ANALYTICAL CHEMISTRY King, E. J., and Watson, J. L., Mikrochemie, 20, 49-56 (1936). Koenig, E. W.,IND.EKG.CHEM.,ANAL.ED., 11, 532-5 (1939). Kurtz, L. T., Ibid., 14,191 (1942). Lorenz, N. Von., L a n d w . Vers.-Sta., 55, 183-220, 278 (1901). Lundell, G. E . F., and Hoffman, J. I., “Outlines of Methods of Chemical Analysis,” pp. 220-6, New York, John Wile) 8Sons, 1938. Matsevitch, V. S.,Zaaodskaya Lab., 9, No. 2, 229-30 (1940). Merz, J. A , , Svensk K e m . Tid., 53, 374-84 (1941). Panfilov, 1’. K., Chemisation Socialistic Agr. (U.S.S.R.), 9, NO.5,54-5 (1940). Scharrer, K., Biochem. Z.,261, 444-9 (1933). Schrenk, W, T., and Ode, W. H., IXD. ENG.CHEM.,AXIL. E D . , 1,201-2 (1929). Shik, I. R., Zaaodskaya Lab., 8, No. 10-11, 1179-81 (1939)
(25) Swinehart, C. F., and Flisik, H. F.. IXD.ENG.CHEM., ANAL.ED., 16,419-22 (1944). (26) Thorne, P. C. L., and Roberts, E . R., “Fritz Ephraim Inorganic Chemistry.” 4th ed., R . 817, New York, Nordeman Publishing Co., 1943; (27) Vasil’ev,K. A., and Barinova, 0. D., Zavodskaya Lab., 8 , 916-20 (1939). (28) Volynets, M. I., Ukrain. K h e m . Zhzir., 11, Wiss. Tl.,18-22 (1936). (29) Volynets, M.I., Zavodskaya Lab., 5, 162-4 (1936). (30) Volynets, ,M.I., and Bernsht,ein,S. S.,I b i d . , 5,1071-2 (1936). Ibid., 3,214-16 (1934). (31) Zhukovskaya, S. S., and Bernshtein, S. S., RECEIVED October 10, 1947. Presented before the Division of Analytical and Micro Chemistry a t the 112th Meeting of the AXERIGAXCHEMICAL SOCIETY,S e w York, S . Y.
Spectroscopic Determination of Cyclopentadiene and MethyIcyclopentadiene -
Butadiene Production D e p a r t m e n t , Southern California Gas C o m p a n y , Los Angeles, Calif. An ultraviolet spectroscopic method is described for determining cyclopentadiene and methylcyclopentadiene in the presence of each other. The sample for analysis is first depolymerized by passing it through a heated tube to convert the cyclopentadiene and methylcyclopentadiene to the monomeric form. The ultravidet absorption of the ilepolymerized sample is determined at two different wave lengths, from which the composition of the sample is
T
HE ultraviolet absorption spectrum of cycloperitadierie has been determined by Pickett, Paddock, and Sackter (S), but the literat,ure gives no method for determining cyclopentadiene by ultraviolet spectroscopy, and no data on the ultraviolet absorption of methylcyc1opent.adiene. The authors postulated that, there would be a shift in the ultraviolet absorption peak of met.hylcyclopentadiene to a higher xvave length than the peak for cyclopent’adiene and proof of this was undertaken. Pure cyclopentadiene and met~hylcyclopent~adiene were separated from hydrocarbon fractions containing these compounds produced by the thermal cracking of petroleum. It was indicated that the methylcyclopentadiene obtained was the l-niethyland 2-methylcyclo-1,3-pentadiene isomers (1). The physical constants of the cyclopentadiene were: boiling point, 41 ’ C.; d20 - 0.7982; and n;’ = 1.4465. The physical constants of the methyl cyclopentadiene were: boiling point., 73” C.; d:3 = 0.8112; and n;’ = 1.4609. The ultraviolet absorption spectra of the cyclopentadiene and methylcyclopentadiene and the spectra of their dimers were determined. The spectra obtained are given in Figure 1. I t will be observed that the absorption peak for cyclopentadiene is a t 240 mp and that for methylcycloprntadiene a t 247 mp, and that the absorption by their dimers is comparatively very small. By making use of these differences in ultraviolet absorption, the following method was developed for determining cyclopentadiene and methylcyclopentadiene in the presence of each other. REAGENTS
Iso-octane, Eastman technical grade. Activated silica gel, 28to 200-mesh ( 2 ) .
