Analysis Of Determining The Concentration Of Citric Acid

Determining the Concentration of Citric Acid

Introduction

Titrations are performed to calculate the unknown concentration of solutions using standard solutions. A solution of known concentration and volume is added to a solution of known volume and unknown concentration, a burette is used to find the exact amount of the known solution is required for the reaction to come to completion. A pH indicator is used to determine when a reaction has completed.

Aim

To determine the concentration of Citric acid in both fresh and bottled lemon juice.

Materials

Burette Pipette Retort stand Clamp Funnel Beakers Conical Flask White tile Vacuum pump Buchner Flask Distilled water Universal Indicator Phenolphthalein Hydrochloric acid solution

HCl

C=1M n=CV

V=0.01887L n=(1)(0.01887) n= ?moles n=0.01887 moles

The ratio of HCl to NaOH is 1:1 so they have the same number of moles

NaOH

C= ?M C=n/V

V=0.02L C=0.02/0.01887 n=0.01887 moles C=1.06M

Bottled Lemon Juice + Sodium Hydroxide Volume (mL) Titre Volume Initial Final mL L

Rough 0.0 3.9 3.9 0.0039

Titration 1 3.9 7.6 3.7 0.0037

Titration 2 7.6 13.7 6.1 0.0061

Titration 3 13.7 19.1 5.4 0.0054

Average in Litres (not including rough) 0.00567 L

C6H8O7 +3NaOH  3H2O + Na3C6H5O7

3NaOH

C=1.06M n=CV

V=0.02L n=(1.06)(0.02) n= ?moles n=0.0212 moles

The ratio of sodium hydroxide to citric acid is 3:1 so the number of moles in citric acid is:

0.0212÷3=7.067×〖10〗^(-3)

C6H8O7

C=?M C=n/V

V=0.00567L C=(7.067×〖10〗^(-3))/0.00567 n=7.067×〖10〗^(-3) moles C=1.25M Fresh Lemon Juice + Sodium Hydroxide Volume (mL) Titre Volume Initial Final mL L

Rough

Titration 1 0.0 5.5 5.5 0.0055

Titration 2 4.9 0.0049

Titration 3

Average in Litres (not including rough) 0.0052 L

C6H8O7 +3NaOH  3H2O + Na3C6H5O7

3NaOH

C=1.06M n=CV

V=0.02L n=(1.06)(0.02) n= ?moles n=0.0212 moles

The ratio of sodium hydroxide to citric acid is 3:1 so the number of moles in citric acid is:

0.0212÷3=7.067×〖10〗^(-3)

C6H8O7

C=?M

When the titration was performed to find the concentration of the sodium hydroxide solution, phenolphthalein was used to determine when the pH reached 7. The disadvantage of using phenolphthalein is that it changes from pink to transparent when the solution has a pH of less than 8, meaning that most of the time the point at which the colour change occurs is not the same as the point at which the solution has become neutral.

Whilst both of the calculated values for the concentration of the lemon juices are quite far from the actual values for the concentration, there are a number of errors that may have occurred while performing the titrations that may have led to these inaccuracies. For the bottled lemon juice titration, universal indicator was used to measure the pH and to determine when the solution had reacted completely and reached a pH of 7, indicating neutrality. This indicator is good to use because the colour change is obvious as it changes from purple to green. The downside and the factor that may have led to errors is that it is best used in solutions that are transparent. As the solution being added to the sodium hydroxide solution was not a transparent solution, the yellow tinge in the lemon juice made it difficult to tell at what point the solution had reacted completely and turned green. This may have led to more or less acid