95. Solving a chemistry problem

6(4). To determine thorium in a solution using the fixed concentration method, the oxidation reaction of iodide ion with hydrogen peroxide was used, catalyzed by thorium compounds. 2.00 ml of the test solution were taken and diluted to 100.0 ml. The necessary reagents and 5.0 diluted thorium were added to a 50.0 ml volumetric flask. The concentration of thorium in the test solution was determined if the following results were obtained when measuring the time T required to achieve a fixed optical density:
standard solutions test solutions
1 2 3 4 5 I II III IV
C*106, mol/l 3.0 4.5 6.5 8.2 12.0 - - - -
t, min 12.5 6.7 5.0 3.7 2.5 4.3 8.3 2.9 6.1