Iodometry Lab Report

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Experiment Summary
The purpose of this experiment was to determine the copper content of brass using iodometry. This was accomplished by creating a thiosulfate solution and standardizing it by titrating using a known amount of KIO3. The second part of the experiment involved preparing a brass sample and allowing copper to react with excess iodide to form I2. The resulting I2 was then titrated with the thiosulfate standard. Both the molar concentration of thiosulfate and the weight % copper was calculated using stoichiometry.
Introduction & Background
In this experiment, the analyte is the amount of copper in an unknown brass sample. The typical range of copper in brass is 59% to 95%. Iodometry is used to determine the amount of copper
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This will dissolve and oxidize copper into Cu2+. The solution was then heated to remove the brown NOx fume. This is necessary since NOx acts as an oxidizing agent and will oxidize I in the redox reaction to be observed. When all NOx fume is removed, H2SO4 was added and the solution was heated until white SO3 fumes appear. The addition of H2SO4 will drives off all nitrogen oxide gas that can interfere with the redox reactions. The SO3 vapor that is observed is an indication that all NOx has been driven off. The solution was cooled in ice bath and DI water is added, followed by the addition of NH3 drop wise. The purpose of this step is to create a slightly acidic to neutral solution, which is an ideal condition for the redox reaction as mentioned above. Addition of NH3 will form Cu(NH3)42+,a copper tetramine complex with a dark-blue color, indicating a slightly basic solution. Adding H2SO4 will bring the solution to a lighter blue color, indicating that it is slightly acidic to neutral. As discussed previously, this will prevent iodide from being oxidized by atmospheric oxygen in the presence of excess acid. In addition, it also prevent Cu(OH)2 from precipitating in the solution due to an highly basic environment. Finally, H3PO4 is added to the solution to prevent Fe3+ in brass from being reduced to Fe2+ by iodide since phosphate will form a complex with Fe3+. The solution was heated before the titration process in …show more content…
The standard was prepared by weighing and dissolving 25 grams of Na2S2O3∙5H2O in boiling water. It is important to sterilize the solution by bringing it to a boil for 5 minutes since common bacterial strains can decompose the thiosulfate solution into sulfite, sulfate, and elemental sulfur. In addition, NaCO3 was also added to create a slightly basic solution, protecting it from rapid decomposition into sulfite and sulfur. The thiosulfate standard was then wrapped and stored in the dark since light can catalyze the decomposition of thiosulfate into sulfite and elemental sulfur. In order to standardize the titrant, KIO3 is dried for 1 hour. The purpose of this step is to remove any water that adheres to the compound that would lead to an inaccurate mass. A known mass of about 0.12xx grams of KIO3 is then weighed and dissolved with water in an Erlenmeyer flask. Excess KI and HCl was added to react with KIO3 to form I2 in the following equation: KIO3 + KI + HCl  KCl + I2 + H2O. Since KI and HCl is added in excess, the reaction will be forced to the right due to Le Chatalier Principle. It is important for the titration process to be carried immediately after HCl is added since iodide oxidizes readily in the presence of oxygen in air due to an acidic environment. The resulting I2 is then titrated with the thiosulfate standard. To calculate the molarity of the thiosulfate standard, the mass of KIO3 is

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