methyldiborane, and 1 mole % residual trimethylborane. As seen in Table I, the consistency is good. Monomethyldiborane is the least stable of the methyl derivatives. I n the preceding example the largest quantity of monompthyldiborane was about ISYO, a n amount which could decompose considerably with a minor over-all effect, A check of the stability of monomethylidoborane iri the fractometer shoned that this need not be considered as only about 0.27, decomposition occurred for each pass through the system. Consistency of
results in cases where monomethyldiborane was the principal reactant was in accord with this observation. Table IV shows the data on equilibria between monomethyldiborane and its decomposition products. LITERATURE CITED
(1) Ambrose, Douglas, Keulemans, A. I. M., Purnell, J. H., ANAL. CHEM.30, 1582 (1958). (2) Desty, D. H., ed., “Vapour Phase Chromatography,” p. xu, Academic Press, New York, 1957. ( 3 ) Johnson, H. W.,Stross, F. H., ANAL. CHEM.30,1586 (1958).
( 4 ) Kaufman, J. J., Todd, J. E., Koski, LT. S., Ibid., 29, 103%(1957). (5) Parsons, T. D.> Silverman, 11. B., Ritter. D. 11..J . A m . Chevi. Sac. 79.
5091 (‘1957). RECEIVEDfor review April 2 , 1959. Accepted August 10, 1959. Research supported by the United States Air Force through the Air Force Office of Scientific Research of the Research and Development Command, under Contract S o . AF 18(600)-1541. Reproduction in whole o r in part is permitted for any purpose of the Cnited States Government. Work begun by G. R . Seely during tenure as postdoctorate fellow in the academic year 1957-58 and continued by J. 1’. Oliver as a portion of his 1’h.D. thesis research.
Polarographic Determination of Hexaethyldilead in Tetraethyllead JOHN E. De VRIES, ALLEN LAUW-ZECHA, and ANA PELLECER Department of Chemistry, Stanford Research Institute, Menlo Park, Calif.
b
A procedure was developed for determining hexaethyldilead in tetraethyllead samples. The method involves a direct polarographic measurement of the concentration of hexaethyldilead, using an organic solvent medium. There is no interference b y tetraethyllead or other lead compounds. Concentrations of hexaethyldilead equivalent to the 0.1% level in tetraethyllead samples were determined rapidly and accurately. The concentration of trieihyllead chloride can b e determined from the same polarogram, as it exhibits a separate polarographic wave.
I
development of analytical methods for side products in preparations of tetraethyllead (TEL), polarography \$as investigated as a means of determining hexaethyldilcad (HED) in the presence of TEL. Although H E D is not encountered in commercially produced T E L , experimental preparations of T E L might contain this material as an interrnediatc or as :t side product. Only a few polarographic studies of metal alkyls h a w been reported. Kolthoff and Lingane (4)reviewed the organometallic compounds re+.ucible at the dropping mercury electrode and listed only tridhyllead chloride (EtrPbCI) and diethyltin dichloride as representing this entire class of compounds. I n subsequent reviews, Wawzonek (6) summarized the polarographic behavior of several additional metal alkyls and their cations. The polarography of the metal alkyls of the H E D N THE
