Nitrites react in warm acid solution (ca 40 °C) with permanganate solution in accordance with the equation:
2MnO4-+ 5NO2-+6H+
= 2Mn2++ 5NO3- + 3H2O
If a solution of a nitrite is
titrated in the ordinary way with potassium permanganate, poor results are
obtained, because the nitrite solution has first to be acidified with dilute
sulphuric acid. Nitrous acid is liberated, which being volatile and unstable,
is partially lost. If, however, a measured volume of standard potassium
permanganate solution, acidified with dilute sulphuric acid, is treated with
the nitrite solution, added from a burette, until the permanganate is just
decolorised, results accurate to 0.5-1 per cent may be obtained. This is due to
the fact that nitrous acid does not react instantaneously with the permanganate.
This method may be used to determine the purity of commercial nitrites.
Potassium permanganate solutions
may be standardised using arsenic(III) oxide or sodium oxalate as primary
standards: secondary standards include metallic iron, and iron(II) ethylenediammonium
sulphate (or ethylenediamine iron(II) sulphate), FeSO4,C2H4(NH3)2SO4,4H2O.
Weigh out about 3.2-3.25 g
potassium permanganate, transfer it to a 1000mL beaker, add 1 L water, cover the
beaker with a clockglass, heat the solution to boiling, boil gently for 15-30
minutes and allow the solution to cool to the laboratory temperature. Filter
the solution through a funnel containing a plug of purified glass wool, or,
more simply, through a sintered-glass or porcelain filtering crucible or
funnel. Collect the filtrate in a vessel which has been cleaned with chromic
acid mixture and then thoroughly washed with distilled water. The filtered
solution should be stored in a clean, glass-stoppered bottle, and kept in the
dark or in diffuse light except when in use: alternatively, it may be kept in a
dark brown glass bottle.
Sodium oxalate is readily obtained
pure and anhydrous, and the ordinary material has a purity of at least 99.9 per
cent. The best way is to make a more
rapid addition of 90-95 per cent of the permanganate solution (about 25-35
mL.min -1) to a solution
of sodium oxalate in 1 M sulphuric acid at 25-30 °C, the solution is then
warmed to 55-60 °C and the titration completed, the last 0.5- 1 mL portion
being added dropwise. The method is accurate to 0.06 per cent. Experimental
details are given below.
2MnO4- +5H2C2O4
+ 6H+ = 2Mn2+ + 10CO2 + 8H2O
It should be mentioned that if oxalate is to be determined it is often not convenient to use the room temperature technique for unknown amounts of oxalate. The permanganate solution may then be standardised against sodium oxalate at about 80 °C using the same procedure in the standardisation as in the analysis.
Dry some analytical grade sodium oxalate at 105-110 °C for 2 hours, and allow it to cool in a covered vessel in a desiccator. Weigh out accurately from a weighing bottle about 0.3 g of the dry sodium oxalate into a 600 mL beaker, add 240 mL of recently prepared distilled water, and 250 mL of 1M sulphuric acid. Cool to 25-30 °C and stir until the oxalate has dissolved. Add 90-95 per cent of the required quantity of permanganate solution from a burette at a rate of 25-35 mLmin-1 while stirring slowly. Heat to 55-60 °C (use a thermometer as stirring rod), and complete the titration by adding permanganate solution until a faint pink colour persists for 30 seconds. Add the last 0.5-1 mL dropwise, with particular care to allow each drop to become decolorised before the next is introduced. For the most exact work, it is necessary to determine the excess of permanganate solution required to impart a pink colour to the solution. This is done by matching the colour produced by adding permanganate solution to the same volume of boiled and cooled dilute sulphuric acid at 55-60 °C. This correction usually amounts to 0.03-0.05 mL. Repeat the determination with two other similar quantities of sodium oxalate.
Weigh out accurately about 1.1 g of commercial nitrite, dissolve it in cold water, and dilute to 250 mL in a graduated flask. Shake well. Measure out 25.0 mL of standard 0.1N potassium permanganate into a 500 mL flask, add 225 mL of 0.5M sulphuric acid, and heat to 40 °C. Place the nitrite solution in the burette, and add it slowly and with constant stirring until the permanganate solution is just decolorised. Better results are obtained by allowing the tip of the burette to dip under the surface of the diluted permanganate solution. Towards the end the reaction is sluggish, so that the nitrite solution must be added very slowly.
More accurate results may be secured by adding the nitrite to an acidified solution in which permanganate is present in excess (the tip of the pipette containing the nitrite solution should be below the surface of the liquid during the addition), and back-titrating the excess potassium permanganate with a solution of ammonium iron (II) sulphate which has recently been compared with the permanganate solution.