374 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Estimation of Iodine. 



On account of the minute quantities of iodine present in tissues special 

 methods have to be adopted for its detection and estimation. Cameron 1 

 has reviewed these methods and finds that the method given by Hunter is 

 very accurate. A very similar method is adopted by Kendall. 2 



The material ('5 gm. of thyroid gland) is placed in a 5*9 cm. nickel 

 -crucible and moistened with 5-6 c.c. of 30 per cent, sodium hydroxide ; 

 10-15 gm. of stick sodium hydroxide, broken into small pieces, are added 

 and the crucible heated on a hot plate till the excess of water has evaporated 

 and the contents are syrupy. Spattering, which may take place if only small 

 quantities of organic matter are present, is prevented by adding a small amount 

 of gallic acid. The crucible is placed in a larger nickel crucible, 7-8 cm., 

 containing a layer '5 cm. deep of sand. The crucibles are heated over a 

 15*6 cm. Meker burner in a special apparatus so that the bottom of the 

 larger crucible attains a dull-red heat. Overheating produces creeping of 

 the fused alkali, underheating prevents complete oxidation of the organic 

 matter. The melted mass at first foams, but this ceases in 5-10 minutes 

 and the melt settles to the bottom with evolution of only a few gas bubbles. 

 The small crucible is now removed and partially cooled by agitating the con- 

 tents with a rotary motion ; 5-10 mgm. of potassium nitrate are added. This 

 oxidises the remaining organic matter with evolution of bubbles. One or more 

 additions of nitrate must be made until no more bubbles of gas are given off. 

 The oxidation should be complete in 10-15 minutes. The fused mass is 

 poured into the cover of the small crucible and allowed to cool. 



The cold material in the crucible and cover are put into a tall beaker of 

 600-800 c.c. capacity with some talcum powder and 125-150 c.c. of water and 

 dissolved by heating on a hot plate. The solution is transferred to a 500 c.c. 

 conical flask and should be a clear colourless solution of about 200 c.c. in 

 volume, i c.c. of 10 per cent, sodium bisulphite and a few drops of methyl 

 orange are added. The bisulphite has a reducing action preventing loss of 

 iodine, retaining it as hydriodic acid. The solution is cooled and neutralised 

 by running in 85 per cent, phosphoric acid, the flask being shaken with a 

 rotary motion to expel carbon dioxide. Only a few drops more acid should 

 be added after the indicator has changed colour. A few drops of bromine 

 are added with shaking until the solution becomes distinctly yellow. The 

 volume is diluted to between 250 and 300 c.c. and boiled for 8-10 minutes 

 upon a hot plate until the solution becomes colourless; 5-10 drops of 5 

 per cent, sodium salicylate solution (prepared by dissolving 5 gm. of salicylic 

 acid in dilute sodium hydroxide and diluting to 100 c.c. : the solution should 

 be only slightly alkaline) are added and the flask cooled in water. The 

 volume must not be less than 175-200 c.c. When cold 5 c.c. of loper cent, 

 potassium iodide solution are added and the liberated iodine titrated with 

 005 N thiosulphate solution using a few drops of '5 per cent, soluble starch 

 solution as indicator. If there be not an immediate liberation of iodine, 

 3-4 c.c. of the phosphoric acid are added. 



1 J. Biol. Chem., 1914, 18, 335. 2 Ibid., 19, 251. 



