MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 275 



indication of the probable presence of K, Rb, Cs or NH4, can be 

 obtained at the same time Na is being searched for. 



In the presence of magnesium there will be obtained in addi- 

 tion to the tetrahedra of the sodium double salt large monoclinic 

 crystals of a triple salt 



NaC 2 H 3 2 . Mg(C 2 H30 2 ) 2 3 (UO 2 (C 2 H 3 2 ) 2 ) 9 H 2 0, 



taking the form of rhombs or appearing to be octahedra, dodeca- 

 hedra or having a more or less triangular outline with incurving 

 sides. When, however, the amount of sodium is very small 

 with reference to that of magnesium, only the triple salt will 

 appear. 



As might be expected any of the other elements in the magne- 

 sium group in the Periodic System, Gl, Zn, Cd, can replace Mg 

 in the triple salt. 



Precautions. 



Too much free acid interferes with the test a further reason 

 for evaporation to dryness before applying the reagent. 



Much magnesium gives rise to a film of salts so hygroscopic 

 that a dry film cannot be obtained unless the salts are first 

 converted into sulphates by evaporation with a little dilute 

 sulphuric acid. 



Members of the calcium group often cause trouble. If, there- 

 fore, an unsatisfactory test for sodium is obtained and subse- 

 quent testing reveals the presence of Ca, Sr or Ba, these ele- 

 ments should be removed by precipitation with sulphuric acid, 

 the solution filtered or decanted from the precipitate and the 

 filtrate evaporated to dryness on platinum (why ?) and again 

 tested for sodium. 



Any compounds present in the material to be tested which 

 will yield an insoluble precipitate with uranyl acetate, as, for 

 example, phosphates, will naturally seriously interfere with the 

 test or may absolutely prevent the detection of Na. In such an 

 event the amount of uranyl acetate employed must be slightly 

 more than sufficient to satisfy all the PC>4 present and to unite 

 with the sodium to form the double salt. Under these condi- 

 tions this test becomes unsatisfactory as applied above since it 



