratios higher than 2.5 parts of titanium or molybdenum to 1 part of tantalum give high apparent recoveries. By performing a double extraction, as described in the recommended procedure, 1 mg. of tantalum may be determined successfully in the presence of 30 mg. of molybdenum or 100 mg. of titanium. Because 60 to 70% of the niobium added was extracted with the tantalum, even a double extraction failed to eliminate interference caused by niobium, unless the weight ratio of tantalum to niobium was a t least 100 to 1. For ratios of 10 to 1, the values for tantalum found were 10 to 15% high. Xobium is the only metal that interferes seriously with the determination of tantalum. In addition to the metals studied for interference with the extraction and colorimetric determination of tantalum, milligram quantities of arsenic, beryllium, barium, zinc, cobalt, zirconium, thallium(I), aluminum, sodium, potassium, and ammonium ions had no effect on development of the tantalum-hydroquinone color. However, thallium(II1) reacts with hydroquinone and causes a positive interference; VOL. 2 9 , NO. 10, OCTOBER 1957
1477
Table 111.
Tantalum Taken, Mg. 0.500 0.640 0.922 1.000" 1.5OOb 2. o o o c 2.500 3.000
Foreign Element,
a.
1.000
0.676" 0 . 79lC 0.428 0.638 0.597 0,760 1.000
0.5 1.0
Zirconium Zirconium Cobalt Thallium Thallium Vanadium Vanadium Vanadium Vanadium
1.300 1.950 0,500 0.640 1.000 1.073 1.073 1.073 2.000 0. 69OC 0.602c 0,506" 0.822O 0.7470 3,390
a
%
80.0
1.000 2.000
Tantalum Found, 100.2 100.9 100.0 99.6 100.7
i .000
1,000
Table IV.
Tantalum in Known Solutions
Foreign Element Added
1.0
15.0 40.0
10.0
30.0 20.0 5.0 5.0
. d
1 0
i
k I m
n 0
P
a 7
100.4
99.5 101.3
100.4 101.0
99.8 101.3 100.0
E
h
100.0 101.0 99.0 102.1 100.5
100.0
99.0 100.8 99.5 98.7 100.5 99.3 101.5 100.2 98.1 101.2 98.8 Average tantalum found, % 100.2 Standard deviation, yo 1.3
6 determinations.
* 3 determinations.
2 determinations. Uranium, 40.0 mg.; zirconium, 2.0 mg.; molybdenum, 1.0 mg. ' Iron, 0.05 mg.; molybdenum, 1.0 mg.; zirconium, 2.0 mg.; cobalt, 3.0 mg.; plutonium, 50.0 mg. Uranium, 92.0 mg.; tungsten, 0.5 mg. 0 Uranium, 105 mg.; tungsten, 0.5 mg. Molybdenum, 0.5 mg.; zirconium, 2.0 mg.; cobalt, 6.0 mg. Cerium, 1.0 mg.; neodymium, 1.0 mg. i Cobalt, 3.0 mg.; neodymium, 2.0 mg.; zirconium, 1.0 mg.; cerium, 1.0 mg.; ruthenium, 0.5 mg. Cobalt, 3.0 mg.; cerium, 1.0 mg.; zirconium, 1.0 mg.; molybdenum, 0.5 mg.; ruthenium, 0.5 mg. Cobalt, 9.0 mg.; zirconium, 1.0 mg.; molybdenum, 0.5 mg. Chromium, 20.0 mg.; strontium, 20.0 mg.; aluminum, 20.0 mg.; potassium, 15.0 mg. Chromium, 12.0 mg.; strontium, 8.0 mg.; aluminum, 4.0 mg.; potassium, 9.0 mg. Chromium, 8.0 mg.; strontium, 4.0 mg.; aluminum, 12.0 mg.; potassium, 6.0 mg. Chromium, 10.0 mg.; strontium, 5.0 mg.; aluminum, 5.0 mg.; potassium, 7.5 mg. a Chromium, 4.0 mg.; strontium, 12.0 mg.; aluminum, 8.0 mg.; potassium, 3.0 mg. 7 Plutonium, 80.0 mg.; iron, 0.08 mg.
