cmy 383 exp 5 report

8/12/2019 CMY 383 Exp 5 Report

1/13

CMY 383

EXPERIMENT 5: Direct

Determination of Ascorbic

Acid in a Commercial Fruit

Juice

ALISSA KRIEL

11123002

BSc Physics

Practical Initiated on 28

February 2013


2/13

The use of Analytical Voltammetry to determine the

Vitamin C content of a Commercial Fruit Juice by

means of Differential Pulse Polarography in

Conjunction with Standard addition

Abstract:

In this quantitative polarographic chemical analysis, the content of Vitamin C within a

commercial juice samplewhich is claimed to contain 48mg/100mLis investigated

by means of both Calibration Plot Differential Pulse Polarography and Standard

addition Differential Pulse Polarography. The result from the calibration curvemethod correlated well with the 48mg/100mL claim by yielding a Vitamin C

concentration of 46mg/100mL , whilst the standard addition curve was not entirely

linear, although it yielded a concentration of 39mg/100mL.

Furthermore the instrument parameters for a DPP and DCTASTwere investigated by

considering the effects they had upon a recorded Polarogram of the same solution.

Method:

The preparation of the cell solution and instrument settings as described in the

CMY 383 laboratory manual was followed without significant deviation.

The Instrument settings were as follows, unless stated otherwise as from

figure 6- figure 9And then only singular changes at a time were made.


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Results:

1. Firstly, DP Polarograms were recorded for a 0.020 L background electrolyte

(0.1M CH3COO- buffer) and consecutive 50l standard 1g/L ascorbic acid

additions up to a total of 200l Standard AA solution present.

These polarograms are represented on a single graph in Figure 1:

Except for the slight unexpected pre-peak response on the green (DP Polarogram

after 100l std Vit C addition), the Polarograms all peak at the same Potential :

Setting the appropriate Peak Potential Ep.

From these peak heights, a calibration curve was constructed ply plotting the various

consecutive standard solution concentrations versus their recorded Peak heights (Ip):


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Calculation of Vit C content in 200L juice using recorded Ip which is gained further

on in figure 3, (Ip= 1.97 x10-7A),

=4.623 ppm

2. Secondly a standard addition techniquewhere 3 consecutive 50L 1g/L Vit

C soln was added to 200L of Juice sample within the 0.02L BE solution

yielded the following DPP Polarograms in Figure 3:


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Once again, the consecutive peaks all share the same Peak Potential, Ep=0.0682V.

BE was very successfully purged as continuum is exceptionally flat.

From This data a Standard Addition Curve was generated by Matching concentration

to Peak Height:


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3. Table 1: Data obtained from the Calibration plot polarogram collection

Volume Std

Vit C soln

Added (L)

*and 0.02L

BE

Concentration

of Vit C in

Polarographic

Cell (ppm)

Peak

Potential

Ep(V)

Peak Height

Ip(A)

Area under

Peak

50 2.49 0.0642 9.48 x 10-8 5.02 x 10-9

100 4.98 0.0642 2.06 x 10-7 1.07 x 10-8

150 7.44 0.0642 3.17 x 10-7 1.67 x 10-8

200 9.90 0.0679 4.28x 10-7 2.25 x 10-8

200l Juice

(No Std soln) 4.62 0.0682 1.97 x 10-7 9.99 x 10-9


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b) Since the Vit C concentration in the 200L juice sample was interpolated as 4.623

ppm = 4.623 mg/L (also by substitution into the linear trend line equation) ,

It follows: The Vit C Concentration per 100mL of Juice is

4.623 mg/L /0.1mL

=46.2 mg/100mL of Juice

4. Table 2: Data obtained from standard addition polarograms

Volume of soln

added (L)

*0.02L BE soln

Concentration

of Vit C in

Polarographic

Cell (ppm)

FROMSTANDARDS

Peak Potential

Ep(V)

Peak Height

Ip(A)

200L Juice

Solution0 0.0682 1.97 X 10-7

Juice soln + 50L

Std 0.000.0682 3.4 X 10-7

Juice soln +

100L Std 2.470.0682 4.47 X 10-7

Juice soln + 150

Std 4.950.0682 5.58 X 10-7

From figure 4 it is clear by the equation of the trendline fitted onto the standard

addition data points that the Vit C concentration in the sample (200L) will equal to

3.93 ppm = 3.93 mg/L.


