Analysis via Functional Groups
Some of the pagers presented in the Symposium on Analysis via Punctional Groups, Division of Analytical Chemistry, 139th M e e t ing, American Chemical Society, St. Louis, Mo., March 1961.
The Role of Separations in Organic Analysis L. D. METCALFE Arrnour Industrial Chemical Co., Research laboratory, McCook, 111.
b The analysis of substances in mixtures is often greatly simplified by a preliminary separation. A number of separation procedures available to the analyst are described briefly. These include both classical and newer separation techniques. Examples of the use of these separation procedures in the recent literature are reviewed. Where possible, special reference is made to methods using functional group analysis.
A
will determine a component directly using only a portion of the original sample. Unfortunately, such a direct approach is not always possible. The available analytical m e t h d s may not be selective enough or the concentration of the material to be estimated may be too low. Frequently the analytical problem can be simplified greatly by the use of a preliminary separation. Almost without exception the analytical methods required for very complex mixtures will necessitate proper cleanup procedures or separations to concentrate the materials in question. Once the separation is made, the specific procedure is applied. Whether it be functional group analysis or some instrumental method will depend, of course, on the nature of the chemical. Separations fall into two broad categories: the classical techniques and the more recent techniques. N IDEAL ANALYTICAL SYSTEM
CLASSICAL SEPARATION TECHNIQUES
Extensive use has been made of the classical separation techniques in functional group analytical procedures. Recent literature references will be reviewed briefly. Distillation. Distillation, although considerably popular in many publications, is used far more than recent
literature would indicate. Steam distillation (30) was used for concentrating biphenyl and o-phenylphenol in concentrated orange juice. The distillate was extracted with chloroform, the chloroform was mixed with Celite, and the Celite was analyzed by gas chromatography. Ketone bodies in blood were determined by converting them to acetone (55). The acetone was distilled and reacted with salicylaldehyde in alkaline solution to give a color reaction. I n the analysis of o-xylene oxidation products, several distillation steps were used in a complex procedure (43). An analytical assay for diosgenin was described using fractional sublimation (55). The crude sample was subjected to a high vacuum (0.1 mm.) a t 180" C. a t which the diosgenin and some impurities sublime. The sublimation was repeated on the sublimate a t 160" C. The impurities sublime leaving behind pure disosgenin. Extraction. Of the classical techniques, solvent extraction far outnumbers the others in the analytical literature. The device appears so often t h a t only a few examples can be mentioned. Nitrofurazon in feed mixes has been determined by extracting the feed with petroleum ether, then chloroform, and finally acetone (14). The active ingredient was determined colorimetrically in the acetone extract. Gallates in edible fats were determined by first extracting the fat directly with 95% methanol a t 40" to 45' C. using a special extraction vessel (11). The extracted gallate was measured colorimetrically. 1Naphthyl methylcarbamate residue in apples was extracted with methylene chloride (26). The color was developed by diazotization with sulfanilamide. Additives in polyethylene were determined by absorption spectroscopy (64). Carbon disulfide, carbon tetrachloride, and iso-octane were used to extract Ionol, Santonox, and oleamide. A
