Colorimetric Determination of Indene D. A. SKOOG' AND H. D. DUBOIS California Research Corporation, Richmond, Calif.
A method for the analysis of indene in hydrocarbon mixtures is described. The procedure involves reaction of the indene with benzaldehyde in the presence of alkali to produce a highly colored condensation product which is suitable for the colorimetric estimation of the indene. The proposed method is rapid, accurate, and reasonably specific.
I
S CONSECTIOX with the preparation and investigatioii ot
certain aromatic compounds it became necessary to develop a method for the analysis of indene in the presence of other hydrocarbons. Attempts to utilize the ultraviolet absorption of indene for such an analysis were not successful because the absorption of other aromatic compounds present in the samples interfered Hammick and Langrish (1) have suggested the use of brominca addition for the determination of indene, and indeed, excellent results were obtained when an electrometiic method for the determination of bromine numbers was applicd to some of the samples. However, in other cases, the analyses were not satisfactory because olefinic compounds in the samples also absorbed bromine, and for this reason a more specific method was sought. Uhrig, Lynch, and Becker (4) have shown that the reaction between cyclopentadiene and carbonyl compounds in the presence of alkali to produce highly colored fulvenes can be used for the colorimetric determination of cyclopentadiene. Thiele ( 3 ) has observed that indene undergoes a somewhat analogous reaction although a t considerably slower rates, and Uighill ( 2 ) has employed such a reaction with acetone as a qualitative test for indene in light oils. Because this reaction appeared to offer a reasonably specific means for the desired analysis, the condensation of indene with a number of carbonyl compounds n as investigated and, from the standpoint of reproducibility of color, benzaldehyde was found to to be the most satisfactory reagent. According to Thiele (3) the reaction produces l-(a-hydro.;ybenzyl)-3-benzalindene, as 1s shonn by the equntion
Benzaldehyde Solution. Dissolve 5 ml. of U.S.P. grade benzaldehyde in 95y0ethyl alcohol and dilute to 250 ml. Alcoholic Potassium Hydroxide. Dissolve 3.0 grams of C.P. potassium hydroxide in 100 ml. of methanol. Heat to hasten solution. Allow to stand 2 hours and filter through a fine-fritted glass crucible. APPARATUS
A11 measurements were made with a Beckman Model D.U. spectrophotometer using 1-cm. cells. RECOMMENDED PROCEDURE
Preparation of Calibration Curve. Prepare a standard indene solution by weighing approximately 5 grams of indene to the nearest 0.02 gram into a 250-ml. volumetric flask and diluting to the mark with benzene. Transfer a 10-ml. aliquot to a 200-ml. flask and dilute to the mark with petroleum ether. This solution will contain approximately 1 mg. of indene per ml. With a buret measure 1-, 3-, 5-, 7-, 9-, and 11-ml. portions of the standard indene solution into 50-ml. Erlenmeyer flasks. Add 20 ml. of the benzaldehyde solution and 5 ml. of the alcoholic potassium hydroxide. Prepare a blank with the same quantities of benzaldehyde and potassium hydroxide solutions. Bring the solutions to a slow boil and hold them at this temperature for 10 to 12 minutes. Cool to room temperature, add 1 to 2 mi. of glacial acetic acid, transfer to 50-ml. volumetric flasks, and dilute to the mark with 95% ethyl alcohol. Determine the optical density with 1-cm. cells a t 420 mp, correct the optical density readings for the blank, and plot the corrected values against the concentration of indene. -4straight-line curve should result. As an alternative procedure, the color may be developed at room temperature. I n this case transfer aliquots of the standard indene solution to 50-ml. volumetric flasks, add the benzaldehyde and potassium hydroxide reagents, and allow the solutions to stand from 8 to 24 hours. Add 1 to 2 ml. of glacial acetic acid and dilute to the mark. The slope of the calibration curve oh-
OH
The product imparts an intense yellow color to the solution, which is well suited for the colorimetric estimation of indene. REAGENTS
Indene. Colorless; boiling point 180-181 "C.; nZ,O = 1.5768. 1 Present address, Department of Chemistry, Stanford TniTersity, Stanford, Calif.
Figure 1.
