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Analysis of 2 commercial brands of bleaching solution & a determination of the best buy

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EXPERIMENT ( 2 ) Topic : Analysis of 2 commercial brands of bleaching solution & a determination of the best buy Introduction : Sodium chlorate(I) forms the basis of most commercial bleaches. The amount present in bleaching solution can be determined by a volumetric technique. In this experiment, sodium chlorate(I) reacts with excess potassium iodide in the presence of acid to liberate. The iodine is titrated against standard sodium thiosulphate solution. Chemicals : Bleach solution , 0.1 M Na2S2O3 ,1 M H2SO4 , KI , Starch indicator Procedures : 1. Determine the total volume of bleach in the commercial bottles provided. Note the brand name, volume & price of each sample. 2. Pipette 25 cm3 bleach solution into a 250 cm3 volumetric flask. Make up to the mark & mix well. 3. Pipette 25 cm3 of this solution into a conical flask. Add 10 cm3 KI solution & 10 cm3 1 M sulphuric acid. 4. Titrate this mixture against the standard sodium thiosulphate solution provided. ...read more.


1.5L bleach solution = 0.01255 mol � 25 X 1500 = 0.753 mol Price of OCl- per mole in brand A = $12.9 � 0.753mol = $17.13 / mol Results : Titration of Brand B against the standard sodium thiosulphate solution Titration 1 2 3 4 Final Burette Reading (ml) 30.90 32.8 31.90 35.20 Initial Burette Reading (ml) 2.10 3.90 3.30 6.90 Volume of titrant (ml) 28.8 28.90 28.60 28.90 Mean volume of titrant (ml) = __(28.90+28.60+28.90)�3 = 28.8 ml__ Calculation: The Result Of Brand B Sodium chlorate(I) reacts with excess potassium iodide in the Bleaching solution, which is a acid medium. Iodine solution produced. Following ionic equation: 2H+-(aq) + OCl--(aq) + 2I--(aq) --> Cl--(aq) + I2-(aq) + H2O-(l) Secondly, the iodine solution is titrated with sodium thiosulphate solution Following ionic equation: I2(aq) + 2 S2O32- (aq) --> S4O62- (aq) + 2 I-(aq) The Molarity of Na2S2O3 : = 0.1M Number of mole of S2O3- reacted with I2: : = Molarity X Volume = 0.1 X (28.8�1000)= 0.00288 mol The Mole ratio of Na2S2O3:: I2= 2:1 Number of moles of I2 reacted with S2O3- = (1�2) ...read more.


3. Bleaching solutions may deteriorate for 2 reasons : (a) react with CO2 in the air according to the equation : 2 OCl- + CO2 --> CO32-+ H2 + Cl2 (b) what is the other reason ? It must be effected by light. It is because the The hypochlorite ions OCl-( will be decompose quickly under light Following equation:2NaOCl -->2NaCl + O2, .While losing some OCl- ions, the result in calculation will not be accurate 4. What should the starch indicator not be added too early ? The starch solution turns the iodine to blue black because of the formation of starch-iodine complex. Also, the complex is not reversible when the concentration of iodine is high. If we add the starch solution early, the attraction of starch molecules and iodine molecules will attract so strongly. Although we have add standard sodium thiosulphate solution, but we can not do completely finish the real result of titration and effecting the calculation. The above-mentioned tell us that the starch solution should be added when only a few of iodine solution left, near the end point of the titration. Conclusion: The bleach solution of brand B is the best buy. ...read more.

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