95. Solving a chemistry problem

3(4) To determine cobalt, we used the oxidation reaction of alizarin with hydrogen peroxide, catalyzed by Co(II) compounds. 2.00 4.00 6.00 8.00 ml of a standard CoCl solution with TCo=5.89*10-4 mg/ml were added to four 50.00 ml volumetric flasks. The test solutions were placed in the same volumetric flasks. After adding reagents (alizarin, borate buffer and H2O2) and bringing the volumes to the mark, the optical density A was measured as a function of time T for all solutions. Then the slope tangents of the straight lines were determined in lgA-t coordinates for the standard and test solutions. standard solutions (tga) test solutions tga, options
1 2 3 4 I II III IV
0.10 0.195 0.303 0.400 0.160 0.230 0.350 0.280
plot a calibration graph in tga-CCo coordinates and determine the cobalt content (μg) in the test solution.