precipitate gradually formed. The type has not been reported. Based on HED \vas characterized a$ follon i: previous polarographic studies of metal alkyls, HED would be assumed capable AXALYSIS. Calciilsteti for HI.]): (’, of undergoing a two-electron change to 24.19; H, 5.11; Pi), 70.39; f o l i r i d : C, 24.8; H. 5 . 3 2 : €‘ti, 70.46. be converted to two triethyllead cations, FREEZING POIST. Litcrnturr value: As T E L is not reducible a t the drop-76.0” f 0.5’ C . : f o u n d : - 7 7 . 0 ” + ping mercury electrode, no interference l o c. was anticipated in the direct application IXDEX OF REFRACTIOS. Lit(1r:itiirevalrlc.: 1.6’25:3: found: 1.6196, of polarographic procedures to samples of TEL. Pure EtnPbC1\vas prcpnrcd according This paper describes the polaroto the method of C‘alingncrt, Dykstr:i, graphic procedures t h a t were dcveloped and Shapiro ( 1 ) . for determining H E D and EtaPbC1 in The org:inic solvrnt Ivitli supporting samples of TEL The solvent system electrolyte \vas prcparctl hy tiissolving consisted of a mixture of equal volumes lithium chloritlc (to n i a k r a find ronof benzene and methanol. n i t h lithium centration in tlit: niixcd solvent of chloride as supporting electrolyte. 0.3.V) in niiliJdrous nicttiariol :ind H E D exhibits a n anodic wave a t t h r adding a n approxiriiatcly rqiial volume half-wave potential of -0.24 volt of thiophenc-frw b c n z r n c ~ . l‘lius 12.7 (S.C.E.) vrhile EtsPbCl exhibits a grams of lithium c~lilorid(~K O K ~ tliscathodic n a v e a t -0.98 volt (S.C.E.). solved in 500 nil. of nirthanol; thrii Iienzcnc \v:ts atltlctl to niukv thv final Thus, both H E D and Et3PbC1 wrre volumc 1 1itc.r. determined in a single polarogram. Currrnt-voltagc c u r \ w \ v ( ~ r crccordvd ~ by a S:irgcsnt 1Iotlt.l S S I rcwrding polarograph. ‘ l l c tlropping rncwury MATERIALS AND APPARATUS clectrotle had a const:int ~ n ” ~ t ]value -I ‘2, The vi-value Pure H E D was prepared by the eronrl mcqsurcd in sodium-liquid ammonia procrdure deair with nljen c,irruit. .I sstrtrated scribed by Calingaert and Soroos ( 2 ) (S,(‘,f,:.)\vas uwcl as a calonicl cl(~c~trotlo and by Closson ( 3 ) . The molecular refcrcncc c,lrc.trotl(, ant1 \vas corinccteci distilled product n as kept in foil. to tho dropping nicwurj. (4t.11 ( m i p a r t wrapped borosilicate glass rials (ap( ~ 1 1\vas mrnt by a silt liridgc. proximately 1 ce. of WED in each) inimcrscrl i n a thrrnimtatir bath kept and stored a t dry ice tcnipcmturr to a t 25” i 0.2” ( ’ , ‘I‘hc, ct.11 \ y a p proprcvent ticconipoqition. The pure matrrial as it first convided with a tuhc. ritlicr for huljbling nitrogrn into the saniplr through a densed in the molecular still, appeared fritted-glass disk or for p colorless, but after a fen milltlitc.rs had continuously over the surfact, of the collected the product became yellon sample. To prevent evaporation of and did not undergo any further cliange on storage. Subsequcntl3, 11 hen each solvent during t’he nitrogen purge, the nitrogen nas presaturated with solvent vial was opened for sampling. a n hite ‘I’hcy
VOL. 31, NO. 12, DECEMBER 1 9 5 9
1995
vapors by scrubbing i t through a bottle containing the solvent mixture. POLAROGRAPHIC PROCEDURE
The sample of T E L to be analyzed for H E D was measured directly into the solvent-electrolyte in a volumetric flask, and diluted to the mark. The volumes chosen nere such that the concentration of H E D in the electrolyte n-as in the range 1.0 X to 1.0 X 10-3.M. .4 convenient amount of diluted sample was transferred to the polarographic cell (usually 3 to 5 ml.). The sample 11 as deoxygenated by bubbling nitrogen through the solution for 10 minutes. During the actual recording of the polarogram. a stream of nitrogen was directed over the surface of the sample.
09
10
-Edmc V O L T S
Figure 1. Table I.
Calibration Data
E , :? Concentration, (S.C.E.), .II x 104 T-olt, Hesnet hyldilead -0 23 -0.25 -0.24 -0.24 -0.24 -0.24 -0.25 -0.23 -0.25 -0.24 -0.22 -0.22 -0.21
1.34 8.04 2.68 5.36 5 36 1 34 6.70 6.70 1.34 1 .:34 6 .i o 5 8.005 9 . 405
Io 2.79 2.48 2.90 3.03 2.90 2.88 3.08 2.96 3.04 2.70 2.70 3.20 3.14
Triethyllead chloride 3.16 6.33 1.58 6.19 1.24 01 = Cm
id
-0.97 -0.99 -0.97 -0.97 -0.98
2.29 2.74 2.31 2.75 2.35
2!3t1/6
Average Z value, 2.90. Standard deviation, 0.20. Standard deviation of mean, 0.06. Approximately 1.O ml. TEL added per
25 ml. of solution.