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Hence, the Vit C concentration per 100mL of Juice is found to be

39.3 mg/100mL.

*Please see figure 4 where the graphical determination was used to mark linear

trend line intercept with the x-axis which marks the sample concentration.

Investigation of instrumental parameters on recorded Polarograms:

Following are 5 figures wherein one specific parameter was changed (depending on

whether a DCTASTPolarogram was to be recorded or a DP Polarogram).

An analyses of what effect takes place and why follows each.

0.00E+00

5.00E-08

1.00E-07

1.50E-07

2.00E-07

2.50E-07

3.00E-07

3.50E-07

4.00E-07

-0.2 -0.1 0 0.1 0.2 0.3

Current(A

)

U(V)

Figure 5: DCTAST wave Polarograms of a

solution using different Voltage Step Times

DCTAST Polarogram obtained

using 0.8 s Voltage Step time for

200ul Juice Solution + 150ul Std

Vit C Soln

DCTAST Polarogram of Same So lnwith Voltage step time = 1.5s


9/13

There is a slight increase in the Height (Id) as the Voltage step time has been

increased. This may have been caused by the fact that voltage steps are now made

farther apart, and since the halfwave potential is also marked a bit farther toward the

positive potentials, it reinforces that the change in step time caused the DC wave to

reach its equivalent points later on.

0.00E+00

5.00E-08

1.00E-07

1.50E-07

2.00E-07

2.50E-07

3.00E-07

3.50E-07

4.00E-07

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

Figure 6: DCTAST wave Polarograms with a

change in Voltage Step Value

DCTAST Polarogram obtainedusing juice + Std solution Voltage

step = 0.004V

DCTAST Polarogram obtained

using same Soln Voltage

Step=0.012V

There is a slight decrease in height when the Voltage step was increased by a factor

2. The diffusion current may not even have been affected by the Voltage step being

altered since the change seems relatively small and the two Polarograms seem

indistinguishable.


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There is a significant decrease in peak height as the Pulse amplitude is halved. In

fact, the height (Ip) seems to also have halved, creating the possibility that the

resultant peak height is directly proportional to the Pulse amplitude implemented.


11/13

The aforementioned direct proportionality between peak height and applied Pulse

amplitude does not seem to carry through successfully here since the increase in

peak height (Ip) is not the expected value exactly, although there is still the general

trend that with larger Pulse amplitudes, larger peak heights are recorded.


12/13

When the pulse time is increased, the peak height is decreased by an amount. This

may be due to the fact that pulse times are now extended and the pulse applied over

a longer period of time. Thus the amplitude will not reach as high a value as before

thus not such high peaksas the measurement is elapsed/completed.


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Discuss ion

The Juice Producer claims that 48 mg/100mL of Vitiman C is present in the juice.

The results obtained here, both by calibration plot method of DP Polarography and a

standard addition method, are somewhat lowerone being ~46mg/100mL and the

standard addition method yielding ~39 mg/100mL . Firstly, we note that both

methods also yielded significantly different results.

It is also considered that in the Standard addition technique, we assume that the

response is linear (Skoog et al, 2004, 210). The trendline shows some deviation and

was also set to intersect the y-axis at the Ip(Current peak) of the 200L Juice

sample. This affects the credibility of the ultimate intercept with the x-axis- and hence

the obtained concentration of Vitamin C in the juice sample.

If the Calibration curvewhich was obtained as inherently linear, is to be considered

as more credible, then the difference between obtained concentration and

manufacturers claimed concentration is minimal. 2mg/100mL.

This also reinforces the ability of the Polarogrophy analytical method to accurately

quantitatively analyse the organic content within a sample matrix.

References:

Skoog et al, 2004, Fundamentals of Analytical Chemistry, 8thEdition,

Brooks/Cole, 210-212

Kealey D. And Haines P.J. , 2005, Instant Notes in Analytical Chemistry,2nd

Edition , BIOS Scientific Publishers Ltd., 46.

J. N. Miller and J. C. Miller, Statistics and chemometrics for analytical

chemistry,2000, 4th Edition, Prentice Hall

cmy 383 exp 5 report

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