26 MICROSCOPIC TECHNIQUES 



aminobenzylidene rhodanine reagent for the precipitation and vis- 

 ualization of silver in tissue sections. The fact that not only silver, 

 but also copper, gold, mercury, platinum, and palladium are like- 

 wise precipitated by this reagent offers little ground for concern 

 since these elements are not apt to exist in significant amounts in 

 tissues unless introduced experimentally or perhaps by accidental 

 poisoning. In these cases only one of the elements at a time is likely 

 to be present. However, certain differentiations can be made, if it is 

 assumed that more than one is present, on the basis of the varying 

 solubility behavior of the precipitated compounds. Thus divalent 

 copper reacts with the reagent only in neutral solution, whereas 

 monovalent copper and the other metals will react in either neutral 

 or acid solution. Furthermore the mercury precipitate is soluble in 

 dilute hydrochloric acid, the silver compound in potassium bromide 

 solution, and gold compound in potassium nitrite solution. Neutral 

 stannous chloride solution reduces the palladium precipitate to the 

 free metal which can then be converted to the chloride by means of 

 chlorine gas; this cannot be done with the platinum compound. Based 

 on these facts, possible separations have been suggested by Okamoto 

 et al. 



Okamoto et al. Procedure for Silver 



SPECIAL REAGENTS 



Precipitation Reagent I. Add 3-5 ml. of saturated soln. of p-di- 

 methylaminobenzylidene rhodanine in absolute alcohol to 1-3 ml. 

 1 N nitric acid and 100 ml. distilled water. 



Precipitation Reagent II. Combine 10-20 ml. of the saturated alco- 

 holic soln. of the rhodanine compound with 1-3 ml. 1 N nitric acid, 

 5-10 ml. 3% hydrogen peroxide, and 100 ml. distilled water. 



Precipitation Reagent III. Add 2-5 ml. of the soln. of the rhoda- 

 nine derivative to 2 ml. 0.1 N hydrochloric acid, 3-5 ml. 1 N nitric 

 acid, and 100 ml. distilled water. 



PROCEDURE 



1. After fixing the tissue in absolute alcohol or neutral formalin, 

 prepare either celloidin, paraffin, or frozen sections. 



2. Place the sections in precipitation reagent I for 24 hr. at 36°, 

 keeping the vessel closed. 



