RadiochemicaI Determination of Ca rbon-Li t hiu m Bonds in Lithium-Terminated Polymers Using Tritiated Alcohols D. R. CAMPBELL and W. C. WARNER Research and Development Center, The General Tire & Rubber Co., Akron 9, Ohio The carbon-lithium bond reacts rapidly and irreversibly with aliphatic alcohols to form a carbon-hydrogen bond. When the alcohols contain hydroxylic tritium, labeled products are obtained which usually may b e isolated for radioassay. Excess alcohol is removable by washing with water if the tritium in the products is not labile. These conditions are realized in the case of polybutadienyllithium for which a quantitative radiochemical assay method has been developed. The hydrogen kinetic isotope effect in the termination reaction is negligible as determined both by the analysis of polymers having known C-Li contents and by the titration technique. Results using the radiochemical method agree well with those by carboxylation. Precise reaction temperature control in the range of 25-65" C. is not required and any of several alcohols may b e employed. The method appears applicable in the presence of a number of common sample contaminants and to certain other anionic polymerization systems.
S
titrimetric methods based on diverse analytical reactions have been developed for the determination of the carbon-lithium bond in organolithium compounds. Early techniques involving carboxylation (9) or alkalimetry following hydrolysis (8, 9) were subsequently refined in a double titration procedure (8) which has been examined critically (5, 1 2 ) . More recent approaches have applied redox principles and procedures have been described in which direct oxidation ( d ) , iodination (S), and the reduction of vanadium pentoxide (4) were utilized for the analysis of organo-lithium derivatives. In addition, a thermometric titration has been reported (6). Although suitable assay methods for the rarbon-lithium bond in many known species have thus been well established, the accurate determination of this structure in materials of relatively high molecular weight, such as mono- and dilithio derivatives of polydienes, requires an analytical technique having EVERAL
276
0
ANALYTICAL CHEMISTRY
broader generic applicability and greater sensitivity. The inaccuracy and imprecision which have been obtained in these laboratories with the vanadium pentoxide procedure for alkyllithium when applied to solutions of lithiumsubstituted polymers are attributable principally to the physical nature of the samples and to the low concentrations of the C-Li bond. Gaseous and solid carbon dioxide have been used extensively for the conversion of the carbon-lithium bond to a carboxyl group. However, this method of derivatization requires meticulous care to suppress side reactions which may significantly reduce the yield of carboxylic acids and result in the formation of carbonyl and hydroxyl groups in the products (10). In addition, the carboxylated materials must be subjected to extensive posttreatment for conversion of the lithium salts to the free acids and for the removal of lithium ions and solvent. The reaction with carbon dioxide also involves the resolution of complex acid mixtures when used for the determination of polymeric carbonlithium bonds in the presence of residual n-butyllithium or other initiators. The importance of lithium derivatives of polymers as reactive intermediates for the preparation of functionally terminated analogs necessitates an independent assay method for carbonlithium content prior t o the introduction of the functional groups. An assay of this type serves to establish the efficiencies of the derivatizing processes and, moreover, permits an evaluation of the effect of polymerization conditions on the degree of functionality of the lithium-containing precursor. Radiochemical techniques have been used for both the qualitative and quantitative characterization of ionic polymerization systems, and values for the hydrogen kinetic isotope effect, K H I K T , have been determined in the termination reactions of several polymers prepared with organometallic compounds. Carbon-metal bonds in polyethylene initiated with titanium tetrachloridealkylaluminum catalysts were found to react with hydroxyl-tritiated methanol a t 68' C. with an isotope effect of 3.7
( 7 ) . Conversely, no significant isotope effect could be detected when butanol, correspondingly labeled, was employed for the termination a t 50" C. of polymerizations of propylene initiated& similar catalysts ( 1 ) . The polymerization of propylene by organometallic substances has also been studied using tritiated methanol ( I S ) . The mechanism of the initiation of methyl methacrylate polymerization by g-fluorenyllithium has been elucidated through the use of carboxyl-tritiated acetic acid, wherein an isotope effect of 1.9 a t -60' C. was reported (11). I n addition, possible limitations of the quantitative use of terminators containing labile tritium have been discussed (15). I n the present method, the carbonlithium bond of polybutadienyllithium is converted to a carbon-hydrogen bond by treatment of the polymer with a low molecular weight aliphatic alcohol containing tritium in the hydroxyl group.
