Analysis of a spring water

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Determination of the permanganate index with a colorimetric titration

Equipment and reagents

  • Standard laboratory equipment, in particular: graduated burette, glassware, pipette filler.
  • A sand-bath hot plate
  • Concentrated sulfuric acid at ½ dilution
  • Saturated solution of monosodium carbonate NaHCO3
  • Titrated solution of potassium permanganate KMNO4 at 0.0025 mol/L (i.e. N/80)
  • Iron and ammonium sulfate solution (FeSO4(NH4)2SO4, 6H2O) (Mohr’s salt; M = 392.14 g/mol) at about 5 g/L (slightly sulfuric) – This solution is difficult to keep- it must be compared to the potassium permanganate solution every day.

Comparison of the oxidizing and reducing solutions:

In 200 mL of distilled water, add 5 mL of sulfuric acid, 20 mL of KMNO4 at 0.0025 mol/L, then 20 mL of Mohr salt.
Titrate the excess of Mohr salt with the solution of KMNO4 at 0.0025 mol/L, until a pink color appears (persistent for 1 min).
This excess of Mohr salt must exist and remain small, corresponding to maximum 0.5 mL of potassium permanganate.
Let V0 be the volume (mL) of potassium permanganate solution used to titrate this excess of Mohr salt.

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