We know the typical experiment of determining the concentration of $\ce{Fe^2+}$, where we titrate $\ce{MnO4^-}$ which has dark purple color and watch the color of the bottle.

Can we also determine the $\ce{MnO4^-}$ concentration using $\ce{Fe^2+}$ as titrant and how?

representation of a titration setup

You can see that the iron is inside the beaker and we titrate with $\ce{MnO4^-}$. Can we do it backward, i.e. having $\ce{MnO4^-}$ inside the beaker with purple color and we titrate $\ce{Fe^2+}$.

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    $\begingroup$ Yes and no. The titrant is picked to give a good end point detection. It is easier to see a color appearing than to see a color disappearing. $\endgroup$ – MaxW May 29 '20 at 10:20
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    $\begingroup$ For human eyes, it is much easier to observe colourless -> colour, than the other way around - so in this case, I'd say it won't work, but there might be other cases where it could by switching to a different indicator. $\endgroup$ – Martin - マーチン May 29 '20 at 10:20
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    $\begingroup$ Also, the equivalence point is light pink when using $\ce{MnO4-}$. Observing pink to light pink is way harder than observing colourless to light pink. $\endgroup$ – Aniruddha Deb May 29 '20 at 12:03
  • $\begingroup$ Refer primary standard solution $\endgroup$ – user600016 May 29 '20 at 13:34
  • $\begingroup$ Additional complication: the unknown substance has an unknown concentration. If you add it to the titrant, what if you don't have enough? $\endgroup$ – Zhe May 29 '20 at 16:05

The titration reaction does not know or care whether one reagent is in the buret and one is in the conical flask (Erlenmeyer flask, it is not a beaker in your figure). And certainly you can standardize/titrate potassium permanganate with ferrous ammonium sulfate solution to determine the concentration of permanganate ion. Ferrous ammonium sulfate hexahydrate is a primary standard in analytical chemistry.

If you are extraordinary careful, you can fill the iron (II) solution in the buret and have permanganate in the conical flask. The end-point will be slightly vague (deep purple to colorless). You really have to be extra-cautious. The question is why bother with this painful eye-stressing procedure? Always take the permanganate in the buret.

A dark solution can be used in the conical flask. For example, in iodometric titrations, we start with a deep brown solution, which fades, to nearly a colorless solution upon titration with thiosulfate. In order to confirm the end-point, you would put a drop of starch before your expected equivalence point. Again, the color change is from deep blue to colorless.

  • $\begingroup$ I disagree that there is an equivalence. If you add all of the unknown via the burret, but it's not enough, what do you do then? $\endgroup$ – Zhe May 29 '20 at 17:03
  • $\begingroup$ Well, a trained analytical chemist will never start randomly. There is something called as "pilot reading" when you have no clue of the sample concentration, which roughly tells us what should be the volume of the titrant. You always have some idea of the sample analyte concentration and they would adjust the concentrations so that they are within the buret volumes. $\endgroup$ – M. Farooq May 29 '20 at 17:08

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