-CH2eLi@
+ ROH3+
-CHzH3
+ ROLi
All available evidence indicates the absence of significant isotopic discrimination in the conversion reaction as performed by the addition of the lithium-terminated polymer to an excess of the tritiated alcohol in an aprotic solvent. EXPERIMENTAL
Equipment. A Nuclear-Chicago Model 6000 dynamic condenser electrometer (Nuclear-Chicago Corp., Des Plaines, Ill.) was employed in conjunction with 250-ml. ionization chambers and a Sargent Model SR recorder (E. H. Sargent Co., Chicago, Ill.) for all specific activity determinations. Reductions were conducted in No. 1720 Pyrex glass tubes, 10-mm. 0.d. (Corning Glass Works, Corning, N. Y.) within a Temco Model F1625 furnace maintained a t 640' C. by a Temco Model CP505T controller. (Thermoelectric Manufacturing Co. , Dubuque, Iowa). Reagents. Ethanol, n-propanol, and iso-butanol were refluxed 16 hours over freshly ignited calcium oxide and distilled. n-Propanol was also used after further drying with Linde Type 4A Molecular Sieve.
Reagent grade methanol (Baker's) was used as received. The water content of each alcohol was determined by the direct titration method of Karl Fischer prior to the addition of the tritiated water. The tritiated reactants were prepared by adding sufficient tritiated water of specific activity one curie per gram (New England ru'uclear Corp.) to the pure alcohols (16) to provide an accurately-determinable tritium concentration in the radioactive polydienes. The specific activities of the labeled alcohols thus prepared ranged from 19 to 26 microcuries per mmole, with the exception of the isobutanol, the specific activity of which was 49 microcuries per mmole. The solvents hexane and benzene were dried by refluxing over sodium metal or calcium hydride and distilling. Radiochemical Method. Solutions of 0.20 ml. of the1 tritiated alcohols in 1 t o 2 ml. of dried solvent were prepared in 5-ml. glass-stoppered Erlenmeyer flasks for those analyses conducted a t room temperature a n d in 10-ml. glass-stoppered tubes for those determinations performed at higher temperatures. Reactions at temperatures of 45-65" C. were carried out in a thermostatted bath. Benzene was employed as the solvent for those reactions involving methanol, and hexane was used with the homologous alcohols. An aliquot of 0.4 to 1.0 ml. of the polymer solution was slowly introduced by means of a hypodermic syringe beneath the surface of the reagent solution with continuous agitation. The reaction mixture was subsequently transferred with 10 to 12 ml. of hexane to a separatory funnel having a Teflon stopcock, and the hydrocarbon solution was washed with two 50-ml. portions of distilled water and then with saturated sodium chloride solution. After centrifugation of the organic phase, the solvent was evaporated on a steam bath and the polymer was dried 30 minutes at 60" C. and 5 mm. Hg. The specific activities of all reactants and polymers were determined by the reduction of 10 to 12 nig. of the materials to a mixture of elemental tritium and tritiated methane in evacuated and sealed glass tubes over zinc and nickel oxide a t 640' C. (17) in the presence of approximately 6 mg. of water. Zirconium silicate microcrucibles were employed as sample containers for the polymers. The mixture of gaseous reduction products was transferred by vacuum technique to a 250-ml. ionization chamber and the activity present n-as assayed using the rate of charge mode of the electrometer (18). The labeled alcohols employed in the analytical procedure were suitably diluted with the purified compounds prior to radioassay. The majority of the analyses, together with all specific activity determinations, were performed in duplicate. No significant corrections for variations in instrument efficiency were necessary during the course of this work as determined by the use of a standard ionization chamber. Carboxylation Procedure. Lithium salts of t h e carboxylic acids formed
