Ferric dipyridyl complex

The above compensating reactions are attractive because of the success of similar schemes in the halide catalysis, but proof in this case is more difficult. Thus it was possible to show in the halide systems that halogen and halide are present simultaneously. Evidence for the presence of ferrous ion in the ferric catalysis would support a similar interpretation. Manchot and Lehmann (44) claimed to have proved that ferrous ion is formed from ferric ion in the presence of peroxide since the addition of <, < -dipyridyl to the mixture resulted in the slow formation of the red ferrous tris-dipyridyl ion Fe(Dipy)3++. However, later work (65,66), which will be discussed when these systems are considered in more detail (IV,6), indicates that the ferrous complex ion may be formed by reduction not of the ferric ion, but of a ferric dipyridyl complex. Similar conclusions on the presence of ferrous ion were drawn by Simon and Haufe (67) from the observation that on addition of ferri-cyanide to the system Prussian blue is formed. This again is ambiguous, since peroxide is known to reduce ferricyanide to ferrocyanide and the latter with ferric ion will of course give Prussian blue (53). 

Manchot and Lehmann (44) observed that in the presence of the bases a,< -dipyridyl (Dipy) or 1 10 phenanthroline (Phen), ferric ion can be transformed quantitatively into the ferrous complex Fe(Dipy)3++ or Fe(Phen)s++ by hydrogen peroxide. It was concluded that the ferric ion... 

X 10 3 M a,a -dipyridyl the maximum amount of Fe(Dipy)3++ is produced in about ten minutes. During this time oxygen is evolved at a rate which is about one hundred times greater than that given by the ferric salt alone and about twenty times greater than that of the complex ion which is formed finally (Fig. 3). After this initial burst, which is usually referred to as the "Katalasestoss the rate of evolution falls to that due to catalytic decomposition by the complex ion. Experiments with various combinations of the reactants (see Fig. 3) established that... 

Observations were also made on the rate of formation of the complex ion in various conditions. In the absence of peroxide with ferrous ion and <, < -dipyridyl concentrations about 10 3 to 10-4 M, complex formation is almost instantaneous at pH 4. If a ferrous or ferric salt is added to dipyridyl plus peroxide the reaction may occur over a period of minutes and goes at the same rate whether ferrous or ferric salts are used. The rate of formation increases with increase of peroxide concentration and pH, and below pH 3.3 no complex ion is formed with the above reagent concentrations. In comparable conditions the complex forms about three times more quickly with <, a -dipyridyl than with phenanthroline, and twice as much oxygen is evolved in the initial burst. 

Because of this difference between the two bases Kuhn and Wasser-mann concluded that the reduction of ferric to ferrous cannot be the ratecontrolling step in the formation of the complex. They suggested that after ferrous ion has been formed in this way, it can either be oxidized by peroxide or react with the base to give the complex. If the rate of the latter reaction is faster with dipyridyl than with phenanthroline this will lead to the observed difference between them. Simon and Haufe (67) do not agree with this and hold the view that the production of ferrous ion by the reaction... 

The intermediate Fe (Dipy) 2++ may then add more base to give Fe(Dipy) 3++ or be oxidized back by peroxide to give a hydroxyl radical in a reaction analogous to (o) above. The radical can then initiate reaction chains similar to those in the simple ferrous and ferric ion systems. The maximum and final fall off in the oxygen evolution may result from the fact that when all the ferric ion is combined as the brown complex further increase in dipyridyl will not increase the initiation step. However, since more of the intermediates will be removed as Fe(Dipy)3++ the number of decomposition chains will decrease, and hence also the amount of oxygen evolved. In addition, the breaking of reaction chains by the oxidation reaction of hydroxyl with dipyridyl will act in this direction. 

The oxidizing agents used include gold chloride (potentiometric determination of the end point of the extremely slow oxidation) (Karrerefal, 1938) ceric sulfate (determination of the end point of the almost immediate reaction, with diphenylamine as indicator) (Kofler, 1941) lead tetraacetate (Kofler, 1941) but mostly ferric chloride in presence of a Fe -complex-forming compound, such as a,a -dipyridyl (Emmerie and Engel, 1938). The red color produced in this reaction is used for colorimetric determination. 

The color complex produced with ferric chloride-dipyridyl is equally sensitive (0.5 jag a-tocopherol detectable) and unspecific. The color reaction can be used, however, for quantitative determination of a-tocopherol in pharmaceutical forms and concentrates after localization of the spot in UV light and elution from the adsorbant (Bolliger, 1962). 

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