4. To determine thorium in a solution using the fixed concentration method, we used the oxidation reaction of iodide ion with hydrogen peroxide and thorium compounds. We took 2.00 ml of the test solution, diluted it to 100.0 ml in a volumetric flask, and after adding the necessary reagents, the solution was diluted another 10 times.

Determine the concentration of thorium (mol/l) in the test solution if, when measuring the time t required to achieve a fixed optical density, we obtained the following data:
Parameters Standard solutions Test solutions
СTh*106 g/l 3.0 4.5 6.0 8.2 12.0 - - - -
t, min 10.0 6.7 5.0 3.7 2.5 4.3 8.3 2.9 6.1
Solution for option 1

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