Permanganometry

Permanganometry is one of the techniques used in chemical quantitative analysis. It is a redox titration that involves the use of permanganates to measure the amount of analyte present in unknown chemical samples.[1] It involves two steps, namely the titration of the analyte with potassium permanganate solution and then the standardization of potassium permanganate solution with standard sodium oxalate solution. The titration involves volumetric manipulations to prepare the analyte solutions.[2]

Depending on how the titration is performed, the permanganate ion is reduced to Mn^x, where x is 2+, 3+, 4+ or 6+. Using permanganometry we can estimate the quantitative presence of Fe²⁺, Mn²⁺, Fe²⁺ and Mn²⁺ when they are both present in a mixture, C₂O₄²⁻, NO₂⁻, H₂O₂ etc.

In most cases permanganometry is performed in a very acidic solution in which the following reaction occurs:[3]

^{${\textstyle {\ce {MnO4- + 8H+ + 5e- -> Mn^2+ + 4H2O}}}$}

The standard potential of this electrochemical reaction is:[4]

E^o=+1.51 V

which shows that KMnO₄ (in an acidic medium) is a very strong oxidizing agent. With this method we can oxidize:

Fe⁺² (E^o_{Fe⁺³/Fe⁺²}=+0.77 V)
Sn⁺² (E^o_{Sn⁺⁴/Sn⁺²}=+0.2 V)

and even

Cl⁻ ( E^o_Cl₂/Cl⁻=+1.36 V) etc.

In weak acidic medium MnO₄⁻ can not accept 5 electrons to form Mn⁺², this time it accepts only 3 electrons and forms MnO₂(s) by the following electrochemical reaction:

${\ce {MnO4- + 4H+ + 3e- -> MnO2 + 2H2O}}$

The standard potential is E^o=+1.69 V.

And if the solution has a concentration c_(NaOH)>1 mol dm⁻³ the following reaction occurs:

MnO₄⁻ + e⁻ → MnO₄²⁻ E^o=+0.56 V.[5]

References

Redox titrations: Permanganometry. In: University Chemistry, Vol. 1. C. Parameshwara Murthy. New Age International, 2008. ISBN 81-224-0742-0. p.632
Louis Rosenfeld. Four Centuries of Clinical Chemistry. CRC Press, 1999, p. 130-175.
https://books.google.com/books?id=XQIIAQAAIAAJ Volumetric analysis, Vol 2. Izaak Maurits Kolthoff, Heinrich Menzel, Nathaniel Howell Furman. J. Wiley & Sons, inc., 1929. page 297
Table of standard reduction potentials. In: Chemistry and chemical reactivity. John C. Kotz, Paul Treichel, John R. Townsend. Cengage Learning, 2008. ISBN 0-495-38703-7. p. 920
Louis Rosenfeld. Four Centuries of Clinical Chemistry. CRC Press, 1999, p. 72-75.

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[1] Redox titrations: Permanganometry. In: University Chemistry, Vol. 1. C. Parameshwara Murthy. New Age International, 2008. ISBN 81-224-0742-0. p.632

[2] Louis Rosenfeld. Four Centuries of Clinical Chemistry. CRC Press, 1999, p. 130-175.

[3] ttps://books.google.com/books?id=XQIIAQAAIAAJ Volumetric analysis, Vol 2. Izaak Maurits Kolthoff, Heinrich Menzel, Nathaniel Howell Furman. J. Wiley & Sons, inc., 1929. page 297

[4] Table of standard reduction potentials. In: Chemistry and chemical reactivity. John C. Kotz, Paul Treichel, John R. Townsend. Cengage Learning, 2008. ISBN 0-495-38703-7. p. 920

[5] Louis Rosenfeld. Four Centuries of Clinical Chemistry. CRC Press, 1999, p. 72-75.