Object: The purpose of this experiment is to check the purity of commercial nitrite by titration.
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
1. Around ( (should be a little more for the impurity)) 0.90g sodium nitrite powder was transferred and weighed to the nearest 0.01g in a weighting bottle.
2. The sodium nitrite powder was dissolved in distilled water in volumetric flask of 250cm3 where the molarity was about 0.05M.
3. 25cm3 0.02M potassium permanganate solution was pipette into a conical flask, 15cm3 dilute sulphuric acid was added for acidification.
4. Sodium nitrite solution was transferred to burette for titration.
5. The initial burette reading was taken.
6. Sodium nitrite solution was slowly added to the acidified permanganate solution with continual swirling until the colour of the permanganate solution was just become colourless.
7. Final burette reading was taken.
8. The steps above were repeated for at least 3 times to obtain more results.
9. Calculation should be done.
The permanganate solution changed from purple to colourless under the reaction.
Initial reading (cm3)
Final reading (cm3)
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%
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..
1. What is the meaning of a primary standard?
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.
2. Explain why potassium permanganate is not a suitable primary standard. Suggest a way to standardize potassium permanganate solution.
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-.
3. Dilute HCl (aq) and HNO3 (aq) are not used to acidify KMnO4. Explain.
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.
4. In weakly acidic, weakly alkaline and neutral medium, KMnO4 reduces to form the brown solid MnO2 instead of Mn2+. Write the equation for this reduction.
MnO4 + 4H+ + 4e- –>MnO2 +2H2O
5. Explain why neutral KMnO4 (aq) is seldom used in redox titration.
It is because the colour change is not significant, and so it is hard to judge the end point.
6. Suggest one advantage and one disadvantage of using acidified KMnO4 in redox titration.
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%.