The Iodoform Reaction by Methods of Microscopy HAROLD F. SCHAEFFER Waynesburg College, Waynesburg, Pennsylvania
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OME technicians have frequent occasion to detect the presence of ethanol, acetone, or certain other compounds by resorting to the use of the wellknown iodoform reaction. As a rule considerable time could be saved by adapting this test to the methods of microscopy. A further advantage of such an adaptation is the f a d that a very minute quantity of sample will suffice. When the customary procedure is applied to colored solutions which contain only small concentrations of the compound sought, the iodoform precipitate may be so faint as to cast doubt upon the interpretation of results. On the other hand, since the microscopic test depends upon the formation of characteristic crystals, visual identification of any iodoform produced, even in a colored solution, is conclusive. It has been claimed inadvisable to perform the test on a microscope slide because of the volatility of the iodoform.' However, by introducing certain modifications, including the use of a hanging drop slide, the test has proved quite satisfactory even with solutions containing less than one per cent of ethanol or its equivalent. To perform the iodoform reaction under the microscope the followingshould be provided. 1. A hanging drop slide having a concavity 1.75 mm. deep. 2. An ordinary microscope slide. 3. A flat-topped steam hot plate. This may be a simple type such as shown in Figure 1.
4. Capillary pipets, drawn from 5- or 6-mm. glass tubing. These are so made that the outside diameter of the lower end measures just about 1.5 mm. 5. A cooling block.
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F I G ~ ETYPICAL GROUP OP IODOFORM CRYSTALS AS F O W D ON THE COVER SLIDE. (ABOUT 2 3 5 X )
The sample consists of only two small drops as delivered from one of the capillary pipets. The latter are filled by capillary action rather than by suction. Holding the pipet in a nearly horizontal position, the tip is applied to the surface of the liquid. The amount which flows into the tube can be readily controlled by varying the angle of inclination. A finger placed over the mouth of the pipet retains the liquid until released. When the finger is removed not all of the liquid flows out of the pipet, even when the latter is in a vertical position. After a bit of practice, however, the analyst will be able to ascertain the correct amount of sample to take into the capillary in order that the desired twodrop sample may be delivered. To the sample on the reaction slide a pinch of anhydrous sodium carbonate is added. A very convenient spatula for taking up an appropriate amount of the carbonate is made by flattening the end of a short length of 20-gage platinum wire. In order partly to disperse the sodium carbonate in F r c u n ~1.-SIMPLIFIED STEAM HOTPLATE, SHOWING RELATIVE POSITIONS O F THE HANGING DROPSLIDE AND THE COYER the liquid the slide is rotated, after which there are SLIDE. ( T H E STEAM HOT PLATECONSISTS MERELYOR A added, from a microuiuet, two droas of a solution of . 250-ML. BEAKER PROVIDED WITH A "TIN CAN"LID) iodine in potassium iodide. This reagent should be FOULKE AND SCHNEIDER, "Microtechnique of organic qualitaabout 0.2 N . At this stagei t is important that none of tive analysis," Ind. Eng. Chew., Anal. Ed., 12, 555 (1941). sur:E M ~ ~ - S C H N E I D E R , u~icrachemica~ laboratory manual,H the readants be permitted to spread to the John Wiley and Sons. Inc., New York City, 1932, p. 119. face of the hanging drop slide. 15
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To prevent loss of the liquid by evaporation an ordinary microslide is placed over the reaction mixture in such manner as to leave a small opening to serve as a vent. (Figure 1.) The combination is then carefully transferred to the hot plate. The water in the latter should be boiling vigorously. After three minutes the slides are removed to a cooling block. In the absence of a regulation cooling block a pile of glass plates may be used for the purpose. After a short interval to allow the mixture to cool, the upper slide is carefully lifted from the reaction slide and inverted. This cover slide is then examined for microscopic crystals of iodoform which may have sublimed from the reaction mixture. As a rule the
crystals resemble six-pointed "snow stars" but occasionally they may appear as thin hexagonal plates. While i t is true that the crystals are frequently imperfect, they can always be identified as belonging to the hexagonal system. A typical group is shown in Figure 2. If none of the iodoform has sublimed to the cover slide the crystals can be found in the residual reaction mixture on the hanging drop slide. Students in chemical microscopy who have applied the foregoing procedure to "unknowns" containing not over two per cent of an appropriate organic ingredient have found i t fully as reliable as most qualitative tests, even when working under the stress of a "practical" finalexamination.
