Determination of water by the classical Fischer method involves titration of a sample solution (or suspension) in methanol by a methanolic iodine solution, containing also an excess of SO2 and pyridine (C5H5N, Py) - Fischer reagent. The following reactions occur during the titration:
SO2 + CH3OH + H2O + I2 => 2 HI + CH3OSO3H
Py + HI => PyH+ I -
Py + CH3OSO3H => PyH+CH3OSO3 -
Iodine content is usually expressed in mg of water reacting with 1 cm3 of the titrant solution (hereunder T, mg cm^-3), which equals the mass of water (mg) reacting with 1.00 cm^3 of the iodine solution. T is determined experimentally by titration of a sample with a known water content. The sample may be, for example, a hydrated compound or a standard solution of water in methanol. In the latter case, it should be taken into account that methanol itself can contain a certain amount of water. In all calculations, please use the atomic masses accurate to 2 decimal points.
1.1 Sometimes titration of water is performed in pyridine medium without methanol. How would the reaction of I2 with SO2 and H2O occur in this case? Write down the balanced reaction equation. Calculate the T values of the iodine solution in each of the following cases:
1.2 12.20 cm^3 of Fischer reagent solution were used for titration of 1.352 g of sodium tartrate dihydrate Na2C4H4O6 â‹… 2 H2O.
1.3 A known amount of water (21.537 g) was placed into a 1.000 dm^3 volumetric flask which was filled by methanol up to the mark. For titration of 10.00 cm^3 of the obtained solution, 22.70 cm^3 of Fischer reagent solution were needed, whereas 2.20 cm^3 of iodine were used for titration of 25.00 cm^3 of methanol.
1.4 5.624 g of water were diluted by methanol up to a total volume of 1.000 dm^3 (solution A); 22.45 cm^3 of this solution were used for titration of 15.00 cm^3 of a Fischer reagent (solution B). Then 25.00 cm^3 of methanol (of the same batch as used for the preparation of solution A) and 10.00 cm^3 of solution B were mixed, and the mixture was titrated by the solution A. 10.79 cm^3 of the latter solution were spent.
