To fond out how much vitamin C in orange and lemon juice

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To fond out how much
vitamin C in orange and
lemon juice
Objective

To find out how much
vitamin C in orange and
lemon juice
Theory(1)

• Vitamin C is ascorbic acid. This is rapidly
and quantitatively oxidized by iodine in an
acid medium according to the following
equation.
Theory(2)

• The standard method for determining ascorbic
acid prevent in a sample is to titrate against a
standard iodine solution. But the low solubility of
the iodine makes this procedure less than ideal.
• In the experiment, a known excess of iodine is
created in site by the reaction between an iodide
and an iodate and those not reacting with the
ascorbic acid is titrated against standard sodium
thiosulphate solution.
Theory(3)

• During the experiment, there are 2 more
equations are involved,
• 1. I2+2S2O322I-+S4O62-
• 2.6H++5I-+IO3-3I2+3H2O
•   Washing bottle        •   Stand and clamp
•   Squeezer              •   Dropper
•   2*25.00cm3pipette     •   1*breaker
•   4*conical flash       •   Test tubes
•   250.00cm3volumetric   •   Test tube holder
•   Orange juice
•   Lemon juice
•   0.5M sulphuric acid
•   Starch solution
•   Sodium thiosulphate
Procedure
•   A)     Prepare the iodate solution
•   1. Accurately weigh out 0.62g of potassium iodate. Dissolve in distilled water and make up to
the mark in the 250cm3 volumetric flask.
•
•   B) Prepare the fruit solution
•   1. Weighed the mass of lemon/ orange
•   2. Extract all the juice out of the lemon. Orange.
•   3. Mix the juice with 150cm3 0.5M sulphuric acid in a breaker.
•   4. Transfer this solution into a 250cm3 volumetric flask and make up to the mark with
distilled water.
•   C) Mix the iodate solution and fruit solution
•   1. Pipette 25.00cm3 lemon/ orange solution into a clean conical flask and add 5cm3 1M KI
solution.
•   2. Pipette 25.00cm3 iodate solution the flask containing the orange/ lemon solution and KI
solution
•   D) Back titration
•   1. Back titrate the excess iodine against the standardized sodium thiosulphate solution.
•
Calculation
No of mole of (KIO3) in 250.00cm 3 =mass/molar
=0.62/39.1+126.9+16*3 =2.897*0.002897mol

No of mole (KIO3) in 25.00cm 3=0.002897*10
=0.0002897mol

No of mole of KI in 5.00cm 3 =molarity*volume
= 1*(5/1000) =0.005mol
6H+ + IO3- + 5I- 3I2+3H2O

No of mole of I2 form from 25cm3 iodine solution
=no of mole of KIO3*3
=0.0002897*3 = 0.0008692mol
A)Find in the percentage by mass of vitamin C in lemon
No of mole of S2 O3 2-use in back titration
=molarity*volume
=0.1(17.195/1000) =0.0017195mol

I2+2S2 O32-2I-+S 4O6 2-
Iodine remain after reaction of 25cm3 iodine and ascorbic acid
=no of mole of S2 O32 /2
=0.0017195/2 = 0.00085975mol
No of mole of iodine reaction with ascorbic acid in 25cm3 Lemon Solution
=0.0008692 - 0.00085975
=0.00000945mol

No of mole of ascorbic acid in 25.00cm 3 lemon solution
=no of mole of iodine =0.00000945mol
No of mole of ascorbic acid in 250.00cm 3 lemon solution
=0.00000945*10 =0.0000945mol

Mass of ascorbic acid C6H8O6
=no of mole * molar mass
=0.0000945*(12*6+1*8+16*6) =0.016632g

Percentage by mass of vitamin C in lemon
=0.016632/115.62*100% =0.012%
B) Find the percentage by mass of vitamin C in orange:

No of mole of 2S2 O32- use in back titration
=0.1(17.3/1000) =0.00173mol

No of mole of I2 remain after reaction of iodine and ascorbic acid
=0.00173/2 =0.000865mol

No of mole of I2 reaction with ascorbic acid in 25cm3 orange solution
=(0.0008692-0.000865)mol =0.0000042mol

No of mole of ascorbic acid in 25.00cm3 orange solution
=o.ooooo42mol
No of mole of ascorbic in 250.00cm 3 orange solution
=0.0000042*10 =0.000042mol
Mass of ascorbic acid C6H8O6
=0.000042*(12*6+1*8+16*6) +0.007392g

Percentage by mass of vitamin C in orange
=0.007392/120.9*100% =0.006114%
Percentage of vitamin C in an orange is
0.00614% and the percentage of vitamin C
in a lemon is 0.014%.
Thus, orange has lower percentage of vitamin
C than lemon.
• During the “iodometric titration”, starch solution is the
most commonly used indicator to indicate the end point at
which iodine in solution has just reacted completely with
thiosulphate solution added. Since starch solution forms a
dark blue colour indicates that is no iodine in the reaction
mixture. This signals the end point of the titration.
• From the calculation in front, we found out that orange
juice contain less vitamin C than lemon juice. This means
that if we want to get more vitamin C(ascorbic acid), we
can prefer to drink lemon juice.
• Compare the percentage of vitamin C contained in fruits
with those in vitamin C tablet(what I did last time), its
shows that vitamin C tablets contain more ascorbic acid
than natural fruits. It may right because tablets is artificial,
they can add how much ascorbic acid they like in the tablet.
• The titration must be carried out immediately after the
addition of sulphuric acid, it is because the solution is
easily be oxidized by sulphuric acid. If the solution is
oxidized then the result is not accurate any more.
• Titration involve.KI and KIO3 which produce iodine which
is mild oxidized agent are often called “iodemetric
titration”. On the other hand, the titration is also “back
titration” in order to find the mass of ascorbic acid in
vitamin C tablets.
• Squeezer may not squeeze enough fruit juice from the fruit,
then there may be some vitamin C still left on the fruit but
not in the juice. Then the calculation of vitamin C left in
juice may not accurate. It is better for us to extract as much
as fruit juice form the squeezer as possible in order to
accurate the result.
• Fruit, KIO, in the powder form and it is needed to mix with
water to form mixture in volumetric flask of 250.00cm3. It
must be stir the powder thoroughly in the water. Before
this mixture mix well with lemon /orange solution and KI
solution, we should stopper the volumetric flask of water
and KIO3. Also stopper another the volumetric flask of
mixture of lemon/ orange solution and iodate solution and
KI. Then shake them up and down in order to make sure to
make the mixture uniformly. The the result is more
accurate.

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 views: 1315 posted: 4/12/2010 language: English pages: 18