Polarograms were recorded for the H E D solutions and also for the solventelectrolyte alone. so that the two polarograms could be superimposed for easy measurement of the diffusion current. The residual current correction thus was conveniently accomplished, as illustrated in Figure 1. The method was calibrated by measuring pure compounds directly into the solvent-electrolyte, diluting to a known volume (usually 25 ml.), and polarographing as described above. Lambda pipets were used to measure the pure HED. Samples of pure EtsPbCl were weighed on a Cahn elec robalance. The polarographic cell was cleaned thoroughly between runs because con-
f
1996 *
ANALYTICAL CHEMISTRY
VI
I 1
12
I3
14
I5
I
I6
5 C E
Polarograms of hexaethyldilead and triethyllead chloride
1 . Solvent-electrolyte blank 2. HED (approximately 3.0 X 1 O-4M) in solvent-electrolyte 3. Et3PbCI (approximately 1.5 X 10-‘M) in solvent-electrolyte Sargent Model XXI polaragraph sensitivity: 0.01 5 po./mm. Average i d = mm. wave height X 0.015 X 6/7
tamination of samples by lead residues was a constant problem. The direct rinsing of the cell with benzene-methanol solvent was not satisfactory. An effective procedure to rinse the cell with acetone, water, and benzenemethanol, in that order, The water rinse was necessary to crystals of potassium chloride which formed on the glass frit that separated the salt bridge from the solvent-electrolyte; crystallization occurred gradually during the course of a run. As a further precaution against contamination, the salt bridge adjacent to the electrode compartment was emptied frequently and replaced with fresh potassium chloride solution. An allglass apparatus is recommended, as the organic solvents tend t o leach out organic matter from stoppers and plastic tubing. RESULTS AND DISCUSSION
Typical polarograms and the method of measuring diffusion current are illustrated in Figure 1. Distinct waves are produced for H E D and for Et3PbC1. The former (curve 2) ex’libits an anodic wave at the half-wave potential of -0.24 volt (S.C.E.); the latter (curve 3) appears as a cathodic wave a t -0.98 volt (S.C.E.) followed closely by a wave withamaximum a t - 1.40volts (S.C.E.). Thus, both H E D and EtsPbCl can be determined simultaneously on a single polarogram. The diffusion current constants, I , and half-wave potentials, E,!,, are presented in Table I. The diffusion current constants obtained for H E D and Et3PbCl are comparable to those obtained for similar type compounds in
like solvents-Le., for ferrocene, ruthenocene, and cobalticinium perchlorate (6) impurity in HED and EtapbCl would not be expected to yield diffusion current constants of this magnitude. Varying levels of impurities between vials of HED would also lead to variations in diffusion current constants. The data do not indicate such variations. Values of the diffusion current constnnt shorn good agreement. Many of the early runs on K E D were completed only after laborious washing and rewashing of the electrolysis cell before the contamination problem was fully understood. These earlier values are included in the calibration data. The runs with a large excess of T E L added do not deviate significantly from the average; a ratio of 1000 to 1, T E L to H E D , did not affect the HED wave. The lower limit of HED determined by the method is approximately 2 mg. of H E D per 25 ml. Thas a 2.0-gram sample of T E L containing O.lyo H E D represents the lower limit for accurate analysis; by increasing the T E L sample size, samples containing as low as 0.03Yo H E D could be detected by this method with diminished accuracy. The polarographic blank of T E L alone, using 2 ml. of T E L in 25 ml. of solvent-electrolyte, gave no significant deviation from the blank of the electrolyte. The solutions of H E D gradually decomposed. The decrease in the polarographic wave of H E D was appreciable in solutions which were allowed to stand in the polarographic cell. The decomposition of H E D was such that, in practically every sample of H E D ana-
lysed, there was a very small wave a t a potential of approximately -0.98 volt (S.C.E.). Because subsequent runs with pure EtaPbCl resulted in well defined waves a t -0.98 volt, it is concluded that EtrPbCl was a decomposition product of HED. To establish the approximate rate of decomposition of HED, samples were removed periodically from a volumetric flask containing 6.7 X lO-4M H E D and polarographed. The flask was maintained a t 25” C. and exposed to normal laboratory lighting. The decomposition reached 2.9% in 30 minutes, 4.4% in 60 minutes, and 20.8y0 after 3 days. As EtaPbCl is a common impurity in H E D samples, several calibration runs were made on this material. The results (Table I) indicate that samples containing H E D can be analyzed simultaneously for EtaPbC1. Oxygen is a
common contaminant and is reduced in the voltage range corresponding to the El,* of EtaPbC1. Care must be exercised to remove oxygen completely for this determination. The second wave for EtaPbCl exhibited a maximum which could be s u p pressed by Triton X-100. No use was made of a suppressor in the measurements for EtsPbCl as the prewave a t - 0.98 volt could be measured without interference from the maximum. Various supporting electrolytes were tested in the H E D procedure, including lithium nitrate, bromide, sulfate, iodide, and sodium sulfate. Solubility limitations prevented consideration of certain other common salts. All of the electrolytes tested with added H E D resulted in the usual anodic wave for H E D , but none gave the ideal wave obtained for the lithium chloride electrolyte.
