Practical organic and bio-chemistry / by R.H.A. Plimmer.

  • Robert Plimmer
Date:
1915
Catalogue details

Licence: Public Domain Mark

Credit: Practical organic and bio-chemistry / by R.H.A. Plimmer. Source: Wellcome Collection.

    592/660 page 572
    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 ! has reviewed these methods and finds that the method given by Hunter is very accurate. A very similar method is adopted by Kendall.* 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 ; To-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. Meéker 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. in volume. 1 c.c. of ro 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), and the flask cooled in water. The volume must not be less than 175-200 c.c. When cold 5 cc. of 10 per cent. potassium iodide solution are added and the liberated iodine titrated with ‘oo5N thiosulphate solution using a few drops of *5 per cent. soluble starch solution as indicator. If there is not an immediate liberation of iodine 3-4 c.c. of the phosphoric acid are added. 1], Biol. Chem., 1914, 18, 335. ' Ibid., 19, 251.
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