Experiment 16
The Determination of the Oxalate Content in K2[Cu(C2O4)2].2H2O

Theory

In acid solution, permanganate, MnO4- [Mn (VII)] is reduced to Mn(II). The half reaction is:

MnO4- + 8H+ + 5e- →      Mn2+ + 4 H2O     Eo = 1.51 V
In basic or neutral solution, permanganate is reduced to manganese(IV) oxide. The half reaction is

MnO4- + 4H+ + 3e- →       MnO2 + 2H2O     Eo = 1.70 V
Potassium permanganate is often contaminated with MnO2, which causes autodecomposition of the solution on standing. The consequences of this decomposition are: 1) it is essential to filter solutions through a sinter before use, 2) permanganate is unsuitable as a primary standard and 3) reaction in acidic media is preferred.

Sodium oxalate or oxalic acid is often used to standardise permanganate. The relevant half reaction is

C2O42- →       2CO2 + 2e-           Eo = -0.49 V
and therefore the reaction with permanganate is favourable, i.e.,

2 MnO4- + 5C2O42- + 16 H+ →       2Mn2+ + 10CO2 + 8H2O, E ° = 1.02 V

Iron (II), arsenic (III), antimony (III), hydrogen peroxide, nitrite and oxalate can all be oxidised by permanganate as described in Vogel.

Method


Into 400 cm3 beakers weigh out to the nearest 0.1 mg duplicate portions, using an analytical balance, about 0.15 g of K2[Cu(C2O4)2].2H2O. Add about 50 cm3 water and 50 cm3 of dil. H2SO4. Heat to about 70 °C and titrate slowly with the standardized permanganate* solution, 'K', shaking the flask well.
If the solution remains pink after adding the first drop or two, wait until it clears before continuing the titration. Continue the titration slowly until a faint, permanent pink tinge is obtained. The temperature should not be less than 60° C at the end point. If it falls below this during the titration, the solution must be reheated.
Calculate the percentage oxalate in the complex. The results of the two analyses should be consistent. Otherwise a 3rd determination should be done. Compare your results with the theoretical value.
  1. B. The permanganate solution, 'K', should be standardized using the calculated mass of sodium oxalate on the worksheet, and utilizing similar experimental conditions as in the analysis of K2[Cu(C2O4 ) 2].2H2O.


Complementary Work


Carry out the following test. Note and explain, with equations where possible, your observations.

Prepare an acidic (use dilute sulfuric acid) aqueous solution of your complex and add to it KI. Add 1-2 cm3 chloroform and shake the mixture gently.

Questions

  1. In order to determine the oxalate content of the chromium complexes, one would normally boil solutions of the complexes with KOH solution before titrating against permanganate. Why?
  2. Outline a method to determine the copper content in the copper complex.
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The Department of Chemistry, University of the West Indies,
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Created Nov 2002. Links checked and/or last modified 22nd October 2005.
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