by addition of the solutions of polybutadienyllithium to large excesses of carbon dioxide a t approximately -78' C. were converted to the free acids and purified with ion exchange resins; t h e solvent was removed under vacuum. The majority of the carboxylations were conducted with crushed dry ice in heptane slurry; the remainder were performed with liquid carbon dioxide. The carboxyl contents of the materials were determined by nonaqueous titration in pyridine solution with a 0.01N solution of sodium methoxide in methanol as titrant and thymolphthalein as indicator. Polymerization Procedure. Standa r d polymers designated A a n d B were prepared from commercial butadiene of high purity containing less t h a n 10 p.p.m. of water using a commercial heptane solution of n-butyllithium as initiator. Polymerizations were conducted in an inert atmosphere, to complete conversion at 50' C. in purified hexane or heptane containing less than 10 p.p.m. of water. The volume of hydrocarbon solvent used was reduced to as small an amount as practicable to minimize the loss of carbon-lithium bonds by reaction with impurities. The concentrations of the polymers in these preparations were approximately 250 grams per liter.
advantageous-Le., thermostatically controlled baths are obviated. Carbonlithium contents of polybutadienyllithium determined with the four
Table 1. Reactions Conducted at Different Temperatures
Sample
Reaction tepz:,
NO.
0.375 0.375 0.369 0.370 0.336 0.336 0.335 0.417 0.418 0.416 0.416 0.374 0.377 0.379 0.380 0,393 0,395 0.397 0.404 0.252 0.255 0.248 0.249 0.251 0.253
60 24
RT 65
RT
25
65 26
RT 65
27
RT 60
28
RT 45 65
Room
Labeled alcohol MeOHS
EtOHa EtOH3
EtOH3
i-BuOH'
EtOHa
temperature, approximately
25' C. ; reactions a t this temperature were
not thermostatted.
Table II. Determinations Performed with Several Alcohols at Approximately
25' C." Carbonlithium bond content
+
where W1 and W Zare the atomic weights of lithium and hydrogen, respectively. For purposes of comparison, the carboxyl contents of the acids were converted to the equivalent concentrations of carbon-lithium bonds by a similar calculation. Effect of Alcohol and Temperature. Values observed for the concentration of C-Li bond in polybutadienyllithium by reaction with the four aliphatic alcohols employed were, within experimental error, independent of the substrate and of temperature within the approximate range of 25' t o 65' C. Analyses which indicate the absence of a significant temperature effect are presented in Table I and include data obtained with methanol, ethanol, and isobutanol. Since the termination reaction is exothermic, the insensitivity of results to temperature is experimentally
of
polymer, mmole/g.
RTa
17
RESULTS AND DISCUSSION
Expression of Results. T h e number of milliequivalents of hydrogen ( E ) present per gram of labeled polydiene introduced b y reaction with t h e tritiated alcohols was calculated from the equation E = (8,) (103)/(&), where S, is the specific activity of the polymer in microcuries per milligram and S , is the specific activity of the alcohol in microcuries per millimole. The carbon-lithium bond content (C), expressed as millimoles per gram, of the polymer in solution was then obtained from the equation (1000) ( E ) C= 1000 ( E )( W , - W,)
Carbonlithium bond content
of
Sample No.
polymer, mmole/g.
7
0.375 0.375 0.388 0.366 0.341 0.349 0.339 0.342 0.520 0.556 0.546 0.524
8
9
Labeled alcohol MeOHa E~OH~ ~-RUOH~ MeOH3 EtOH3 n-PrOH3 i-BuOH' MeOH3 EtOH3 n-PrOHa i-BuOH3
a Tables I11 and IT' also contain relevant data which are not given here.
