V O L U M E 2 5 , NO. 10, O C T O B E R 1 9 5 3
1515
spectively, using the indicators employed by Kolthoff, Carr, and Carr The polymer content was determined by Tyeighing the extracted specimen Sample Sample Sample Control Test after i t had been dried to constant weight in a X-418BL, Sample A o . 4, X-558, X-603, % 70 7c _ _ _ _%_ vacuum a t 50" C. The time required for drying S o h - Extrac- Solu- ExtracSolu- ExtracSolu- Extracwas about 23 hours. The nonpolymer content Constitrtrnt tion tion tion tion tion tion tion tion was determined by difference with corrections Organic acid 4 80 1.72 5 . 1 2 5.12 5 66 .j 59 5.85 .5 80 Soap 0 098 0 . 1 4 6 0 . 2 2 1 0.254 0 00.5 0 038 0.048 0 086 applied for the ash content of the extracted niateetabilizcr 1.314 1 . 1 4 0 1.105 0 . 9 4 3 1 283 1.104 1.183 1 094 Moisture or volatile rial using the values for inorganic saks givrn matter 0 00 , , . 0.00 ... 000 ... 0.00 ... in Table V. Ash (total) 0 98 ... 0.42 . .. 0 78 ... 0.85 ... Ash (froin soap As a check on the completeness of extraction, the 0 017 _. 0.038 . . . 0.009 .. 0 0083 . . . calcd.) refractive index of the dried specimen was measAsh (inorganic salts by difference) 0 96 0.38 ... 0 78 ... 0 84 . ured and the corresponding bound styrene content Totalnonpolyiiier 7.17 7'26 6.83 6 83 7 73 7.93 7 92 8. . 13 Total polynier 02.83 .,. 93.17 ... 92.27 ... 92.08 .. determined. The mean values obtained for these Bortnd atyrene (in polyitier) 23.99 23 90 24 01 23 94 20 22 1 9 . 9 5 2 3 . 4 5 23 87 tests are given in Table V under the columns Bound styrule (in labeled extraction. Consistent,lyhigher values were rubber) 22 27 ., 22 37 ... 18 66 ... 21.59 .., Bound bittadiene obtained for soap, probably because a different (in rubtier)" 70.56 .. , 70 80 .. . 73.61 , .. 70.49 ... indicator x-as used in the extraction procedure. a This fignre inclrtdes boitnd modifier which usually aiiiounts t o about 0.67,. The consistently low values obtained for hoth _ _ _ _ _ ~ _ ~ _ _ _ _ ~ _ ~ _ . ~ stabilizers were probably due to incomplete extraction. Consistently higher values were 011Addition of the value for inorganic ash to the values for nioistairied for total nonpolymer by the extraction procedure. These ture, organic acid, soap, and stabilizer gives a value for the total probably were due to the small quantity of low polymer that the authors of the procedure indicate is removed by the extrarnonpolymer portion of the sample. The polymer portion is detion solvent. S o consistent differences existed for organic acid termined by subtracting this value from 100. The bound strrene or bound styrene content. in the polymer, as determined by experiment, is then reduced to Further attempts to check the values for the total polynier conthe bound styrene in the whole rubber. tent of t,hesesamples were made using a modification of the method For comparison these samples were analyzed using the extracof Kolthoff, Carr, and Carr (4)for determining the low-polytion procedure suggested by Kolthoff, Carr, and Carr (4) for the mer content of ethanol-toluene-r~-aterextracts by precipitating determination of the polymer and nonpolymer portions of GR-S. from a chloroform solution with iodine chloride and subsequeiitly T h e extraction solvent was prepared by adding 10 parts by volume weighing the dried precipitate. In this modification, t,he whole of water to 100 parts of ethanol-toluene azeotrope, composed of 70 sample was dissolved in chloroform and treated with iodine chloparts of ethanol and 30 of toluene. The thinly sheeted rubber ride. These values were grnerally ahout 3Oj, lower than those sample was cut into small strips and duplicate tests were made b>calculated by difference or ohtained experimentally liy the estracextracting 6-gram portions for 2 hours with 100 ml. of this solvent. tion procedure. T h e extract was poured into a 250-ml. volumetric flask, 100 ml. of This discrepancy is undouhtedly an effect that depends on the fresh solvent were added to the extraction flask, and the extracquantity of material analyzed hecause the test has been used very tion continued for an additional 2 hours. The second 100-ml. successfully for estimating small quantities of low polymer. The portion was added to the volumetric flask and the extract made discrepancy is obviously far greater than could reasonably be esup to volume with fresh solvent. pected. Accordingly, the iodine chloride precipitation method The stabilizer content of the samples was determined spectre is not suitah]? for the determination of the total polymer portion photometrically on a 3-ml. aliquot of the extract. Organic acid and soap were determined on 100- and 147-mI. aliquots, reof GR-S. Table V. Comparison of Analyses of Gross Chemical Constituents of GR-S by the Complete-Solution Procedure and Extraction Method (4)
(e).
