This experiment is about testing vinegar and bleach, which are close to our lives. To test foods and other household products is important because not all of them are quality products, there must be some defective products. These defective products may not useful or even have negative effects to human. To test them can check if they are quality. For these reasons, food inspection agency is set up in every country.
Vinegar and bleach are familiar things in kitchen. Vinegar is produced by bacteria through anaerobic respiration and its main chemical component is acetic acid, which can also called ethanoic acid, CH3COOH. The chemical composition of household bleach is sodium hypochlorite, NaClO. It is nocuous and its solution is alkaline. In this experiment, the sodium hydroxide solution was used to titrate the vinegar. According to Liam, 2010, the reaction equation of titration of vinegar is: CH3COOH + NaOH -> CH3COONa + H2O. The reaction equation of titration of bleach is: ClO- + 2I- + 2H+ -> Cl- + I 2+ H2O and I2 + 2S2O32- -> 2I- + S4O62-. Phenolphthalein solution and a starch solution were used as indicators in the experiment. Indicators are weak acid or weak base which will change their colour by the PH change. According to University of Southern Maine, 2010, it includes a redox reaction. In the titration of bleach experiment, the potassium iodide, sodium hypochlorite and dilute sulphuric acid will form iodine, which will react with starch solution and the colour will become blue. Then sulphate acid offered hydrogen cation to react with ClO- and I- and form I2. After that, the sodium thiosulphate will react with the iodine and make it back to iodine cation. This is the reason for the colour become back to colourless after the titration.
Method
Apparatus: test tubes, funnel, conical flask, pipette, vinegar solution, bleach solution, NaOH solution (0.1M), phenolphthalein indicator solution, sodium thiosulphate solution (1M), potassium iodide solution(1M), dilute sulphuric acid (0.1M), starch indicator solution (1M).
1. The reaction of phenolphthalein indicator solution with acid and base.
A dropping pipette was used to drop a little NaOH into a test tube and drop a little vinegar in to another test tube. One drop of phenolphthalein indicator solution was added into each test tube. The observation of each test tube was noted.
2. Titration of vinegar
The NaOH solution (0.1M) was put into the burette using funnel. The NaOH solution was poured out of the pipette until the level was read as 0.0cm3 and the exact initial burette was recorded.
A burette was used to put 2.5cm3 of vinegar into a conical flask. The initial volume and final volume were record. 20cm3 of water and four drops of indicator was poured into the flask. The flask was put under the burette and was shaken when the NaOH solutions (0.1M) were added into the flask. Addition of base was made slowly until some obvious colour change was noted. When a colour change occurred the final burette reading was recorded. These steps were repeated for three times.
Titration of bleach
A burette was filled with sodium thiosulphate solution (1M) by funnel.The level of the solution was adjusted to the 0.0cm3 and the exact value recorded. A burette was used to added 1cm3 bleach solution to the conical flask. The initial volume and the final volume were recorded. 10.0 cm3 of the potassium iodide solution (1M) and 10cm3 of dilute sulphuric acid (0.1M) were added into the conical flask. Sodium thiosulphate solution (1M) was added from the burette to the flask. The flask was shaken as soon as the sodium thiosulphate solution was added into the flask. When the colour changed to pale yellow, five drops of starch solution was added into the flask and the colour change was noted. Sodium thiosulphate solution (1M) was added into the flask until a colour change was noted. The final burette value was recorded. These Steps were repeated twice.
Result
Table 1 shows the colour change of phenolphthalein indicator with vinegar and base.
Reactants
Observation
Vinegar and phenolphthalein
All solutions and mixture were colourless
Sodium hydroxide and Phenolphthalein
Colourless changed to pink
Table 1 - Colour Change
Table 2 is the data of the titration of vinegar.
Agent
First time
Second time
Third time
Volume of used NaOH (cm3)
39.1
38.9
Initial volume (cm3)
0.2
0.1
Final volume (cm3)
39.3
39.0
Volume of vinegar (cm3)
2.5
2.5
Initial volume (cm3)
0.3
0.1
Final volume (cm3)
2.8
2.6
Table 2 Titration of Vinegar
The phenolphthalein was forgotten to add into the flask so the data of the second time did not make sense and had not been marked.
Table 3 shows the titration of bleach.
Agent
First time
Second time
sodium thiosulphate solution (cm3)
3.8
3.6
Initial volume (cm3)
0.0
0.2
Final volume (cm3)
3.8
3.8
Table 3 Titration of Bleach
Discussion
By the data, the concentration of ethanoic acid and sodium hypochlorite can be calculated.
Titration of vinegar
The mole of base is: 0.0389dm3-0.1M = 0.00389 mole
1 mole NaOH (cm3) solution titrates 1 mole vinegar so the mole of vinegar is the same as the mole of the NaOH (cm3).
The volume of the vinegar is 0.0025dm3.
Then the concentration of vinegar is: 0.00389 ÷ 0.0025 = 1.556 mole/dm3
The observation shows that to add 25cm3 of NaOH is not enough. Apart from the second time of doing this experiment, in the first time and third time of this experiment, more than 25cm3 NaOH was added to occur the excepted observation.
In the third time, after a drop was added in the flask the colour became pink and did not change back, about one minute latter the colour change back to colourless and the addition was continued. This time the colour did not change back.
Titration of bleach
The mole of sodium thiosulphate is 0.0036dm3-1M=0.0036 mole.
2 mole of Na2S2O3 reacts with 1 mole I2 so 0.0036 mole of Na2S2O3 reacts with 0.0018mole I2.
1mole ClO- takes place to the reaction and forms 1 mole I2. 0.0018 mole I2 needs 0.0018 mole ClO-.
The concentration of bleach is: 0.0018 ÷ 0.001 = 1.8mole/dm3
This reaction is weak acid titrates with strong base. It's easy to miss the appearance of the pale yellow if dropped to quickly. When the colour shows pale yellow, the starch solution was added and a blue colour can be seen.
Several mistakes were made during the experiment, which is cause by careless and can be avoided by reading the instructions carefully and implement it attentively.
Conclusion
The experiment this time shows us the way to test vinegar and bleach through titration, and it is based on a series of steps and a lot of data. In general, titration is a type of experiment that can make an exactly calculation.
