The reaction Fe2++ 2H2O –Fe(OH)2+ H+occurs readily. Why would this be a problem in the determination of the amount of iron and what procedure was used in this experiment to prevent this reaction from interfering with the determination of the percent iron in the unknown sample?
Once the Fe(OH)2 forms the it readily precipitated and would not be able to react with the reagents used in in the estimation process which converts Fe2+toFe3+(like permanganate or dichromate).Beside this the water used in the lab may contain trace amount of iron as Fe(OH)2.So we get errors in calculations.
To solve this problem conc Sulphuric acid is added to increase the conc of H+ to shift the equilibrium of the reaction Fe2++2H2O-->Fe(OH)2 + 2H+ towards left hand side(Le Chatelier's principle)and H3PO4 is added to form FePO4 complex to shift the reaction Fe2+-->Fe3+towards right hand side by reducing the formal potential of that system.
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