Experiment 8 Lab Report Analysis of Oxidizing Capacity of Bleach The goal of this lab is to perform a titration of a sample of bleach with thiosulfate, figuring the end point with a starch indicator. We will determine the percent mass of sodium hypochlorite in bleach by using the volume of sodium thiosulfate needed to reach the end point. This value represents the oxidizing capacity of the unknown bleach. At the end, we will create two redox reactions to determine the overall chemical reaction that occurs during this process. I think that my oxidizing capacity will be 5.0g. Most bleach contains salts with the hypochlorite ion, the oxidizing agent that removes stains and whitens them. The hypochlorite ion usually comes in the form of sodium hypochlorite, NaOCl, or calcium hypochlorite. The two oxidation reduction reactions that occur during this experiment are first the oxidation of iodide to iodine, I2(aq), by the hypochlorite ion and then making the aqueous iodine reduce back to iodide by titrating it with sodium thiosulfate. The overall stoichiometry of the reaction is one mole of hypochlorous acid, formed by placing the sodium hypochlorite into acidic solution, plus two moles of the thiosulfate ion plus hydrogen ions from the solution yielding one mole of chlorite anions, one mole of the tetrasulfur hexoxide anion and one mole of liquid water. We began the titration of aqueous iodine by adding .05M sodium thiosulfate until the brown colored solution began to turn a bright yellow. As the solution proceeded to turn yellow, we mixed thoroughly to ensure that all the iodine indeed dissolved in solution. After the solution became yellow, we added starch as an indicator and the solution turned a dark black color. We then added more thiosulfate, this time drop by drop, until the solution turned colorless meaning the end point had been reached. The dominant form of iodine before the starch is added is the iodide anion because the iodine reacted almost completely with the thiosulfate. After the end point is reached, the iodine will be more prevalent as enough iodide has been reached to fully reduce the hypochlorite ions in solution. If the starch is added too soon, the large amount of triiodide that is formed in solution gets absorbed very quickly by the starch, causing a large complex to be formed that does not dissociate as rapidly as we would want it to for the sake of time in this experiment. It also would be very difficult to determine the end point of the reaction because it is not easily titrated. We had to take care of adding the ammonium heptamolybdate immediately after adding the sulfuric acid to use as a catalyst for the reaction. We added the sulfuric acid to contribute hydrogen ions so that the solution would be acidic and the reaction could go faster. If the catalyst was not added right after the sulfuric acid addition, then the iodine dissolved in solution would be so great that is would exceed its molar solubility and precipitate out as solid iodine. As more solid iodine forms it will clump together, making it more challenging to dissolve and titrate, causing the titration to be scrapped. The oxidizing capacity is the amount or extent to which an oxidizing agent such as the hypochlorite ion can oxidize the reducing agent, in this case iodide anion. The strong acid present in solution with a hypochlorite anion causes hypochlorous acid to form because of the opposite charges associated with the hydrogen ion and hypochlorite ion. They create a weak acid which dissociates very little, making the majority of substance present in solution to be the weak acid form. I determined the oxidizing capacity of my unknown sample by using the volume of sodium thiosulfate needed to reach the endpoint and multiplying it by its molarity. Then by using the mole ration between thiosulfate and the sodium hypochlorite, found the moles of the sodium hypochlorite. Finally, I multiplied by its molecular weight and divided that number by the mass of bleach used in the titration. My oxidizing capacity was 2.55±.01%. Some errors that might affect the results are in the time it took to add the catalyst after adding the sulfuric acid to solution. Even if there is very little solid iodine left, the titration would have a little error because not all the iodine will have reacted. Also, when titrating the solution of iodide until it becomes yellow, adding too much thiosulfate will cause the solution to turn colorless even without the addition of starch, so the volume used to reach that point may not be the true end point. Another source of error is the iodide in solution before starch is added can be oxidized by the air, causing some to use less thiosulfate than expected to turn the solution a bright yellow as some of the iodide already formed iodine.