1190 Journal of Applied Microscopy 



Remove the slide from the watch glass and turn it over. Cause the con- 

 densed moisture to unite in one drop by stirring with a glass rod. This drop 

 will contain the ammonia which has been expelled from the substance. The 

 danger of a possible loss has been guarded against by the drop of dilute hydro- 

 chloric acid employed. 



In order to test for ammonium in the drop thus obtained, it is only necessary 



to add a suitable reagent. Since it is extremely unlikely that any compound 



other than ammonium chloride is present, a number of methods are available. 



There are two, however, which will be found more satisfactory than the others. 



I. Chlorplatinic Acid (Platinum Chloride). 



II. Magnesium Acetate and Sodium Phosphate in alkaline 



solution. 



In practice Method I is the most convenient, simple, and satisfactory. 

 It is essential that a blank experiment be always performed to ascertain 

 whether the reagents employed are free from ammonium salts. 



/. Chlorplatinic acid added to solutions of Animonimn salts precipitates Ammo- 

 nium Chlorplatinate. 



2NH4CI + HgPtCle = (NH4)2PtCl6 + 2HC1. 



Method. Cause a drop of platinum chloride to flow into the drop obtained 

 after the manner described above. In a few moments yellow octahedral crystals 

 of (NH4)2 PtClg are obtained. These crystals resemble those of the corres- 

 ponding potassium compound in size, form, and color. The reader is therefore 

 referred to Potassium Method I, for a discussion of the appearance of the crys- 

 tals and to Fig. 29 for a representation of their form. 



Remarks. When much ammonium is present there is apt to be an immediate 

 precipitation of the chlorplatinate in the form of very minute or skeleton crystals. 

 It is then advisable to add a drop of water and recrystallize by heating. If, on 

 the other hand, the amount of ammonium is small, no crystals will appear until 

 the liquid has been concentrated by gentle heat. 



//. The addition of Magnesium Acetate and Sodium Phosphate to alkaline solu- 

 tions of Ammonium salts gives rise to the formation of Magnesium Ammotiiutn 

 Phosphate. 



NH4CI + Mg(C2H302)2 + HNa2P04 + NaOH = 



MgNH^PO^ . 6H2O + NaCl + 2Na(C2H302) + H2O. 



Method. To the test drop add a fragment of sodium phosphate and a very 

 little magnesium acetate ; stir thoroughly. Beside the drop, place a drop of a 

 moderately concentrated solution of primary sodium carbonate (HNaCOg) or of 

 sodium hydroxide. Cause the drops to flow together. There is generally an 

 amorphous precipitate immediately produced, which soon partly changes into 

 star- and X-like crystallites, see Fig. 40. A little farther away, roof- and envelope- 

 like crystals are obtained. 



Remarks. In dilute solutions the Xs and stars are generally absent, being 

 replaced by prismatic forms. 