colorimetric micromethod for biphenyl in paper wrappers was developed (55). A color reaction was run on a chloroform extract. Glutethemide in biological fluids was extracted with chloroform (22). The chloroform was evaporated and the ultraviolet absorbance of the residue was determined in 0.2N KOH. Captan in plant extracts was determined by first dipping the plant in benzene (63). The benzene was filtered through powdered charcoal and the powdered charcoal was extracted with warm benzene. A colorimetric procedure was run on the evaporated filtrate. Often, extraction is used in conjunction with chromatographic columns to separate organic compounds for analysis. Spray paper was extracted with benzene to determine dieldrin (2). The benzene extract was passed over an alumina column for further purification. Hexachlorocyclohexane residues in foodstuffs were extracted with hexane. The hexane was treated with concentrated fuming sulfuric acid ( 7 1 ) . The hexane layer was passed through a Celitesulfuric acid column. A colorimetric method was finally used on the residue from the evaporated hexane. Furazolidone and nitrofurazone were determined in liver, fat, and muscle (18). The liver was ground with metaphosphoric acid and phenylhydrazine was added. The system was extracted with toluene and the extract chromatographed on alumina. The red zone was collected and read a t 400 mp. Solvent extraction and alumina columns were used to determine Warfarin in aqueous solutions (41). A device for making repeated extractions of organic compounds labeled with the same tracer atom has been described (58). Partition solvent systems were used and extensive mathematical treatment was given. Dialysis and Electrodialysis. ( )F ganic compounds ma)' be separatec VOL 33, NO. 1 1 , OCTOBER 1961
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by diffusion through a suitable membrane. This type of separation is called dialysis. Electrodialysis is a variation of this technique and is readily applied t o ionized or charged organic molecules and is much faster. Analytical papers using dialysis as a separation procedure are rare although the technique is commonly used by biochemists. A rubber membrane has been used to rapidly separate phospholipides from other lipides (4). Phospholipides dissolved in petroleum ether will not pass through the membrane but triglycerides, sterols, and sterol esters will. The recovered phospholipides were estimated by phosphorus content. Precipitation and Crystallization. Tlie use of precipitation and crystallization for the separation of organic compounds is found fairly often in recent literature. Piperazine was determined in crude mixtures by precipitating i t as the diacetate salt and weighing the crystalline product (8) Clilorendic acid was determined in fire retardant paint by precipitating the acid as the potassium salt after saponification in isopropyl alcohol (19). The recovered salt was dissolved in water and acidified. The free acid was extracted with ether and titrated with standard base. Sometimes i t is possible to precipitate interfering substances rather than material being determined. Thus, silver nitrate was used to precipitate sulfides and chlorides in refinery waste waters making it possible to determine phenolics colorimetrically by the 4-aminoantipyrine method without the usual distillation step (23). Cholesterol has been deterniined quantitatively using tomatine as a precipitating agent (33). Aerosols may be precipitated electrostatically, to separate them for further study or analysis. The use of a miniature electrostatic precipitator for sampling aerosols was proposed (57). I
NEWER SEPARATION TECHNIQUES
I n the past 15 years there has been a revolution in analytical chemistry through the development of instrumentation and nonaqueous solvent methods along with these new separation techniques. Recent literature makes extensive use of the newer separation techniques in functional group analysis. Ion Exchange. Recently, ion exchange resins have been used extensively t o separate organic compounds or groups of compounds. Most organic compounds which are ionic or which can be converted into ionic derivatives can be separated from nonionic materials by this technique. Ion exchangers in analytical separa1560
ANALYTICAL CHEMISTRY