calculated. Aromatic hydrocarbons and conjugated diolefins interfere. However, by dimerizing the sample, the cyclopentadiene and methylcyclopentadiene are converted to dimers which have little ultraviolet absorption, and a blank ultraviolet absorption value is obtained which corrects for these interferences, if the concentration of interferences is small compared to the concentrations of cyclopentadiene and methylcyclopentadiene. APPARATUS
Beckman quartz photoelectric spectrophotometer equipped for ultraviolet spectroscopy. Silica gel column. Depolymerization apparatus (4). Volumetric flasks, 50- and 100-ml. Pipet,s, 5 , lo-, 15-, and 25-ml. PROCEDURE
Purification of Iso-octane. Pass the iso-octane repeatedly through the silica gel column until it shows no absorption a t 240 mp. Iso-octane which has been used in the analysis can be reclaimed by silica gel treatment. Preparation of Sample. If aromatics or conjugated diolefins other than cyclopentadiene and rnethylcyclopentadiene are present and the cyclopentadiene and methylcyclopentadiene are not all in the dimeric form, heat the sample to 100”C. for 6 hours in a closed container capable of withstanding a pressure of 10 atmospheres. This operation is performed to prepare a blank sample for determining the ultraviolet absorption of the interferences in the sample, inasmuch as the dimers of cyclopentadiene and methylcyclopentadiene have no absorption at the wave lengths used; i t is not necessary if neither of these conditions exists. Pipet approximately 5 nil. of sample into a tared 100-ml. volumetric flask and weigh. Dilute to the mark with iso-octane. Place the outlet tube of the depolymeiization apparatus in a 100ml. volumetric flask containing about 30 ml. of iso-octane. Cool the flask in an acetone-dry ice bath. Adjust the power input to the depolymerization apparatus in order to have the depolymerized sample vapor leave the heating coil at 340 O to 360 O C. This is accomplished by predetermining the setting of a variable voltage transformer to give the above temperature of vapors immediately leaving the heating coil as indicated by a test thermocouple inserted within the depolymerization tube. Transfer 5 ml. of the diluted sample into the funnel a t the top of the depolymerization apparatus. When the funnel is empty, add 5 ml. of iso-octane to the funnel, washing down the sides during the addition. Remove
511
V O L U M E 20, NO. 6, J U N E 1 9 4 8 the volumetric flask from the acetone-dry ice bath, allow it to come to room temperature, and dilute to the mark with isooctane. Dilute a portion of the contents according to the, following table: Dilutions 1st 2nd 5,:lOO 5/50 51100 10/50 5il00 25/30
Wt. % of Cyclopentadiene Plus Methyl Cyclopentadiene In Sample 82-100 28-89 11-33 6-17 3-5
_.
-
..
5:100
... ...
5/50 15,!30
0-3
mately 49 and 36 and a t 258 nip of 17 and 28, respectively. Aromatics have extinction coefficients below 3 a t t8hese wave lengths, and their interference is eliminated bv using a reference blank of the material dimerized to convert the cyclopentadiene and methylcyclopentadiene t'o t,he dimers that. have negligible absorption. Indene, however, has extinction coefficients a t 240 and 258 mp of 57 and 49, respectively, and, therefore, present's a definite inkrference if present, in considerable quantity. If indene is present to any extent, it sho~lltllw rrxmoved by fractional distillation.
Ultraviolet Absorption. Prepare a blank reference sample by diluting the dimerized sample if dimerization were performrd to the same degree as n-as done for the depolymerized sample. With the spectrophotometer, determine the optical density of the depolymerized diluted sample a t 240 and 258 mp with the spectrophotometer set a t 10070 transmittance on the blank diluted sample a t each wave length, If the sample contains no interferences and dimerization JTas not performed, use a blank of iso-oc tane. Calculations. Calculate the extinction coeficients from tht: optical densities observed a t the two wave lengths;, w i n g tht, following equation:
v-11ere K = extinction coeficient D = optical density C = concent#rationof sample iii grimis per litrr
Oi
solutioii
L = optical path in em. making
K l = extinction coefficient at' 240 nip li, = extinction coefficient at' 258 mp
Calculate the weight, per cent, of t,he cyclopentadiene anti methylcyclopentadiene from the t,wo equations: X = weight of cyclopentadiene = 100 (k1A-1 - k2K?) Y = weight 70of met'hyl cyclopentadiene = 100 (k& - k,K,) Determine the constants k , , k l r k 3 , and k* in these equations bj' carrying out the above procedure, using pure cyclopentadiene and methylcyclopentadiene dimers separately for the czracking trratmerit, determining the estinction coefficients for the two nionorners, and substituting these values in the following equations:
so
k, k, k, kq
= = = =
Kdd &A K,d K,A
ahere
'4 K, g b
= =
1 h'oh'd
- KbK,
extinction coefficient of cyclopenradiene a t 240 mp coefficient of methvlcrclopentadiene a t
= extinction
Table I. Results of Analysis of Synthetic Samples of alixtures of Cyclopentadiene and 3Iethylcyclopentadiene Cyclopentadiene Present, Found; 90.8 31.7 57.9
89.3 31.3 58.3
Figure 1. Ultraviolet Absorption Spectra of Cyclopentadiene, Rlethylcyclopentadiene, and Their Dimers
Other diolefins may interfere if present in large quantity but the blank absorption determined on the dimerized sample will minimize this interference. They may also cross-polymerize during the dimerization with cyclopentadiene and methylcyclopentadiene and not decompose during the subsequent depolymerization, thus causing an error in the analysis.
Methylcyclopentadiene Preqent, Found. 9.2 68.3 42.1
ACKNOWLEDGMENT
8.5 60.2 43.3
The authors wish to acknowledge the able assistance of Althea
M. Christopher in the experimental work. ~~~
LITERATURE CITED
ACCUR4CY OF METHOD
The results of a series of analyses on synthetic samples containing both compounds in known concentrations are given in Table I. As indicated, the widest deviation from the actual percentage present is l.5yO. Most other compounds that would be present in a hydrocarbon fraction containing cyclopentadiene and methylcyclopentadiene do not interfere appreciably. Cyclopentadiene and methylcyclopentadiene have extinction coefficients a t 240 mH of approxi-
(1) Edaon, K. C., Powell, J. S., and Fisher. E. L., IWD. ENO.CHEM., t o be published. (2) Graff, O'Connor, and Skau, TND. Ex-o. CHEM.,r\N.4L. ED , 16,556 (1944). (3) Piokett, L. W., Paddock, E., and Sackter, E., J . Am. ChPm. SOC., 63, 1073 (1941). (4) Powell, J. S., Edson, K. C., arid Fisher, E. L., ANAL.CHEM.,20, 213 (1948).
RECEIVED July 12, 1947.
