SECTION TWO 



SILVER NITRATE 

 For ascorbic acid 



Reagents required: 



A. Silver nitrate, 5% aqueous . . 100 ml. 

 Glacial acetic acid . . . . 5 ml. 



B. Sodium thiosulphate, I % aqueous 



Techiique: 



1. Small, thin slices of fresh tissue are immersed directly in 

 reagent A, m the dark, for about half an hour. 



2. Wash thoroughly in repeated changes of reagent B. 



3. Wash thoroughly in several changes of distilled water. 



4. Dehydrate in dioxane {see p. 512) and embed in paraffin wax. 



5. Press the wax sections on to slides and dewax with dioxane. 



6. Mount the dewaxed sections in D.P.X., Clearmount, or 

 Emexel. 



Results: 



Ascorbic acid is indicated by a black granular precipitate. 



Notes: 



(a) Localization is only approximate. 



(b) Ascorbic acid is a sugar derivative which is soluble in water 

 and in alcohol, and which is unstable to alkali, light and oxidation. 

 It is a fairly strong aliphatic organic acid and possesses strong 

 reducing properties. In biochemistry, silver nitrate, ferric 

 chloride, ferricyanides, iodine, methylene blue and 2 :6-dichloro- 

 phenol-indophenol have been used for the determination of the 

 reduction capacity of ascorbic acid-containing substances. In 

 histology, ascorbic acid is identified solely on its properties as a 

 reducing agent: in the technique described above, ascorbic acid, 

 if present, reduces the silver nitrate to metallic silver, which is 

 observed as a black precipitate. 



Reference, Girond, A., Leblond, C. P., Ratsimananga, R. & Rabowicz, M. 

 (1936). 



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