Dennis H. Evans
University of Wisconsin Madison 53706
Coulometric Titration of Cyclohexene with Bromine
Faraday's iaws of electrolysis are almost universally discussed in introductory chemistry courses in connection with chemical stoichiometry and the electrical nature of matter. The topic is also important in chemical analysis since these laws form the basis of the various coulometric methods. Therefore, a coulometric titratio11 (constant current coulometry) seemed an appropriate quantitative experiment to incorporate into an introductory course. The goal of the experiment may be either the measurement of the faraday by the titration of a known quantity of a pure suhstance or the analytical goal of determining the amount of a given substance in a sample. The latter approach was adopted in the experiment to he described. Several coulometric titrations for the student lahoratory have been developed (1-4). The most frequently encountered is the titration of As(II1) with electrogenerated iodine. This system does not adequately illustrate one of the principal advantages of coulometric methods, viz., the use of reagents such as Cu(1) or bromine which are d i c u l t to employ as standard titrant solutions. Consequently, the coulometric titration of an olefin with electrogenerated bromine was chosen. This system is also an example of a coulometric method which has actually found industrial application (5) in the determination of trace unsaturation in petroleum products. Since amperometry is used for end point detection, the student is introduced to this powerful analytical technique as well. Lingane's monograph (6) remains the best single source concerning the principles of coulometric titrations and amperometry. For an example of a very high precision coulometric titration the reader is directed to a recent work on the titration of primary standard acids (7) and for a modern determination of the faraday by a coulometric method the aork of the National Bureau of Standards group should be consulted (8).
bar which was rotated at a speed just sufficient to produce a vortex in the liquid. An ordinary variable speed magnetic stirrer was employed. The generating anode and cathode were 20 and 16 cm lengths of 24 gauge platinum wire, respectively. These current carrying electrodes were wound in helical form and sealed through the walls of the electrode probe as shown in the figure. This probe oomprised a 14/20 standard taper joint with a 10-mm glass tube extending about 5 cm into the cell. The tube was terminated at this point and a 6-mm tuhe was ring-sealed within the larger tube. The smaller tuhe extended from outside the cell to about 3 cm beyond the end of the larger tube. The cathode was sealed into the end of the smaller tube and the anode was sealed into the annular space between the tubes a t the lower end of the larger tube. In this way the generating electrode pair was constructed on a single probe which could be inserted through the side port. A few centimeters of mercury was placed within each tuhe for contact to the leads to the power supply. The amperometric indication electrode pair was constructed in a similar way. These were a pair of 1-cm2 platinum foil electrodes mounted parallel to one another with a separation of about 4 mm. Wire electrodes were also found to he suitable for indicator electrodes. The 19/38 center port was used solely for the addition of sample solution. The exact position of the electrodes is not critical. Considerably less elaborate methods of mounting the electrodes should be suitable, e.g., the electrodes could be sealed in straight glass tubes which in turn are inserted through a two-hole rubber stopper. The electrolyte used was the same as that used by Leiey and Grutsch (5), 60:26:14 (volume per cent) glacial acetic acid-methanol-water, 0.15M potassium
The Experiment
In this experiment, 1-2 mg of cyclohexene in a methanol solution was determined by reaction with bromine. The bromine was generated by constant current electrolysis of an acidic potassium bromide solution. Amperometry with two polarized electrodes was used to detect the end point. The apparatus used in this experiment is a simplified form of that designed by Leisey and Grutsch (5). The cell is shown in the figure. It was constructed from a 100 cc round bottom flask the bottom of which was flattened and provided with a circular indented region which served as a guide for a 1-in. Teflon coated stirring 88
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ENERATING ANODE
INDICATOR ELECTRODES
I Cell for coulometric titrotions.
bromide and 1 g mercuric acetate per liter. The mercuric acetate was found to be essential for rapid addition of bromine to the olefin. The constant current and timing apparatus used in this experiment was the Sargent Model IV Coulometric Current Source. Other commercial apparatus for coulometry at controlled current or the simple devices described by Waser (4) or Reilley (2) should also be satisfactory. A particularly appealing constant current device would be a programmable regulated power supply operated in the constant current mode (9). The amperometric end point detection system r e quires that a small, constant voltage be applied between the two electrodes and that the resulting current be monitored. A Sargent Model XV polarograph served this purpose admirably as would any manual or recording polarograph. A much less expensive polarizing unit was constructed by using a 1000 ohm, one-turn potentiometer as a voltage divider with a 1.35 volt battery (Mallory RM42R) as the power source. The continuously variable voltage from the potentiometer was applied to the indicator electrodes with a sensitive microammeter (Simpson Model 1327C, 0-25 microamperes) in series. The components were assembled in a small cabinet (Bud No. AC-1613) which was placed next to the cell. In operation the polarizing voltage was set to the desired value with the single turn potentiometer which was calibrated by measuring the output at various settings with a high impedance voltmeter. With this unit polarizing voltages between 0 and 1.0 v and currents up to 25 microamps could he used. This apparatus proved to be very satisfactory for student use and its operation was easily comprehended after a brief examination of the components and the wiring. To carry out a titration, enough electrolyte was added to the cell to cover the electrodes. The cell was closed, the stirring was initiated and the polarizing voltage (0.25 v) was applied to the indicator electrodes. The indicator current at this point was less than 1 microamp. The coulometric current source was then switched on. The generation of bromine was terminated when a steady indicator