Experiment 8 Unique # 51070 The main purpose of the experiment involves two oxidation-reduction reactions to calculate the oxidizing capacity of a sample of unknown bleach. In order to determine the volume of Na2S2O3 added, students will conduct a titration of bleach with thiosulfate with addition of a starch indicator to find the end point of the titration. Moreover, the oxidizing capacity of bleach is calculated with the percentage by mass of NaOCl in the unknown bleach sample. The overall chemical reaction throughout the experiment will be balanced with the two oxidation-reduction reactions.
Within the household chlorine bleaches, sodium hypochlorite (NaOCl) and calcium hypochlorite Ca(ClO)2 are the oxidizing agents. Before titration, the mass of the bleach is weighed out for each trial and recorded, approximately 0. 4-0. 6 grams. After the bleach sample is weighed, bleach is poured into a beaker with several aliquots of distilled water to completely wash the bleach out. Next, 3 M KI is added to the beaker, then 3 M H2SO4 is poured into the bleach with 5 drops of 3% ammonium molybdate catalyst immediately subsequent of acid addition.
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Titration begins with Na2S2O3 added into the bleach solution until the solution turns completely yellow, then drops of starch indicator is added and mixed. The starch will turn the solution into a dark blue-black color, and then titration should continue until the solution turns colorless at the end point and the buret reading will be recorded. Before the starch is added, triiodide is the form of iodide that is present; after reaching endpoint, iodine reduces and returns to iodide ions.
A precautionary procedure to be considered is to ensure the color of the solution is transformed into a complete yellow color before adding the starch. If the starch is added before it is yellow, the reaction of the dark-blue complex would be difficult to reverse and would require excessive amount of time to dissociate; thus it would be hard to determine the end point. In addition, starch has the ability to hydrolyze in acidic solution, which would eliminate its indicator potentials. Another important step to nsure after addition of sulfuric acid, the addition of 5 drops ammonium heptamolybdate and titration should begin immediately. Ammonium heptamolybdate acts as a catalyst to speed up the reaction of iodide ions and the oxidizing agents. Therefore, it is crucial to add ammonium heptamolybdate so that sufficient amount of iodine to react with the sodium thiosulfate while titrating. Moreover, the titration had to start immediately subsequent the addition of ammonium heptamolybdate and sulfuric acid to prevent formation of iodine crystal solids.
If the crystals formed with the amount of iodine exceeding the solubility of the solution, then it could result in clumping of the insoluble crystals that does not react with sodium thiosulfate, which would largely affect the end point. In addition, if less sodium thiosulfate reacted, the ending result of the oxidizing capacity of the bleach would be lower than the acceptable value. Simply, the oxidizing capacity is the bleaching strength of an oxidizing agent. When a strong acid is present, hypochlorite ion changes to hypochlorous acid when the strong acid donates the H+ when it dissociates in the solution.
In order to calculate the oxidizing capacity, the mass of NaOCl present in each trial should be determined using the volume of Na2S2O3 added in each trial multiply by the concentration to find the number of moles of Na2S2O3. Next, use the number of moles of Na2S2O3 mutiply by the mole ratio of NaOCl to Na2S2O3, which is 1 to 2, and then multiply by the molecular weight of NaOCl (74. 55g/mol). Then use the mass of NaOCl and divide it by the mass of the bleach to calculate the percentage of the NaOCl in the unknown bleach sample. The average of the percentages from the three trials would be the oxidizing capacity, which was 0. 089% +- 0. 1497 (calculated standard deviation). Several procedures that could consist of errors include titrating to the exact color change, and then add the chemicals in the order by the procedures. Also, as mentioned earlier, starch needed to be added when the solution turned yellow and titration stopped exactly when the solution turns colorless. However, for most of the trials, it was difficult to determine the exact point to stop titrating because the color turns colorless after mixing, which does not happen instantaneously.
In addition, dilution of Na2S2O3 may not be exactly 0. 05M due to slightly inaccurate measures of water and Na2S2O3 (May be caused by usage the less accurate graduated cylinder). Another error could have resulted from transfer of bleach to the 250mL beaker; some sample might have adhered to the sides of the beaker instead of being transferred to the beaker for titration. Moreover, during some trials, it was difficult to manage to titrate immediately after the reaction has been acidified, thus producing unwanted errors.