aggregates below the CMC, such as those suggested by Mukerjee, Mysels, and Dulin (IS), would explain the rise in D value a t low concentrations as well as decrease, but probably not eliminate, the discrepancy between the two sets of micelle size values. Distribution Coefficient of OPE,. It is concluded t h a t concentrations a little below the CRIC are suitable for the determination of the desired distribution coefficient. If small aggregates are present, some confusion may result. Since the purpose is t o compare different S.4AJs, difficulties mill be avoided by consistency in choice of concentrations below the CRlC or below the concentration a t which the small aggregates form. The data in Table I show that D increases with 5 in a reasonable fashion, as would be expected of the hydrophile-lipophile ratio of the members of this family. Equation 4 shows that 7 units in 2 produce a tenfold change in D, from which one can conclude that the standard free energy of transport of one mole of ethylene oxide units from the iso-
octane to the water phase is ea. 200 cal. This value is probably for hydrated ether units, since both vapor pressure data (4) and some direct calorimetric measurements of heat of solution in water give values of about 1 kcal. per mole. J t is particularly interesting to note that D has the value 1 when z is 10. This is the member of the series which is by far the most widely used as a commercial SAA. The validity of this method for the determination of the osmotic coefficient of nonionic surfactants in water deserves further investigation. ACKNOWLEDGMENl
The authors thank George L. Brown for helpful advice during the course of this investigation and Karol J. Mysels for offering a number of suggestions upon reviewing the manuscript. LITERATURE CITED
(1) Cockbain, E. G., MeRoberts, T. S., J . Colloid Sci. 8, 440 (1953). (2) Cohen, M., Mem. services chim. dtat Paris 36, 93-114, 207-17 (1951).
(3) Cross, J. M., Oficial Proceedings, 36th Mid-Year Meeting, Chemical Specialties Manufacturers ASSOC., June 13, 1950. (4) Greenwald, H. L., Brown, G. L., J . Phys. Chem. 58,825 (1954). (5) Greenn-ald, H. L., Brown, G. L., Fineman. M. N..ANAL. CHEY. 28. \ - - - - I
(6) Grieger, P. F., Kraus, C. A,, J . Am. C h q . SOC.71, 1455 (1949). (7) . . Griffith. J. C., Chem. & Ind. (London)‘1957, 1041. (8) Harkins, W. D., Brown, F. E., J . Am. Chem. SOC.41. 499 (1919): 38. 248 (lQl6). \----/.
(9) Hsiao, L., Dunning, H. N., Lorenz, P. B., J . Phys. Chem. 60,657 (1956). 110) Kellv. J.. Greenmald. H. L.. Zbid.. . 62, 1096’(1958). (11) Kushner, L. M., Hubbard, W. D., Ibid., 58, 1163 (1954). (12) Mankowich, A. M., Ind. Eng. Chem. 47, 2175 (1955). (13) Mukerjee, P., Mysels, K. J., Dulin, C. I.. J . Phys. Chem. 62,1390 (1958). (14) Philippoff, R., Discussions Faraday SOC. No. 11,96 (1951). (15) Weeks, L. E., Lewis, J. T., Ginn, M. E., J . Am. Oil Chemists’ SOC.35, 149 (1958).
RECEIVEDfor review August 4, 1960. Accepted Sovember 9, 1960.
Oxid imetri c Determination of n - B uty I Iithi um in Hydrocarbons Using Vanadium Pentoxide PETER F. COLLINS, CONRAD W. KAMIENSKI, DONALD L. ESMAY, and R.
B. ELLESTAD
Research laboratories, lithium Corp. of America, Inc., Bessemer City, N. C.
b When a solution of n-butyllithium in n-heptane is treated with vanadium pentoxide, a rapid reaction takes place in which reduced vanadium compounds are formed in amounts equivalent to the n-butyllithium originally present. The reduced vanadium is determined b y titration with standard sulfatoceric acid and from this the concentration of n-butyllithium can b e accurately ascertained. The method yields reasonably accurate results even in the presence of large amounts of alkoxide. Under these conditions, the usual double-titration method has been found to give values which are low.
