Kjeldahl titration with iodine

Ewins s Method o-i-o-2 gm. of the substance is mixed in a 300 ml. Kjeldahl flask with 10 gm. potassium sulphate, o>2-o-3 gm. starch and 20 ml. concentrated sulphuric acid. This mixture is then heated by means of a Bunsen burner, first moderately for 10-15 minutes, then more vigorously for about 4 hours, until decomposition is complete. The liquid is cooled, transferred to a 350 ml. flask and made alkaline to litmus paper with sodium hydroxide. It is then cooled to 30° to 40° and sulphuric acid added drop by drop until the solution is faintly acid. A saturated solution of sodium bicarbonate is then added until the solution is again alkaline, 5-10 ml. being added in excess. The arsenious acid formed is then titrated with iodine solution using starch as indicator. 

Ammonia is determined by the Kjeldahl method. Cobalt is determined by decomposing the salt with aqueous sodium hydroxide, dissolving the cobalt(III) oxide in acidified potassium iodide, and titrating the liberated iodine with thiosulfate. Anal. Calcd. for Co(NH3)3H20(C1)2C1 Co, 25.0 NH3, 21.6. Found Co, 24.6 NH3, 21.8. 

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