Why do you use sulfuric acid in the standardisation of potassium permanganate?

Answer 1

For preparation of standard solution of Mohr salt {FeSO4.(NH4)2SO4.6H2O}, it's necessary to add dilute H2SO4 to prevent the Fe2+ ions of Mohr salt solution from undergoing oxidation (to Fe3+).

Just try to prepare the standard solution without adding this acid. You'll observe that your "Mohr solution" will become yellow in colour, i.e., it won't remain "Mohr salt solution" anymore, hence can't be used in titration.

And while doing the titration of the Mohr salt solution with KMnO4 (or even with K2Cr2O7), you need to add dilute H2SO4. The reason is well explained by the chemical equation involved:

reduction: (MnO4)- + 8H+ + 5e ----> Mn2+ + 4H2O

oxidation: Fe2+ ----> Fe3+ + e

net equation:

(MnO4)- + 8H+ + 5Fe2+ ----> Mn2+ + 4H2O + 5Fe3+

There is involvement of H+ ions in this reaction. This redox titration requires acidic medium. Moreover, KMnO4 acts as a very good oxidizing agent in the presence of acidic medium, than in the basic and neutral medium.

NOTE: Now a query may arise that for providing acidic medium, can HCl or HNO3 be used? (Let me tell you it's one of the favourite questions of external examiners)

The answer is NO, because HNO3 is a very strong oxidizing agent. So, the Fe2+ present will be oxidized both by KMnO4 and HNO3, which will result in an error in determination of volume of KMnO4 reacting with Mohr salt solution.

Also, HCl can't be used because Cl- from HCl will be oxidized by KMnO4:

HCl (aq) ---> H+ (aq) +Cl- (aq)

oxidation by KMnO4=> 2Cl- ----> Cl2 + 2e

So, some extra KMnO4 will be used up because it will oxidize both- Cl- as well as Fe2+, => error in titration

Answer 2

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Answer 3

Potassium permanganate is not a primary standard. It is not easily obtained in perfectly pure form which is completely free from manganese dioxide. Also the aqueous solutions are not stable for long as the ordinary distilled water usually contains reducing substances (traces of organic matter, etc.) which react with permanganate resulting in precipitation of MnO2. This reaction is catalysed by light. Presence of manganese dioxide is totally undesirable as it catalyses the auto decomposition of the permanganate ion on standing:

4MnO4¯ + 2H2O → 4MnO2 + 3O2 + 4 OH¯

Permanganate solution decompose in presence of manganous ions also:

2MnO4¯ + 3 Mn2+ + 2H2O = 5MnO2 + 4H+

The reaction is quite slow in acidic medium but attains a fast rate in neutral solutions.

Thus Permanganate solution can not be considered as a primary standard solution by simply making the calculations based on the amount of permanganate weighed and dissolved. The permanganate solutions once prepared are either left for a day or two or the freshly prepared solutions are boiled for an hour. In both the cases the resulting solution is filtered through sintered glass crucibles. The clear solutions so obtained are then standardized by titrating with a primary standard solution

Answer 4

Firstly, the reaction between Iron(II) Oxalate and Potassium Manganate(VII) is a redox reaction where Iron(II) ions oxidize to Iron(III) ions and; Manganate(VII) ions reduce to Manganese(II) ions or Manganate(IV) ions in acidic and neutral/alkaline media respectively.

In fact, Sulphuric(VI) Acid is used as a reagent when this experiment takes place in an acidic medium. The main reason, or the obvious reason for using Sulphuric(VI) Acid is the addition of H+ ions to the system (i.e. to the titration flask). In case of this reaction, there is another important reason for using sulphuric acid.

Assume that a different acid such as Nitric(V) Acid, Hydrochloric Acid or Phosphoric(V) Acid. These acids contain ions such as NO3-, Cl-, PO43- which would also react with the added permanganate ions. Then, an excess amount of permanganate ions will be used, and the burette reading becomes erroneous. The sulphate ions will not interfere the reaction in standard conditions, hence Sulphuric(VI) Acid is used in this redox titration.

Answer 5

so that the color becomes clear so that when you actually dothe titration with the permanganate, you will be able to see the color change when titration complete