0
THE various methods which have been proposed for the assay of alkyllithium solutions, the most widely used is the double-titration method of Gilman and Haubein (4). A recent investigation (6) of this method has shown that reproducible results can be obtained for hydrocarbon solutions of n-butyllithium only if the reagents have been rigorously purified and the experimental conditions are
B
468
ANALYTICAL CHEMISTRY
G. Frederick Smith Chemical Co., Columbus, Ohio. Place a 3-liter beaker containing 184 ml. of sulfuric acid on a magnetic stirrer and slowly add 130 grams of ceric hydroxide with good stirring to assure wetting all the ceric hydroxide. The mixture will begin to expand; a t this point, add 500 ml. of water as rapidly as possible. The mixture becomes hot, but there is no spattering. On continued stirring, practically all the ceric hydroxide dissolves. When cool, dilute the solution to 2 liters and filter using a filter aid. To standardize, accurately weigh 0.5 to 0.6 gram of dried primary standard arsenious oxide into a 400-ml. beaker. Rinse the walls of the beaker with 10 ml. of water and add 2 grams of sodium hydroxide. Swirl the solution to dissolve the arsenious oxide and when complete solution is obtained, dilute to 100 ml. and add 30 ml. of dilute sulfuric acid (1 to 10). Add 2 drops of osmium tetroxide solution (0.25 gram of Os04 dissolved in 100 ml. of 0.1N HzS04) EXPERIMENTAL and 2 drops of 0.0131 ferrous 1 , l O Reagents. STANDARDSULFATO- phenanthroline sulfate, then titrate with the cerate solution. As the end CERIC ACID, 0.3N in 0.75F HtSOa. point is approached, add the cerate For this preparation it is necessary to solution dropwise with good stirring use ceric hydroxide which is finely until 1 drop changes the color from ground; this is available from the
strictly controlled; consequently, the double-titration method is time consuming. The iodination method recently described by Clifford and Olsen (1) is unattractive because of the practical difficulties of using a standard solution of iodine in diethyl ether, possible interference by alkoxides, and the inherent error due to a coupling side reaction, the extent of which may be dependent on experimental conditions. With the objective of developing a relatively simple method which would circumvent some of the difficulties encountered with the known procedures, a study of the redox reactions of nbutyllithium with various inorganic compounds was undertaken. This paper describes the application of the reaction of n-butyllithium with vanadium pentoxide to the determination of nbutyllithium in hydrocarbon solvents.
pink to a very faint blue. For a O.1N sulfatoceric acid solution, dilute 333 ml. of the 0.3N solution to 1 liter with 0.75JI sulfuric acid and standardize as above, using 0.20 to 0.25 gram of arsenious oxide. This procedure for the preparation of sulfatoceric acid solutions is a modification of that proposed by Diehl and Smith (2). VANADIUMPESTOXIDE. Dry C.P. vanadium pentovide (Vanadium Corp. of America) a t 150" C. for several hours. Reagent grade vanadium pentoxide obtained from some companies contains appreciable amounts of reduced vanadium compounds and should not be used in this procedure. ~ H E P T A X Dry E . n-heptane (Phillips Pure) over 5 h Molecular Sieves (Linde). Apparatus. Equipment for carrying out a potentiometric titration is rrquired. For this work a Beckman &lode1 H-2 pH meter or a vacuum tube voltmeter d c s i g n d by Garman and Droz ( 3 ) .r\ as used for all titrations. The electrode system consisted of a bright platinum indicator electrode and a normal calomel or silver-silver chloride reference electrode. A sudden increase in meter reading was observed with the Garniaii and Droz voltmeter a t the end point, but with less sensitive instruments it may be necessary to make a plot of potential us. milliliters of titrant to determine the end point accuratelv. The point of inflection of the titration curve occurs a t the equivalence point. A 250-nil. Erlenmcyer flask fitted with a 2-hole rubber stopper is a suitable reaction vessel. A glass tube extending 3 inches into the flask is inserted through the stopper for the introduction of argon. The other hole in the stopper is for adding the sample by means of a pipet and the diameter of this hole should be such that the pipet may be easily inserted. The flasks and pipets must be dried prior to running the determination. This is best done by heating in an oven a t 110' C. for a t least 1 hour and then flushing with dry argon until cool. The dried glassware should then be used immediately. Procedure. This procedure is applicable for solutions less than 2.5M in alkyllithium. For more concentrated solutions, a smaller sample should be used. Place 6 grams of dried vanadium pentoxide and 30 ml. of n-heptane in a dry 250-ml. Erlenmeyer flask. Insert the stopper carrying the argon inlet tube and flush with a rapid stream of argon fcr 10 minutes. Add a 5.00-ml. sample of the alkyllithium solution using a pipet and rubber bulb. Swirl the flask vigorously to mix and allow to stand for 10 minutes, swirling the flask occasionally. An atmosphere of argon must be maintained in the flask throughout the entire reaction time. Add 15 ml. of 1 to 10 sulfuric acid and 100 ml. of boiling water, and mix to dissolve any reduced vanadium. Titrate the mixture while still hot with standard sulfatoceric acid, stirring continuously with a magnetic stirrer. Use a 0.3-V
Table 1.
