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Analysis of Two Commercial Bleaches

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Introduction

Date of experiment: 13/10/2010 Name: Cheung Lap Kwan (6) Po Leung Kuk Ngan Po Ling College 2010-2011 Form 6 Chemistry Laboratory Report Title: Analysis of Two Commercial Bleaches Introduction: Most of the commercial bleaches we can buy in the markets have the same main active ingredient, Sodium hypochlorite [sodium chlorate(I)]. If we want to estimate the actual amount of sodium hypochlorite in the bleach, we can do the titration in the laboratory. In this experiment, the active ingredient, sodium chlorate (I), is allowed to react with an excess of potassium iodide solution in the presence of hydrogen ion, liberating iodine. The chemical reaction is: ClO- (aq) +2 I- (aq)+ 2 H+ I2(aq) + H2O + Cl-(aq) The iodine produced is then titrated with standard sodium thiosulphate solution. Near the end-point during titration, we use starch solution as an indicator. This reaction we call"Iodometic titration". The chemical reaction is: I2(aq) + 2 S2O32-(aq) 2 I-(aq) + S4O62-(aq) By taking and analysing the data of volume of standard sodium thiosulphate solution, we can estimate the amount of sodium hypochlorite in the target bleach. Purposes: To analyse and compare two commercial bleaches, "Kao" Bleach and "Clorox" Bleach and then find out which one is cheaper upon their bleaching strength. Apparatus and Reagents Used: Apparatus used: 2 100ml beakers (One for "Kao" Bleach, one for "Clorox" Bleach), 2 25.0cm3 pipettes (Washed by deionized water first, then washed by bleaches), 2 10.0cm3 pipettes (Washed by deionized water ...read more.

Middle

of mole of ClO- in the original bleach = 5.875 �10-3 mol mass of NaClO in the original bleach = 5.875 �10-3 � (23+35.5+16) = 0.438 g ?The amount of NaClO available in gdm-3 = 0.438 � 0.025 = 17.5075 gdm-3 ?The cost per gram of this compound = 7.12 � 17507.5 = $ 0.4067 g-1 From the above results, the "Clorox" Bleach is much cheaper than "Kao" Bleach upon their bleaching strength. Discussions: Answer to Question 1: The potassium iodide solution present should be in excess because we have to ensure that all of the hypochlorite ions are reacted completely after the reaction between sodium hypochlorite and potassium iodide. If the quantity of potassium iodide is inadequate, not all of the iodide ions are converted to iodine. Therefore the experimental concentration of bleach less than the theoretical concentration. Answer to Question 2: The reaction between sodium hypochlorite and potassium iodide need the present of hydrogen ion. Therefore the function of sulphuric acid is to supply hydrogen ion for the reaction. Answer to Question 3: We add the starch indicator near the end-point of titration because the colour change of the solution near the end-point is not obvious, which is from pale yellow to colourless. After adding the starch indicator, starch will form a blue black aqueous complex with iodine. The sudden disappearance of the blue black colour accurately indicates that there is no iodine in the reaction mixture, i.e. ...read more.

Conclusion

They wrongly mixed 10.0cm3 of thiosulphate solution with bleach first. After searching some information in the Internet, I found that there is a chemical reaction between sulphuric acid and sodium thiosulphate: Na2S2O3 + H2SO4 Na2SO4 + S + SO2 + H2O During the reaction, sulphur is produced and it is insoluble in water. It formed the white precipitate in the solution. So this is why the solution became milky. (ii) Most of our classmates found that the 10.0cm3 pipettes are difficult to use because the graduation mark on 25.0cm3 pipette is different from that on 10.0cm3 pipette. Maybe we cannot get the accurate 10.0cm3 of needed solution. I suggest that we can still use 25.0cm3 pipette to attract the solution. However, if we attract more solution, we will also waste more chemicals. So I also suggest that the solution can be more diluted so the chemical waste can be reduced. Conclusion: From this experiment, after the analysing and comparing the data we calculated, we knew that the "Clorox" Bleach is cheaper upon their bleaching strength than the other brand, "Kao" Bleach. Reference: 1. E. Cheng, J. Chow, K. K. Lai & W. H. Wong (2008) Chemistry A Modern View 2, Page 256 & 285. Hong Kong ARISTO ISBN 978-962-469-531-1 2. Y. C. Wong & C. T. Wong (2005) New Way Chemistry for Hong Kong A-Level 1, Page 65. Hong Kong MANHATTAN PRESS ISBN 978-988-204-467-8 3. Baidu(tm) Zhidao: http://zhidao.baidu.com/question/35658287 ; http://zhidao.baidu.com/question/56317221 & http://zhidao.baidu.com/question/19019365.html --End of the Report-- ?? ?? ?? ?? Page 1 of 6 ...read more.

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