'
large amounts of cobalt precipitate in the concentrated sulfuric acid and cause low absorbances for the tantalum. Water, fluoride, and phosphate cause a decrease in absorbance of the tantalum-hydroquinone color, nearly linear with the amount of substance added. Water caused 0.019% decrease in the tantalum-hydroquinone absorbance for every milligram of water present in the 10 ml. of solution; phosphate, a 0.024% decrease per mg.; and fluoride, about 6% per mg. It is obvious that even 1478
ANALYTICAL CHEMISTRY
though the effect of water and phosphate is small, it is essential to use reagents that are free from phosphate and are protected from atmospheric moisture, to obtain reproducible results. Because fluoride exerts such a pronounced effect, all fluoride must be removed before the color with hydroquinone is developed. This is achieved by heating the sulfuric acid solution of the tantalum to very strong fumes of sulfur trioxide for 3 to 5 minutes. Reliability of Method. Because no
Tantalum in Uranium Alloys
Tantalum Found, 70 Colori- GraviDifference Sample metric metric between No. method method Methods, % 1 1.31 1.34 +o. 03 2 0.98 0.95 -0.03 3 1.04 1.03 -0.01 4 0.99 0.98 -0.01 5 0.98 0.97 -0.01 6 1.02 0.98 -0,04 7 0.97 0.98 +o. 01 8 0.95 0.95 0.00 9 0.84 0.86 $0.02 10 0.91 0.88 -0.03 11 0.91 0.90 -0.01 12 0.94 0.89 -0.05 13 0.84 0.82 -0.02 14 0.51 0.50 -0.01 15 0.53 0.53 0.00 16 0.52 0.53 $0.01 17 0.54 0.53 -0.01 18 0.47 0.47 0.00 19 0.53 0.53 0.00 20 0.53 0.56 +0.03 21 0.54 0.56 +0.02 22 0.56 0.56 0.00 23 0.55 0.55 0.00 24 0.56 0.56 0.00 25 0.57 0.56 -0.01 Av. -0.005
standard samples of uranium-tantalum alloys were available, t h e reliability of t h e method was estimated from analytical results for solutions containing known amounts of tantalum and comparison of results obtained by the described colorimetric procedure and by a gravimetric method for actual uranium samples. Data for 50 representative determinations of tantalum in known solutions containing various amounts of other elements, as given in Table 111, show an average for the tantalum found of 100.2%, with a standard deviation of 1.3%. Because the method was developed for determining tantalum in uranium and plutonium alloys, large amounts of the two latter elements were added in some of these determinations. These solutions were fumed with sulfuric acid and treated as actual solutions of samples, and the results should indicate the reliability of the method for samples. However, because fewer individual analyses are run on an actual sample and the variation in the color development itself is about 1.5Yo for duplicates, a reproducibility of about 2% is usually obtained for samples. The comparison analyses were performed on uranium-tantalum and uranium-tantalum-tungsten alloys containing 0.5 to 1.3% tantalum. T o eliminate sampling errors, aliquots were taken from solutions containing 8 to 20 grams of the sample and analyzed by the two methods. The first 13 results in Table IV are for the binary alloys; the remaining 12 are for the ternary alloys. Because of the low
tantalum content, aliquots containing 3 to 5 grams of the alloy were required for each gravimetric analysis, in which the tantalum was precipitated by acid hydrolysis, ignited to the oxide, and weighed. For the ternary alloys, which contained about 0.5% of tungsten, a separation was required in the gravimetric method. Analyses of filtrate solutions from the gravimetric precipitations showed that a small amount of tantalum remained in solution. The extraction-colorimetric procedure described was ideally suited for the low percentages of tantalum, and no interference was caused by the tungsten. Because the results for the gravimetric determination of tantalum in the binary alloys are slightly lower than the colorimetric results, although the two methods give comparable results with ternary alloys, it is possible that the tantalum is not completely precipitated,
as shown by the analysis of the filtrate solution, and that a small positive interference is caused by tungsten in the gravimetric method. I n any event, the difference between the two methods is small, considering the low concentrations of tantalum used. ACKNOWLEDGMENT
Gravimetric analyses were performed by Owen Kriege and Ross Gardner of this laboratory.
