Analysis of
Vitamin C in Fruit
Juices
Instrument
Metrohm Polarograph
Rebekah Livesley, 2008
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 Juices
Expected concentration
Expected concentrations of Vitamin C in Fruit Juices
700
600
500
400
Apple Juice
300
Orange Juice
200
100
0
McCance and
Widdoson’s The
Composition of
Foods (7)
Berlitz, Grosh,
Scheiberle Food
Chemistry (8)
StaplesDrinks (9)
Samples
Popper Juice
Apple and
Blackcurrant
Popper Juice
Orange Juice
Popper Juice
Apple Juice
And V8 Vegetable Juice
Popper Juice
Apple And
Mango Juice
WESTCLIFF
Apple Juice
Metrohm Polarograph
Photographed by Shay
Procedures
Differential 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%
Relative Error
51%
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
-
Concentration Range of Vitamin C
To Find the Concentration Range for the Metrohm
Polarograph
(values from 0.1 - 100mg/L)
Peak Height (nA)
4000
y = 29.61x - 66.298
R2 = 0.9865
3000
2000
1000
0
-1000 0
20
40
60
80
Concentration (mg/L)
100
120
Concentration (mg/L)
To Show Rate of Oxidation of 20 times diluted westcliff
apple juice samples
210
205
200
195
190
185
0
10
20
30
40
Time (minutes)
50
60
70
Concentration
(mg/L)
Chart of the concentration of WESTCLIFF apple juice in an
open popper as a funtion of time
280
278
276
274
272
270
268
266
0
20
40
60
time (minutes)
80
100
120
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