Spectrophotometric Titration of Olefins with Electrically Generated Bromine JOHN W. MILLER1 and DONALD D. DeFORD Department o f Chemistry, Northwestern University, Evanston, 111.
b A spectrophotometric titration method employing electrically generated bromine has been developed for the determination of olefins. Titrations are carried out in a 3 to 1 mixture of glacial acetic acid and methanol, with a small amount of potassium bromide and hydrochloric acid added. Mercuric chloride must be used as a catalyst to permit direct titrations. Bromine numbers of 13 olefins of various structures were determined with on a precision of better than samples ranging from 3.4 to 1 1 mg. Results obtained by this method are compared with values obtained by two standard procedures, both of which use bromine as the titrant.
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HE USE of bromine to determine unsaturation has been discussed by many authors. The side reaction of substitution has been the main objection to its use. Nearly all the modifications described in the literature have sought either to suppress the substitution reaction (6, 9), to accelerate the addition reactions (2, 8 ) , or to do both (6). Most of the direct titration procedures described previously have employed an empirically adjusted end point. This has been necessary because substitution reactions slomly use up bromine when it is present in a slight excess. Such empirically set end points are undesirable when a mixture containing several olefin types is to be analyzed. The general usefulness of coulometric titrations in routine analysis has been pointed out (4). The use of electrically generated chlorine for the titration of long-chain unsa$urated acids has been described by CPlta and KuEera ( 3 ) . These authors found that bromine gave less exact results because the break in the amperometric titration curve was not sharp enough. Iodine added too slowly to be of any value. Shortly after this work was first begun, another group undertook an investigation on this same problem (7). The only significant difference in the
Present address, Research Division, Phillips Petroleum Co., Bartlesville, Okla.
tn-o procedures is the method of end point detection. This paper describes the results of studies on the determination of olefins with electrically generated bromine. The use of a spectrophotometric end point has advantages which are not present in other end point methods. First, the dependence of the extinction coefficient on wave length permits a degree of selectivity which is not present in other end point methods. The sensitivity can be varied simply by a variation in wave length setting. Second, the trouble caused by electrical coupling between the indicating circuit and the generating circuit in automatic coulometric determinations is eliminated. Third, the exact end point in titrations which are slow in the vicinity of the end point can be determined by extrapolation of the linear portions of the titration curve before and after the end point. APPARATUS
A Cary Model 11 recording spectrophotometer capable of plotting absorbance vs. wave length or time was used in all the titrations. The only modification was a new cover for the sample cell compartment. A new type of modified cell holder was constructed so that a 100-ml. electrolysis beaker could be used as the titrating vessel. The tn-o pieces of new apparatus are pictured in Figure 1. Cell Holder. The base platform, A , was made of 0.5-inch aluminum, 10.0 by 11.5 cm. The bottom of base A was notched so that when the regular cell holder was removed, the base of the new cell holder would fit tightly into position on the platform pegs of the spectrophotometer sample cell compartment. Four holes were drilled to a depth of l/8 inch on the top of the platform base, the centers being 1.2 em. from each edge. These mere threaded with an 8-32 tap. Four brass rods, each 10 em. long ands',1 inch in diameter, were threaded on each end and then screwed into these four holes. The adapter, B , for the beaker was constructed of l/s-inch Bakelite with a hole cut exactly in the center to fit a lOO-ml., tall form beaker. The adapter was 10.5 by 11.5 em. Four holes were drilled 1.4 em. from each edge a t the
