Theory: Sodium nitrite (commercial nitrite) solution is oxidized by potassium permanganate to nitrate: 2MnO4- + 6H+ + 5NO2- –> 2Mn2+ 5NO3- +3H2O
As potassium permanganate itself changes colour in this reaction, so no indicator is required. However, once sodium nitrite is acidified, nitrous acid will form and decompose (*). Thus, sodium nitrite is placed in burette and added to acidified potassium permanganate.
*2NaNO2 + H2SO4 –>Na2SO4 + 2HNO2
3HNO2–>HNO3 + H2O + 2NO
The permanganate solution changed from purple to colourless under the reaction.
Data
Trial
1st
2nd
3rd
Initial reading (cm3)
0.20
0.00
0.06
12.34
Final reading (cm3)
25.20
25.00
25.08
37.40
Titre(cm3)
25.00
25.00
25.02
25.06
Mean titre:
Calculation:
Mol of titre used:
As only 25.02cm3 titre was used for titration, mol of the total 250cm3 sodium nitrite solution:
Thus, there is pure sodium nitrite:
Finally, the purity of the commercial nitrite:
Result: The purity of the commercial nitrite is 95.8%
Discussion:
The purity was found.
There was just a small difference between the reference answer (94%) and the answer calculated.
The 3rd time reading was larger than the previous readings due to the slow reaction of this experiment. Thus more sodium nitrite solution was added and might affect the result. To solve this problem, sodium nitrite MUST be added slowly and swirling should be faster..
Primary standard is a pure compound from which a standard solution of accurately known concentration can be prepared directly by dissolving a known mass of the compound in a solvent and making up the solution to a known volume.
A primary standard should be stable over long periods of time, but potassium permanganate decomposes when exposed to light. To standardize it, potassium permanganate is titrated with strongly acidic solution of C2H62-.
When HCl is added to KMnO4, chlorine gas, which is harmful for human, may produce. When HNO3 is added to KMnO4, NO2 is formed which is toxic.
MnO4 + 4H+ + 4e- –>MnO2 +2H2O
It is because the colour change is not significant, and so it is hard to judge the end point.
The advantage of using acidified KMnO4 in redox titration is the great colour change and so it can be indicator itself. The disadvantage of it is that it cannot react with compound containing chloride ion or high concentration acid.
After the experiment and calculation, the purity found is 95.8%.
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