OBSERVING IMICKOCRYSTALS 







^**t 











1. ^ 



t^^^ i"^ ^.^* ^" ^ ^ 



V, 





■+4» ^ If 







V'V^ ## # 







Fig. 2f. Trimethylaniine HCl with iodine-KI 

 (5:80) in H3PO4 (2:1) (iodine-KI reagent B-1). 

 (direct addition), crystals black, opaque. lOOX. 



fere more or less with the ammonia reaction. 

 The recommendation of the test, therefore, 

 is still for cases where ammonia is the only 

 product coming off, or at least the chief one. 



Proceed as previously described, but in- 

 stead of fixing the evolved ammonia with 

 dilute HCl, use a hanging drop of neutral, 

 1% formaldehyde solution (0.1 ml of the 

 usual concentrated formaldehyde, 37 % by 

 weight, diluted to 4 ml with water). After 

 exposure above the alkaline solution, rein- 

 vert the hanging drop and allow it to dry. 

 Methenamine gives branching isotropic 

 crystallization, frequently with tripartite 

 forms. Examine for a trace soon after drying, 

 as it has a slight volatility. In a blank test 

 there is at most a very slight deposit of tri- 

 oxymethylene left from the formaldehyde 

 solution itself, which usually does not show 

 anything definite microscopically, and does 

 not react in the following crystal tests. 



To confirm methenamine, redissolve the 

 deposit in a little droplet of water, add a 

 droplet of iodine-KI solution (1:1 g in 100 

 ml), and preferably rein vert o\'er a cavit}^ 



containing a little crystal of iodine — ^this 

 prevents evaporation of iodine from the test- 

 drop, and the crystals can be examined at 

 relative leisure. Characteristic birofringent 

 crystals form at once. 



Methenamine can also be confirmed with 

 reagent 3 above, HAuBrj in 9H.3P04-2H.>0- 

 15HBr. Put a droplet of the reagent on a 

 small cover-glass and invert on the dry resi- 

 due. Allow a short time for the formation of 

 characteristic crystals with a very small 

 amount . 



Of the lower amines, methylamine gives 

 the most noticeable results, including minute 

 microcrystals, but it could not possibly be 

 confused with ammonia if more than a trace 

 of the latter is concerned, or if the character- 

 istic crystals are obtained and observed. 



The formaldehyde test for ammonia, al- 

 though confirmed by the usual sort of basic- 

 nitrogen microcrystal tests, in the formation 

 of methenamine illustrates the use of ciuite 

 a different reaction for microscopical chem- 

 istry. 



Charles C. Fulton 



OBSERVING MICROCRYSTALS 



To make full use of microcrystals, the 

 chemist-analyst must learn to see and under- 

 stand all the characteristics that may possi- 

 bly distinguish them. In studjdng a particu- 

 lar test, not every detail descriptive of the 

 crystals needs to be permanently recorded, 

 but everything that can readily be observed 

 should be observed, in deciding what is 

 worth writing down for a formal description, 

 and what should be the points of compari- 

 son to establish identity between known and 

 unknown. Too often those who try such tests 

 content themselves with a very superficial 

 observation of the crystal forms alone. It is 

 surprising how much can be done with such 

 observations, often using mereh^ the ordi- 

 nary microscope; it is not nearly as bad as 

 failing to use the microscope at all, but still 



51 