tions are increasingly popular in recent literature. I n the biochemical field great use has been made of modified cellulose ion exchangers (52) in the exchange and separation of proteins and other macromolecules on DEAE (diethylaminoethy1)cellulose. Also, long-chain quaternary ammonium compounds have been separated from interfering substances in chloroform extracts of animal feeds using partially carboxymethylated cellulose (45). Enzymes exchanged on cellulose ion exchangers do not lose their biological activity as they do on resin exchangers (47). Ion exchange membranes, papers treated with ion exchange resins or papers that have been modified chemically, are beginning to find use in organic separations and analytical procedures Free fatty acids in fat were determined by exchanging the acids on a strong anion exchanger ( S I ) . The acids were later removed as methyl esters from the ion exchanger and gas chromatographed. Dowex 50 has been used to separate cationic ethylene oxide condensates from polyoxyethylene glycols (36). Choline, betaine, and other natural occurring nitrogenous compounds were separated on Dowex 50 columns before eventual colorimetric determination (12). Ion cwhange materials were used as a practical means of separating amino acids and similar materials (10). A mixed bed of cation and anion exchangers was used to remove interfering substances in the determination of betaine in sugar-beet juices and plant material (15). Trisaturated glycerides in fats were determined with mercaptoacetic acid. This method made use of solvent extraction and a DEAE-cellulose column in the separation step (18). Countercurrent Extraction. A simple liquid-liquid partition extraction system works well only when the difference in t h e partition coefficients of the solutes for t h e two liquids is very large. When t h e difference in t h e partition coefficients is small, t h e separation can often be accomplished by a process called countercurrent extraction or distribution. Although countercurrent distribution (CCD) is widely used in the fractionation of naturally occurring biological materials, the number of papers that deal with functional group analysis by this technique are fen. Countercurrent distribution has been used for the fractionation of high-boiling phenols (34). The various components were identified by infrared or ultraviolet spectroscopy. The use of CCD for the fractionation of polymers (polyglycols) has been discussed in a series of papers (1). For those interested in this field. a most authoritative review
and bibliography has been published ( I S ) . Nearly 400 nearly-immiscible solvent systems have been listed in another paper (46). Inclusion Complexes. A numbei of substances have t h e ability tc form inclusion complexes with organic compounds. The formation of these adducts depends on the size and shape of t h e constituents involved. Two general types of complexes are formed: channel complexes, in which one constituent forms a cylindrical channel or framework in which the second constituent is enclosed; and clathrate or cage complexes, in which molecules of one constituent fit into molecular cages formed by the second constituent. These adducts have been used for separation of organic molecules by extractive crystallization. Deoxycholic acid forms channel complexes with fatty acids, normal paraffins, camphor, and other compounds. Urea will adduct with long-chain compounds like normal paraffins, straight chain aCidS, alcoliole, and esters. Thiourea will addurt with normal, branched, and cyclic caompounds. Hydroquinone will form clathrate or cage complexes with methanol, acetonitrile, and similar compourids. Tlie well-known starch-iodine reaction is the result of the formation of an inclusion compound. Pure linoleic acid has been prepared through the use of urea adducts. Branched chain fatty acids have been separated from normal acids using adduct formation. d very complete review of the use of urea and thiourea in separating organic compounds has been given (66). Although this paper is several years old, it is still one of the best reviews in this field. A recent paper deals with ureahydrocarbon complexes (37). The composition of hydrocarbon-urea complexes wab determined by the amount of urea present in the complexusing 9-xanthenol. Molecular Sieves. Molecular Sieves are related t o t h e inclusion complexes in t h a t they are synthetic or naturally occurring substances t h a t have channels of given sizes t h a t may hold molecules of the correct molecular size. Zeolite has channels of 5 to 6 A. which can hold the hydrocarbons methane through heptane. Recently synthetic. zeolites having Molecular Sieve action have become commercially available. The most common pore sizes available are 5A. and 4A. diameters. These materials have also been used for the fractionation of gaseous mixtures and have found wide use as column packings for the gas chromatographic separation of gases. A Molecular Sieve of 5A. was used to determine the normal paraffin content of olefin-free petroleum distillates (40). The hydrocarbons are allowed to stand with the Molecular Sieve which absorbs the paraffins. The decrease in volume
types is by far the most popular is measured and used to quantitate the separating device in organic analysis straight chain hydrocarbon present. today. M a n y hundreds of papers Molecular Sieve absorption has been applied t o hydrocarbon-type analysis appear every year in this field; therefore, only a few of each type and par(50). The hydrocarbons were separated ticularly those t h a t have some reinto saturates, olefins, and aromatics on lationship to functional group analysis silica gel columns. The separated hydrocarbons were passed over 5-4. Molecuwill be cited. Column chromatography using alular Sieve columns. The absorbcd hymina has been used to separate air-borne drocarbons were weighed. Mass specpolynuclear aromatic hydrocarbons (GO), trometry was later used to identify the hydrocarbons. nonionic surface-active agents in oil Electrophoresis. ’ 1 ’ 1 1 ~ t c i . i i i (3lwtroand solvent extracts from no01 (29), nonionic gasoline additives (61), and phoresis is applied to the niigi-‘i t,’ 1011 fatty acids from unsaponifiable matof charged particlw under tlip influence of a11 elect’rica field. Polar ter (59). In this last paper, the fatty moleculcs in organic iriisturc>> (’ai1 acids are recovered as the esters by be separatcd using t ~ l r ~ c t ~ i ~ n p’ h o ~ c refluxing with methanolic HCI and devices. E:Iectrophoresis call hv u s ( d extracting with petroleum ether. This to separate mixtures t h a t cannot bc paper presents a cleanup device that separated by electrodialysis. Strongly could very well be used in gas chromapolar organic molecules such as carboxtographic procedures. ylic acid, amines, phenols, and their Silicic acid columns have been used salts can be separated from ottirr orto determine organic acids in river ganic molecules that arc not 1)i)I:ir or water (49),aldehydeb in air (SZ), and are only slightly polar. 2,4-dinitrophenylhydrazine derivatives d very comprehensive book c)n the, of highly oxygenated carbonyl comtheory, methods, and applications of Complex mixtures of pounds (YO). electrophoresis has been published (6). polycyclic aromatic hydrocarbons have been separated on magnesia-Celite colA study of the fractionation conditions for the quantitative analysis of hriinaii umns (42). Volatile compounds have serum protein fractions by cAc,llulose been adsorbed on a charcoal column, acetate electrophoresis has bwn 111:ide removed by a stream of gas, and identi(9). The uptake of the dye, TAiss:iniine fied by gas chromatography (16). Green SF 150 is used for drtrrmining Liquid-liquid partition chromatogthe concentrations of thc fractions. raphy has been used to determine -4rapid procedure for thr elt‘ctrophorcsis chemical antioxidants in fats ( 5 ) , of alkaloids was recently reported (69). 