corners to allow the adapter to be set over the jour brass rods which mere attached t o the platform base. Each brass rod bad a piece of brass tubing placed ovei. it to hold the d a p t e r 7 cni. above the top of the platform base. Shorter pieces of tubing mere placed over the rods after the adapter was in position, and these were held tightly in place by four nuts. The whole assembly was painted flat black. Although only a 100-ml. beaker was used in this work, titration vessels of other sizes could be accommodztted by using appropriate adapters. Sample Cell Compartment Cover. The cover (Figure 1) was identical with the siandard cover on the Cary spectrophotometer except for three holes which were drilled near the center. The center hole held the stirring shaft of the motor, which mas mounted on the cover, and the other two holes Fermitted the introduction of the generation electrodes. A glass paddle stirring rod was attached to the motor by means of a No. 1, one-hole rubber stopper. The stirrer was long enough to reach just above the light path. By pushing the rubber stopper firmly against the cover, room light was prevented from entering the sample compartment. A small rheostat was attached to the motor to vary the speed of stirring. I n the final work with the apparatus, a large hole was drilled in the cover to allow the introduction of sample without removing the whole cover. This hole was closed satisfactorily by a No. 6 rubber stopper, as shown in Figure 1. Constant Current Source. A IiayLab Meter Calibrator, Model No. MlOAlO (loaned by Kalbfell Laboratories, San Diego, Calif.) was used as a source of constant current. This instrument is capable of delivering from 0.1 to 100.0 ma. with an accuracy of 5 parts in 10,000. Electrodes. The anode used in all determinations mas a platinum foil electrode (1.O byl.4 em.) ; the cathode was a platinum wire,which was placed in a compsrrtment isolated from the bulk of the r,olution by means of a. 1-em. sintered glass disk. The cathode conipartment TWS tapered a t the top to fit through a 3/ls-incli hole in the cover. REAGENTS
A11 chemicals employed were VOL. 29,. NO. 4, APRIL 1957
C.P.
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reagent grade. The arsenite samples were aliquots of standard solutions prepared in the conventional way from weighed quantities of arsenic trioxide. The olefins were from several sources. 1-Hexene, Zmethyl-Zhutene, 4-vinyl-lcyclohexene, and Zmethyl-l,3-butadiene (Phillips Petroleum Co., Bartlesville, Okla.) had a minimum purity of 99 mole yo. 4-Methyl-cis-Z-pentene, 4methyl-trans-2-pentene, 2,4,4-trimethyl-2-pentene, and 1-phenyl-%butene were also from Phillips Petroleum CO.; these samples had a minimum purity of 95 mole yo. Cyclohexene was from Eastman Kodak Co., White Label No. 1043. 2-Methyl-1-heptene (boiling point, 119-21' C.), 2,6-dimethyl-1heptene (boiling point 141-3" C.), 1,5hexadiene (boiling point 58-60" C.), and 2,5-dimethyl-1,5-hexadiene were supplied by Peninsular ChemResearch, (Gainesville, Fla.); the purity of these samples was not known (Table I, column 2). Standard solutions of the olefins were prepared by u-eighing a known amount of sample into u. volumetric flask and diluting to the mark with carbon tetrachloride. The standard solutions were prepared so that 0.500 ml. of the sample solution would take up 13 mg. of bromine. The generator electrolyte was essentially a nonaqueous mixture similar to that employed by Sweetser and Bricker (11) for the spectrophotometric titration of olefins with a standard bromatebromide solution. A stock solution of the generator clectrolytc mas prepared by mising 646 ml. of glacial acetic acid, 256 ml. of methyl alcohol, 16 ml. of concentrated aqueous hydrochloric acid, and 30 ml. of 40y0 (by weight) aqueous potassium bromide. A 15% (by weight) mercuric chloride solution in methanol x a s prepared as a catalyst.
Figure 1. Titrotion cell holder and sample cell compartment with electrodes and stirrer attached
PROCEDURE
The spcctrophotomcter and constant current source wcre allowed to warm up 15 to 20 minntcs before the titrations were begun. A wave length of 360 mp was selected when mercuric chloride was employed as a catalyst. This was the shortest wave length that could be used in the presence of mercuric chloridc, which has a high absorbance in the ultraviolet. A rate of bromine generation was selected so that the total time of generation vould be a t least 250 seconds. When the standard solutions were prepared as described above, a current of 50.00 ma. was employed. The cell holder was placed in the sample cell compartment, and the stirring paddle and electrodes were attached to the cover. The cathode compartment was filled with 1N hydrochloric acid, as the nonaqueous solvent ran out too rapidly. A standard quartz sample cell was filled rt-ith distillcd watcr and placed in the reference beam; 95 ml. of generator electrolyte and 8 ml. of the mercuric chloride solution were then added to the electrolysis beaker. The sides of the beaker were dried and polished with lens tissue. The olefin
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