,
(Chemical Analysis of GR-S by Complete Solution Procedures)
Titration of Mineral and Organic Acids in Toluene-Ethanol Solution FREDERIC J. LINNIG AND ALICE SCHNEIDER, .Vutional Bureau of Standards, Wushington, D. C .
I
N T H I S method for the determination of mineral acid and or-
ganic acid, which are sometimes found together in GR-S synthetic rubber, both types of acid are determined in a solution of the sample by making a single titration to two different end points with the same indicator. The presence of mineral acid, in addition to the organic acid always present in GR-S, is caused by the use of a greater quantity of sulfuric acid during the coagulation than is required for reaction Kith the soap in the latex. This excess mineral acid is known to retard the rate of cure of the polymer and thus to change the processing characteristics of the material. PROCEDURE
A 2-gram specimen of sheeted GR-S rubber is dissolved in 140 ml. of toluene in the manner described above. When the rubber is completely dissolved, the solution is allowed to cool to room temperature, and 30 ml. of 95% ethanol are added with constant swirling of the flask during the addition to prevent large quanti-
ties of polymer from precipitating. The flask should be swirled until all flocks of precipitated polymer have completed redissolved without heating. About 7 drops of the m-cresol purple indicator solution are added, and the solution titrated with approximately 0.1 X alcoholic sodium hydroxide. A pink color indicates the presence of mineral acid, and the titration for this acid proceeds until the color definitely reaches the yellow range. From this end point, the titration for organic acid continues to the first change toward purple that appears as a darkening of the solution which persists after swirling the flask. The titrations are conveniently performed with a 5-ml. buret graduated in 0.01 ml. For the determination of the blank corrections, 140 ml. of solvent are refluxed and treated in the same manner. The first blank titration is made by adding standard 0.05 Y alcoholic hydrochloric acid until the solution changes from the yellow color, normally obtained with the indicator and solvent, to a pale salmon. The second blank is determined by back-titrating the same portion of solvent with 0.1 6 alcoholic sodium hydroxide to the first change to purple which does not disappear on swirling the flask.
ANALYTICAL CHEMISTRY
1516 CALCULATIONS
Mineral acid (as sulfuric acid), Organic acid,
By
=
NB = A0 =
NA Bp
= =
Bo
=
w =
K
=
4.90
=
yo =
yo =
+ A o N A )4.90
(BYNB
(Bp - By
W
- Bo) NBK W
milliliters of standard base (sodium hydroxide) used to titrate the rubber solution to the yellow color normality of the standard base (sodium hydroxide) milliliters of standard acid (hydrochloric) used to titrate the blank to the first change from yellow to a pale salmon color normality of the standard acid (hydrochloric) milliliters of standard base (sodium hydroxide) used to titrate to the second change (faint purple end point) milliliters of standard base sodium hydroxide required to titrate the blank from the pale salmon color to the faint purple weight of the original dry sample 28.4 when the organic acid is determined as stearic acid; 34.6 (determined empirically) when determined as rosin acid ( 1 1 ) milliequivalent weight of sulfuric acid X 100 DISCUSSION OF PROCEDURE
The 5 to 1 mixture of toluene and absolute ethanol used for the analysis of GR-S containing soap cannot be employed when sulfuric acid is present because it will react with ethanol a t the boiling temperature of the mixture. Sulfuric acid will also catalyze the esterification of the organic acid with the ethanol, thus leading to low results for each constituent. Toluene by itself was found to be satisfactory for dissolving the rubber. However, the presence of ethanol in the solution is necessary to dissolve the indicator and to prevent phase separation during the titration. Moreover, in order to make possible the complete recovery of the mineral acid it has been found necessary to add 1.5 ml. of water to the quantity of solution used here. Only a portion of the acid is titrated when less than 1.5 ml. of water are present, while an increase in this quantity to 2.0 ml. does not alter the amount of mineral acid determined. Phase separation results if more than 2 ml. of water are present. The minimum quantity of water necessary (1.5 ml.) is conveniently supplied by adding the indicated amount (30 ml.) of 95% ethanol. The color change from pink t o yellow is sufficiently sharp for visual detection even in solutions of rubbers containing dark stabilizers such as BLE, the darkest in color of a number of antioxidants used in the rubber industry. It should be emphasized, however, that the titration should be continued to the. yellow color range of the indicator which on addition of small increments of sodium hydroxide does not undergo further change toward yellow. It would, of course, be possible to devise a