7. To determine iodide ions in water by the colorimetric method, a calibration characteristic is first established. To do this, a certain volume of the working solution, the concentration of which is 0.0002 mg1-/ml, is poured into a series of 50 ml measuring flasks and diluted to the mark with distilled water. Mix well and take 10 ml of the prepared solutions and process them in the same way as the sample. Then measure the optical density of the obtained solutions and plot a graph in todides (D-D0) from C(I-). The results are given in the following table:
V(I-), ml 20 40 60 80
C(I-), mg/ml
D 0.25 0.5 0.75 1
Place 10 ml of the sample and the necessary reagents in a reaction tube. After 30 minutes, measure the optical density D=0.6. Determine the concentration of iodide in the water sample.

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