ACKNOWLEDGMENT
The authors are indebted to the Ethyl Corp., Detroit, Mich., for support of this work. LITERATURE CITED
(1) Calingaert, G., Dykstra, F. J., Shapiro, H., J . A m . Chem. SOC.67, 190 11945). (2j-Calhgaert, G., Soroos, H., J. Org. Chem. 2, 535 (1938). (3) Closson, R. D., Ethyl Corp., Ferndale 20, Detroit, Mich.,. private communica-
tion. (4) Kolthoff, I. hl., Lingane, J. J., “Polarography,” 2nd ed., Interscience, New York, 1952. (5) Page, J. A., Wilkinson, G., J . A m . Chem. SOC.74, 6149 (1952). (6) Waazonek, S., ANAL.CHEM.24, 32 (1952); 26, 65 (1954); 28, 638 (1956); 30,661 (1958). RECEIVEDfor review May 19, 1959. Accepted August 4, 1959.
Paper Chromatography of 2,4-Dinitrophenylhydrazones Estimation of 2-Alkanone, n-AlkanaI, Alk-2-ena1, and AI k-2,4-d ienaI Derivatives REX ELLIS and A. M. GADDIS Meat laboratory, Eastern Utilization Research a d Development Division, Agriculfural Research Service, U. S. Deparfment of Agriculture, Belfsville, Md.
b The application of paper chromatographic methods of separating 2alkanone, n-alkanal, alk-2-enal, and a I k- 2 , 4 d i en a I 2,4-dinitrophenylhydrazones into .individual compounds has been examined. In experiments with four mixtures the mean recoveries were consistent and showed small variations. In over-all recovery, average deviation from the mean was &3.4%. Recovery decreased from the 2-alkanones to the alk-2,4-dienals because of differences in stability of the classes. However, this was reflected in but small error in proportions of classes and the ratio of individual compounds found.
-
R
publications have described rapid paper chromatographic methods of separating mixtures of 2-alkanone, n-alkanal, alk-2-ena1, and alk2,4-dienal 2,4-dinitrophenylhydrazones into classes ( 2 , 4 , 6 ) and each class into individual compounds (1-4). These methods have been applied to the determination of changes in the proportions of steam-volatile monocarbonyl classes with the autoxidation of pork fat (6), ECENT
and to the identification of such compounds volatilized from a rancid pork fat (3). In the latter study, tentative quantitative data were reported for the classes and individual compounds. Aside from the inevitable mechanical losses in the manipulations, it was recognized that the most serious obstacle to quantitative application was the variation in stability of the different classes (2). The alk-2,4-dienal derivatives are particularly sensitive to light and air (8,6). This work was undertaken to determine the quantitative capabilities of the methods. EXPERIMENTAL
Solvents, reagents, materials, and equipment were the same as used in similar operations described in earlier papers (1-6). Authentic monocarbonyl 2,4-dinitrophenylhydrazones employed have been described (1, 8). Stock solutions of each hydrazone in carbon tetrachloride were prepared containing the equivalent of approximately 25 mg. per liter (25 y per ml.). Suitable volumes were taken from each stock solution to make up 100ml. solutions of 30 kmoles per liter con-
centration. Aliquots of the various solutions were used to prepare the mixtures used in the experiments. Separation into classes (8, 6) and resolution of the classes into individual compounds (1-5) were performed as described in earlier papers. Proportions of classes (2, 6) could be determined from spots extracted from three paper strips and measured spectrophotometrically in 3.00 ml. of carbon tetrachloride. However, it was necessary to accumulate a number of paper strips from the class separation to provide sufficient material for estimation of the individual compounds (3). Spectrophotometric measurements of absorbance were usually made a t the wave length of maximum absorption in 3.00 ml. of carbon tetrachloride. However, it was necessary to measure the spectra of compounds separated on vaselineimpregnated paper in alcoholic alkali (S), because of the presence of a persistent impurity (1, 3) that absorbed at the lower wave lengths. Every practical effort was made to protect the carbonyl derivatives from the effects of light and air. Spotted paper strips were chromatographed immediately, and separated spots extracted and measured as quickly as possible. Solvent was removed from VOL. 31, N O . 12, DECEMBER 1959
1997
