and Laboratory Methods. 



137^ 



Tests applied to pure salts and simple mixtures are quite satisfactory, and 

 would scarcely lead the worker to anticipate the annoyances and difficulties 

 which may beset him in other cases. 



In ordinary practice three reagents will be found useful : 



I. Secondary Sodium Phosphate in Ammoniacal solution. 

 II. Potassium Antimonate, 

 III. Uranyl Acetate with Sodium Acetate. 



/. Jhc addition of Scconda?y Sodium Phosphate to Ammo7iiacal solutionis am- 

 tainiug Magnesium pj'ecipitatcs Ammoniiim Magnesium Phosphate. 



MgSO^ + HNa2P04 + NH^OH = NH4MgP04 • GH2O + Na2S04 + HgO. 



Method. — Two methods are available ; the choice of procedure depending 

 upon the nature of the salts present in the drop to be tested. In all cases where 

 there is a doubt as to the probable composition of the material to be examined, 

 it is best to have recourse at once to the modification B.* 



A. To the solution of the material to be tested, which must not be too con- 

 centrated, add several fragments of ammonium chloride ; stir, then a very slight 

 excess of ammonium hydroxide, and warm the preparation. (If a precipitate 

 results it is best to draw off the clear solution.) To the warm solution add a 

 small crystal of secondary sodium phosphate. Crystals of ammonium magnesium 

 phosphate soon appear. 



B. To the solution to be tested add a fragment or two of citric acid, then an 

 excess of ammonium hydroxide. Evaporate to dryness. To the residue add 

 dilute ammonium hydroxide. Warm, then add a very little solid secondary 

 sodium phosphate. Crystals of ammonium magnesium phosphate separate. 



The crystals of the ammonium magnesium phosphate separate as skeletons 

 and hemimorphic forms of the orthorhombic system (see Figs. 40 and 66). 



Remarks. — It should be remembered that a number of elements are pre- 

 cipitated by phosphates in alkaline solution ; the most 

 frequently met with in the course of micro-chemical 

 /|. analyses, either in the substance to be tested, or present 

 as reagents from previous tests, are doubtless, lithium, 

 members of the calcium and magnesium groups, triva- 

 lent metals, manganese, nickel, cobalt, tin, lead, silver, 

 copper, uranium. t Of these elements, lithium, iron, 

 manganese, cobalt, and nickel form, with ammonium 

 and phosphoric acid, salts of similar composition to 

 and isomorphous with the magnesium salt. 



The ammonium glucinum phosphate, ammonium 

 zinc phosphate, and ammonium cadmium phosphate 

 are not precipitated in crystal form. 



Fig. 60. 



* Romijn, Zeit. anal. Chem. 37, 300. 



t Most of these elements will have been removed in the progress of the analysis before the 

 addition of the sodium phosphate. 



