Investigating Chemical Concentration In Vinegar Biology Essay


The purpose of this practical is to test chemical concentration, especially ethanoic acid in vinegar and sodium hypochlorite in bleach.

While components of food or household products are often listed on the container label, the concentration of chemicals in these products might vary from the data given by producers. Therefore, testing chemical concentration of food and household products is important for health and environment protection. This practical will focus on testing the concentration of ethanoic acid in vinegar and the concentration of sodium hypochlorite in bleach solution.

According to Lane (2010a), acid contributes hydrogen ions and base contains hydroxide ions. Ethanoic acid (CH3COOH (aq)), also called acetic acid, contains hydrogen ion and it is responsible for the acidity of vinegar (Babb and Perona, 2010). Ethanoic acid can react with a base, sodium hydroxide (NaOH (aq)), following the equation below to produce water and salt (Lane, 2010b).

NaOH (aq) + CH3COOH (aq) → CH3COONa (aq) + H2O (aq)

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This reaction can be used in acid-base titration to test the concentration of ethanoic acid. The reaction is completed when it reaches equivalence, when all acid hydrogen has been neutralized (Lane, 2010b). An indicator is needed to show whether the equivalence point is reached. It is stated by Babb and Perona (2010) that an indicator is a substance that changes its colour as the acidity of a solution varies. For this reaction, phenolphthalein, a colourless weak acid, is used as indicator since it has apparent and quick colour change from colourless to violet when it meets base (Lane, 2010a). It remains colourless when acid is added. Its structure is showed in Figure 2 (Lane, 2010a).

Figure 2 Structure of Phenolphthalein (Lane, 2010a)

Since phenolphthalein will change from colourless to violet under basic conditions, when all ethanoic acid has reacted with sodium hydroxide, one excess drop of sodium hydroxide into the solution will result in instant colour change. Therefore, adding phenolphthalein into the tested solution - vinegar before adding sodium hydroxide can help to indicate when the equivalence point is reached and how much sodium hydroxide is needed to react with certain amount of ethanoic acid. With given concentration of sodium hydroxide and given volume of vinegar, the concentration of ethanoic acid in vinegar can be calculated by knowing the amount of sodium hydroxide used in the experiment.

In most bleach solution, there is sodium hypochlorite. This substance contains hypochlorite ion and its concentration can be determined by a redox titration (Lane, 2010b). A redox titration is based on a redox reaction. This kind of reaction is essentially the exchange of electrons between species (Shodor, 2008) Oxidation number of elements in redox reactions shows the number of electrons that have been removed from the element or gained by the element. In redox reaction, oxidation corresponds with an increase of oxidation number while in reduction, oxidation number decreases (Clark, 2000). The oxidation number of each element is Na = +1, O = -2, Cl = -1, K = +1, H = +1, S = +6. According to Lane (2010), hypochlorite ion can react with iodide under acidic condition to produce iodine and water, as the following equation (Lane, 2010b):

ClO- + 2I- + 2H+ → Cl- + I2 +H2O

Hence, using sodium hypochlorite, potassium iodide and diluted sulphuric acid, the equation can be written as:

NaOCl (aq) + 2KI (aq) + H2SO4 (aq) → NaCl (aq) + I2 (aq) + K2SO4 (aq) + H2O (aq)

This reaction will result in pale yellow (the colour of iodine solution with colourless NaCl (aq) and K2SO4 (aq) ). Then, using sodium thiosulfate solution, iodine will change into iodide again, as the following equations (Lane, 2010b):

I2 + 2S2O32- → 2I- + S4O62-

2Na2S2O3 (aq) + I2 (aq) → Na2S4O6 (aq) + 2NaI (aq)

An indicator is needed to add into the solution when it appears pale yellow to detect whether iodine has all transferred into ions. Starch solution will appear black-blue when it meets iodine but it will remain unchanged when it meets iodide (Elmhurst College, 2003). Therefore, when all iodine has reacted with sodium thiosulfate, the solution which appears blue-black with starch solution inside should turn into colourless. This change of colour indicates that the equivalence point of the reaction has been reached.


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Test tubes, funnel, beakers, burette, conical flask, pipette, pipette filler.


Vinegar, bleach, sodium hydroxide (NaOH (aq)), phenolphthalein, sodium thiosulphate solution (Na2S2O3 (aq)), dilute sulphuric acid (H2SO4 (aq)), starch solution, potassium iodide (KI (aq)).

Indicator Experiment

2 cm3 NaOH was put into a test tube and 2 cm3 vinegar was put into another tube.

1 drop of phenolphthalein indicator solution was added into each tube.

Titration of vinegar

A funnel was put on the top of a burette and NaOH was poured into the funnel.

The NaOH bottle was placed under the burette and some NaOH was poured back into the bottle until the level could be read as 0 cm3.

Some vinegar was poured into a small beaker.

Using the 5 cm3 pipette, 2.5 cm3 vinegar was put into a conical flask.

20 cm3 water was poured into the flask.

4 drops of phenolphthalein indicator was added into the solution.

The flask was put under the burette with a piece of white paper under it.

The flask was shaken to mix the solutions as NaOH was added into the flask.

When the burette reading went down to 25 cm3, more NaOH was added into the burette until the reading reached 0 cm3.

More NaOH was added slowly after some colour change appeared and disappeared as the flask was shaken.

NaOH was added 1 drop at a time near the end point.

After 1 drop of NaOH that had caused a colour change of the solution last to more than 20 seconds, the final burette reading was recorded.

Titration of Bleach

1. Using a funnel, a burette was filled with sodium thiosulfate solution and adjusted to 0 cm3.

2. Using a pipette filler, a 2 cm3 pipette was filled with some bleach solution and 1 cm3 bleach solution was transferred to a conical flask.

