Official Method: Thyroid

Contact: BMORS Enquiries


Published by authority of the Minister of Health

Date: 1981/10/15

Administrative Changes Date: 2003/11/17

Health Products and Food Branch

I. Application

This method shall be used to assay Thyroid as referred to in Section C.06.251 of the Food and Drug Regulations.

II. Reagents

ACS Reagent Grade chemicals or equivalent shall be used.

  1. Anhydrous Potassium Carbonate;

  2. Dilute Phosphoric Acid (1 in 2);

  3. Phenol Solution (1 in 20);

  4. Bromine Solution. A saturated solution of bromine prepared by agitating 2 to 3 mL of bromine with l00 mL of cold distilled water in a glass-stoppered bottle, the stopper of which should be lubricated with petrolatum. Store cold, protected from light;

  5. Potassium Iodide Solution. Dissolve 16.5 g of potassium iodide in distilled water to make 100 mL. Store in light-resistant containers;

  6. Starch Solution. Mix 1 g of soluble starch with 10 mg red mercuric iodide and sufficient distilled water to make a thin paste. Add 200 mL of boiling distilled water, and boil for 1 minute with continuous stirring. Cool and use only the clear solution.

  7. 0.01 N Sodium Thiosulfate. Dissolve about 26 g of sodium thiosulfate and 200 mg of sodium carbonate in 1000 mL of recently boiled and cooled water. Standardize the solution as follows: Accurately weigh about 210 mg of primary standard potassium dichromate, previously pulverized and dried at 120 C for 4 hours, and dissolve in 100 mL of water in a glass-stoppered, 500 mL flask. Swirl to dissolve the solid, remove the stopper and quickly add 3 g of potassium iodide, 2 g of sodium bicarbonate, and 5 mL of hydrochloric acid. Insert the stopper in the flask, swirl to mix, and allow to stand in the dark for 10 minutes. Rinse the stopper and the inner walls of the flask with water, and titrate the liberated iodine with the sodium thiosulfate solution until the solution is yellowish green in color. Add 3 mL of Starch Solution, and continue the titration to the discharge of the blue color. Calculate the normality. Quantitatively dilute this solution 1 to 10 (0.01 N Sodium Thiosulfate).

III. Laboratory Equipment

  1. Muffle Furnace;

  2. Porcelain Crucibles;

  3. General Laboratory Glassware;

  4. Starch Iodide Test Paper.

IV. Procedure

  1. Weigh accurately 1 g of Thyroid, or proportionately less, if the iodine content is greater than 0.20 percent, in a porcelain crucible;

  2. add 7 g of Anhydrous Potassium Carbonate;

  3. mix carefully, and gently tap the crucible several times to compact the mixture;

  4. overlay with 10 g more of Anhydrous Potassium Carbonate and compact the material thoroughly by tapping;

  5. ignite the mixture for 25 minutes at 675 to 700 C in a muffle furnace preheated to that temperature;

  6. cool, add 20 mL of distilled water, or more if necessary;

  7. heat gently to boiling, and decant through a filter into a conical flask of a suitable size;

  8. repeat the extraction by boiling with 20 mL of distilled water;

  9. wash the crucible and the char on the filter with hot distilled water until the filtrate measures approximately 200 mL;

  10. add 7 mL of freshly prepared Bromine Solution;

  11. then slowly add 40 mL of Dilute Phosphoric Acid (1 in 2) and boil until starch iodide paper is no longer coloured blue by the vapours;

  12. during the boiling add distilled water from time to time, as necessary, to maintain a volume of not less than 200 mL;

  13. wash down the walls of the flask with distilled water, and continue the boiling for 5 minutes;

  14. cool, add 5 mL of Phenol Solution (1 in 20);

  15. rinse the walls of the flask, and allow to stand for 5 minutes;

  16. add 2 mL of Dilute Phosphoric Acid (1 in 2) and 5 mL of Potassium Iodide Solution;

  17. titrate immediately with 0.01 N Sodium Thiosulfate, adding 3 mL of Starch Solution as the end-point is approached;

  18. perform a blank determination, and make any necessary correction;

  19. each mL of 0.0100 N sodium thiosulfate is equivalent to 0.2115 mg of iodine (I).

V. Reference

  1. The Pharmacopeia of the United States of America,

Twentieth Revision, 1980, Mack Printing Company, Eastman, PA 18042, U.S.A.

The method described above, being comprised of 3 pages and identified as DO-26, Thyroid and dated October 15, 1981 is hereby designated the "official method" referred to in Sections C.06.251 of the Food and Drug Regulations.

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