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Redox

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  Redox Titration Objectives: * To standardize potassium permanganate solution using primary standard sodium oxalate. * To determine the amount of calcium in limestone.  Introduction: A redox titration is based on oxidation-reduction reaction between analyte (Na  2  C  2  O  4  ) and titrant (KMnO  4  ). Potassium permanganate is a powerful oxidizing agent and serves as a self indicator in the titration with reducing agent. Sodium oxalate is used for standardization of potassium permanganate as the following reaction show  )(2)(2)(2 )(4)(4 22  281025  aql  g aqaq  MnO H CO MnOOC   +−− ++→+ 1  And this is the reaction which used in this experiment.  Methods: 1-Standardization of Potassium Permanganate. Two samples of Na  2  C  2  O  4   were weighed and transferred to 250 ml conical  flasks. Then 50 ml of H   2  SO  4   was added to each of the flasks and then heated to 80-90 0  C.After that titrated against KMnO  4   immediately when it is reached that temperature. 2-Analysis of the unknown Limestone. A sample of dry limestone were weighted and transferred to 250 ml beaker. 10ml of water was added and covered with watch glass. Then 10ml of concentrated HCl was added. After that 5 drops of saturated bromine was added followed by boiling in the hood for 5 minutes to remove the excess Br  2  . Then it was diluted to about 50 ml with water and heated to boiling. 100 ml of hot 6% ammonium oxalate solution were added in the hood 3 or 4 drops of methyl red indicator were added and precipitate CaC  2  O  4   by slowly adding 6M NH   3   and continue adding 1 drop every 3 to 4 second until the solution turn to yellow orange color. The solution was allowed to cool fro 30 minutes 2  then it was filtered. Then the precipitate was transferred to 250 ml conical  flask. Finally titrated it against KMnO  4  .  Apparatus: Two 250 ml conical flasks, 250 ml beaker, 100 ml graduated cylinder, burette, filter flask, funnel.  Materials: KMnO  4 solution, Na  2  C  2  O  4  , Limestone, Bromine H   2  O, Concentration HCL, (NH   4  )  2  C  2  O  4   solution, Methyl red indicator and dilute H   2  SO  4.  Results: Asample 1sample 2m (NaC2O4)  -(g)0.233 ± 0.00010.2345 ± 0.0001V i(KMnO4)  -(ml)0.10 ± 0.0511.91 ± 0.05V f(KMnO4) -(ml)11.91 ± 0.0523.69 ± 0.05V (KMnO4)  -a-(ml)11.!1 ± 0.0 11. ! ± 0.05M (KMnO4)  -(m#l$l )0.05!!9 ± 0.00010.05916 ± 0.0001 Table 1: Data taken from the experiment. %sample 1sample 2m-(g)0.2590 ± 0.00010.2593 ± 0.00013  V i(KMnO4) -(ml)32.45 ± 0.0523.69 ± 0.05V f(KMnO4)  -(ml)42.32 ± 0.0532.23 ± 0.05V (KMnO4)  -a-(ml)9.! ± 0.05!.45 ± 0.05& mass CaO31.62 .03& mass Ca22.5!22.55 Table 2: Data taken from the experiment. Calculations: See the appendix.  Discussion:  We must get rid of other cations such as Fe  2+  & Ni  2+  to avoid forming of insoluble oxalates or co-precipitate of the unknown with calcium oxalate making it impure.   Alternative methods to determine Ca  2+  is substitution titration in which the calcium oxalate titrated to get color changing from red to blue.   The advantage in using my method is in the detection of color change  from colorless to pink at the end point is easy, but in substitution method the color changes from red to blue and that difficult to detect.  The advantage of using substitution method is the product solubility so it is easily to be filtered. The disadvantage of this method cannot be used to any sample except which contain magnesium or alkali metals.  4
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