VOL. 37, NO. 2, FEBRUARY 1965
a
277
alcohols a t room temperature and which illustrate the nondependency of results upon the molecular weight of the compound containing hydroxylic tritium are shown in Table 11. Additional relevant data also appear in Tables 111 and IV. The results of analyses with the several alcohols indicate the range in C-Li content for a given polymer which may be expected through the influence of experimental technique in the tritium assays of polymer and substrates by the reduction procedure. Estimation of Isotope Effect. T h e presence of a normal hydrogen isotope effect in the termination reaction will yield a polymer having a tritium concentration less than t h a t of the alcohol on a n equivalent weight basis. Therefore, the true degree of isotopic discrimination in the reaction must be known for the method to be useful for the determination of absolute concentrations of lithium bound to carbon. For the evaluation of the isotope effect two methods mere employed. I n each of these approaches the lithiumterminated polymers were prepared
Table 111.
from butadiene with an alkyllithium derivative as initiator. The monomer is readily polymerized in aliphatic hydrocarbon solution by low concentrations of substances such as butyllithium ( 1 4 ) . Also, an essentially one-to-one molecular correspondence exists between organolithiuin initiators of this type and polydienes of relatively low molecular weight produced by polymerization. The initiator solutions were assayed by the vanadium pentoxide method ( 4 ) and known amounts were used to polymerize weighed quantities of dry butadiene to complete conversion in dry hydrocarbon solution. Through this technique the carbon-lithium contents and the approximate number-average molecular weights of the polymers were known. The concentrations of the C-Li bond in each of the polymers were then determined with the tritiated alcohols. The isotope effect, KH/’KT, was calculated by the procedures described subsequen tly . The preparations of polybutadienyllithium designated A and B were formulated to contain 0.273 f 0.008 and 1.36 f 0.04 mmoles of C-Li bond per
Isotope Effect, K H / K T , by Analysis of Polybutadienyllithium Standards
Designation of preparation
Apparent Isotope Mmoles of C-Li/g. mol. wt. effect, Addeda Found of polymerb K H / K T ~ A m 0 ~ 3 0.273 i 0.008 0.269 3700 1.01 0.272 EtOH3 0.282 3540 0.97 0.282 n-PrOH3 0.275 3640 0.99 0.275 0.266 3740 1.02 i-BuOH3 0.269 B n-PrOH3 1 . 3 6 f 0.04 1.32 758 1.03 1.32 1.33d 1.32d a Based upon relative amounts of butadiene and n-butyllithium employed. * Calculated on basis of assumed monofunctionality of polymer; predicted value, 3660 f 110 for preparation A and 735 f 22 for B . c Expressed as ratio of predicted C-Li content to that found. d Assays of preparation after storage for 1 month at room temperature. Labeled alcohol
Table IV. Isotope Effect, K x / K T , by Reaction of Polybutadienyllithium with Excess and Stoichiometric Amounts of Hydroxyl-Tritiated Alcohols C-Li content of polymer, mmole/g. Isotope
Designation
of preparation
c
Labeled alcohol
llleOH3
With excess alcohol 0.884 0.891 0.900 0.919 0.474 0.484 0.484 0.492 0 308
n-PrOH3
0,308
m
0
~
3
n-PrOH3
D
~vie0~3 n-PrOH3
E
0 305
B titration 0.931
7
effect, KH/KT~ 1.06
0,900
0.99
0.489
1.02
0.478
0.98
0 317
1 03
0.308 0.99 0.311 Ratio of C-Li content obtained with a stoichiometric amount of alcohol-i.e., titratiori-to that observed with a large excess of alcohol. (1
278
ANALYTICAL CHEMISTRY
by
gram of polymer, respectively. The carbon-lithium bond contents of additional polydiene preparations designated C, D, and E were between those of A and B. The procedure applied initially for estimation of the isotope effect involved the direct determination of the C-Li bond in the standard polymers A and B using the technique described in the experimental section. Preparation A was analyzed with the four tritiated alcohols; sample B was analyzed only with n-PrOH3. The isotope effect was then evaluated as the ratio of the predicted carbon-lithium content of each polymer to that obtained with each of the alcohols. The results of