rebekah livesley, 2008 analysis of vitamin c in fruit juices instrument metrohm polarograph
TRANSCRIPT
Rebekah Livesley, 2008
Analysis of Vitamin C in Fruit Juices
InstrumentMetrohm Polarograph
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
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
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
Fruit JuicesExpected concentrations of Vitamin C in Fruit Juices
0
100
200
300
400
500
600
700
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
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
Metrohm Polarograph
Photographed by Shay
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
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%
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 -
Com parisson of Results to the concentration of vitam in c specified in the Nutritional Inform ation
0
100
200
300
400
500
600
700
800
900
Co
nc
en
tra
tio
n m
g/L
average concentration mg/L nutritional information concentration mg/L
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
0
1000
2000
3000
4000
0 20 40 60 80 100 120
Concentration (mg/L)
Pea
k H
eig
ht
(nA
)
To Show Rate of Oxidation of 20 times diluted westcliff apple juice samples
185
190
195
200
205
210
0 10 20 30 40 50 60 70
Time (minutes)
Con
cent
ratio
n (m
g/L)
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)
Choice of method
normal calibration simpler for sample delivery (no pipetting required) more consistent results better RC values
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
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
Polarography
Disadvantages
Not time efficient
Must use a buffer solution
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