CHEMICAL MICKOCKYSTAL IDENTIFICATIONS 



on a slide as with several ml in a test-tube, 

 before using the microscope at all. Color 

 tests on the spot-plate are a useful adjunct. 

 Accustom oneself to working on some such 

 scale, which will probably be found more 

 convenient than the "usual" one, anyway. 



Keep a polarizing microscope at hand, and 

 use it. If a polarizing microscope cannot be 

 used, polarizing attachments for the ordi- 

 nary microscope are helpful, but are at best 

 a poor makeshift. Observation of the micro- 

 crystal tests is almost always by transmitted 

 light, although it would be possible to use 

 reflected light in many cases. "Low Power" 

 of 80-100 X is usually sufficient for the 

 tests, at least for all the initial observations. 

 If the crystals are very small, a higher power 

 is then used. 



Make a practice of looking at the result 

 of a precipitation test microscopically, 

 whether the textbook mentions crystals or 

 not. For example, the iodoform test for ethyl 

 alcohol is made quite definite for the product 

 iodoform (although in any case it is not 

 specific for ethanol) by microscopic observa- 

 tion, although textbooks often fail to men- 

 tion this and may only refer to the odor. 



Pictures that can be consulted are helpful 

 in dealing with a test not yet familiar, and 

 when time permits or the case is important 

 enough the analyst should take them of his 

 own results, as a reminder to himself and 

 information to others. 



There is a real art in taking photomicro- 

 graphs of known crystals so that they will 

 most plainly show the truly characteristic 

 forms that should be looked for. Pictures for 

 reference, however, valuable as they are, are 

 no substitute for at least a little actual ex- 

 perience, and no substitute, either, for actual 

 comparison of the results given by an un- 

 known with those given by a known com- 

 pound. 



Most of the familiar tests, for organic as 

 well as inorganic substances, are made with 

 aqueous solutions. Usually the ordinary flat 

 microscope slide is quite satisfactory. Com- 



monly, a cover-glass is not used, unless or 

 until examination by high power is wanted. 

 The test is observed while evaporation takes 

 place. This time can be prolonged by setting 

 a petri-dish over the slide when it is not 

 under immediate examination. Cavity slides 

 are less convenient for focusing, but the 

 evaporation is more gradual, and can be 

 stopped by simply laying a slide over the 

 one on which the test is made. Cavity slides 

 are also more convenient for mixing three or 

 four ordinary drops, in more complex tests. 



The reagent drop may be added directly 

 to the drop tested, or placed beside it on a 

 plain slide and the two drops allowed to flow 

 together. Reagent is usually added in about 

 the same size drop as the solution tested. 

 The actual concentration of substance tested 

 is therefore reduced by about half in the 

 test-drop, but it is customary to report the 

 sensitivity or optimum concentration, etc., 

 in terms of the solution tested, before it is 

 mixed with reagent. 



A full drop of water (let fall from a fairly 

 wide opening) is about 0.05 ml. Use smaller 

 rather than larger "drops", by touching the 

 pipet to the slide and letting just a little flow 

 out, by dropping from a narrow orifice, or 

 by using a rod to transfer a drop. In most 

 tests with no especial need to conserve ma- 

 terial ordinary 1-ml pipet s may be used for 

 handling both the solutions to be tested and 

 the reagents. 



Not only crystals, but also amorphous 

 precipitation, or absence of precipitation, are 

 observed. Amorphous precipitates may be 

 finely divided or curdy, or in drops, the latter 

 often a prelude to the formation of large 

 crystals. Crystals may form directly, or from 

 an amorphous precipitate. As evaporation is 

 usually not controlled, the time during which 

 a test is observed is then only that required 

 for the drop to dry up. Test-crystals often 

 form around the edge, especially with dilute 

 solutions, as the drop dries. The reagent it- 

 self may crystallize out when evaporation 

 has gone far enough, and a reagent-blank 



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