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Determination of Acetic Acid in Vinegar Investigatory Project

 

Investigatory Project on Determination of Acetic Acid in Vinegar

 

Introduction

 

Acetic acid is a weak acid. It is widely used in industrial chemistry as glacial acetic acid. In food industry, it is mainly used as vinegar. Vinegar is a dilute solution of glacial acetic acid. Commercial vinegar is prepared by fermentation and it contains 4 - 5% of acetic acid.

 

In this project, the amount of acetic acid present in commercial vinegar is determined by a simple volumetric method. Here we titrate vinegar, a weak acid, with a standard solution of a strong base such as sodium hydroxide. Sodium hydroxide is not a primary standard. Therefore it should be standardised using a suitable primary standard such as oxalic acid using phenolphthalein indicator.

 

According to the theory of acid-base titrations, the end point in the titration of vinegar with sodium hydroxide will be observed between pH 8 and 10. Therefore, in this titration also, phenolphthalein is a siutable indicator. The reaction between vinegar and sodium hydroxide is given by,

 

CH3 - COOH + NaOH —> CH3COONa + H2O

 

By knowing the volume of vinegar solution reacting with a definite volume of NaOH solution, the strength of vinegar solution can be calculated.

 

Aim of the Investigatory Project

 

The aim of the project is to estimate the quantity of acetic acid present in commercial vinegar sample.

 

Apparatus and Chemicals Required

 

Burette and burette stand, Conical flasks, Pipette, Standard flask, Funnel, Weighing bottle, 0.1N NaOH solution (dissolving 4g NaOH in one litre of pure water), Phenolphthalein indicator (dissolving 0.4g phenolphthalein in 500 millilitre of ethanol and 500 millilitre of H2O by continuously stirring), Vinegar sample and Oxalic acid (AR)

 

Procedure

(i) Preparation of standard oxalic acid solution

 

Carefully weigh the bottle with around 0.63g of oxalic acid crystals taken in a chemical balance. After that transfer it into a funnel fixed over a 100 millilitre standard flask. Find out the weight of the bottle once more and check the accurate mass of oxalic acid moved by considering the variation amid the two weights. Then the oxalic acid must be washed down into the standard flask by some water (approximately 20 millilitre). Then rotate the flask till the oxalic acid is dissolved. The funnel and the stem are then washed down into the standard flask and then the funnel must be removed. Then solution should be make up to the mark and shake it to acquire a uniform concentration.

 

(ii) Standardisation of NaOH solution

 

Wash the burette with a few drops of oxalic acid and after that fill the acid until zero mark. Then pipette out 20 millilitre of standard NaOH solution in a 250 millilitre conical flask. Mix two or three drops of phenolphthalein indicator to produce a pink colour for the solution. Titrate the obtained solution against oxalic acid from the burette until the pink colour gone. Observe the burette reading and do again the titration to obtain concordant titre values.

 

(iii) Detemination of acetic acid in vinegar

 

Pipette out the 20 millilitre of comm. vinegar in a 100 millilitre standard flask. Fill the solution up to the mark of the flask with distilled water. Wash a fresh burette with some solution and fill it until zero mark with the same solution. Then pipette out 20 millilitre of NaOH solution in a 250 millilitre conical flask and put two or three drops of phenolphthalein indicator in the solution. The obtained solution is then titrated alongside vinegar solution from the burette until the pink colour gone. Observe the burette reading and do again the titration to obtain concordant titre values.

 

Observations

 

Mass of bottle + Oxalic acid crystals = m1 g

Mass of bottle after the transfer of salt = m2 g

Then, Mass of Oxalic acid transferred = m1 – m2 = m g

 

Oxalic acid Vs Sodium hydroxide solution – Phenolphthalein indicator


No

Volume of NaOH (mL)

Initial burette reading (mL)

Final burette reading (mL)

Volume of Oxalic acid

1

20

 

 

 

2

20

 

 

 

3

20

 

 

 

 

Vinegar solution Vs Sodium hydroxide solution – Phenolphthalein indicator


No

Volume of NaOH (mL)

Initial burette reading (mL)

Final burette reading (mL)

Volume of Vinegar Soln

1

20

 

 

 

2

20

 

 

 

3

20

 

 

 

 

Calculations

 

(i) Estimation of strength of NaOH soln

 

Equivalent Mass of oxalic acid = 126/2 = 63

Mass of of oxalic acid put in the 100 mL soln = m g

Normality of oxalic acid soln, N1 = (m x 1000) / (63 x 100) = _______ N

Volume of Sodium Hydroxide soln (V2) = 20 mL

Volume of oxalic acid needed to react with 20 mL Sodium Hydroxide soln = V1 mL

We know that, Normality of Sodium Hydroxide soln is N2

Therefore, N1V1 = N2V2

i.e., N2 = _______ N

 

(ii) Determination of strength of Vinegar soln

 

Normality of Sodium Hydroxide soln, N2 = _______ N

Volume of Sodium Hydroxide soln, V2 = 20 mL

Volume of vinegar soln reacting with 20 mL Sodium Hydroxide soln = V3 mL

We know that normality of vinegar soln, N3,

N2V2 = N3V3

N3 = ______ N

Here 20 mL of vinegar is diluted to 100 mL, so the normality of comm. vinegar = N3 x (100/20) = ______ N

Equivalent mass of acetic acid = 60

i.e., Strength of comm. vinegar = 5N3 x 60 = ______ gL-1

 

Result

 

Normality of vinegar soln = ______

Normality of comm. vinegar = _____

Strength of comm. Vinegar = _____

 

Viva Questions and Answers

 

1. What is vinegar?

Ans: Vinegar is dilute solution of acetic acid

 

2. How much acetic acid is normally present in vinegar?

Ans: Vinegar contains 4 - 5% acetic acid.

 

3. Why is acetic acid a weak acid ?

Ans: It is incompletely ionised in solution. It has low Ka (1.8 x 10-5).

 

4. What is a standard solution ? What is the standard solution used in this experiment ?

Ans: A standard solution is one whose normality is known. In this experiment crystalline oxalic acid is used to make standard solution.

 

5. What is the amount of crystalline oxalic acid used to make 100 millilitre of 0.1N oxalic acid solution? 

Ans: Equivalent mass of oxalic acid is 63. Therefore 0.63 g oxalic acid in 100 millilitre solution forms 0.1N solution.

 

6. How will you find the equivalent mass of oxalic acid ?

Ans: Molecular mass of crystalline oxalic acid (H2C2O4 . 2H20) is 126. Its basicity is two.

Therefore equivalent mass = molar mass/basicity = 126/2 = 63

 

7. Indicate the reaction between oxalic acid and NaOH ?

Ans: Neutralisation.

H2C2O4 + 2NaOH  Na2C2O4 + 2H2O

 

8. Why do you choose phenolphthalein as indicator as the titration of vinegar against NaOH ?

Ans: It is a weak acid - strong base reaction for which the end point appears between pH 8 and 10. So phenolphthalein is suitable.

 

9. Why can't you use methyl orange as indicator in this experiment?

Ans: Methyl orange is suitable for weak base-strong acid titration. The pH range of methyl orange is from 3 to 4.

 

10. Why can't you prepare a standard solution of NaOH directly to titrate against vinegar solution?

Ans: Sodium hydroxide is not a primary standard. It is not available in the pure state. So to standardise it we need a primary standard (oxalic acid).

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