and Laboratory Methods. 1123 



like aggregates (Fig. 34 A). Rarely, tiny four-sided prisms, with pyramidal 

 ends, are formed. 



If a sufficiently high power is employed, there is generally no difficulty in 

 distinguishing the double salt of potassium from that of sodium. 



Owing to the fact that higher powers must be employed than is usual in 

 micro-chemical work, it is necessary that the drop be spread out in a thin layer, 

 it being impossible to examine a well-rounded deep drop. The usefulness of the 

 test is therefore restricted. 



VIII. With Sodium-Cobalt Nitrite. 



This reagent produces, in neutral solutions of potassium salts or solutions 

 acidified with acetic acid, a very difficultly soluble double nitrite of potassium and 

 cobalt of the formula 3KNO2 • Co(N02)3. 



It is greatly to be regretted that this interesting and delicate reaction can 

 seldom be made to yield more than very minute globular grains. While it is a 

 convenient and generally reliable reaction for potassium in ordinary qualitative 

 analysis in the wet way, it is not to be recommended for micro-chemical testing. 



Either the standard reagent, all prepared, can be employed, or what is per- 

 haps more convenient for our purposes, the sodium nitrite, is added to the 

 neutral test drop and a solution of cobalt acetate, w^eakly acidified with acetic 

 acid, is allowed to flow in. The formation of yellow spheroids, octahedra, or the 

 skeletons of octahedra, will indicate the presence of potassium, providing that 

 ammonium, rubidium, and cesium are absent, these elements giving an identical 

 reaction. 



IX. With Sodium Tarti-ate or Tartaric Acid. 



The reaction taking place can be expressed as follows : KCl + HNaC4H406 

 = HKC4H40e -]- NaCl. Since the product of the reaction, primary potassium 

 tartrate (potassium bi-tartrate), requires a neutral or only slightly acid solution, 

 and is, moreover, fairly soluble,. it is convenient to proceed as follows : Evaporate 



the test drop so as to obtain a thin uniform film 

 of material. Place, near by, a drop of water into 

 which introduce a little tartaric acid and a slightly 

 greater quantity of sodium tartrate, stir until all 

 has dissolved ; then draw the reagent thus pre- 

 pared across the film of substance. If no crys- 

 tals appear after a short time, add a drop of weak 

 alcohol. The potassium salt separates as trans- 

 parent, highly refractive prisms. The crystal 

 forms are quite varied, those most frequently 

 obtained are shown in Fig. 35. Primary potas- 

 *"^- 35- slum tartrate crystallizes in the orthorhombic 



system, and exhibits a great tendency to assume hemihedral and skeleton forms. 

 Tartaric acid alone can be employed with good results, but primary sodium 

 tartrate is better. The addition of the free acid suggested above is for the pur- 

 pose of assuring the presence of the primary compound. 