these analyses are presented in Table 111. The mean of the values calculated for K H I K T by this method for polymer iz with all of the labeled reactants is 1.00 and for polymer B the isotope effect is 1.03. For the subsequent determination of isotopic discrimination in the termination reaction by a technique which would be insensitive to possible losses of carbon-lithium bonds during polymerization the preparations designated C, D, and E were employed. hliquots of these solutions were added to large excesses of MeOH3 and n-PrOH3 in benzene and hexane, respectively. To separate portions of the polymer solutions the labeled alcohols were added a t rates sufficiently slow to assure the complete consumption of all added tritiated reagent before excess of the latter was present. The isotope effect in this case was evaluated as the ratio of the specific activity of the polydiene obtained by the careful addition of alcohol to polymer solution to that yielded when the labeled reactant was initially present in large excess. The results of these analyses appear in Table IV. Using this technique, the mean value for the isotope effect with these three preparations is 1.01 with a range of 0.98 to 1.06. Effect of C-Li Concentration of Solutions. The determination of the influence of volumetric concentration of the carbon-lithium bond in hydrocarbon solutions on results involves the estimation of the approximate minimum concentration of C-Li that may be handled by the syringe technique without loss of active functionality in transfer. The carbon-lithium molarities of the preparations d and B , with which essentially quantitative recoveries were obtained, were 0.07 and 0.45, respectively. The molarities of the majority of polymer solutions analyzed, and with which good precision was obtained, were in the range of 0.05 to 0.08. However, in the case of samples having C-Li concentrations less than approximately 0.0351, use of the syringe method was accompanied frequently
by the loss of significant amounts of organolithium as evidenced by a decrease in repeatability. For this reason the addition of a slight excess of the labeled alcohol to the polymer solution is preferred as the means for quenching preparations having low volumetric C-Li concentrations. Removal of Excess Tritiated Alcohol. T h e adequacy of t h e washing technique for t h e removal of tritium activity not chemically incorporated in polymers was established by terminating the lithium derivatives with excesseb of nonradioactive alcohols, adding the labeled alcohols, and subjecting the solutions to the procedure described for isolation of the polymer. The ion currents obtained upon assay of the polydienes used in these experiments were equivalent to that of background. Comparison with Carboxylation. Results of t h e application of both the radiochemical method and carboxylation to 23 lithium-twminated polydienes, ranging in molecular weight from 600 to approximately 6000, are summarized in Table V. The majority of these samples were carboxylated and reacted with tritiated alcohols within a period of 24 hours. The values obtained for C-Li content using the tritium technique exhibit relative deviations from those yielded by carboxylation which range from -6.2 to +9.6%; the average relative deviation, without regard to sign, is 3.4%. I n the analysis of a second series of 12 preparations the radiochemical procedure indicated carbon-lithium contents 10 to 20% higher than those observed by reaction with carbon dioxide. This disparity is believed to be attributable to incomplete conversion of the C-Li bonds in these polymers to carboxyl groups. Accuracy and Precision. Accuracy of t h e isotopic procedure as described in the experimental part is limited by the accuracy with which the isotope effect may be determined. An examination of the data of Tables I11 and IV indicates a possible variation from unity of approximately *3% for this factor. Specific activity determinations of substrate and polymer reproducible to &ly0 may further decrease the certainty of absolute accuracy to an estimated value of k 5 % . However, when polymers are terminated by the slow addition of alcohol to sample-i.e., by titration--the effect of even slight isotopic discrimination is circumvented and the accuracy is determined by the precision of the radioassay method. T o estimate the reproducibility of the isotopic method, duplicate analyses of sample number 1 of Table V were performed with tritiated ethanol on four consecutive days; the results of these determinations appear in Table VI. The mean carbon-lithium content
