Principle: Iodometry
Oxidation of the methylthio radical of methionine by iodine in excess and assay of the excess reagent with sodium thiosulfate.

Reagents
- Phosphate buffer obtained by mixing 1 liter of dipotassium phosphate solution at 100g/l and 400 ml of potassium dihydrogen phosphate solution at 100 g/l
- Potassium iodide solution at 67 g/l
- About 0.1 N iodine solution
- 0.1 N sodium thiosulfate solution precisely standardized
- Thiodene (starch-indicator in iodometry)

Procedure
- Into a 250 ml conical flask provided with a ground stopper, introduce a sample close to 0.3 g, weighed to within 0.1 mg.
- Add 70 ml of phosphate buffer and 30 ml of potassium iodide solution. Dissolve.
- Add 50 ml of iodine solution, stopper and leave in the dark for 30 minutes.
- Titrate with thiosulfate until a pale yellow colour is obtained, add approximately 100 mg of thiodene and continue the assay until decoloration is obtained.
- Carry out a blank assay under the same conditions.

Calculation
Let:
- V1 be the volume of thiosulfate for the assay
- V2 the volume of thiosulfate for the blank assay
- E the mass, in grams, of the sample
- T the thiosulfate titer