2,3,6-trichlorobenzoic acid in herbicide Preparative zone electrophorwis for formulations (,?I), C4 through CI2 samples up to 1 gram has been dtwribed dibasic acids (64,and 0,O-dimethyl-S(AV-methylcarban~oyIrnethyl)phosphoro(56). This type of approach might bey most useful for t’he applicatioii of fun(*thiolothionate (17). tional group analysis to sampks sepaThe ion exchange. chromatography rated by electrophoresis. Thr elccof amino acids rontinues to be of trophoresis of amino acids on c~cllulose t (20, 24, 25). Free purines, powder has been described (48). Thc dines, and nucleodes were sepaelectrophoresis of nucleic acids in silica rated and determined in cod muscle gel media has been reported (27). using a Dowex 1 chromatographic Chromatography. T h e greatest adcolumn (32). A series of modified vances in recent years in separations celluloses for the ion exchange of anof organic compounds have taken ionic materials was described (58). place in bhe realm of chromatography. DEAE-cellulose was used to separate Chromatography may be defined as a and purify folic acid analogs (51). separation technique by which coniI n the field of paper chromatography, ponents in a mixture are distributed hexahydro - 1,3,5 - trinitro - S - triazine between two phases in a column: one (RDX) and octahydro-1,3,5,7-tetraniphase is stationary a n d has a large tro-5’-tetrazine (HMX) have been sepasurface; the other phase is mobile. rated on ST’hatman 31111 paper (72). The substances to be separated will The RDX was migrated off the paper move through t.he column at different and determined colorimetrically. Carrates of speed depending on their bonyl compounds were paper chromatoaffinity for either the stationary or graphed as the 2,4-dinitrophenylhydramoving phase. The various classifizones ( 7 ) . After being separated, the cations of chroniatographic techniques derivatives were titrated with titanous are: column (liquid), which includes chloride. adsorption, ion exchange, and partition Gas-liquid chroniatography (GLC) types; paper, which includes normal separated compounds which were then phase, reverse phase, and thin layer collected and identified through functypes; gas, which includes gas-solid tional group analysis. Isomeric aldeand gas-liquid chromatography. hydes were separated by this technique Recent Applications of Chromaand the fractions collected (44). The tography. Chromatography of all aldehydes underwent a Wolff-Kishner
reduction to hydrocarbons and wcrc compared with API standards to identify the isomers. Gas-chromatographed fractions were treated with reagents to remove certain functional groups ( 3 ) . The treated fractions were rechromatographed and the disappearance of peaks was noted. Oxygen compounds in GLC fractions were identified by catalytic deoxygenation (67). Low boiling carbonyls were precipitated with 2,4-dinitrophenylhydrazine. The recovered hydrazone was mixed with a-ketoglutaric acid and heatflashed as the carbonyl compound into a GLC instrument (65). 12 qualitative and quantitative analysis was made in this manner. A similar procedure was published earlier (64). Components separated by gas chromatography were identified by passing the effluent stream into various reagents t h a t classified the components into functional group classes (68). This appears t o be a very promising approach to the use of gas chromatography and functional group analysis. This completes a cyclc because this is essentially n h a t Martin and James did when they first s e p a r a t d fatty acids and amines by gas chromatography and titrated them automatically. LITERATURE CITED
(1) Almin, K. E., Acta Chem. Scand. 13, 1263, 1274, 1278, 1287, 1293 (1959). ( 2 ) Baker, E. A,, Skarrett, E. J., Analyst 85. 184 (1960). (3) Basette, R.,’ Whitnah, C. H., ANAL. CHEX.32, 1098 (1960). ( 4 ) Beers, G. J. van, DeIongh, H., Boldingh, J., Proc. 4th Intern. Conf. Lipide; Essential Fatty Acids, p. 43, Butterworths. London. 1957. (5) Berger, K.’ G . , Sylvester, N. D., Haines, D. M., AnaIyst 85, 341 (1960). (6) Bier, M., “Electrophoresis-Theory,
Methods and Applications,” AcadenGc Press, New York, 1959. ( 7 ) Blom, L., Caris, J., Nature 184, 1313 (\19.59). - - - - , -
