THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 313 



When a precipitate is to be recrystallized from hot concen- 

 trated sulphuric acid, it must be placed or formed at the corner of 

 the object slide and any supernatant aqueous solution decanted. 

 A moderate sized drop of the concentrated acid is then placed 

 upon the precipitate and the slide immediately inclined at an ang!e 

 of at least 30 degrees, to prevent the acid from spreading. Heat 

 from a tiny flame is then applied to the object slide just below 

 the upper edge of the drop, and as the acid fumes off the flame 

 is brought nearer and nearer to the corner. As soon as it appears 

 that sufficient material has passed into solution, the preparation 

 is removed from the flame and allowed to cool for a few seconds, 

 while still held in an inclined position. The inclined slide is then 

 tipped so as to cause a slow flow to the adjacent corner (see page 

 280, Decantation), thus decanting the clear acid from the remain- 

 ing insoluble precipitate, the channel of flow is cut off with filter 

 paper and the slide inclined until it is almost vertical, thus 

 causing the clear drop of acid to gather at the very corner of the 

 slide. This corner is then touched to a clean slide and through a 

 touch with a glass rod or platinum wire the drop is made to flow 

 from the inclined slide to the horizontal one. A small clear drop 

 is thus obtained. 



This system of attack can be employed in all cases involving 

 re-solution in strong reagents. Where constituents dissolving 

 from the glass slide are objectionable platinum foil can be em- 

 ployed, eventually transferring as above to a glass slide. 



The second case mentioned arises most often in the analysis 

 of organic compounds, as, for example, in the separation of a 

 free base from its salts by means of an alkali. Although the 

 amorphous appearing material will eventually crystallize sponta- 

 neously if given sufficient time, it is usually desirable to hasten 

 the formation of typical crystals. This can be accomplished by 

 taking upon a platinum needle the most minute fragment 

 possible from a portion of the pure base believed to be present 

 and drawing it through the amorphous mass, crushing it at 

 the same time. Crystallization of the amorphous material is 

 almost always immediately started and proceeds with great 

 rapidity. 