of this polymer is 0.349 mmole/gram and the range of values obtained is 0,009. The standard deviation of the method, as estimated from these limited data, is 0.0027, corresponding to a relative standard deviation of approximately 0.8%. Interferences and Applicability. T h e radiochemical method for the determination of the C-Li bond in polymers is subject to interference from other organolithium derivatives which yield nonvolatile compounds when reacted with alcohols. I n addition, precautions must be observed in the isolation of the mdioactive products to avoid nonquantitative recoveries attributable to the volatilization of relatively low molecular weight species. Although the present method has been applied only to polybutadienyllithium in the lower range of molecular weight, it is considered to be equally suitable for the analysis of higher polymers. The procedure is applicable generally to anionic polymerization systems in which the polymeric carbanion is a sufficiently strong base to react instantly and irreversibly with hydroxylic hydrogen. However, the rate of a given termination reaction is determined principally by the strength of the aliphatic alcohol as a proton donor relative to that of the carbanion as a proton acceptor. Furthermore, for analytical purposes, the practical characterization of this acid-base relationship in the termination of a given anion involves the estimation of the hydrogen kinetic isotope effect in the reaction. Application of the radiochemical procedure to polymerization systems other than that concerned in the present investigation therefore requires the antecedent determination of the magnitude of possible isotopic discrimination; the evaluation of this isotopic factor is most conveniently performed by the titration technique. Knowledge of isotope effects in specific termination reactions may then permit the calculation of absolute concentrations of carbonmetal bonds in the respective polymers and thereby extend the scope of the method to carbanions less basic toward alcohols than that derived from polybutadienyllithium. The method appears applicable in the presence of residual n-butyllithium, phenyllithium, lithium alkoxides and phenoxides, and lithium metal and hydride. ACKNOWLEDGMENl
The authors acknowledge the helpful suggestions and assistance of many coworkers in the implementation of the several experimental facets of this work. Particular recognition is extended to W. L. Shannon, who performed the
Table V. Carbon-Lithium Bond Determinations by Radiochemical Method and by Carboxylation
Carbon-lithium content per cent Sam- of polymer, mmole/g. relative Ple Carboxyla- deviaNo. Tritiationb.c tiond tion0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 a
0.349" 0.248 0.440 0.364 0.394 0.441 0.505 0.257 0.068 0.379 0.252 0.453 0.364 0.388 0.396 0.400 0.377J 0.342J 0.536' 0.3430 0.292 1.47@ 1.6@
0.355 0.232 0.471 0.381 0.391 0.462 0.514 0.259 0.067 0.381 0.230 0.457 0.376 0.405 0.368 0.384 0.369 0.334 0.519 0.323 0.281 1.44 1.69
-2.0 +6 9 -6.2 -4.5 +0.8 -4.5 -1.7 -0.8 +1.5 -0.5 +9.6 -0.9 -3.2 -4.2 +7.6 +4.2 +2.2 +2.4 +3.3 +6.2 +3.9 +2.1 -1.8
Per cent deviation of values observed
by the radiochemical technique from those
by carboxylation. All reactions were performed with EtOHa a t approximately 25" C., except where otherwise indicated. c Mean of two analyses, except m indicated. d Single determinations. e Mean of eight determinations. Average of results from determinations performed with methanol, ethanol, and isobutanol. 0 Analyses conducted with n-propanol.
Table VI. Reproducibility of Radiochemical Method
Relative order Of
analyses" 1
2
3 4
Carbonlithium bond content of polymer, mmole/g.