(8) Bond, G. R., Jr., ANAL. CHEM.32, 1332 (1960). (9) Brackenridge, C. J., Zbid., 32, 1357 (1960). (10) Buchanan, D. L., Zbid., 32, 1400 (1960). (11) Cassidy, W., Fisher, A. J., Analyst 85, 295 (1960). (12) Christianson, D. D., ANAL.CHEM. 32, 874 (1960). (13) Craig, L,., C., “Technique of Organic Chemistrv. 2nd ed.. Vol. 111. Part I. pp. 149-332, Intersdence, New York’> 19.56. (14) Cross, A. H. J., Hendey, R., Stevens, S. G. E., Analyst 85, 355 (1960). (15) Curruthers, A,, Oldfield, J. F. T., Teanue. H. J.. Ibid.. 85, 272 11960). (16) Ghont, J. ’H., Weurman, ‘C., Zbid , 85, 419 (1960). (17) Dupee, L. F., Gardner, K., Newton, P., Zbid., 85, 177 (1960). (18) Eshelman, L. R., Manzo, E. Y Marcus, S. J., Decoteau, A. E., Hammond, E. G., ANAL. CHEM. 32, 844 (1960). (19) Esposito, G. G., Swann, M. H., Zbid., 32, 680 (1960). Arch. Biochent. (20) Feiltelson, J., Riophys. 79, 691 (1959). ~
VOL. 33, NO. 1 1 , OCTOBER 1961
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(21) Gardner, K.,Overton, K. D., A n a l . Chim. Acta 23,271 (1960). ’ (22) Goldbaum, L. R., Williams, A., Kofpangi, T., ANAL. CHEM. 32, 81 (1960). (23)Gordon, G.E.,Zbid., 32,1325(1960). (24) Hamilton, P. B., Zbid., 32, 1779 (1960). (25) Hamilton, P. B., Bogue, D. C., Anderson, R. A., Zbid., 32, 1782 (1960). (26) Hardon, H. J., Brunink, H., VanAnalyst 85, 187 (1960). DerPol, E. W., (27) Harris, I). N., Davis, F. F., Biochem. et Bzophys. Acta 40, 373 (1960). (28) Herrett, R. J., Duyarce, J. A., ANAL. CHEM. 32, 1677 (1960). (29) Hobson, B. C., Hartley, R. S., Analyst 85, 193 (1960). (30) Homas, R. T., Zbid., 85, 551 (1960). (31) Hornstein, I., Alford, J. A., Elliot, L. E., Crove, P. F., ANAL. CHEW 32, 540 (1960). (32)Hughes, E. E., Lias, S. G., Zbid., 32, 707 (1960). (33) Kabora, J. J., McLaughlin, J. T., Riegel, C. A., Ibid., 33, 305 (1961). (34)Karr, C., Jr., Zbid.,. 32, 463 (1960). (35) Kaufman, S.,Medina, J. C., Zapata, C., Ibid., 32, 192 (1960). (36) Killheffer, J. V., Jungermann, E., Ibid 32, 1178 (1960). (37) i’iser, R. Shetlar, M. D., Johnson, G. D., B i d . , 33, 315 (1961). (38) Jakubovic, A. O.,Nature 184, 1065 (1059).
w.,
(39)Jones, M.R., Analyst 85,111 (1960). (40) Larson, L. P., Becker, H. C., ANAL. CHEM.32, 1215 (1960). (41) Leahy, J. S., Waterhouse, C. E., Analyst 85,492(1960). (42) Lijinsky, W.,ANAL.CHEW32, 684 (1960). (43) Lumokin. H.E..Xicholson. D. E.. ’ Ibid., 3 i , 74’(1960).’ (44) Mathews, J. S., Burow, F. H., Snyder, R. E., Ibid., 32, 691 (1960). 45) Metcalfe, L. D., Ibid., 32, 70 (1960). 46) Metzsch, F. A. V., Angew. Chem. 65, 586 (1953). (47) Mita, M. A., Schleuter, R. J., J . Am. Chem. SOC.81, 4024 (1959). (48) Montant, C., Touze-Soulet, J. M., BuZl. Soc. Chem. Biol. 42, 161 (1960). (49) Mueller, H. F., Larson, T. E., Ferretti, M., ANAL. CHEM. 32, 687 (1960). (50) O’Connor, J. G., Xorris, S. hl., ioid., 32, 701 (1960). (51) Oliverio, V. T.,Ibid., 33, 273 (1961). (52) Peterson, E. A,, Sober, H. A., J . Am. Chem. SOC.78,751 (1956). (53) Rajzman, 8.,Analyst 85, 116 (1960). 154) Rall. J. IT-.. ANAL. CHEM.32. 332
I
(55) Rdd, R. L., Analyst 85,265(1960). (56) Reuter, W.,Biochern. 2. 331, 337 (1959). (57) Robinson, M., ANAL. CHEM. 33, 109 (1961). (58) Rudstam, G., Zbid., 32, 1664 (1960).
(59) Sammons, H. G., Wiggs, S. M., Analyst 85,417 (1960). ( 6 0 ) Sawicki, E., Elbert, W., Stanley, T. W., Hauser, T. R., Fox, F T., ANAL.CHEM. 32, sin ( I R A O ) (61) Sherwood, R. fi Jr.. Zbid.. 32. 1131 (1960).(62) Smith, E: D .., Ibid.. 32. 1301 (1960). (63) Somers. E..Richmond. D. V.. A nalyst 85,440 ( i k m / . (64) Spell, CHEM. 32, 1811 (1’ (65) SteDhens. R.L., \ - - ,
\ - - - - ,
(67)Thompson, C. J., Coleman, H. J., Hopkins, R. L., Ward, C. C., Rall, H. T., ANAL.CHEM.32, 1762 (1960). (68) Walsh, J. T..Merritt, C.,. Jr.,. Ibid., . 32, 1378 (1960).’ (69) Williams. L. A.. Brusock. Y . M.. ’ Zak. B.. Zbid., 32, 1883 (1960).’ rom, M. L., Arsenault, G. P., < (1960). ) Wood, R., Analyst 85,21 (1960). (72) Yasuda, S. K., Rogers, R. K., ANAL.CHEM.32, 911 (1960).
