Experiment 4B

Experiment 4B - EXPERIMENT 4B Analysis of Oxalate Ligands...

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Experiment 4B- Page 1 EXPERIMENT 4B Analysis of Oxalate Ligands in an Iron (III) Oxalate Complex INTRODUCTION In this experiment, volumetric analysis will be used to determine the weight percentage of oxalate ligands in the K w [Fe x (C 2 O 4 ) y ] z H 2 O product from the synthesis in Experiment #4A. This experiment involves redox titrations rather than the acid-base titrations performed in Experiment #2. In addition, since one of the species used (in this case, it is the titrant) changes color itself as it reacts, a secondary indicator is unneeded (unlike Experiment #2). Specifically in this experiment, the exact amount of oxalate ions (from the iron-oxalate complex synthesized in Experiment #4A) can be determined by an oxidation-reduction titration with the permanganate ion, MnO 4 , in an acidic aqueous solution. Permanganate ion is a strong oxidizing agent capable of converting (i.e., oxidizing) oxalic acid to carbon dioxide and water: _ MnO 4 _ H 2 C 2 O 4 _ H _ Mn 2 _ CO 2 _ H 2 O Permanganate ion is also capable of oxidizing clothing to rags if spilled or splashed. Wear old clothes ! The above reaction occurs best in a strong acid solution, which means that the oxalate ions (i.e., C 2 O 4 2- from the original iron-oxalate complex) will actually be fully protonated and will exist as oxalic acid (i.e., as H 2 C 2 O 4 as shown in the reaction equation above). The potassium permanganate (KMnO 4 ) solution that you will use has already been prepared and standardized by the storeroom personnel for you (thank them!). Still, you should already have an idea on how to prepare a standard solution of potassium permanganate from your efforts in Experiment #2. Specifically, pure sodium oxalate, Na 2 C 2 O 4 , was used to determine the standardized concentration of permanganate ion in much the same way you standardized the sodium hydroxide solution using potassium acid phthalate (KHP) in Experiment #2. The preparation and storage of permanganate ion solutions require special precautions. Solid manganese dioxide, MnO 2 , must be removed completely from the reagent and the solution must be stored in the dark after standardization. For these reasons, a sufficient amount of standardized KMnO 4 solution has been prepared for everyone in advance. The deep purple color of a KMnO 4 solution is so intense that it can serve as its own indicator in an oxidation- reduction titration. The product, Mn 2 ( aq ), has such a faint pink color that it is essentially colorless. The reaction of permanganate with oxalate ions or oxalic acid is more complex than the balanced equation indicates. Even at high temperatures, the process occurs slowly unless some Mn 2 is present as a catalyst (a catalyst speeds up a reaction without being consumed). This means the reaction is slower at the beginning of the titration, before any Mn 2 is produced, than at the end where there is a lot of Mn 2 . To speed the process, you will heat the titration flask when first
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This note was uploaded on 03/12/2010 for the course CHEM CHEM 6B taught by Professor Christinajohnson during the Fall '07 term at UCSD.

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Experiment 4B - EXPERIMENT 4B Analysis of Oxalate Ligands...

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