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determination of the percentage of oxalate in iron (II) oxalate by redox titration

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F.6A Lam Pik Sum (10) Title: determination of the percentage of oxalate in iron (II) oxalate by redox titration Date: 31/10/2008 Objective: to find out the percentage of oxalate in iron (II) oxalate by redox titration. Introduction Potassium manganate (VII) is not suitable for use as a primary standard as its solution decimoises in solution, especially in sunlight, and readuky oxidizes organic matter. Moreover, brown deposits of manganese (IV) oxide are often present in its solution. In fact, potassium manganate (VII), being not a primary standard, is usually standardized by standard solution of sodium thiosulphate. Permanganate ion, MnO4(aq), is a strong oxidant. Since permanganate ion is intensely coloured and its reduction product, Mn2+(aq), is almost colourless, a self-indicating titration is possible. The addition of the first drop of permanganate solution in excess imparts a pink colour to the solution. Potassium manganate(VII) reacts quantitatively with many reducing agents. Oxalates are readily determined by titration with standard potassium manganate (VII) in warm acidic solution: C2O42- � 2CO2 + 2e- In strongly acidic medium, permanganate undergoes a 5 electrons reduction to manganese(II) ion: MnO4(aq) + 8H+(aq) + 5e- � Mn2+(aq) + 4H2O(aq) Potassium permanganate does not oxides oxalates in cold solution. A temperature of about 70�C is necessary to cause the reaction to occur rapidly. If coloured or oxidized cations are present, the procedure will be more complicated. ...read more.


x 100% = 61.2% Mass of FeC2O4 in the sample = 88 x 0.014467125 = 1.273107g Calculated percentage of oxalate in the sample = (1.273107 � 2.632) x 100% = 48.4% The chemical formula of the hydrated iron (II) oxalate We assume all the impurity in the sample are water of crystallization, then the water of crystallization by mass in the compound is 21% and the chemical formula of hydrated iron (II) oxalate is known to be FeC2O4?xH2O Relative formula mass of FeC2O4?xH2O = 55.8 +12x2 + 16x4+ (1x2 +16)x = 143.8 + 18x Relative molecular mass of water of crystallization = 18x 18x � (143.8 + 18x) = 21 � 100 1800x = 3019.8 + 378x 1422x = 3019.8 x = 2.124 �2 Therefore, the chemical formula of the hydrated iron (II) oxalate is FeC2O4?2H2O. Conclusion The percentage of oxalate in iron (II) oxalate is 48.4% which is smaller than the predicted one. Discussion Precaution 1) Handle potassium permanganate with great care because it is toxic and irritation to skin 2) Handle manganese (IV) oxide with great care because it can cause serious eye injury, is a skin and inhalation irritant, and can be fatal if swallowed Sources of error 1) KMnO4 is too reactive that light may induce the decomposition of KMnO4. It may lead to the error in accurate determination of the concentration of oxalate. ...read more.


In the initial stages of the titration, only a small amount of Mn2+ is formed, therefore the reaction is slow and the Mn2+ is discharged at a slow rate and the colour of Mn2+ is covered by the green Fe2+. 7) You are given a standard solution of potassium iodate, some solid potassium iodide, a bottle of dilute sulphuric acid and some starch indicator. Describe briefly how you could standardize a solution of sodium thiosulphate. You may assume that all common titration apparatus is available. We can standardize sodium thiosulphate solution through titration of iodine with thiosulphate, therefore we need to prepare a standard iodine solution. A standard iodine solution cannot be prepared directly by accurately weighing a certain amount of iodine, but it can be prepared by dissolving a known amount of pure potassium iodate solid into an acidic medium containing excess iodide. We first dissolve weighed amount of KI into dilute sulphuric acid. The number of mole of it should be at least 5 times that of potassium iodate. After adding potassium iodate solution into acidified KI, iodine would be formed. Just use a small portion of the mixture and titrate with thiosulphate solution until the brown color nearly fade, then add starch solution and continue to titrate until the mixture becomes colourless. If Potassium iodate is in excess rather than KI, a slight addtion of KI to the resulting mixture would form a brown to dark brown mixture indicating that I2 is formed. End of Report ?? ?? ?? ?? 1 ...read more.

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