3. 10 cm3 was added of potassium iodide solution and 10 cm3 of dilute sulphuric acid was added into the conical flask.

4. With a piece of white paper placed under the conical flask, the solution from the burette was added into the flask.

5. The flask was swirled continuously.

6. 6 drops of the starch indicator were added into the solution when the colour of the solution faded to pale yellow.

7. The thiosulfate solution was added dropwise while swirling.

8. When the colour of the solution changed from blue-black to colourless, the dropping of thiosulfate solution was stopped. The final burette reading was recorded.


Table 1 below shows the observation for the simple indicator experiment.



NaOH (aq) + Phenolphthalein Indicator

Colour changed from clear to pink.

Vinegar + Phenolphthalein Indicator

No colour change.

Table 1 Indicator Experiment

Table 2 shows the data and observation of the Titration of Vinegar.

Trial 1

Trial 2

Trial 3

Initial Burette Reading / cm3




Final Burette Reading / cm3




Volume of NaOH (aq) used / cm3





After approximately 30 cm3 had been added before any colour seen.

Then some pink appeared but turned back to colourless as the flask was shaken

Finally the entire solution changed to pink and remained.

Table 2 Titration of Vinegar

Table 3 below shows the data and observation of titration of bleach.

Trial 1

Trial 2

Initial Burette Reading / cm3



Final Burette Reading / cm3



Volume of Na2S2O3 (aq) used / cm3




The mixture of KI and bleach (NaOCl) was light yellow.

The solution turned into yellow brown when diluted H2SO4 was added.

The solution faded to pale yellow gradually as Na2S2O3 was added.

Blue black appeared as starch indicator was added.

With shaking and addition of Na2S2O3 (aq), the solution changed into colourless.

Table 3 Titration of Bleach


Calculation for Titration of Vinegar

Average Volume of NaOH (aq) = (43.0 + 40.8 + 40.5) / 3=41.43 cm3

NaOH (aq) + CH3COOH (aq) → CH3COONa (aq) + H2O (aq)

Concentration 0.1 mol dm-3 x mol dm-3

Number of mole 1 : 1

Volume 41.43 cm3 2.50 cm3

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x = (0.1 - 41.43 -10-3) / (2.5 -10-3)

≈ 1.66 mol dm-3

The concentration of vinegar is 1.66 mol dm-3.

Calculation for Titration of Bleach:

Average volume of Na2S2O3 (aq) = (4.0 + 6.4) / 2 = 5.2 cm3

NaOCl (aq) + 2KI (aq) + H2SO4 (aq) → NaCl (aq) + I2 (aq) + K2SO4 (aq) + H2O (aq)

Concentration x mol dm-3 1 mol dm-3

Volume 10 cm3

2Na2S2O3 (aq) + I2 (aq) → Na2S4O6 (aq) + 2NaI (aq)

Concentration 0.1 mol dm-3

Volume 5.2 cm3

n (Na2S2O3) : n (I2) = 2 : 1 , n (KI) : n (I2) = 2 : 1

n (Na2S2O3): n (KI) = 1 : 1, n (NaOCl) : n (KI) = 1 : 2

n (Na2S2O3) = n (KI) = c - v = 1-10-10-3 = 0.01 mol, n (NaOCl) = 0.005 mole

Concentration of NaOCl = n / v = 0.005 / (5.2-103) ≈ 0.96 mol dm-3

Serious errors during this experiment have affected the result.

The burette for both experiments was not checked for liquid leaks before the experiment. It is very likely that a certain amount of tested solution leaked out of the burette while reading. This leads to imprecise control of solution volume.

Equipments were not cleaned properly, including beakers, tubes, pipette and funnel. The impurity of equipments results in slower reaction rate and concentration difference.

Some equipment such as beaker was washed after first time used, there may have been water residue on them which led to change of concentration of solution.

The reading of burette was not done by one person. In terms of consistency, two different people reading the burette can result in different readings. Meanwhile, the method to read the burette was not correct every time, leading to parallax.

The judgement of end point colour might not be accurate due to the lack of colorimeter.

The rubber tube of burette might contain air bubbles inside during titration. Therefore, when titrating, the bubbles took up space but disappeared when the solution in the burette came down. The volume of these bubbles was counted as the volume of NaOH or NaOCl. Thus, the actually needed volume might be over estimated.

The rubber tube of burette was not good to control dropping of solutions. More than 1 drop was put in each time. Therefore, the end point of the titrations might actually need less solution to obtain.

While swirling the flask, there might be lose of solution occurring due to random error. The volume hence was affected.

The chemicals used in these experiments might not be freshly made. This results in impurity of chemicals, affecting the result of the experiments.

Measurement of chemical volumes varies, using measuring cylinders, pipettes, and beakers with scale on them. This will lead to

Improvement for future experiments should be made. First, check purity of chemicals and equipments. After washing equipments, use organic solvent and blow dry them. Be consistent for data measuring, which means that only one person should be in charge of data reading for one experiment. Meanwhile, be aware of the constants in experiments, use correct equipment for the level of accuracy. Follow procedures accurately, pay attention to the correct method for reading or measuring data.


In conclusion, the concentration of ethanoic acid in vinegar can be determined as 1.66 mol dm-3 using the equation:

NaOH (aq) + CH3COOH (aq) → CH3COONa (aq) + H2O (aq).

The concentration of sodium hypochlorite in bleach solution is approximately 0.96 mol dm-3, using these two equations:

NaOCl (aq) + 2KI (aq) + H2SO4 (aq) → NaCl (aq) + I2 (aq) + K2SO4 (aq) + H2O (aq)

2Na2S2O3 (aq) + I2 (aq) → Na2S4O6 (aq) + 2NaI (aq)