Analysis of Solutions of n- and tert-Butyllithium in Various Solvents
Compound
Solvent
BuLi by V2O6 Total Method, M Base, M
Difference, %
n-Butyllithium
n-Heptane %-Heptane %-Heptane Cyclohexane Tolu-Sol Tolu-Sol
1.67 2.74 1.23 2.58 1.20 1.19
1.70 2.80 1.24 2.62 1.21 1.23
1.8 2.1 0.8 1.5 0.8 3.2
tert-Butyllithium
n-Pentane
1.46
1.52
4.0
solution of sulfatoceric acid if the molarity of the alkyllithium solution is 1 or greater, and a 0.1N solution if less than 1. Determine the end point potentiometrically. -4sharp increase in potential should be observed a t the end point. Calculation :
Table II. Comparison of Analyses of Solutions of n-Butyllithium and Lithium n-Butoxide in n-Heptane b y Two Methods
n-BuLi OrigiRelanaiiy nPres- B ~ O Hn-BuLi, Mmoles tive ent. Added. ReError. MmolesMmoles maining Found % '
RESULTS AND DISCUSSION
The estimation of the accuracy of any analytical method for alkyllithium compounds is difficult since none of the available methods have been shown to yield accurate results and no compounds of known purity are readily available. However, indirect evidence hich is described in the following paragraphs indicates that the accuracy of the vanadium pentoxide method exceeds that of the double-titration method, especially in the presence of appreciable amounts of alkoxide. Results obtained by the vanadium pentoxide method and for total base are shown in Table I for solutions of n- and tert-butyllithium in various solvents. The total base values were obtained by a base titration after hydrolysis of the butyllithium solution in a n excess of standard hydrochloric acid. I n all cases, the concentration of butyllithium as determined by the proposed method is less than the concentration of total base. This is to be expected since any air oxidation of nbutyllithium results in the formation of lithium n-butoxide which is soluble in solutions of n-butyllithium (7). However, in every case the difference between the two values is 47, or less which indicates the presence of only a small amount of soluble base other than butyllithium. The preparation, handling, and storage of the n- and tertbutyllithium solutions were carried out under dry argon to exclude air; therefore, the amount of soluble base other than butyllithium would be expected to be quite low. Using similar solutions of n-butyllithium in n-heptane, Kamienski and Esmay (6) obtained results with the double-titration method which indicated that 4 to 5% of the total
Proposed Vanadium Pentoxide Method 54.80 10.78 10.78 16.17 16.17 68.50 26.95 26.95
44.02 44.02 38.63 38.63 41.55 41.55
43.5 43.8 38.2 38.6 41.4 41.4
-
-
-
1.1 0.5 1.0 0.0 0.5 0.5
Double-Titration Method 9.20
a
b
1.72 2.59 3.45 4.31 4.60
7.48 6.61 5 75 4.89 4.60
-
6.81a 9.0 6.04 8.6 5.12 -11.0 4.05 -17.2 4.02b -12.6
Average of 4 values. Average of 3 values.
base present was nonbutyllithium base. This blank was fairly constant and the authors concluded that i t may be inherent to the double-titration method under the experimental conditions used. Further evidence of the accuracy of the vanadium pentoxide method was obtained from an investigation of possible interference by relatively large A amounts of lithium n-butoxide. series of solutions was prepared in which known amounts of a standard solution of 1-butanol in n-heptane were added to known volumes of a n-heptane solution of n-butyllithium which had been analyzed by the vanadium pentoxide method. The concentration of the n-butyllithium solution as determined by the vanadium pentoxide method was within 2y0 of the total base value and was assumed to be correct. The resulting solutions of n-butyllithium and lithium n-butoxide were then analyzed for n-butyllithium by the proposed method. Solutions were preVOL. 33,
NO. 3, MARCH 1961
469
pared in a similar manner and analyzed by the double-titration method. The results are summarized in Table I1 and these data indicate that in the presence of lithium n-butoxide, the vanadium pentoxide method is of greater accuracy (average relative error = -0.67,) than the double-titration method (average relative error = - 11.770). The vanadium pentoside method was used by West and Glaze for the analysis of crystalline ethyllithium which they prepared as follows (8): 4 solution of ethyllithium was prepared from lithium dispersion and ethyl bromide in pentane under nitrogen. The pentane was renioved by evaporation and the residue mas extracted with hot benzene. On cooling the benzene extract, crystals of ethyllithium were obtained which were filtered, washed, and dried. A weighed sample of ethyllithium was then dissolved in benzene and this solution mas analyzed by the vanadium pentoxide method and for total base. These operations were carried out under argon in a dry box to minimize oxidation of Table 111. Reaction of n-Butyllithium with Various Compounds
n-BuLi Oxidized by
Compound Ti02 V206
CrLh MnOz FelOs Co3O4
cuo
Com-
pound, %a
E 0 81
37
81
-48205
28 4
MOO3
c
SnO? Ba02
0
Bi203 CrC13 AeBr H&1,
KzCr207
(NH4)4Ce(SO,) 4
K2S208
%a
80 29 2
30 100 16 4 0 1
0 \TO3 I13Fe(CS)e 0 mea. of reduced Calculated from: - species mmoles of n-BuLi x 100 = yo BuLi oxidized by compound. Value obtained using Baker's analyzed reagent TiO?. Other samples gave lower values. c Molybdenum Blue was formed, but the amount could not be determined readily.
