256 ELEMENTARY CHEMICAL MICROSCOPY 



EXPERIMENTS. 



a. Obtain a thin uniform film of NaCl as described above. 



b. Near the residue (2 to 3 millimeters) place a drop of distilled water; acidify 

 the drop by touching with a drawn-out glass rod which has been dipped in dilute 

 HC 2 H 3 O 2 ; introduce a tiny fragment of UO2(C 2 H 3 O 2 )2. Warm the drop gently to 

 facilitate solution, but do not evaporate. Cool. By a single, rapid stroke of a 

 glass rod or platinum wire, draw a streak or channel of the reagent across the 

 center of the dry material. Place the preparation upon the stage of the micro- 

 scope and search the edges of the streak of liquid at once. Tiny faintly yellow 

 triangular and tetrahedral crystals of NaC 2 H 3 O 2 UO 2 (C 2 H3O 2 ) 2 will be seen. 



Analytical applications Na, Mg, U, acetates. 



V. Upon failure to obtain a decisive test owing to the unsatis- 

 factory separation of crystals, the delicacy of the reaction can be 

 increased through the addition of another reagent which will produce 

 a less soluble salt of the same nature. 



The chemical reactions involved in the practical application 

 of this method of increasing the delicacy of microchemical iden- 

 tity-tests are among the most interesting and instructive with 

 which we have to deal. To properly apply and interpret them 

 or to devise new tests to meet special conditions requires, in 

 inorganic chemistry, a good working knowledge of the Periodic 

 System of Mendelejeff : while in the case of reactions in the field 

 of organic chemistry success can only follow a profound knowl- 

 edge of the chemical and physical properties of the compounds 

 to be studied. 



Considering the method only from the viewpoint of inorganic 

 analysis, the delicacy of a test can be increased by introducing 

 into the test drop, in which no separation of a crystalline phase 

 has taken place, a salt whose base will form a less soluble com- 

 pound than that originally present. For example, suppose a 

 test for the presence of chlorides is being made by means of 

 platinum sulphate and a salt of potassium; with much chlorine, 

 potassium chloroplatinate will separate, but if we obtain no 

 crystals, we may add a little rubidium sulphate to the drop. 

 Should this yield no result, it can be followed by a little cesium 

 sulphate and finally carried to the limit by the introduction of a 

 thallous salt. With the potassium salt the limit of the test is 7~ 4 

 milligrams of chlorine, but with thallium 4~ 6 milligrams (Beh- 



