Download - Rebekah Livesley, 2008 Analysis of Vitamin C in Fruit Juices Instrument Metrohm Polarograph
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Rebekah Livesley, 2008
Analysis of Vitamin C in Fruit Juices
InstrumentMetrohm Polarograph
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Vitamin C (Ascorbic Acid)
formula weight 176.13g
Water Soluble
melting point 190-192° C,
unstable in solutions containing oxygen as it is readily oxidized
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Why is Vitamin C important? It is involved in over 300 biological processes
in the body for example Antioxidant Collagen manufacture & neurotransmitters Immune function
It is a Nutrient not synthesized by the body Complete deficiency results in scurvy Deficiency symptoms
Bleeding gums Joint pain Bruising & wounds that don’t heal Fatigue & muscular weaknesses
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Recommended Daily intake is 30-60mg
The RDI is what you need to consume to prevent a deficiency
100ml (which is less than ½ a cup) of the Popper Juice orange Juice is all you would need to consume
excess vitamin C is usually eliminated through the urine
An excess of Vitamin C can cause diarrhea as water is pulled into the colon from the circulatory system
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Fruit JuicesExpected concentrations of Vitamin C in Fruit Juices
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McCance andWiddoson’s TheComposition of
Foods (7)
Berlitz, Grosh,Scheiberle Food
Chemistry (8)
StaplesDrinks (9)
Exp
ecte
d c
on
cen
trat
ion
Apple Juice
Orange Juice
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Samples
Popper Juice Apple and Blackcurrant
Popper Juice Orange Juice
Popper Juice Apple Juice
Popper Juice Apple And Mango Juice
WESTCLIFF Apple Juice
And V8 Vegetable Juice
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Metrohm Polarograph
Photographed by Shay
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ProceduresDifferential Pulse Polarography
Standard Addition Method Dilute sample accurately to approximate concentration of
40mg/L
Place in sample cell with water and buffer solution
Run scan -0.4 to +0.1 V
3 standard additions
Normal Calibration Method Prepare 0, 5, 10, 25mg/L standard solutions in buffer
solution Dilute sample accurately to an approximate concentration
of 10mg/L in buffer Runs scans as above
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Differential Pulse Polarography
Standard Addition Results
Sample WESTCLIFF Apple Juice
200mg/L standard Solution of Ascorbic acid
Average concentration
263 mg/L 235 mg/L
Recovery Check 174.1 % recovery
Standard Deviation 68% 51%
Relative Error 17%
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Differential Pulse Polarography
Normal Calibration Results
Sample Average concentration
(mg/L)
Recovery Check
(% recovery)
Standard Deviation
WESTCLIFF Apple Juice 273 117 6%
JUST JUICE Apple Juice 174 91 -
POPPER JUICE – Orange Juice 811 108 -
POPPER JUICE – Apple and Blackcurrant Juice
633 - -
POPPER JUICE – Apple and Mango Juice
731 - -
POPPER JUICE – Apple Juice 662 - -
V8 – Vegetable Juice 332 155 -
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Com parisson of Results to the concentration of vitam in c specified in the Nutritional Inform ation
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Co
nc
en
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tio
n m
g/L
average concentration mg/L nutritional information concentration mg/L
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Concentration Range of Vitamin C
To Find the Concentration Range for the Metrohm Polarograph
(values from 0.1 - 100mg/L)
y = 29.61x - 66.298
R2 = 0.9865
-1000
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Concentration (mg/L)
Pea
k H
eig
ht
(nA
)
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To Show Rate of Oxidation of 20 times diluted westcliff apple juice samples
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0 10 20 30 40 50 60 70
Time (minutes)
Con
cent
ratio
n (m
g/L)
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Chart of the concentration of WESTCLIFF apple juice in an open popper as a funtion of time
266268270272274276278280
0 20 40 60 80 100 120
time (minutes)
Conc
entra
tion
(mg/
L)
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Choice of method
normal calibration simpler for sample delivery (no pipetting required) more consistent results better RC values
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What went wrong
Standard Addition Method
Lack of precision in the results obtained Caused by
Micropipettes Human error 3 aliquots to be transferred to sample cell per run
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What went wrong
Normal Calibration Method An aliquot of the V8 vegetable juice could not be taken by
pipette as the juice was so viscous Had to weigh a mass and do a density calculation before
diluting the juice JUST JUICE – Apple Juice concentration was outside the
concentration range of the standards used in the calibration graph
Easily fixed – just recalibrate
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Polarography
Disadvantages
Not time efficient
Must use a buffer solution
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Conclusion The normal Calibration polarographic method for the
analysis of vitamin C is better than the standard addition method
All sample analyzed had considerably more vitamin C than specified on the packet except Just Juice apple juice which had about a 1/5