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ib chemistry hl notes
IB CHEMISTRY HL

Rancidity of Polyunsaturated Oils

UPDATED ON - 30 APR 2020
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Investigating the Oxidative Rancidity of Polyunsaturated Oils

Table of content:

 

Research question

Prediction

Hypothesis

Procedure

Safety Precautions

Data Collection and Processing

Conclusion and Evaluation

 

 

Research Question

How do different storage temperatures (-20.0, 4.0, 25.0, 40.0, 60.0 ºC) of sunflower oil affect the relative concentration of hydroperoxides produced as a result of its oxidative rancidity, as determined by the absorbance of red (635 nm) light of the oxidized oil after it is reacted with potassium dichromate (IV) (K2Cr2O7), and consequently its rate  (hr-1) of oxidation? 1

 

Comment: Very well-focused research question.

 

Prediction
If the storage temperature (ºC) of sunflower oil increases, then the rate (time-1) of oxidation  will increase with a slope of 1.

 

Comment: This is a bit sudden. Not well sequenced.

 

Figure 1: Predicted Relationship Between Temperature (ºC) and Rate (hr-1)

 

Comment: These are not the units of rate. This has become a very common point of confusion.

 

The above graph represents a prediction of the relationship between the storage temperature of sunflower oil and the rate of oxidation. The predicted line of best fit has a slope of 1.

 

 

1 Due to the lengthy nature of this experiment, the SI unit of seconds (s) has been replaced with the larger unit,
hours (hr) for ease of calculation.

 

Hypothesis

Sunflower oil is a polyunsaturated oil compressed from sunflower (Helianthus Annuus) seeds. Like most polyunsaturated fats, it is most commonly a triglyceride composed of glycerol and three unsaturated fatty acid chains. An example of a sunflower oil triglyceride is shown below. (Thomas, 2002)

 

Figure 2: A sample sunflower oil triglyceride with 2 linoleic acids (doubly unsaturated) fatty acid chains and one oleic (monounsaturated) fatty acid chain. 

                                                (Smokefoot, 2011)
The term “unsaturated” refers to the presence of one or more carbon-carbon double bonds in the fatty acid hydrocarbon chain. Triglycerides with these unsaturated fatty acids undergo oxidative rancidity, a process that follows a free radical reaction mechanism.
Free radicals are formed by homolytic fission. In the context of fatty acids, the C-H bond in the fatty acid chain undergoes homolytic fission to form two free radicals:

RH ⇒ R. + H . This step is called initiation.2

Next, the radicals undergo a free radical chain reaction, in which O2 from the air is added to the fatty acid-free radical (R•) to form ROO•. ROO• then reacts with a fatty acid molecule to form R• and a hydroperoxide (ROOH). 

            R. + O2 ⇒ ROO.

            ROO. + RH ⇒ ROOH + R.

In the final step, termination, the ROOH hydroperoxide decomposes into RO• + OH•.

 

Comment: This is not the termination step.

 

These, along with the other free radicals present, react to form non-radical products, including aldehydes, ketones, alcohols, esters, and alkanes. The products are collectively referred to as hydroperoxides. (Derry, 2009)

 

Comment: These are not called hydroperoxides.



2 The symbol “•” is used to denote a free radical. “R” represents the part of the hydrocarbon chain attached to the H

 

 The first step of this reaction, initiation, requires energy to begin. Because of this, heat can act as a catalyst for the reaction. (Derry, 2009) In order to test for the amount of oxidation in a sample of sunflower oil, the number of hydroperoxides produced can be found. Hydroperoxides, particularly alcohols and aldehydes,

 

Comment: Continued wrong use of nomenclature.

 


can be oxidized by acidified potassium dichromate (IV). When oxidation occurs in this Reaction, the orange solution containing dichromate (IV) ions are reduced to a green solution containing chromium (III) ions. The electron half-equation for this reaction is as follows:

Cr2O72- + 14H++ 6e -⇒ 2Cr3+ + 7HO2

(orange)                                 (green)

The more sunflower oil is oxidized, the more hydroperoxides are produced. (Clark, 2003) Consequently, the more hydroperoxides are produced, the more green light is emitted and red light (its complementary color) is absorbed by the potassium dichromate (IV) solution. This is expressed in the form of:      A = ebc

in Beer’s Law, where A is absorbance, e is the molar absorbtivity (Lmol-1cm-1), b is the path length of the sample, and c is the concentration of the compound in the solution (molL-1). Beers law indicates a directly proportional relationship between absorbance and concentration of absorbing compounds. Thus, the absorbance of red (630 - 700 nm) light by a solution of potassium dichromate (IV) and oxidized sunflower oil is directly proportional to the concentration of hydroperoxides in the solution. (SHU, n.d.)

Given this relationship, colorimetry can be used to determine the relative concentration of hydroperoxides produced in sunflower oil at different temperatures. As stated above, heat can act as a catalyst for the reaction as initiation requires energy to begin. Therefore, as storage temperature increases, so should the concentration of hydroperoxides, and consequently the absorbance of red light over a controlled period of time.

