Question

Explain why in Winkler DO procedure, 1 mole of oxygen corresponds to 4 moles of thiosulfate.  

Answer 1

Winkler was the first scientist to determine the method of chemical determination of oxygen concentration on sea water .Oxygen in the water sample oxidizes iodide (I-) to iodine(I2) quantitatively. The amount of iodine generated is then determined by titration with sodium thiosulfate solution , the end point is determined by using starch as the indicator .

Procedure of Winkler DO method : At the Time of sampling dissolved oxygen is fixed by addition of Mn(II) under basic conditions results in brown precipitate [MnO(OH)2] sample is then acidified to dissolve the precipitate which generate Mn(III)+ which oxidizes iodide to Iodine by which Iodate ion (I3-) generated which is titrated with sodium thiosulfate to produce sodium tetrathionate . The whole Ionic chemical reaction is shown in the image attached and I also explained why 1 mole of O2 corresponds to 4 mole of thiosulfate.