14 MICROSCOPIC TECHNIQUES 



solution is placed on the section, and to avoid evaporating when 

 prolonged contact between the liquid and the section is necessary, 

 a hanging-drop slide is placed over the section so that the drop is 

 enclosed by, but does not touch, the walls of the depression. The 

 two slides are bound together with a rubber band and carefully 

 inverted so that the section is covered by the hanging-drop. 



B. INORGANIC ELEMENTS AND RADICALS 



POTASSIUM 



Macallum's original method for the histochemical detection of 

 potassium based on the precipitation of sodium potassium cobalti- 

 nitrite has been subject to modifications over the past thirty years. 

 The relatively recent modification of Gersh ( 1938) will be included 

 in the present work as well as the method of Carere-Comes ( 1938) , 

 which depends on the development of an orange color with Siena 

 orange. 



Gersh Modification of Macallum Method for Potassium 



SPECIAL REAGENTS 



Anhydrous Petroleum Ether freshly distilled over sodium {20-40° 



b.p.). 

 Dried Paraffin {Grilbler, 50-52° ni.p.). Just before use heat at 



100° or more in vacuo for about 15 min. or until bubbling stops. 



12% Sodium Cobaltinitrite Solution. Dissolve 150 g. sodium nitrite 



in 150 ml. hot water, cool to 40° (some crystals appear), add 50 



g. cobalt nitrate crystals, while stirring rapidly add 50 ml. 50% 



acetic acid in small portions, stopper, and shake well. Pass a 



rapid stream of air through the solution and let stand overnight. 



Siphon off clear hquid and filter. Add 200 ml. alcohol in small 



portions to the filtrate with stirring. After a few hr. filter off 



precipitate by suction. Wash precipitate four times with 25 ml, 



portions of alcohol followed by two times with ether. Recrystal- 



lize by dissolving each 10 g. solid in 15 ml. water and precipitate 



with 35 ml. alcohol. Make up the 12% aqueous solution fresh 



before use. 