scheme of analysis of the gross chemical constituents using toluene alone as the original solvent instead of toluene-et,hanol. However, the aliquots of the toluene solution used for the organic acid and soap tests would have to be diluted with mixtures of toluene and ethanol to produce a satisfactory titrating medium. Similarly, tests for stabilizer and bound styrene could be worked out. Some such scheme of analysis might indeed be very desirable where experience indicates the presence of mineral acid in a fair proportion of the samples tested. Usually only very few samples contain mineral acid, and in view of the greater operational difficulties presented by such a modified procedure it would seem desirable to employ the original technique for the other constituents and to make an additional determination for mineral acid only when titration of the soap aliquot indicates its possible presence. RESULTS
Using established methods (7), the accuracy and precision of these tests were studied. Precision. The data illustrating the precision of these tests
are given in Table VI. Samples A and B are two samples of GR-S stabilized with BLE for which the test for soap wm negative. The precision of the mineral acid test is definitely better than that of the soap titration and should be satisfactory to estimate even the small quantities of mineral acid which may be present in GR-S. The precision of the test for organic acid is about the same as that obtained when this constituent is titrated in the absence of mineral acid. There is an indication of a real though, very small day-to-day variability for both tests. Accuracy. In view of the small quantities of mineral acid usually encountered in this work the experiments were planned to determine within 0.01 ml. of reagent (0.0025%) whether the blank titration serves as a correction for the constant type of error in this test. The data indicated the blank titration to be adequate within this limit. The study of the relative type of error showed the method to give results which were about 6.5y0 lowi.e., low by 6.5% of the total quantity present. Accordingly, in the analysis of samples requiring 0.1 ml. (0.0260J0), the titration would be low by no more than about 0.0065 ml., which is the equivalent of about 0.0015 % mineral acid. This quantity of 0.025% exceeds the mineral acid content of any of the samples tested thus far in this laboratory.
Table VI. Run 1
2 3 4 5
Mean sib Ed 8 4
Mineral Acid a n d Organic Acid in GR-So
Sample A , % Mineral acid Organic acid 0.013 0.012 0.015 0.015 0.015 0.015 0.014 0.014 0.013 0.015 0.0141 0.0010 0.0012 0.0016
4.21 4.72 4.74 4.69 4.73 4.73 4.76 4.78 4.69 4.69 4.724 0.025 0.028 0.038
Sample B, % ’ Mineral acid Organic acid 0 011 0.012 0.015 0.018 0.015 0.015 0.014 0.015 0.012 0.012 0.0139 0.0010 0.0012 0.0016
4.79 4.79 4.83 4.83 4.83 4.83 4.85 4.83 4.83 4.77 4.818 0.025 0.028 0.038
a D a t a obtained for the mineral acid content of both samples were analyzed statistically as a unit and the d a t a for organic acid were treated similarly. b 81 is the standard deviation corresponding to intrinsic variability. C sd is the standard deviation corresponding t o day-to-day variability. d 8 , is the standard deviation of a single random determination.
Thus, for practical purposes the method is accurate within the limits of the precision of measurement. The study also showed that the results obtained for organic acid in this test were accurate within the limits of the precision of measurement and were not appreciably affected by the presence of different quantities of mineral acid. ACKNOWLEDGMENT
This work was performed as part of the research project sponsored by the Reconstruction Finance Corp., Office of Synthetic Rubber, in connection with the Government Synthetic Rubber Program. LITERATURE CITED
Arnold, Aurelia, Madorsky, Irving, and Wood, L. A,, . ~ N A L . CHEW,23, 1656-9 (1951). Banes, F. W., and Lund, A. J., private communication to the Reconstruction Finance Corp., Office of Synthetic Rubber. Frieden. Earl. I b i d . Kolthoff, I. hi., Carr, C. W., and Carr, Betty J . , J . Polvmer Sci., 2, 63742(1947). Laitinen, H. A , , Nelson, J. S., Jennings, W. P., and Parks, T. D., private communication to the Reconstruction Finance Corp., Office of Synthetic Rubber. Laitinen, H. il., Parks, T. D., Xelson, J. S., and Jennings, W. P.,
Ibid. Linnig, F. J., Illandel, John, and Peterson, J. M., to be published in ANAL.CHEM.
V O L U M E 25, NO. 10, O C T O B E R 1 9 5 3 (8) Linnig, F. J., Milliken, L. T., and Cohen, R. I., J . Research Natl. Bur. Standards, 47, 135-8 (1951). (9) hlaron, S. H., Llevitch, I. N., and Elder, M. E., ANAL.CHEM.,
24, 1068-70 (1952). (10) Moses, F. L., private communication to the Reconstruction
Finance Corp., Office of Synthetic Rubber. (11) Reconstruction Finance Corp., Office of Synthetic Rubber, “Specifications for Government Synthetic Rubbers” (Revised Edition, Oct. 1, 1952). (12) Tryon, Max, J . Research Natl. Bur. Standards, 45, 362-6 (1951).