Procedure

1. Plugin and turn on the colorimeter. Allow 0.5 hr for the machine to warm up.
2. Using a 10.0 mL glass pipette and pipette filler, fill 1 test tube with 8.5 mL of 1M
potassium dichromate (IV).
3. Using a 5.0 mL glass pipette and pipette filler, add 1.0 mL of .01M sulfuric acid to the test 
tube to acidify the solution.

 

Comment: A stock solution of greater volume should be prepared.

 

Acidifies the solution and provides the H+ ions necessary to catalyze the oxidation of hydroperoxides in the sunflower oil.
4. Rinse the pipette. Then, using a new 5.0 mL glass pipette, add 0.5 mL sunflower oil to the test tube.
A new pipette is used to avoid cross-contamination.

5. Move the test tube in a figure-eight motion to allow the reactants to mix.
6. Using a 10.0 mL glass pipette and pipette filler, fill a cuvette with 8.0 mL of the mixed solution.
7. Zero the colorimeter.

Controls the base of measurement for the colorimeter.

8. Select a wavelength of 635 nm.

 

Comment: 635 or 670 nm? Not clear.

 

Controls the wavelength of light absorbed by the solution.

9. Polish the cuvette and insert it into the chamber of the colorimeter. Record the absorbance of 670 nm light. This is the baseline absorbance of sunflower oil.

 

CommentConfusion over wavelength again.

 

The cuvette is polished in order to minimize the interference of dirt on the absorption of light by the sample. A baseline is recorded to control the relative measurement of the later samples.

10. Using a thermometer (ºC), determine and record the temperature of the refrigerator, the freezer, and the room.
11. Using a 100.0 mL measuring cylinder, measure out 50.0 mL of sunflower oil. Pour into a 100.0 mL glass beaker. 

Controls the volume (mL) of  sunflower oil, and the volume (mL) of the beakers

12. Repeat step eleven for all fifteen beakers.

Ensures that all fifteen beakers are under the same conditions.
 

13. Wrap all beakers in aluminum foil.

Controls the exposure to light of the sunflower oil. Because light exposure can also affect or catalyze the reaction, this control is crucial to isolating just one independent variable.

14. Prepare a 40 ºC water bath and a 60 ºC water bath.

15. Place 3 foil-wrapped beakers into each water bath, and 3 foil-wrapped beakers eac into a 4 ºC refrigerator, a -20 ºC freezer, and a box at room temperature.

Sets five different values for the independent variables and three different trials per value in order to decrease the likelihood of random error.
 

16. Over the course of the next 72 hours, refill the water baths so they do not evaporate.

If a water bath dries up, it could pose a potential fire hazard.

17. After 71.5 hours, plugin and turn on colorimeter so as to allow .5 hr for warmup.
18. Using a 10 mL glass pipette and pipette filler, fill 15 test tubes with 8.5 mL of 0.1M potassium dichromate (IV).
Controls the volume of 1M potassium dichromate (IV) in the test tubes.

19. Using a 5 mL glass pipette and pipette filler, add 1 mL of 2 M sulfuric acid to each test tube to acidify the solution.

 

Comment: Should make up a stock of this solution.

 

Controls the volume of 2 M sulfuric acid in the test tubes.

20. Rinse the pipette. Then, using the same 5.0 mL glass pipette, add 0.5 mL of oxidized

 

Comment: Not the best control of volumes.


Sunflower oil from each beaker into its corresponding test tube.

 

Comment: How long is the mixture allowed to react before measuring absorbance?

 

Be sure to rinse the pipette thoroughly between samples to avoid cross-contamination.

Rinsing controls the purity of the samples.

21. Place all test tubes on the test tube rack.Move test tube rack in a figure-eight motion to allow for the mixing of reactants.

 

Comment: Not an ideal method of mixing.

 


Moving the entire rack together controls the path of movement of the test tubes, thus allowing the movement of the reactants within the tubes to be controlled to an extent.

22. Using a 10.0 mL glass pipette and pipette filler, fill each of 15.0 cuvettes with 8.0 mL of the corresponding mixed solution.

Controls the volume (mL) of oxidized hydroperoxide solution in each cuvette.

23. Zero the colorimeter.

Controls the base of measurement for the colorimeter.

24. Select a wavelength of 635 nm.

Controls the wavelength of light absorbed by different samples.

25. Polish each cuvette and insert it into the chamber of the colorimeter. Record the absorption of 635 nm wavelength light.

The cuvette is polished in order to minimize the interference of dirt on the absorption of light by the sample.

26. Remove the cuvette from the machine for each sample. Clean up station, washing all materials and very carefully washing and storing colorimeter cuvettes away from other test tubes. 

- For each of 3 trials of 5 different independent trial values, the absorbance (A) of 670 nm wavelength light will be recorded.

Comment: Or 635? 


- To find the rate of reaction for each trial, the absorbance will be set over the total amount of Time of the reaction.

- Given that Beer's Law states that absorbance is proportional to concentration, 

A/hr ∝ Concentration/hr, and so in terms of rate the units will be (hr-1).


