30 Microscopic Histochemistry 



Potassium— This element exists partly (probably to a small 

 extent) in a poorly ionized, protein-bound, nondiffusible 

 form, while the bulk of it is diffusible and ionized. Whether 

 or not both fractions are demonstrable histochemically is 

 not known. The reagent used is sodium cobaltinitrite, which 

 forms with potassium a microcrystalhne precipitate of potas- 

 sium cobaltinitrite, orange in color. For better visibility it is 

 transformed in a second step into black cobalt sulfide. The 

 reaction is quite specific for potassium. Originally it was 

 believed that creatine could give a spurious reaction; more 

 recent findings, however, indicate that the precipitate was 

 caused by traces of potassium in the creatine sample used^ 

 Pure creatine gives no precipitate. Localization is fair to 

 good. 



Method (Macallurns,^ modified) 



Reagent -Solution A, dissolve 5 g. of cobalt nitrate in a 

 mixture of 10 ml. of distilled water and 2.5 ml. of acetic acid; 

 solution B, dissolve 15 g. of sodium nitrite in 25 ml. of dis- 

 tilled water. For use, pour solutions A and B together, shake 

 for a few minutes until the bulk of the nitrous fumes which 

 develop on mixing has escaped. Chill the reagent, one dish 

 of distilled water and three to four dishes of 50-70 per cent 

 alcohol in an ice bath. Place small pieces of fresh tissues in 

 the reagent for about 2 minutes. Rinse briefly in distilled 

 water, followed by thorough rinses in the alcohols. Transfer 

 to a dilute solution of yellow ammonium sulfide (about 1 

 drop to each 5 or 10 ml. of distilled water) for about 2 min- 

 utes. Wash, counters tain as desired, dehydrate, and mount. 

 A black, granular precipitate indicates the sites of potassium. 



Thorough rinsing of the tissue in dilute alcohol is very 

 important if one wants to avoid a disturbing gray back- 

 ground due to adsorbed cobalt. The mixed reagent can be 

 kept for only a few days. 



2. Macallum, A. B.: Australian J. Exper. Biol., 9:159, 1932. 



3. MacaUum, A. B.: J. Physiol., 32:95, 1905. 



