MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 305 



The detection of magnesium in simple salts is comparatively 

 easy and rapid, since characteristic crystals are readily obtained, 

 but its microchemical identification in complex mixtures is usu- 

 ally a matter of not a little difficulty, in as much as this element 

 is commonly associated with others, closely related, which are 

 prone to interfere with or prevent the formation of typical 

 crystals with the reagents employed for its recognition. 



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 II. 1 



I. To the solution of the material to be tested, which must 

 not be too concentrated, add several fragments of ammonium 

 chloride; stir; then add a very slight excess of ammonium hy- 

 droxide, 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 am- 

 monium magnesium phosphate NH4MgP04 6 H 2 O soon appear. 



II. To the solution to be tested add a fragment or two of 

 citric acid, stir until dissolved, then add an excess of ammonium 

 hydroxide. Evaporate to dryness. To the residue add dilute 

 ammonium hydroxide. Warm; then add a very small frag- 

 ment of secondary sodium phosphate. Crystals of ammonium 

 magnesium phosphate separate. 



The crystals of the ammonium magnesium phosphate sepa- 

 rate as skeletons and hemimorphic forms of the orthorhombic 

 system (see Ammonium). 



It should be remembered that a number of elements are 

 precipitated by phosphates in alkaline solution; the most fre- 

 quently met with in the course of microchemical analyses, either 

 in the substance to be tested, or present as reagents from previous 

 tests, are, doubtless, lithium, members of the calcium and mag- 

 nesium groups, trivalent metals, manganese, nickel, cobalt, tin, 

 lead, silver, copper, and uranium. 2 Of these elements, lithium, 



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



2 Most of these elements will generally have been removed in the progress of 

 the analysis before the addition of the sodium phosphate. 



