After 240 seconds, the mixture stopped turning drops of iodine solution blue because the dissolved vitamin C in the orange juice had been completely oxidized by the iodine.

Vitamin C acts as a reducing agent and donates electrons to the iodine molecules, converting them from oxidized iodine (I2) to reduced iodide ions (I-).

As the reaction progresses, the concentration of vitamin C in the solution decreases, and there are fewer reducing agents available to react with iodine. Eventually, all of the vitamin C is consumed, and the reaction stops.