An Application of the Polarizing Microscope to Reaction Studv .J GARRETT W. THIESSEN and LAWRENCE F. BESTE Monmouth College, Monmouth, Illinois
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HIS paper describes a simple experiment in the quahtative . . study of reactions by means of the polarizing microscope. The reactions discussed are electrolytic, but the method is applicable to other types of reaction as well. The indispensable reqnirement is that materials and products shall diier in their effectupon polarized light. This method offm an easy means for illustrating the progress of some reactions which would otherwise require a difficult and tedious procedure. It possesses value in teaching something of the use of the microscope, the physics of light, crystallography, electrolytic cells, and the chemistry of the reactions involved. It may be used as a stimulus to student investigation in applying the method to other reactions. A projecting microscope would make the process visible to an entire class a t once.
polarized light whether any chlorate or bromate is being formed. APPARATUS
We used an inexpensive microscope of 100-200-300 power purchased several years ago for twenty dollars. A polarizer-analyzer was purchased a t the same time for eight dollars. Upon the polarizer is mounted an improvised revolving stage. The analyzer is in a cap which is set loosely on the eyepiece and rotated a t will. The electrolysis was carried out in a vial which was cut off to approximately one-ml. capacity. This vessel was mounted on the vertical spindle of a spring motor which turned a t about eighty R.P.M. Stirring was found not essential in such a small cell, but it gave better yields. Two platinum wires were used as stationary electrodes. The circuit was arranged as in Figure 1,
THEORY
Crystals belonging to the isometric system are optically isotropic, that is, they possess the same refractive index in all directions. All other crystals are birefringent, a t least in some directions (I). Between a "crossed" polarizer and analyzer an anisotropic crystal will be brilliantly lighted against a dark field, showing that i t has broken the light from the polarizer into two components which will pass the analyzer. This effect will not be evident if the crystal happens to lie in a "position of extinction" or if the observer is looking down the unique axis of a tetragonal or hexagonal crystal. However, the double refraction of anisotropic materials is seldom overlooked because of such an orientation (2). There is little danger of oversight if the field includes a number of crystals. First-year chemistry students are familiar with the electrolysis of the alkali metal chlorides according to the equation:
Copper Oxide Rectifier
It is not so well known that chlorine reacts with hydroxyl ion to form hypochlorite, chloride, and water. Then, with the aid of heat, the hypochlorite oxidizes itself still further to the chlorate: 3KC10 + 2KCl
+ KCIOl
The alkali metal bromides behave similarly. It happens that potassium chloride and potassium bromide both crystallize in the isometric system, whereas potassium chlorate and bromate do not. It should be easy, therefore, to determine by means of
allowing the potential across the electrodes to be varied gradually from 0 to 5 volts. A storage battery or four dry cells in series with a variable resistor would provide a satisfactory current, instead of the rectifier.
volts, which was too low to discharge sufficient chloride A saturated solution of the salt was placed in the ves- ion for our purpose, produced an observable amount of sel and a drop removed to a slide with a stirring rod. bromate. For demonstration purposes saturated soluIt was noted each time that the crystals at the begin- tions, heat, 3.5 volts, and thirty minutes' time are recning of the experiment were entirely isotropic. The ves- ommended. sel was held several minutes in a water bath just under TABLE 1 10O0C. and then transferred to the stirrer and electroTempera- Time ir Soft lvra Minuter Vollnr~ Rerxllr lyzed. After fifteen minutes a drop was removed for KC1 Rmm 20 3 . 5 sz 0 . 1 0 examination, the solution was reheated for a moment, KC1 Hot 20 3.5-0.1 2C-f5% KC108 KC1 Hot 30 2.0 - 0 . 1 . O and electrolysis continued. KC1 Hot 30 3.5 - 0 . 1 33% KCIO, Prior to each microscopic examination the solution KBr Hot 30 2 . 0 -0.1 Slight KBr Hot 30 3.2 * 0 . 1 Satirfacto~ was made slightly acid to remove the possibility of hyfor demoortradroxide, carbonate, or bicarbonate among the products. Hydrochloric acid was used for the chloride and hydroI t may be well to add here that the potassium chlobromic for the bromide. The indicator was a drop of rate produced in one run was positively identified by the bromthymol blue mixed with a drop of the solution on a spot plate. Litmus paper may be used for the pur- immersion method (3). As mentioned, hydroxide, carbonate, and bicarbonate are destroyed before examinapose, although the bleaching effect is rapid. tion. There is one other possible anisotropic product of chloride, perchlorate, but F. Winteler has shown that RESULTS Certain rough quantitative estimates may be made of no perchlorate is formed until nearly all the chloride is the results obtained as shown below. I t will interest converted to chlorate (4). METHOD
students to try the effect ofvarying the potential, the time, or the concentration of the salt. Favorable and unfavorable temperatures might also be determined. Table 1gives some data collected. Drops from 0.5 M solutions of potassium chloride and potassium chlorate were mixed in appropriate proportions on a slide to serve as rough standards of comparison for the "results" column. I t might be pointed out that a potential of 2
LITERATURE CITED
(1) FORD."Dana's manual of mineralom." 14th ed...-Tahn Wilev and Sans, Inc., New York City, 1.929, p. 78. (2) CHAMOT AND MASON, "Handbook of chemical microscopy," John Wiley and Sons, Inc., New York City, 1930, Vol. 1, p. 282. (3) Ibid., p. 364. (4) MELLOR."A comprehensive treatise on inorganic and theoretical chemistry," 3rd impression, Longmans, Green and Company, Inc., New York City, 1930, Vol. 2, p. 375.
