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