1528
Rate of Color Development a t Room Temperature
V O L U M E 2 1 , NO. 1 2 , D E C E M B E R 1 9 4 9
1529 Table I.
Analysis of Indene in Synthetic Samples
Approximate Composition of Diluent
toi&ne, ;On naphthalene 32% toluene, 0 % styrene, 63% benzsm 71% toluene, 26% Tetralin 73% toluene, 27% cyclohesene Y3c$ toluene, 7% fluorene
-96%
Figure 2.
2.58 1.89 4.26 1.96
0.7
1.2 1.1 0.2
2.61
1.87 4.27 1.99 2.19
2.14
70
Error,
1. 5
2.3
pared and analyzed by the recommended procedure. There were included in some of the samples various hydrocarbons which it was thought might interfere with the analysis. The results from these tests are given in Table I; excellent results were obtained in each case. The only hydrocarbons which have been encountered which interfere with the procedure are cyclopentadiene and some of the methyl derivatives of indene. The presence of the former in a sample is indicated by the development of a yellow color within 3 minutes after adding the reagents and a t room temperature. The methyl derivatives give a color which is nearly identical with the color formed with indene, and the proposed method has been used also for the analysis of these compounds in certain mixturrq.
Itate of Color Development at Boiling Temperature
tained by t liis n i t s t hot1 will Ijc slightly greater than that obtained by heating the solutions. Analysis of Samples. Weigh out samples and dilute with benzene or petroleum ether to such,a volume that the concentration of indene is between 0.2 and 2 my. per ml. Take 5-ml. aliquots and treat in exactly the same way as the standards. A reagent blank Ghould be run with each series of samples. Reference to the calibration curve gives the milligrams of indene in the aliquot.
Indene, % Present Found J . 69 3,73
1.4 .
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1.2
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DISCUSSION
The color obtained upon mixing benzaldehyde with indene in the presence of alkali is an intense yellon. which has an absorption maximum in the ultraviolet region of the spectrum. However, the quantitative nieasurements described herein were made a t 420 mp because i t was desired to have a method that would be adaptable to the use of visual photometers or colorimeters. The rate of color development is relatively s l o ~at room temperatures. For example, in the presence oi’ 0.3% potassium hydroxide a t least 8 hours are required t o reach the maximum color, and 24 hours are required t o reach the same color with 0.15y0potassium hydroxide (Figure 11. Higher concentrations of alkali give more rapid reactions: however, theseare to be avoided because excessive values for the blank are obtained as :I result of side reactions of t,he benzaldehyde. The reaction rate can be markedly increased by raising the temperat,ure. Thus a t 40” C. 2 hours are required for maximum color, while a t reflux temperatures full color is obtained in less than 10 minutes when the alkali concentration is 0.3%. This is illustrated in Figure 2. The color produced by the reaction was found to follow Beer’s law. However, the slope of the straight line relating optical density and concentration Tvas somewhat less when t,he color was developed a t the boiling temperatures of the solutions than when the reaction was carried on a t room temperatures (Figure 3). From the standpoint of accuracy and reproducibility, the two methods of color developnient are equally satisfact,ory and both have been used for analyses. The stability of the color was found to be excellent, particularly if the escess potassium hydroxide was neutralized with a little acetic acid after the reaction was complete. For example, solutions measured immediat,ely after color development and after 7 2 hours showed no change in optical density. I n order to test, the proposed method of analysis, a series of synthetic samples containing knom-n quantities of indene was pre-
CONC. INDENE -mg. / 50d. Figure 3 .
Calibration Curves for Indene Analysis LITERATURE CITED
(1) Hammick. D. L.. and Lanerish. D.. J. Chem. SOC..1937. 79T-801. ( 2 ) Mighill, T. H., Am. Gas. 154, 33 (1941). (3) Thiele, J., Ber., 33, 3395 (1900).
i,
(4) Uhrig, K., Lynch, E., and Becker, H. C., IXD. ENG. CHEM., . ~ N A L . ED., 18, 550 (1946). R E C E I V EJDu l y 2 5 , 1940
Correction In the article on “Determination of Cottonseed Oil on Tin Plate” [Donelson, J. G., and Keish, R . A., ANAL.CHEY.,21, 1102 (1949)], Figure 2 on page 1104 was printed upside down. The numbers on the horizontal axis should have been 10, 20, 30, 40, 50, and not 10, 20, 40, 60,80 as printed.
