Preparation of Potassium Trioxalatoferrate Trihydrate
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Iron, being a transition metal can be found in different oxidation states, such as 2+ and 3+. Transition metal ions react with neutral or charged ligands, such as Br- or NH3, respectively, to form complex ions or neural compounds. Iron in the Potassium Trioxalatoferrate(III) trihydrate, is found in the +3 oxidation state. This thus forms an octahedral complex that could accommodate up to 6 monodentate ligands around the metal iron. Ligands are Lewis bases that can donate pairs of electrons to the central metal ion, in this case Fe3+ 1. The Oxalate ion C2O42- (as shown in figure 1), however, is a bidentate ligand. This means that is able to donate two pairs of electrons from two of its oxygen atoms. Also bidentate ligands may also be referred to as chelating ligands, since they are able to attach to metal ions at two positions. A complex containing a chelating agent is called a chelate 2
The first step in the synthesis of Potassium Trioxalatoferrate(III) trihydrate, K3[Fe(C2O4)3].3H2O, is the preparation of Iron(II) oxalate. Ferrous ammonium sulfate reacts with oxalic acid to form iron(II) oxalate, FeC2O4, as seen in the equation below.
Fe2+(aq) + H2C2O4(aq) + 2H2O(I) → FeC2O4(s) + 2 H3O+(aq)
The oxalate was introduced as potassium oxalate to replace the water and sulfate ligands coordinated to the Fe2+ ion of the iron(II) oxalate, forming a yellow solid. The hydrogen peroxide was then added to oxidise the iron to the +3 state.3
2 FeC2O4(s) + H2O2(aq) + 2 H+(aq) + C2O42-(aq) → Fe2(C2O4)3(s) + 4 H2O(I)
The temperature is maintained at 40 OC to increase the rate of oxidation of the Fe2+ to Fe3+. This, however, was done slowly since the heat liberated from the addition of the peroxide could be enough to decompose the peroxide itself. Thus the complete oxidation would not be possible and the resulting solution would be a mixture of two complexes. 4
After complete oxidation the excess hydrogen peroxide was removed by additional heat 5. Oxalic acid was then introduced to convert the iron(III) oxalate to trioxalatoferrate (III) ion, as shown in the equation:
Fe2(C2O4)3 (s) + 3 H2C2O4 (aq) + 6 H2O(I) → 2 [Fe(C2O4)3]3 -(aq) + 6 H3O+ (aq)
After being formed the Fe3+ complex reacted with potassium ion to form the complex iron salt Potassium Trioxalatoferrate(III) trihydrate, K3[Fe(C2O4)3].3H2O. Since this is soluble in water it will not be able to precipitate. Thus ethanol is added so that the green crystalline solid could precipitate.
3 K+(aq) + [Fe(C2O4)3]3-(aq) → K3[Fe(C2O4)3)(s)
The solution must be stored in the dark while the salt is crystallizing because visible light will reduce Fe 3+ to Fe 2+ .
During the first titrations of the Iron(II) oxalate, the titre value depicted the amount of permanganate needed to oxidise the C2O42- to CO2, and the Fe2+ to Fe3+. The permanent pink colour indicating the end point. On addition of the zinc dust the iron(III) is reduced to Iron(II), thus with the second titration the amount of permanganate added is directly proportional to the amount needed to oxidise Iron(II) to iron(III) since the oxalate had already been oxidised to carbon dioxide in the previous titration.5
Zn à Zn+ +e-
e- + Fe3+ à Fe2+.
The analysis of the yellow Iron(II) oxalate crystals depicts that the compound contains 3 water molecules (of crystallisation),however, 2 water molecules are actually present. This error could have been due to insufficient drying that led to an increased weight and thus higher value.
Percentage of Oxalate in Potassium Trioxalatoferrate(III) trihydrate was found to be very close to the theoretical mass. In fact there was only a 4.01 % decrease in the final mass. The percentage of iron in the sample decreased by 5.13 compared to the theoretical value. Losses could also be due to losses during transfers.
It could be concluded that the the Iron(II) oxalate contains 2 moles of water (of crystallisation) rather than the calculated 3 moles. Thus this would give the sample an empirical formula FeC2O4.2H2O . it was also concluded that the Iron(ii) Oxalate contained 52.96% and 61.11% of weight by mass of Iron(II) and Oxalate in the sample. Its complex, Potassium Trioxalatoferrate(III) trihydrate, contained 6.28% and 49.76 % weight by mass of iron(III) and Oxalate respectively.
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