In reaction 1 excess KI was added to provide I which forms I 3 Sulfuric acid H

In reaction 1 excess ki was added to provide i which

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reactions are (1) and (2). In reaction (1), excess KI was added to provide ¿ I ¿ which forms I 3 . Sulfuric acid (H 2 SO 4 ) is only added after KI crystals because doing the reverse would jincrease the dissolved oxygen. Sulfuric acid produces H+ ions and this is importance because an acidic environment is needed for the reaction with iodate ( ¿ IO 3 ¿ ) to occur. The whole process produces a dark yellow solution. Standardization was continued in reaction (2) when sodium thiosulfate was standardized against ¿ I 3 ¿ . Titration was performed until pale yellow color formed from the previous dark yellow. Since it is difficult the determine the endpoint through pale yellow color, 1° starch solution was added which turns the solution to deep blue. The rationale behind this is Iodine gets adsorbed on the starch molecule surface and product of adsorption has strong, blue color. Starch was not added prior to titration because starch solution undergo decomposition when mixed with high I 2 concentrations and this results to a higher endpoint. (Titrations.info, 2014) The solution is further titrated until deep blue color turns colorless indicating that the endpoint was reached. Table 1 shows the data gathered from the standardization. The net volume (mL) for all three trials was 37.0 and the calculated molarity of the standard was 0.0113 M
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R. J. C. De Guzman / Chemistry 26.1 (2017) P a g e | 3 Using the reactions (1) and (2), it can be deduced that 1 millimole of IO3- is equivalent to 6 millimoles of S2O3. Pertinent reactions for the analysis of the water sample are (3) and (4) when water sample was treated with MnSO 4 and NaOH with KI and NaN 3 solution. Addition of MNSO 4 solution can precipitate with ¿ O H ¿ . Manganese alone undergoes reduction and causes the volume of titrant used to decrease and thus DO content will decrease. To counter this, KI precipitates + ¿ M n 2 ¿ and produces ¿ I ¿ that will be oxidized to I 2 . NaN 3 on the other hand eliminates nitrites that converts ¿ I ¿ to I 2 in the solution. In reaction (3), + ¿ M n 2 ¿ will liberate ¿ O H ¿ from added NaOH and thus forms Mn ( OH ) 2 The sample was again sealed and then mixed thoroughly and vigorously and formed the precipitate 4MnO(OH) and this fixed oxygen content in the sample. In reaction (5), H3PO4 was added to provide + ¿ H ¿ ions generating an acidic medium which dissolves 2MnO(OH) to + ¿ 2 M n 2 ¿ and I 2 . The sample was again mixed thoroughly and vigorously to completely dissolve the oxygen and accurately account for the DO content of the sample. In reaction (6) I 2 reacts with excess ¿ I ¿ and forms I 3 . Titration performed for the primary standard is repeated this time for the water sample and this is indicated by equation (7). Thiosulfate titrated against I 3 will reduce it to ¿ I ¿ .
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  • Fall '17
  • Sir Jaden Smith
  • R. J. C. De Guzman

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