117. Задачка по химии №011 762

Is it possible: a) by the action of Bertolet´s salt in an acidic medium, oxidize iodide - ions to free iodine; b) chloride - ions to free chlorine c) by the action of potassium dichromate in an acidic medium, oxidize arsenite - ion to arsenate - ion? Why, when oxalic acid (or oxalate) is titrated, do the first drops of a permanganate solution discolour slowly, while in the future this discolouration occurs almost instantly? What can be done so that the first drops of KMnO4 also instantly discolor?

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