1324 Journal of Applied Microscopy 



near the edge of the test drop (Fig. 57). These crystals are clear and trans- 

 parent. By transmitted light they appear to be colorless, but if examined by 

 reflected light they will be seen to have a very faint, almost imperceptible yellow 

 tint. 



Remarks. — The reagent crystallizes in prisms belonging to the monoclinic 

 system, while the barium salt is to be ascribed to the orthorhombic system. The 

 danger of confusing the two salts is, therefore, slight. 

 The crystals of the ferrocyanide of barium and potas- 

 sium extinguish parallel to the diagonal bisecting the 

 acute angles of the rhombs. Many of the crystals of 

 w \ -^X ^ ^ >r^\^^^ barium salt appear to be rectangular plates or even 

 ■-■ \-^ a - \«r^ cubes, according to the position in which they are 



i — *^ Q I ' / ^/^ seen. An examination with crossed nicols will dispel 

 ^— ^y p^ ^^ the illusion. When the test drop is concentrated with 



respect to barium, the crystals of the double ferro- 

 cyanide separate at the point where the reagent was 

 introduced. 



Potassium ferrocyanide, though giving a neat re- 

 action with pure salts of barium, is of little value 

 when dealing with mixtures. It is then often difficult to avoid the precipita- 

 tion of calcium with the barium, particularly if much ammonium chloride is 

 present, or if much sodium acetate has been added to mitigate the action of 

 mineral acids. 



From mixtures, strontium may sometimes be precipitated if the solution is 

 quite concentrated, and may thus interfere with the test. Pure salts of strontium 

 give, in very concentrated solutions, only a granular deposit consisting of globular 

 masses, exhibiting no distinguishable crystal form. 



Magnesium is precipitated from ammoniacal solutions, but neither from acid 

 nor from neutral solutions ; hence the presence of this element will not mask the 

 test for barium. 



In addition to calcium and strontium, there are a number of other elements, 

 which, if present, will either be precipitated in insoluble form or will interfere 

 with the formation of the barium crystals. In this list the most frequently met 

 with will be lead, iron, zinc, rare earths, and less often copper, mercury, uranium, 

 titanium. 



Exercises for Practice. 



Crystallize a little of the reagent alone, and determine its optical properties. 



Try reagent on pure salts of Ca ; Sr ; Ba ; using both dilute and concen- 

 trated solutions. Try again, this time proceeding as directed under Calcium. 



Try the reagent on mixtures, say of Ca and Sr; Ca and Ba; Sr and Ba. 



Try effect of the reagent on salts of Pb, Zn and Fe. Then make mixtures of 

 Ba and these elements, and test. 



Make a preparation of BaK2Fe(CN)g • 5 HgO, measure the angles of the 

 crystals, and determine the optical properties of the compound. 



