and Laboratory Methods. 1293 



Normal potassium chromate (KoCr04) on the other hand, will precipitate 

 strontium at once from neutral or slightly acid solutions, as SrCr04, in the form of 

 slender rod-like prisms of the orthorhombic system. The crystals obtained with 

 KoCrO^ are usually better than those produced by KgCrgO^ or (NH4)2Cr20. ; 

 unfortunately barium is precipitated by both these reagents in either acid or 

 alkaline solution. It thus becomes a decided advantage to use a dichromate 

 in a solution acidified with acetic acid ; under these conditions only barium will 

 be precipitated, the supernatant liquid can then be drawn off, and to it ammo- 

 nium hydroxide added, when strontium will be precipitated. 



Salts of calcium yield no precipitate with ammonium dichromate, whether the 

 solution be acid or alkaline. 



Testing for strontium with dichromate is impossible in the presence of zinc, 

 cadmium or the rare earths. 



Lead and other elements forming insoluble chromates will be precipitated 

 before the ammonium hydroxide is added, but may escape complete precipitation 

 and interfere with the subsequent test for strontium. 



Exercises for Practice. 



See exercises and suggestions given under Barium. 

 V. Primary Sodium Carbonate. 



This reagent precipitates, from very dilute solutions, strontium carbonate in 

 the form of spherulites, often of considerable size. 



When simple salts of the elements Ca, Sr, Ba are employed it is not at all 

 difificult to distinguish between them by testing with primary sodium carbonate 

 (or ammonium carbonate). A drop of the almost saturated solution of the 

 reagent being caused to flow into the dilute neutral test drop, calcium will give 

 well defined, highly refractive grains and rhombohedra, strontium sperulites exhib- 

 iting the usual black cross between crossed nicols, and barium, spindle shaped 

 crystallites and fibrous masses. But if two or more of these elements are pres- 

 ent the reaction fails, characteristic crystals being the exception. 



Elements of the magnesium group must be absent. 



Primary sodium carbonate is of more value as a group reagent than as an 

 identification test. 



BARIUM. 



The most important reagents available for the microchemical detection of 

 barium are as follows : 



I. Sulphuric Acid. 

 II. Oxalic Acid. 



III. Potassium Ferrocyanide. 



IV. Ammonium Fluosilicate. 

 V. Ammonium Dichromate. 



VI. Potassium Antimonyl Tartrate. 

 VII. Primary Sodium Carbonate or Ammonium Carbonate. 



