Determination of potassium nitrite with standard potassium manganate(VII).
Potassium manganate(VII) oxidises nitrite ion to nitrate in acid solution:
5NO2- + 2MnO4- + 6H+ à 5NO3- + 2Mn2+ + 3H2O
The usual method for a manganate(VII) titration is to add potassium manganate(VII) from the burette, since the endpoint is seen when the solution just turns pink. This is easier to see than the reverse change of pink to colourless. The problem in the case of the analysis of nitrite is that the solution of nitrite ions cannot be acidified since the nitrous acid produced decomposes rapidly enough for the analysis to be very inaccurate. Potassium manganate(VII) cannot therefore be run in to an acidified solution of, say, potassium nitrite. The simplest method is to add the solution of sodium nitrite from the burette to a known volume of standard potassium permanganate solution which has been acidified with dilute sulphuric acid. The tip of the burette dips under the liquid surface to prevent any possible loss of nitrogen dioxide as the nitrite ions meet the acid.
Method.
1 Weigh accurately about 1.0 g of potassium nitrite, transfer to a 250 cm3 graduated flask, and dissolve in pure water. Make to the mark with pure water and mix well. Fill a burette with this solution.
2 Pipette 25.0 cm3 of 0.020 mol dm-3 potassium manganate(VII) solution (which has previously been standardised against, say, sodium ethanedioate solution) into an evaporating basin, and add about 25 cm3 of bench sulphuric acid.
3 Arrange for the burette tip to dip into the acidified potassium manganate(VII) solution, and titrate slowly, stirring the mixture with a glass rod, until the solution just turns colourless.
4 Repeat to obtain three consistent readings.
Results.
Mass of potassium nitrite used/g:
| Final volume/cm3 | ||||
| Initial volume/cm3 | ||||
| Titre/cm3 |
Mean titre/cm3:
Hence calculate the percentage purity of the potassium nitrite used.
Other methods exist for the determination of nitrite; see for example Belcher R & Nutten AJ, Quantitative Inorganic Analysis: London, Butterworths 1960, p 262.