1517 (13) Tryon, Max, unpublished work. (14) Tryon, Max, and Arnold, Au&lia, private communication to the Reconstruction Finance Corp., Office of Synthetic Rubber. (15) Yanko, J. A,, J . PoEymm Sci., 3,576-600 (1948); Rubber Chem. and Technol., 22, 494-517 (1949). RBCEIVED for review February 14, 1953. Bccepted August 4, 1953. Presented in part at the 61st Meeting of the Division of Rubber Chemistry of the AMERICANCHEMICAL SOCIETY, Buffalo, N.Y., October 1952.
Separation of Tantalum and Niobium by SoIvent Extraction PETER C. STEVENSON, Radiation Laboratory, University of California, Berkeley, Calif., AND
€LARRY G. HICKS, California Research and Development Co., Livermore, Calif. During a systematic investigation into methods of radiochemical purification of the less familiar elements, the need arose for separating niobium and tantalum from each other and from the remaining elements. The reported extractability of the fluoride complex of tantalum seemed a promising lead. The extractability of the fluoride complexes of tantalum and niobium by diisopropyl ketone was investigated from various mineral acid media as a function of conditions. Both complexes were found to extract, that of tantalum being much more readily extracted than that of niobium. The difference in
extractability was such that the two elements could readily be separated from each other, and the extraction of both elements from a sulfuric acid-hydrofluoric acid medium was found to be specific for the two elements. The rate of extraction was rapid. The fraction extracted under proper conditions was high enough so that the phenomenon should prove useful for a radiochemical analytical method for the two elements, with possible applications in quantitative separation. The process should be applicable to industrial separation and purification of tantalum and niobium elements.
T
ANTALUM and niobium have been found t o extract into certain polar organic solvents from aqueous solutions containing hydrofluoric and hydrochloric acid (1-3). The dependence of per cent extracted on extraction conditions has been studied t o investigate the possibility of a tantalum-niobium separation based on this phenomenon. Diisopropyl ketone was chosen as a solvent for examination, as it does not extract significant amounts of hydrochloric acid. Preliminary investigations indicated that a tantalum-niobium separation was indeed possible, tantalum being far more readily extracted than niobium. In addition to the system tantalumhydrochloric acid-hydrofluoric acid, the systems tantalumsulfuric acid-hydrofluoric acid, tantalum-nitric acid-hydrofluoric acid, and tantalum-perchloric acid-hydrofluoric acid were inves- ; tigated. The system sulfuric acid-hydrofluoric acid seemed t o offer the most nearly specific solvent-extraction separation and purification of tantalum and niobium. EXPERIMENTAL
Tantalum tracer was prepared by dissolving neutron-irradiated tantalum metal in nitric and hydrofluoric acids, adding a large volume of 6 M hydrochloric acid, and extracting twice into diisopropyl ketone. The ketone layers were combined and washed twice with an aqueous solution 6 M in hydrochloric acid and 1 M in hydrofluoric acid. The tantalum was then brought into aqueous medium by bringing the diieopropyl ketone twice into contact with water. The resultant clear aqueous solution had a p H of 1.5 and no tantalum precipitated upon standing 2 or 3 weeks in a glass centrifuge cone. Aliquots (50 PI.) of the above solution containing about 1 mg. of tantalum pentoxide were added t o measured amounts of standardized acids and the total volume was adjusted to 1 ml. in a glass centrifuge cone. One milliliter of Eastman technical grade diisopropyl ketone was added to the cone, and the mivture was stirred well with a platinum stirring wire for 1 minute and centrifuged briefly; then equal aliquots were taken from each phase for counting.
-----=-
e
2ov a
0.40YW
0
0.20M HF
A O.IOp? HF
0
0
LO
21)
3.0
4.0
5.0
6.0
HCI MOLARITY
Figure 1. Tantalum Extracted in System Hydrochloric Acid-Hydrofluoric Acid-Diisopropyl Ketone as a Function of Hydrochloric Acid Concentration
Care was taken to work rapidly and at room temperature to minimize the effects of the hydrofluoric acid on the glass. The rate of extraction was too rapid to measure. If the mineral acid was known t o extract appreciably into diisopropyl ketone, the ketone was pre-equilibrated with the same concentration of the mineral acid used in the extraction. Hydrochloric and sulfuric acids did not extract appreciably, while perchloric, hydrofluoric, and nitric acids did extract. For simplicity and ease of handling, results were based on initial concentration of the hydrofluoric acid in the aqueous phase because the hydrofluoric acid extracted appreciably into the ketone.
