Reagents used: “Kao” Bleach,
“Clorox” Bleach,
1.0 M potassium iodide solution (KI),
1.0 M sulphuric acid (H2SO4),
Standard thiosulphate solution(S2O32-) with conc. = 0.05 M, and
Starch indicator
Chemical Reactions Involved:
ClO- (aq) +2 I- (aq)+ 2 H+ I2(aq) + H2O + Cl-(aq)
I2(aq) + 2 S2O32-(aq) 2 I-(aq) + S4O62-(aq)
Procedures:
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Exactly 25.0 cm3 of one of the bleaches was pipetted into a clean 250 cm3 volumetric flask. The mark was made up by using deionized water. The flask was shook well.
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25.0cm3 of this solution was pipetted to a conical flask, and then 10.0cm3 of 1.0 M potassium iodide solution and 10.0cm3 of 1.0 M sulphuric acid was added into the conical flask.
- The solution was titrated immediately against the standard thiosulphate solution (concentration = 0.05 M). When the titrate became pale yellow, two drops of starch indicator were added and the solution was titrated to the end-point. The results were recorded.
- The titration was carried out 3 times to yield two consistent results.
- Procedures 1 to 4 were repeated for the remaining bleach sample.
Observations:
After adding the potassium iodide solution into bleach solutions, the solution turned from colourless to brown. When the standard thiosulphate solution was added into conical flask, the colour of the solution became paler and then turned to pale yellow. When the starch indicator was added into the solution in conical flask, the solution suddenly changed to blue black colour. When the end-point was reached, the colour changed from blue black to colourless.
Data Analysis:
From the data sheet:
For “Kao” Bleach, the mean volume used = 16.42cm3, Price = $13.17/dm3
For “Clorox” Bleach, the mean volume = 23.5cm3, Price = $7.12/dm3
Calculation of “Kao” Bleach,
no. of mole of Na2S2O3 used = 0.01642 × 0.05 = 8.21 × 10-4 mol
mole ratio of I2 : S2O32- = 1 : 2
∴ no. of mole of I2 = 4.105 × 10-4 mol
∵ no. of mole of I2 = no. of mole of ClO-
∴ no. of mole of ClO- in the diluted bleach = 4.105 × 10-4 mol
no. of mole of ClO- in the original bleach = 4.105 ×10-3 mol
mass of NaClO in the original bleach = 4.105 ×10-3 × (23+35.5+16) = 0.3058 g
∴ The amount of NaClO available in gdm-3 = 0.3058 ÷ 0.025 = 12.2329 gdm-3
∴ The cost per gram of this compound = 13.17 ÷ 12.2329 = $ 1.077 g-1
Calculation of “Clorox” Bleach,
no. of mole of Na2S2O3 used = 0.0235 × 0.05 = 1.175 × 10-3 mol
mole ratio of I2 : S2O32- = 1 : 2
∴ no. of mole of I2 = 5.875 × 10-4 mol
∵ no. of mole of I2 = no. of mole of ClO-
∴ no. of mole of ClO- in the diluted bleach = 5.875 × 10-4 mol
no. 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. the end-point of titration is reached. We don’t add the iodine indicator in the solution first because during titration, the fade away of blue black colour can not be observed obviously. It is difficult for us to prepare to stop the titration at adequate time.
Answer to Question 4:
Sodium chlorate (I) and sulphur dioxide are two common bleaching agents. These are the differences between them:
①From the difference of reaction type, the bleaching reaction of sodium hypochlorite is oxidation, while the bleaching reaction of sulphur dioxide is reduction.
OCl- + Dye Cl- + Dye+O
SO2 + H2O + Dye SO42- + 2H+ + Dye-O
②From the difference of persistence of two bleached products, the bleached product from sodium hypochlorite bleach is permanent, while the bleached product from sulphur dioxide bleach is not permanent, because the product can be oxidized to return its original form by the oxygen in air.
③From the difference of their suitable bleaching targets, sodium hypochlorite bleach is generally used to bleach cotton, linen and plant fibres. It will damage wool and silk. While sulphur dioxide bleach is usually used to bleach certain dyes and natural colours.
④From the difference of their rates of bleaching, the rate of bleaching of sodium hypochlorite bleach is faster and more vigorous, while the rate of bleaching of sulphur dioxide bleach is slower and milder.
Answer to Question 5:
Bleaching solution may be deteriorated by many reasons. One of them is the attack by CO2 in air.
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This is the equation for the reaction between bleaching solution and CO2.
CO2 + H2O + 2NaOCl Na2CO3 + 2HOCl
- There are another reasons on the deterioration of bleaching solution. If the bleach is stored in high temperature space, it will decompose itself under high temperature, to form sodium chlorate (V) and sodium chloride. The effectiveness will be highly reduced. The equation is
3NaOCl NaClO3 + 2NaCl
Discussion of alternative method:
The above answer of the deterioration by carbon dioxide can also be used to estimate the amount of sodium hypochlorite in the household bleach. After pumping the excess carbon dioxide to the diluted bleach or put excess dry ice into the diluted bleach,
①We can estimate the amount of sodium carbonate by reacting with limewater, and then find the mass of calcium hydroxide produced. Thus the number of mole of sodium carbonate produced can be calculated;
②We can also estimate the amount of hypochlorous acid by titration with base such as potassium hydroxide solution. Remember that the indicator used should be phenolphthalein instead of methyl orange because hypochlorous acid is weak acid while potassium hydroxide is strong base.
Discussion of another application:
The method we used in this experiment can be used to estimate other substance. The amount of iodine in seaweed can be estimated by this method. We first attract the iodine in the seaweed by high-temperature-warming with suitable solvents. Then we can find out the amount of iodine in the solution by titration with thiosulphate solution. Thus the amount of iodine in the seaweed can be found.
Errors and Improvements:
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Some of the classmates found that they produced some milky solution after mixing diluted bleach with other chemicals. They claimed that they mixed the wrong chemicals with the bleach before titration. 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™ Zhidao: ;
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