and Laboratory Methods. 



1325 



Fig. 58. 



IV. Amnwuiuni Fluosilicate precipitates Barium Fluosilicate. 



BaCU + (NH^)2SiF6 = BaSiFg + 2NH4CI. 



Method. — Place, on a celluloid slip, a drop of the moderately dilute solution 

 to be tested. Acidify with acetic acid ; spread 

 out the drop a trifle ; add a fragment of am- 

 monium fluosilicate and warm gently. There 

 will immediately form, throughout the test 

 drop, fusiform crystals, either singly, in crosses, 

 or in more or less irregular masses (Fig. 58). 



If the solution is quite dilute, instead of 

 the usual fusiform crystals, well-defined 

 rhombohedra and prismatic crystals are ob- 

 tained. 



Remarks. — See Sodium, Method III. It 

 is important to avoid testing concentrated 

 solutions, since fluosilicates of calcium or 

 strontium may possibly separate, although 

 neither of these elements will be precipitated under the conditions which usually 

 obtain in testing. This caution as to concentration is necessary, because when 

 crystals of calcium fluosilicate CaSiFg • 'iH^O do appear, the forms obtained 

 may resemble the barium salt quite closely. Calcium fluosilicate is to be 

 referred, however, to the monoclinic system. The corresponding strontium salt, 

 SrSiFg . 2H2O, is isomorphous with the calcium compound, and is slightly less 

 soluble than the latter. 



The form of barium fluosilicate varies quite a little according to the concen- 

 tration of the test drop, and to its state of acidity. 



Much free mineral acid is apt to interfere slightly with the precipitation. 



When employing celluloid slips, it is of course essential to use great care in 

 warming the preparation, owing to the inflammability of the material. Under 

 proper precautions there is very little danger of losing the test. The warming 

 should be slight. 



If barium alone is to be searched for, a glass slip may be employed, as the 

 formation of any sodium fluosilicate will not materially affect the test for barium. 



In the absence of ammonium fluosilicate, ammonium fluoride and a little 

 silica may be added to the test drop, or the silica may be suppressed and the 

 test performed on a glass slip. 



Exercises for Practice. 



Test pure salts of Ba ; Sr ; Ca ; first in dilute, then in concentrated, in 

 neutral, and in acid solutions. 



Make a mixture of Ca, Sr, Ba, and test as above. 



Try reaction on mixtures of Na and Ba ; then on Na, Ca, Ba ; Na, Sr, Ba ; 

 varying the concentration of the test drops. 



Test a mixture of Ba and Mg ; then one of Ba and Fe. 



Try the reagent on a salt of Pb. 



