117. Solving a chemistry problem

What is the essence of iodometry? Why are only oxidized and not reduced forms of the corresponding pairs used in permanganatometry and chromatometry? Why, in contrast to this, both of these forms are used in iodometry? What weighed portion of potassium dichromate should be taken so that 20 cm3 of a 0.05 N sodium thiosulfate solution is used to titrate the iodine released after adding a solution of potassium iodide to it? Write reaction equations.