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Analysis of commercial bleaches.

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Tittle: Experiment 2 Analysis of Commercial Bleaches Objectives: To determine the concentration of two bleaching solutions by titration with standardized thiosulphate solution. Introduction: The bleaching action of commercial bleaches is due to its oxidizing power of sodium hypochlorite (NaClO(aq)). We cannot determine the concentration of this ingredient directly. We can firstly add excess acidified potassium iodide to the bleaches. The iodide would reduce to form iodine. NaClO +KI(aq)+ H2SO4(aq) ->I2(aq)+ H2O(l)+ NaCl(aq)+ K2SO4(aq) The concentration of iodine generated in situ is directly proportional to the concentration of the NaClO. The iodine is titrated against standardized sodium thiosulphate solution (2NaS2O3(aq)). I2(aq) +2NaS2O3(aq) ->Na2S4O6(aq) +2NaI(aq) Starch is used as an indicator in this titration. When the concentration of iodine is low (pale yellow), the starch indicator is added and the solution turns bluish black. The titration is continued until the color of the solution becomes colorless. So, the concentration of the active ingredient NaClO(aq) in bleaches can be determined form this back titration. Procedure: 1) 25 cm3 of each of the commercial bleaches (Clorex and Kao) ...read more.


0.00 0.00 0.00 Final reading / cm3 25.90 25.65 25.45 volume of sodium thiosulphate used / cm3 25.90 25.65 25.45 Average titre: 25.67cm3 Questions: 1. Determine the concentration of each of the two brands of bleaches provided. Ans: The molar mass of NaClO =35.5+16+23=74.5 No. of mole of Na2S2O3: No. of mole of NaClO= 2:1 The Clorex conc. is Avg. Na2S2O3 used is 32.7cm3 No. of Mole/volume=M The no. of mole of diluted NaClO 1/2X0.1X32.7/1000=1.635X10-3 The no. of mole of NaClO=1.635X10-3X10 =1.635X10-2mole mole X molar mass/volume=g/cm3 1.635X10-2X74.5/25=0.0487 g/cm3 The Kao conc. is Avg. Na2S2O3 used is 25.67cm3 No. of Mole/volume=M The no. of mole of diluted NaClO 1/2X0.1X25.67/1000=1.28X10-3 The no. of mole of NaClO=1.28X10-3X10 =1.28X10-2mole mole X molar mass/volume=g/cm3 1.28X10-2X74.5/25=0.0382 g/cm3 2. Explain why the thiosulphate solution should always be freshly prepared for titration. Ans: It is because the thiosulphate will react with the carbon dioxide in the atmosphere. The concentration of thiosulphate will decrease and the result of the concentration of the sodium hypochlorite will inaccurate that will have higher that the actual concentration. ...read more.


5. Suggest reasons why bleaches should be stored in cool and dark place. Ans: With the present of the light, the UV inside the light will give rise the energy level of the NaClO that will lead the decomposition of the NaClO ion and chlorine gas and oxygen gas will form. When the temperature is high, the energy for the decomposition occurs boost, the rate of decomposition of the NaClO ion will enhance, so bleaches should be kept in cool and dark place. 2ClO------------>O2 +Cl2 (with the present of UV or high temperature) Discussion: It is a back titration that the concentration of the NaClO cannot be determined directly. The NaClO is a strong oxidizing agent that would oxidize the acidified potassium iodide to iodine. While the amount of iodine can be determine by titration with sodium thiosulphate, the end point can be adding starch indicator when the iodine concentration is very low. Conclusion: The Clorex is more concentration is the active ingredient of NaClO than the brand Kao. However, we do not know what the cost of these two brands are, we cannot determine which brand is the best buy. ...read more.

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