Means of detecting the end point of the reaction between the standard and test solutions. Usually an indicator is used however in redox titrations, the oxidising agents are self indicators which means an indicator is not needed.
In the above reduction half-reaction, manganese has undergone a decrease in oxidation number from +7 to +2.
The pink color of the dilute MnO4- solution indicates the end of the reaction. This is known as the "end point" or equivalence point of the titration.
3. Heat each solution containing the iron sample almost to boiling. SnCl2 solution should be added drop wise taking care such that the yellow Fe(III) color just disappears. Then add only 2 drops excess of SnCl2 solution. The solution is cooled by placing the outside edge under running cold water until held, <40C. HgCl2 solution is rapidly added into the solution. A small quantity of a white precipitate should appear. If there is no appearance of precipitate or if the ppt is black or grey then the experiment should be discarded.
4. After waiting only about 2 to 3 minutes, add 5mlof concentrated sulfuric acid and 7ml of syrupy phosphoric acid to the prereduced solution. Dilute with distilled water to bring the volume to 125 ml.
5. The solution should be cooled again as done in step 3. Add 8 drops of diphenylamine sulfonate indicator and slowly titrate with the standard K2Cr2O7 (Prepared above) solution from a blue-green, through a greyish tinge to the first permanent violet, which is the end point Three trials should be performed.
6. Blank: There is a significant indicator blank for this titration. ...