Deviation from mean
0.350 0.354 0.350 0.348 0.348 0.345 0.347 0.350
+0.001 +0.005 +0.001 -0.001 -0.001 -0,004 -0.002 +0.001
Duplicate analyses performed on four consecutive days. Sample number 1 of Table V. (I
majority of the specific activity determinations. LITERATURE CITED
( 1 ) Bier, G., Hoffmann, W., Lehmann, C., Seydel, G., Makromol. Chemze 58, 1 (1962). ( 2 ) Clifford, A . F., Olson, R. R., ANAL. CHEM.31, 1860 (1959). VOL. 37, NO. 2, FEBRUARY 1965
0
279
(3) Zbid., 32, 544 (1960). Collins, P. F., Kamienski, C. W., Esmay, D. L., Ellestad, R. B., Ibid., 33, 468 (1961). ( 5 ) Eberly, K. C., J . Org. Chem. 26, 1309 (1961). 16) 36, 854 , , Everson. W. L., ANAL. CHEM. (1964). ( 7 ) Feldman, C. F., Perry, E., J . Polymer Sci. 46, 217 (1960).(8) Gilman, H., Haubein, A. H., J . Am. Chem. SOC.66, 1515 (1944). (9) Gilman, H., Langham, W., Moore, F. W., Ibid., 62, 2327 (1940). (4)
(10) Gilman, H., Van Ess, P. R., Zbid., 5 5 , 1258 (1933). (11) Glusker. D. L.. Stiles. E.. Yonkoski. B,, 3. Polyker Sd.49, 297 (1961). (12) Kamienski, C. W., Esmay, D. L., J . Org. Chem. 25, 115 (1960). (13) Kohn, E., Schuurmans, H. J. L., Cavender, J. Y.,Mendelson, R. A., J . Polymer Sci. 58, 681 (1962). (14) Kuntz, I., Gerber, A., Zbid. 42, 299 (1960). (15) Natta, G., Porri, L., Carbonaro, A., Greco, A., Makromol. Chemie 71, 207 (1964). (16) Pro, M. J., Martin, W. L., Etienne, ~
A. D., U . S. A t . Energy Comm. TID 13828 (1961). (17) Wilzbach, K. E., Kaplan, L., Brown, W. G., Science 118, 522 (1953). (18) Wilzbach, K. E., Van Dyken, A. R., Kaplan, L., ANAL. CHEM. 26, 880
(1954).
RECEIVED for review September 14, 1964. Accepted Sovember 16, 1964. Presented in part before the Division of Analytical Chemistry, 147th Meeting, ACS, April 1964. The authors thank The General Tire & Rubber Co. for permission. to publish this work.
Separation and Determination of Rhenium by Anion Excha nge Using the FIuo ride-C hIo rid e System SlLVE KALLMANN and HANS K. OBERTHIN ledoux & Co., leaneck,
N. 1.
b Rhenium is retained by Dowex-1, when various combinations of solutions of HCI, HF, NH4F, and N H L l are passed through the resin b e d to remove constituents of steels, high temperature alloys, refractory metals and alloys, also minerals and ores. The subsequent elution of rhenium with perchloric acid allows the gravimetric determination of rhenium as the metal, as the tetraphenylarsonium perrhenate, or the photometric determination using the thiocyanate complex.