(5) Ikenberry, L., Martin, J. L., Boyer, W. J., ANAL. CHEW 25, 1340 (l9?3): (6) Leddicotte, G. W., Moore, F. L., J. Am. Chem. Soc. 74, 1618 (1952). ( 7 ) Milner, G. W., Barnett, G. A., Smales, A. A., Analyst, 80, 380 (1955). (8) Palillrt, F. C., Adler, N., Hiskey, C. F., ANAL. CHEY. 25, 926 (19.53). (9) Stevenson, P. C., Hicks, H. G . , Ibid., 25, 1517 (1953). (10) Thanheiser, G., Mitt.Kaiser Wilhelm Inst.. Eisenforsch. Dusseldorf 22. 258 '(1940).' (11) Vanossi, R., Annales QSSOC. quim. argentina 42, 59 (1954). (12) Waterbury, G. R., Bricker, C. E., ANAL.CHEM. 29, 129 (1956). (13) Wernig, J. R., Higbie, K. B., Grace, -
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LITERATURE CITED
Brandt, ITT. IT., Purdue University, West Lafayette, Ind., private communication, July 22, 1955. Ellenburgh, J. Y., Leddicotte, G. W., Moore, F. L., ANAL. CHEY. 26, 1045 (1954). Harvey, A. E., Jr., Manning, D. L., J . Am. Chern. SOC. 72, 4488 (1950). Hunt, E. C., Wells, R. A., Analyst 79, 345 (1954).
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I
J. T., Speece, B. F., Gilbert, H. L., Ind. Eng. Chem. 4 6 , 6 4 4 (1954).
RECEIVED for review March 19, 1957. Accepted May 8, 1957. Work done under the auspices of the Atomic Energy Commission.
Determination of Phthalate Esters in Propellants HARRY STALCUP, FRANK McCOLLUM', and C. 1. WHITMAN U. S. Naval Powder Factory, Indian Head, Md. b A general method for determination of phthalate esters in experimental propellants involves hydrolysis of the phthalate by anhydrous ethanolic potassium hydroxide after reduction of nitrate, nitro, and nitramine compounds with powdered zinc and acetic acid. The phthalate i s recovered quantitatively as the dipotassium salt and may b e determined either b y weighing the salt, or by titration with perchloric acid dissolved in glacial acetic. This method has been applied to the determination of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, and dioctyl phthalate, in the presence of nitroglycerin, dinitrotoluene, 2-nitrodiphenylamine, diphenylamine, ethyl centralite (centralite-1 ), and triacetin. Phthalate recovery from synthetic propellant mixtures agrees within 1 .O% of the amount added, except for dimethyl phthalate where losses were 5 to 6%.
S
the introduction of the complicated Thames (14) lead phthalate gravimetric method and the Kavanagh (6) phthalic acid titration, several procedures have become availINCE
* Present address, Quartermaster Research and Development Command, Natick, Mass.
able for the determination of phthalate esters in plastic and propellant formulations. Some procedures have the disadvantage of being designed for specific or limited application. The iodometric method of Butts and associates (1) for diethyl phthalate is based on dichromate oxidation of the ethyl alcohol produced after hydrolysis of the ester with alkali. The chromatographic method developed a t Picatinny Arsenal (IO), in addition to being specific only for diethyl phthalate, has the usual disadvantages of chromatographic techniques in that the absorbent and developing solvents must meet rigid specifications for reproducible results. Several methods based on the use of instruments have been published. The Whitnack and Gantz (15) polarographic technique and the Pristera (9) infrared technique have been successfully applied to propellants. However, they require expensive equipment not readily available in many analytical laboratories. The ultraviolet spectrophotometric method of Shreve and Heether (1.9) and the Shaefer and Becker (12) gravimetric methods appear to give satisfactory results for alkyd resins and lacquers. One of the most difficult problems associated with the analysis of propellants has been the elimination of inter-
ferences due to nitrate esters and aromatic nitro compounds. The Lamond (7) procedure, which employs ammonium sulfide for the reduction of the nitrated compounds, is rather involved. The Mullaly (8) technique, based on the earlier work of Dickson and Easterbrook ( 2 ) involves the reduction of nitroglycerin and nitro compounds with methanolic ferrous chloride. Wight (16) found that nitrate esters could be effectively destroyed by reduction with zinc in the presence of sulfuric acid, after which the phthalate ester could be precipitated quantitatively as the alcoholate of dipotassium phthalate by the Goldberg (3) modification of the Kappelmeier (6) technique. These methods, however, are applicable to only a limited number of propellant formulations. They cannot be used for propellants requiring complicated preliminary separation techniques. More recently Grodzinski (4) has described a method based upon a titanous chloride reduction of nitrate esters and nitroaromatic compounds from which the phthalate is separated by means of a liquid-liquid extraction with petroleum ether. The phthalate is saponified and determined by titration. This method is obviously not applicable to those formulations containing other esters which may be etherextracted, saponified, and titrated. VOL. 29, NO. 10, OCTOBER 1957
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