Iodine displacement titrations

Iodine displacement titrations are used in pharmacopoeial assays of liquefied phenol, methyl hydroxybenzoate, propyl hydroxybenzoate and phenidione. 

The resulting solution has a much lower vapour pressure than a solution of iodine in pure water, and consequently the loss by volatilisation is considerably diminished. Nevertheless, the vapour pressure is still appreciable so that precautions should always be taken to keep vessels containing iodine closed except during the actual titrations. When an iodide solution of iodine is titrated with a reductant, the free iodine reacts with the reducing agent, this displaces the equilibrium to the left, and eventually all the tri-iodide is decomposed the solution therefore behaves as though it were a solution of free iodine. 

For good results, the following experimental conditions must be observed (1) the hydrochloric acid concentration in the final solution should be at least 4M (2) air should be displaced from the titration mixture by adding a little solid sodium hydrogencarbonate (3) the solution must be allowed to stand for at least 5 minutes before the liberated iodine is titrated and (4) constant stirring is essential during the titration to prevent decomposition of the thiosulphate in the strongly acid solution. 

Kinetic Studies. Peracetic Ac id Decomposition. Studies with manganese catalyst were conducted by the capacity-flow method described by Caldin (9). The reactor consisted of a glass tube (5 inches long X 2 inches o.d.), a small centrifugal pump (for stirring by circulation), and a coil for temperature control (usually 1°C.) total liquid volume was 550 ml. Standardized peracetic acid solutions in acetic acid (0.1-0.4M) and catalyst solutions also in acetic acid were metered into the reactor with separate positive displacement pumps. Samples were quenched with aqueous potassium iodide. The liberated iodine was titrated with thiosulfate. Peracetic acid decomposition rates were calculated from the feed rate and the difference between peracetic acid concentration in the feed and exit streams. 

These titrations involve displacement of iodine from iodide by a stronger oxidising agent followed by titration of the displaced iodine with sodium thiosulphate. 

The displaced iodine is then titrated with thiosulphate according to the following equation ... 

Arsenic may be determined iodometricaQy. A dried sample of 150-200 mg. is mixed with 7 ml. of 15 N nitric acid and the solution boiled gently for 5-10 minutes. Three milliliters of 2 AT sodium bromate is then added and the mixture evaporated gently to dryness. The residue is taken up in 150 ml. of 4 M hydrochloric acid in an iodine flask then 1 g. of sodium hydrogen carbonate is cautiously mixed in to displace the air, followed by 1 g. of potassium iodide. The flask is then stoppered and allowed to stand 5 minutes. The contents are rapidly titrated with 0.05 N sodium thiosulfate to the disappearance of the iodine color. 

Chlorine displaces iodine from iodides. The iodine formed can be determined by titration with a standard thiosulphate solution. Chlorate(l) solutions are often used as a source of dlorine as they liberate chlorine readily on reaction with acid ... 

Iodination of proteins has produced measurable changes in the titration curves. In zein (280), insulin (125), and pepsin (6) the region of the titration curves usually assigned to the phenolic group of tyrosine (pK=10) is displaced in iodinated proteins in the direction of increased acidity by nearly 2 pH units. This is apparently not true for iodinated globin (299). 

Chlorinated compounds are all evaluated on their available chlorine content. For estimation the chlorine is liberated with acid in the presence of excess of potassium iodide, from which the chlorine displaces an equivalent of iodine the liberated iodine is then titrated with thiosulphate. 

---