RECEIVEDfor review April 19, 1961. Accepted July 17, 1961. Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
CompIexometric Determination of Aromatic Hy d roca rbo ns with Tetra cya noet hylene GEORGE H. SCHENK and MARA OZOLINS Chemistry Department, Wayne Sfafe University, Detroit 2, Mich. Tetracyanoethylene (TCNE) is used to titrate aromatic hydrocarbonsphotometrically via A complexation. Pure aromatic hydrocarbons as basic as or more basic than naphthalene are estimated in a wholly organic complexometric titration. Mixtwes of a stronger aromatic P base in a weaker aromatic A base are assayed for the stronger base-e.g., naphthalene in phthalic anhydride, benzo(a)pyrene in benzo(e)pyrene, and 2,6-dimethylphenol in 2,6-di-terf-butyl-4-methylphenol. The calculated titration curve agrees fairly well with the experimental titration curve.
F
group approaches to the determination of aromatic hydrocarbons have been limited to such methods as base-catalyzed condensation at a methylene carbon as for cyclopentadiene or indene (16), and DielsAlder reactions as for anthracene (10, 15, 17). General colormetric determinations of aromatic? have been limited to nitration ($1 and the recent phosphorus pentachloride-catalyzed conaensation of UNCTIONAL
1562
ANALYTICAL CHEMISTRY
piperonal (14) with aromatics more basic than toluene. The above reactions depend to some degree on the Lewis or Bronsted basicity of the aromatic hydrocarbon. However, none of these methods are specifically based on taking analytical advantage of the basicity of the ?r electron system, the only “functional group” common t o all aromatic hydrocarbons. For instance, both olefins (7) and sulfides (5) have been determined by complexation with iodine, a Lewis acid. The work of Merrifield and Phillips on the colored T complexes of the strong IT acid, tetracyanoethylene (TCNE), suggested the use of T C N E as titrant in a complexometric titration of aromatic R electron systems. The procedure reported uses pure T C N E as a reference standard. A methylene chloride solution of T C N E titrant is added to a methylene chloride solution of the aromatic hydrocarbon to be estimated, and the end point 1s determined photometrically This procedure depends on the formation of a stable colored ?r complex and appears to be the first wholly organic
complexometric titration systeni. It is also necessary that the titrant be as concentrated as possible as well as a strong T acid. Other possible titrants are 7,7,8,8tetracyanoquinodimethane, whose K value for ?r complexation with pyrene is 78.4 (I), and 2,3,5,6-tetrachlorobenzoquinone, whose K value with pyrene is 23.3 (8). T C N E hm a K value of 29.5 with pyrene ( 8 ) . However, the first two a acids can be prepared only a t one tenth of the concentration of the TCNE titrant descyibed here; this is apparently not satisfactory for complexometric titration. EXPERIMENTAL
Apparatus. T o perform the photometric titrations, use a Bausch & Lomb Spectronic 20 in conjunction n i t h a 50- or 125-m1. Erlenmeyer flask (11). Use a 5- or 10-ml. buret equipped with a Teflon stopcock Reagents. Tetracyanoethylene ( T C S E ) , 0 1M. Synthesjzf, from malononitrile (4) or obtain fiom Eastman Kodak Co Sublime the TCNF and recrystallize (101. T I eizh 01’ exactly 640 nig of TCNF a n n dissolvr