470
0 14
Compound
n-BuLi Oxidized by Compound,
SH~\'OB
ANALYTICAL CHEMISTRY
the ethyllithium. The calculated niolarity of the solution based on the weight of ethyllithium was 0.288. Found by the vanadium pentoxide method, I\I = 0.265; total base, AI = 0.2T6. Consecutive deterniinations on the same solution of n-butyllithium in nheptane indicate the precision obtainable nith the procedure: III = 2.31, 2.31, 2.29, 2.28. 2.31: average = 2.30; average deviation = 0.01. An attempt n.as made to use this procedure for the determination of nbutyllithium in the presence of a large amount of diethyl ethcr by simply substituting purified ether (distilled from lithium aluminum hydride under anhydrous conditions) for the n-heptane normally added in the procedure. The reaction mixture was cooled in a dry ice-acetone bath before adding the nbutyllithium. The molarity determined for the solution was 2.11 compared to a value of 2.26 obtained by the proposed method 15-ith n-heptane as the solvent. It is not known whether the presence of ether directlv affects the reaction with vanadium pentoside or whether the lower value is the result of cleavage of ether by n-butyllithium which would actually reduce the concentration of nbutyllithium. At any rate, no evidence is available which indicates that the method gives correct results for ether solutions of alkyllithium compounds. Only n-, sec-, and tert-butyllithium and ethyllithium solutions have been assayed by the new method, but the procedure should be generally applicable to the determination of any alkyllithium compound in a hydrocarbon solvent. The described procedure cannot be used for the determination of phenyllithium, however. Although phenpllithium rapidly reduces vanadium pentoside, most solutions of phenyllithium contain lithium phenoxide because of air oxidation. On titration with sulfatoceric acid, the phenol is oxidized along with the reduced vanadium. This leads to high results for the phenyllithium content. Samples of tris(isobuty1)aluminum in n-heptane (Hercules P o d e r Co.) and ethyl magnesium bromide in ether were
tested for possible reaction with vanadium pentoside. Very little reduction of the vanadium pentoxide occurred. indicating that this method is unsuitable for these compounds. Various other oxides and salts have been tested for reaction with n-butyllithium in n-heptane. I n each case, a theoretical excess of the dried oxide or salt was reacted with n-butyllithium for approxirnately 15 minutes and the amount of reduced species formed or oxidized compound remaining was then determined by usual analytical methods. The results are summarized in Table 111. The only compounds which were found to react essentially quantitatively TT-ith n-butyllithium were vanadium pentoxide and silver bromide. This reaction with vanadium pentoxide also serves as a convenient qualitative test for the presence of alkyllithium compounds. When the alkyllithium solution is added to vanadium pentoside suspended in n-heptane, the color changes instantly from bright orange to deep green. This test is fully as sensitive as a color test developed by Gilman and Schulze (a). Bdditional studies are under way on the reaction mechanism, the application of the reaction to other organometallic compounds, and the possible use of the reaction for synthetic purposes. LITERATURE CITED
(1) Clifford, A. F., Olsen, R. R., AXAL.
CHEJI.32,544 (1960). (2) Diehl, Harvey, Smith, G. F., Talanta 2,382 (1959). (3) Garman, R. L., Droz, M. E., IND. EKG.CHEM.,~ ~ X A LED. . 11, 398 (1939). (4) Gilman, Henry, Haubein, A. H., J . Am. Chem. SOC.66, 1515 (1944). (5) Gilman, Henry, Schulae, F., Ibid., 47, 2002 (1925). (6) Kamienski, C. W.,Esmay, D. L., J . Org. Chenz. 25, 115 (1960). ( 7 ) Lithium Corp. of America, Inc., Bessemer City, S . C., unpublished data, 1959. (8) West, Robert, Glaze, William, Department of Chemistry, University of Tisconsin, Madison, \Vis., private communication, 1960. '
RECEIVEDfor review July 14, 1960. Accepted December 9, 1960.