current of 20.0 microamps was attained. The current depends linearly upon the bromine concentration reaching 20 microamps with a bromine concentration of about 2 X 10-SM under the conditions used. Reactive impurities in the electrolyte did not exceed 2 X 10-BN. The indicator current remains constant for several minutes. Fluctuations due to irregularities in stirring were about *0.3 microamp. A 2.000-cc volume of sample solution containing 3 X 10-qo 6 X equivalents of olefin was transferred to the cell and the cell was closed. The indicator current decreased immediately as the excess bromine wes consumed. The timer mas set to zero and bromine was again generated until the indicator current had returned to a steady value of 20.0 microamps. The number of equivalents was noted, the timer was returned to zero and the next titration could be carried out immediately. It should be noted that in this procedure there is a significant excess of bromine a t the end point. The accuracy of the method depends upon the exact duplication a t the end point of the bromine concentration
that was present at the beginning of the titration. After extended use the cathode of the generator electrode pair becomes coated with mercury which is formed by electrochemical reduction of the catalyst. The mercury does not seem to affect the results but it may be removed by treatment with 1: 1 nitric acid if desired. Students received solutions of cyclobexene in methanol as samples. A master stock solution containing about 4 mg cyclohexene per cc was prepared. Students were issued known volumes of the master stock solution in the range of 20-30 cc. These were diluted by the student to 100 cc with methanol for the analytical solution. Results were reported as total mg cyclohexene in the sample. The stock solution was standardized by the teaching staff at regular intervals by the same method employed by the students. Since cyclohexene is somewhat volatile, all solutions must be stored in tightly stoppered containers. For example, it was found that carrying out titrations with the cell open to the atmosphere produced results which were several per cent low. A tightly stoppered stock solution is relatively stable with typical decreases in titer of 0.1% per day. Results
The absolute accuracy of the method was iuvestigated since the cell had been simplified by omitting the separate compartment for the current carrying cathode thus leaving both the anode and cathode of the generating electrode pair in the same solution. This configuration was successfully employed by Lingane in the coulometric titration of acids with electrogenerated hydroxide (10). I t seemed that it would be successful in this case since the titration product, 1,2-dibromocyclohexane should not be reduced a t the cathode and the major product a t the cathode, hydrogen gas, should not react rapidly with either cyclobexene or bromine. These expectations were realized. Results obtained with the cathode in a compartment separated from the test solution by a 10 mm medium porosity glass frit were identical with those obtained with the cell as shown in the figure within the precision of the determination. Furthermore, when a standard solution was prepared from pure cyclohexene the average amount of cyclohexene found was 99.9Y0 of that taken. (Looking at this result another way, if the object of the experiment had been to determine the magnitude of the faraday, the result would have been 0.1% low.) Matheson, Coleman, and Bell cyclohexene was used. Any peroxides that may have been present were removed by passing the cyclohexene through a column of silica gel. It was found that samples from 1 to 5 mg of cyclohexene could be titrated with the same accuracy. The accuracy was also unchanged when a generating current of 4.825 milliamps was used instead of the normal 9.65 milliamps. The expedient of putting both the generator anode and cathode in the test solution would undoubtedly lead to high results if the hromination reaction were slow. This would cause an accumulation of bromine during the titration and increase the amount reduced a t the cathode. With cyclohexene the addition of hromine is so rapid that no appreciable accumulation of bromine occurs, as evidenced by the fact that the amVolume 45, Number 2, February 1968
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perometric indicator current remains negligibly small until the end point is approached. The bromination of some olefinsis much slower (5). An experienced worker can obtain a precision of about + 1ppt (e.g., standard deviation 1.3 ppt with six trials) in the determination of 2 mg of cyclohexene by this method. Student precision with the same equipment and procedure varied from 1to 20 ppt in a section of eight freshmen. The accuracy obtained by students wm considerably lower than their precision. The average error of a group of thirteen juniors and seniors was 0.5y0 while that of the section of eight freshmen was 1.4%. In its present form the experiment is of an analytical nature. I t serves as an illustration of the techniques of constant current coulometry and amperometric end point detect,ion. These topics are treated in the lecture portion of the course along xith other aspects of electroanalytical chemistry. This is usually the student's first experience with the analysis of a volatile constituent and one of his first experiences in organic analysis. It is important that a careful presentation of the theory of amperometric end point detection be
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given since this is the most confusing aspect of the experiment to the students. I wish to thank Mr. Stephen H. Hall and Mr. Robert Johnson who did much of the experimental work. Literature Cited
(1) MELOAN, C. E., AND KISER,R. W., "Problems and Experiments in Instrumental Analysis," C. E. Merrill Books, Inc., Columbus, Ohio, 1963, p. 170. (2) REIILEY,C. N., J. CHEM. EDUC.,31, 543 (1984). D. T., "Experiments for (3) REILLEY,C. N., AND SAWYEB, Instrumental IMethods," McGraw-Hill Book Co., Inc., New York, 1961, p. 98. (4) WABEB,J., "Quantitative Chemislry," W. A. Benjamin, Inc., New York, 1961, p. 196. J. F., Anal. Chem., 28, 1533 (5) LEISEY,F. A., A N D GRUTSCH, (1956). J. J., "Eleetmanalytieal Chemistry," (2nd ed.), (6) LINGANE, Interscience Publishers, Inc., (division of John Wiley & Sons, Inc.), New York, 1958, p. 267. p. 484. E. L.,AND SHAFPER, E. W., JR., Anal. Chem., (7) ECIWELDT, 37, 1534 (1965). J. I., I.AI~-,C. A,, AND HAMER, (8) CMIG, D. N., HOFFMAN, W. J., J. Res. !Vat. Bur. Sld., 64A, 381 (1960). (9) BIRMAN,P., "Kepco Power Supply Handbook," Kepco, Inc., Flushing, N.Y., 1965, p. 39. J. J., Anal. Chim. Ada, 11, 283 (1954). (10) LINGANE,
