Citric Acid in fruit juice compared to fresh juice
Being a citrus fruit, lemon and its juice contain citric acid as by far the dominant acid. Indeed, it is this
acid that provides much of the flavour that we recognise as being distinctly lemon, and the citric acid
content of a particular lemon juice can have a great impact on how that juice tastes. The citric acid
content of a lemon juice can be determined through a titration of the juice. This process involves a certain
volume of juice having a base of a known concentration added to it until a point is reached where all of
the acid in the juice is neutralised (the endpoint).
Titration with phenolphthalein indicator and titration curve
From the results of the titration, the amount of citric acid in the juice can be determined.
The citric acid content of lemon juice varies between lemons, with a typical concentration being about
5% by mass. Most bottled lemon juice companies standardise the citric acid content of their juice.
Aim
To volumetrically analyse both freshly squeezed lemon juice and bottled lemon juice.
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Apparatus and materials
Lemons, Lemon squeezer, Bottled lemon juice, Straining gauze.
Sodium hydroxide of approximately 1 mol./L.
Phenolphthalein indicator.
Distilled Water.
Retort stand and clamp.
Burette.
Volumetric flask.
250 mL conical flask, 25 mL pipette & Glass funnel.
1
1.
2.
3.
4.
5.
6.
7.
8.
Method
Titration of freshly squeezed lemon juice with sodium hydroxide (using phenolphthalein indicator)
Squeeze the lemons and strain the lemon juice to remove any pulp.
Measure exactly 25 mL of lemon juice into the 250 mL conical flask using the pipette.
Add about 75 mL of distilled water to the lemon juice.
Add 5 drops of phenolphthalein indicator to the lemon juice and distilled water (clear).
Fill the burette to the top calibration mark with the sodium hydroxide solution.
Add the sodium hydroxide solution to lemon juice until the phenolphthalein indicator turns pink.
Record volume added.
Repeat 2 times
Conduct steps 1 to 7 with the bottled lemon juice
Results
Sample
1
Lemon juice 1 m NaOH added Bottled Lemon Juice 1M NaOH added
mls.
mls.
2
mls.
mls.
3
mls.
mls.
Calculations
C6H8O7
Citric acid
+
3NaOH
Sodium hydroxide
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Conclusion
2
Na3C6H5O7 +
Sodium citrate
3H2O
Water