N
o
SPECIFIC chemical reactions of rhenium are known. I n most instances, it is therefore necessary to carry out preliminary chemical separations before the final determination of rhenium can be achieved. Rhenium can be separated from a number of elements by distillation from a solution containing hydrochloric or hydrobromic acid and a high boiling acid such as sulfuric, perchloric, or phosphoric ( 8 ) . Steam distillation from a sulfuric acid medium (7) a t 270' C. is less effective. Rhenium usually occurs in molybdenum-rich materials, therefore some molybdenum carry-over must be anticipated in the above distillation procedures (12). Other selective reactions involve the precipitation of rhenium as the heptasulfide, Re&, as nitron perrhenate ( S ) , and as tetraphenylarsonium perrhenate (17 ) . Molybdenum interferes with the determination of small amounts of rhenium by the thiocyanate procedure and also forms a n insoluble sulfide in rather strong acid media. Several ion euchange procedures have been proposed to achieve the separation of rhenium from molybdenum (and technetium). The difference in the distribu-
280
e
ANALYTICAL CHEMISTRY
tion coefficients of Re(VII), Mo(VI), and Tc(VI1) in a l;HaCSS medium is large enough to allow a sharp separation based on the early elution of the rhenium (1, 5 ) . If a 10% sodium hydroxide solution of perrhenate and molybdate is passed through Dowex-1 , molybdenum is recovered in the eluate, while rhenium is retained by the resin and is then recovered by elution with 7 to 8 M hydrochloric acid ( 2 ) . A potassium oxalate medium has also been recommended for the separation of molybdenum from rhenium. Rhenium is retained and subsequently is eluted with dilute perchloric acid (11). The phosphate medium has also been investigated (13)*
I n analyzing various iron, nickel, cobalt, tungsten, and tantalum-base alloys containing rhenium by the now widely accepted anion exchange techniques involving the HCl-HF medium (4, 9, fO),no rhenium appeared in any of the fractions. Additional tests indicated that the following eluents used in successive steps did not remove rhenium from the column: 4y0 HF (elutes iron, nickel, cobalt, copper, and manganese); 10% HF-50T0 HCl (elutes tungsten, titanium and zirconium) ; 25YG HC1-20y0 HF (elutes niolybdenum); 4y0 HF-14yG wt./v. NH4C1 (elutes niobium); and 470 HF-14% NH4C1 neutralized with ",OH to pH 5.5 (elutes tantalum). In addition, when mineral, slag, or ore samples containing rhenium were fused in sodium peroxide, and the melt was leached in water, then acidified with sulfuric acid and adjusted with H F and/or HCI to provide any of the media previously mentioned, rhenium remained on the column, while all other constituents of the sample were removed in successive steps.
The above observations naturally offer the pleasant prospects of a separation of rhenium from virtually any combination of elements encountered in metallurgical analysis. EXPERIMENTAL
Apparatus. Polystyrene columns, 1 inch inside diameter containing 10 inches of 100- to 200-mesh Dowex-1 resin, 8 to 10% divinylbenzene crosslinkage (10). Reagents. Various HC1-HF solutions (see Table I ) ; 10% (vol./vol.) perchloric acid; 1% (wt./vol.) tetraphenylarsonium chloride; 20% (wt./ vol.) sodium thiocyanate solution; 0.5Oj, (wt./vol.) stannous chloride solution. Dissolve 5 grams of SnClz. 2 H 2 0 in 50 ml. of hot HCl and dilute to 250 ml. with water. Procedure. SOLUTION O F THE
SAM-
Tungsten, molybdenum, titanium, zirconium, niobium, and/or tantalum base alloys. Dissolve 0.5 to 3 grams of sample in a covered beaker of platinum, polyethylene, or Teflon in 15 to 75 ml. of warm hydrofluoric acid with the occasional addition of a minimum amount of nitric acid. Do not heat longer than absolutely necessary. Iron, Nickel, and/or Cobalt Base Alloys. Dissolve 0.5 to 3 grams of sample without heating in a covered polyethylene or Teflon beaker in 25 to 50 ml. of hydrochloric and 2 to 10 ml. of nitric acid. After the reaction ceases, add 10 ml. of hydrofluoric acid and 0.5 ml. of phosphoric acid and evaporate to dryness on a steam bath. Add 5 ml. of HC1 and 5 ml. of H F and repeat the evaporation. Minerals, Ores, and Concentrates. Fuse 0.5 to 5 grams of sample in an iron, nickel, or zirconium crucible in an appropriate amount of sodium peroxide. Leach the cold melt in a polyethylene beaker in a minimum PLE.
