Iodine with sodium thiosulphate

Among the most important indirect methods of analysis which employ redox reactions are the bromination procedures for the determination of aromatic amines, phenols, and other compounds which undergo stoichiometric bromine substitution or addition. Bromine may be liberated quantitatively by the acidification of a bromate-bromide solution mixed with the sample. The excess, unreacted bromine can then be determined by reaction with iodide ions to liberate iodine, followed by titration of the iodine with sodium thiosulphate. An interesting extension of the bromination method employs 8-hydroxyquinoline (oxine) to effect a separation of a metal by solvent extraction or precipitation. The metal-oxine complex can then be determined by bromine substitution. 

In such estimations, the pharmaceutical substances can be measured either directly or back titration of excess iodine with sodium thiosulphate solution. 

Table 7.2 Substances Assayed by Residual Titration of Excess Iodine with Sodium Thiosulphate...

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

The amperometric titration of iodine with sodium thiosulphate using two indicator electrons. 

Volumetric methods have been suggested, notably that of Klobbie,4 which consists in treating a solution of osmium tetroxide in dilute sulphuric acid with potassium iodide and titrating the liberated iodine with sodium thiosulphate. One molecule of osmium tetroxide liberates four atoms of iodine. The solution becomes dark green, but no further reduction takes place even after several days. 

Perturbation studies Reduction of iodine with sodium thiosulphate (Figure 4 5) The system was same as described for the sequence studies. Two sets of experiments were run under identical conditions. Iodine was completely reduced by adding required amount of sodium thiosulphate(1. 0 x 10 3m), gne set was equilibrated at 30°C and the other at 60 C for 30 minutes respectively. The set maintained at 60°C was cooled to 30°C and both the sets were reiodlnated by adding iodine(BVR) - 0.6 ml followed by equilibration fcr 30 minutes at 30°C. 

Alternatively, a known weight of the pyrolusite may be heated with concentrated hydrochloric acid and the chlorine evolved passed into potassium iodide solution. The iodine liberated is titrated with sodium thiosulphate ... 

In water pollution studies, the oxygen content can be measured by making the water alkaline and shaking a measured volume with an oxygen-free solution containing Mn- (aq). The solution is acidified with sulphuric acid, potassium iodide added and the liberated iodine titrated with sodium thiosulphate. 

The free iodine may then be determined by titration with sodium thiosulphate (Section 10.113). 

B. Iodometric Assays i.e., release of iodine and subsequent titration with sodium thiosulphate. 

In this titration method an excess of iodine solution is added to the solution of the substance and thus, the latter gets oxidized quantitatively. The excess of iodine is subsequently back titrated with sodium thiosulphate using freshly prepared starch solution as indicator with an end-point from violet to colourless. 

Theory Sodium metabisulphite in acidic medium (HC1) yields S02 which reacts with water to produce sulphurous acid. The generated sulphurous acid is quantitatively oxidized by iodine to sulphuric acid, and the excess iodine is subsequently back titrated with sodium thiosulphate. The various reactions can be expressed as shown below ... 

A few pharmaceutical substances can be assayed by titrating the liberated iodine from potassium iodide with sodium thiosulphate as stated in Table 7.3. 

A sample of phenol glycerol injection was dilutee with water and an aliquot was taken and reacted with excess bromine generated from potassium bromide and potassium bromate solutions. The excess bromine remaining after reaction was reacted with potassium iodide and the liberated iodine was titrated with sodium thiosulphate. A blank titration was carried out where the same quantity of bromine was generated as was used in the titration of the diluted injection, potassium iodide was then added and the liberated iodine was titrated with sodium thiosulphate. From the following data calculate the percentage of w/v of the phenol in the injection. 

When the lactam ring is open it will react with iodine. 1 mole of the ring open form of penicillin will react with 8 equivalents of iodine, the intact lactam ring will not react. In this type of titration excess iodine solution is added to a sample of the penicillin and the iodine which is not consumed in the reaction is estimated by titration with sodium thiosulphate. The value obtained for the amount of hydrolysed penicillin in the sample should be no more than 5% of that obtained when all the penicillin in the same amount of sample is completely hydrolysed to the ring-opened form and then reacted with iodine. Most of the pharmacopoeial monographs for penicillins indicate that this test should be carried out. 

Why are various products of sulphur oxidation obtained when chlorine and iodine water react with sodium thiosulphate What forms when bromine water reacts with sodium thiosulphate What is the commercial name of sodium thiosulphate ... 

Hydrogen sulphide is sometimes estimated volumetrically by treating a solution containing less than 0-04 per cent, with excess of standard iodine and then after a short period titrating the residual iodine in the usual manner with sodium thiosulphate or arsenious oxide accurate direct titration is not possible 3... 

By using excess of iodine and titrating after one hour with sodium thiosulphate in the presence of a little carbon disulphide, the reaction may be used for the determination of the selenocyanate.1... 

In a third series of eight experiments a weighed amount of cobalt was dissolved in excess of a standard solution of iodine in potassium iodide, the excess of iodine being then titrated with sodium thiosulphate Ia Co 100-000 23-462 Co = 59-556... 

Both iron and copper interfere. If, however, potassium iodide solution is added, they are reduced to the non-reactive iron(II) and copper(I) ions the liberated iodine may be decolourized with sodium thiosulphate solution. Alternatively, the reduction may be carried out with sodium thiosulphate solution alone on a spot plate, the copper acting as a catalyst for the reduction of the iron ... 

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