A PHOTOBROMINATION EXPERIMENT WAYNE W. UMBREIT University of Wisconsin, Madison, Wisconsin
By the simple application of the colorimeter to the measuremat of bromine contat, difficulties formerly experiaced i n attempting to teach photochemical reactions may be eliminated. The reduction of costs t h w effected will permit the smaller teaching laboratories to include photo-reactions as a subject of study.
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URING recent years there has been an increase in the demand for courses in physical chemistry among students in our smaller colleges and training schools. Inasmuch as the permanency with which a course of this type is retained by the student depends considerably upon the adequacy of laboratory training, the increased demand has produced still greater strains upon the budget-stretching facilities. The author believes that any attempt to reduce laboratory expense without the sacrifice of its spirit is a step toward the wider dissemination of scientific knowledge. A suggestion, which may be of use in that direction, is offered. Among the varied fields with which the student of physical chemistry should be acquainted is that of the photochemical reactions. They serve not only in their own right, but as a most excellent introduction to the theories of radiant energy and of molecular and atomic structure. Because of the expense ordinarily involved in their treatment, they are usually either restricted to the lecture-hour demonstration or neglected entirely as laboratory projects. The author does not believe that expense is any longer a handicap in their investigation and a method is herein outlined whereby an entire class may have first-hand information about photochemical reactions a t no increase in expense to the department. The photobromination of cinnamic acid has received abundant attention in recent years and has, in addition, a thorough scientific grounding. It is an uuusually favorable reaction for elementary experimental study. The cinnamic acid, as well as the bromine, is sufficiently soluble in carbon tetrachloride to allow the reaction to proceed. The cinnamic acid is ordinarily included among those preparations made by the organic classes and with a little purification these "preps" may be used in student work without endangering the results. The bromination proceeds with the addition of bromine to the double-bond of the cinnamic acid (CsHsCH : CHCOOH Brz + CeHsCHBrCHBr.. COOH) a t a reasonably rapid rate. The rate of the dark reaction is such that it may be neglected for student determinations. The only real obiection
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to the reaction is that i t offers technical difficulties to the student limited by inadequate laboratory facilities. As usually conducted the solution of both cinnamic acid and bromine in carbon tetrachloride is placed in a large buret and exposed to the light for definite intervals, samples being withdrawn, run into potassium iodide solution, and the liberated iodine titrated with tbiosulfate. Excellent results have been obtained with tbis method but its use in laboratory teaching is subject to several limitations. In the first place few determinations can be made; the curve of the reaction can thus only be indicated in a very general way. The difficulty of the complete removal of the bromine (and the iodine) from the carbon tetrachloride, in the presence of which it must be titrated, serves more as an analytical problem and diverts the energies and mind of the student from the photochemical reaction to that of the analytical procedure. The withdrawal of samples disturbs the system and often with inexperienced operators radically divergent results are obtained. Aside from wasting time and materials poor results often undo a considerable amount of teaching by creating a feeling of doubt or disappointment with the lecturer's statements when the student finds that he cannot c o d r m them in the laboratory. But in addition to these the greatest difficulty which detracts from the feasibility of the use of this method in student classes is that the expense associated with it is relatively great. Special burets are often necessary; some method must be devised for illuminating them evenly along their entire length; the quantities of bromine, potassium iodide, and carbon tetrachloride consumed are rather large; the process of analysis is complicated and often consumes the greater portion of the time allotted to the experiment; in short, the entire procedure is not easily adaptable to student workers. It is readily seen that the chief difficulty in the application of this photo-reaction to student investigation is the cumbersome method of analysis. The substitution of a more satisfactory analytical method would make the reaction easily applicable. During the course of a minor research the colorimetric method for the estimation of bromine content was investigated. It is believed that this will seme to eliminate most of the difficulties inherent in the former method and thus make i t more widely available. A colorimeter is not an instrument of great cost, and a very simple one will suffice. If it is not independently available it may often be borrowed from the biological or bacteriological departments or may even be built in the laboratow a t
small cost. It has been found as a result of much study that in dilute solutions the colorimetric is actually more accurate than the titration method. Since, in dilute solutions, the reaction follows more nearly the Einstein law of photochemical equivalence (besides using smaller quantities of expensive reagents), the method of color measurement seems particularly fitted for elementary studeut work. The extent of reaction at various depths in the liquid (since the light must enter from the bottom) differs slightly and a small error is introduced if the measurement is made by means of a fixed standard with a movable sample. However, if the sample is kept stationary and the standard moved the error in measurement can be entirely eliminated. A solution of bromine in carbon tetrachloride of a slightly lower concentration than the reacting solution is used for the color standard and is standardized by titration. Beer's law may be applied and from it the concentration of the bromine calculated. Because of the deteriorating effects of the bromine upon the packing of the cells that ordinarily accompany the colonmeter, special cells made entirely of glass should be constructed. They are made simply from one-inch test-tubes, cut off at a depth to fit the colorim-
eter, and with a flat piece of glass sealed across the bottom. The glass may even be cemented on with silicate cement, which is unaffected by either the bromine or the carbon tetrachloride. Since the cells are rarely above 10 cc. in capacity, less than 50 cc. of carbon tetrachloride is required per studeut, though larger quantities may be used to advantage if available. The light source may be a 60-watt frosted bulb placed not over six inches from the colonmeter reflector. I t has been found more advantageous to use a ground-glass reflector, but a plain mirror will serve if the former is not available. In really accurate work corrections must be made for evaporation from cells and partial reflection, but these errors are ordinarily neglected by students without serious effect, although the calculation of the quantum efficiency is somewhat impaired. I t is now apparent that the application of the colorimeter to this photochemical reaction (and to many others which will be suggested by it) ought to make it possible for physical chemistry teachers in the smaller colleges to give to their students as close an experience with photochemical reactions as those in the larger laboratories where more funds are available. It is in this spirit that the suggestion is offered.