The average rate of oxidation for each independent variable value will be calculated using the standard formula for mean: mean = (a + b+ c)/n, where n = the number of values being averaged, in this case, 3.

- These average rates will then be plotted on a graph of Temperature (℃) vs. Rate of Oxidation (hr-1), and the slope of the line of best fit will be found.

Safety Precautions

Students should be cautious in using water baths. If they dry out, they pose a potential fire safety hazard. It is thus important that they always have water in them. In addition, potassium dichromate is one of the most common causes of chromium dermatitis. (Masters, 2003) As with all CrVI compounds, potassium dichromate is carcinogenic, and thus must be handled with gloves and safety goggles, and worked with under a safety hood.

 

Comment: Good consideration of safety issues.

 

Data Collection and Processing

Qualitative Observations

The 15 beakers of sunflower seed oil appeared identical as they were placed into their Respective storage temperatures. The oil was a light yellow color and quite transparent. After a period of 72 hours, some obvious changes in appearance occurred. For example, the oil stored at -20 ºC was frozen solid and had to be thawed before experimented upon. The oil stored at 60 ºC became cloudy and more translucent. When the potassium dichromate wasacidified with 2M sulfuric acid, the reacting solution gave off heat, and the test tubes were hot to the touch. Finally, when the oil was added to acidified potassium dichromate, no obvious color change was observed.

Table 1: Base Measurements for the Absorbance (A) of Sunflower Seed Oil After it has Reacted With Potassium Dichromate (IV) (±.001)3

Solution  Absorbance (A) (±.001)
Distilled Water (H2O)  0.000
Unoxidized Sunflower Seed Oil Reacted with (concentration)
Potassium Dichromate (IV) solution

-0.050

Comment: Is this the baseline used in the end?

Table 2: Absorbance (A) of Oxidized Sunflower Seed Oil After Storage at Different Temperatures and a Reaction with Potassium Dichromate (IV) (±.001) 
 

Storage Temperature  -20.0  4.0  25.0  40.0  60.0
Absorbance (A) (± .001)
Trial 1                                         0.020  0.037  0.039  0.068  0.060
Trial 2 0.030  0.033  0.043  0.061 

0.095

Comment: No comment on the variance and
possibility of outliers.

 

Trial 3 0.022  0.035  0.041  0.059  0.078

Table 3: Average (Mean) Absorbance of Oxidized Sunflower Seed Oil After Storage at Different Temperatures (ºC) and a Reaction with Potassium Dichromate (IV) 

Storage Temperature (℃)  Mean Absorbance (A)
-20.0  0.024 ± 5.00%
4.0  0.035 ± 3.03%
25.0  0.041 ± 2.56%
40.0  0.063 ± 1.69%
60.0  0.078 ± 1.67%

3 Uncertainty of the colorimeter

 

Calculation of the mean absorbance (A):

The formula used: Aavg= ∑A/n, n = number of trials (3 in this case)

Sample calculation: mean absorbance for oil stored at -20.0℃: 

Aavg= ∑A/n

= (0.020 + 0.030 + 0.022) / 3 = 0.024 

Calculation of uncertainty:

In this instance, the highest percent uncertainty for each storage temperature was taken as the uncertainty of the average.

The formula used: %U = Ua/ A ∙ 100, where Ua is the absolute uncertainty and A is the absorbance for each datum collected.

Sample calculation: % uncertainty for oil stored at -20.0℃:

    %U = Ua/ A ∙ 100

    = 100 ((.001/0.020), (.001/0.030), or (.001/0.022))
    = .050 ∙ 100 = 5.00%

Table 4: Relative Rate (hr-1) of Oxidation of Sunflower Seed Oil at Different Storage Temperatures

Storage Temperature (ºC)

Average relative rate of oxidation (hr-1)

-20.0 

3.33 ∙ 10-4± 5.69%

4.0 

4.86 ∙ 10-4± 3.72%

25.0     

5.69 ∙ 10-4± 3.25%

40.0 

8.75 ∙ 10-4± 2.38%

60.0 

1.08 ∙ 10-3± 2.36%

 

 

 

Comment: Too many sig figs compared to input absorbance data.

 

 

Calculation of average relative rate of oxidation:

The formula used: rate = A avg/time Sample calculation: average relative rate of oxidation of oil stored at -20.0℃: 

        rate = A avg/time

        = 0.024 / 72 hr = 3.33 ∙ 10-4 hr-1 

Comment: Should the student be taking into account the unoxidized oil absorbance to calculate a rate of change?

 

Comment: The units should be A.hr-1in this case.

 


Calculation of uncertainty

Formula used: %Urate= %UA+ %U time4

Sample calculation: %U of oil of the average relative rate of oxidation of oil stored at -20.0℃

        %Urate= %UA+ %Utime

        = 5.00% + .694% ≈ 5.69%

4 Absolute uncertainty for time is assumed .5 hr. For %U, .5 / 72.0 * 100 = .694%

Figure 3: Graphical Representation of the Relative Rate of Oxidation (hr-1) of Sunflower Seed Oil Versus its Storage Temperature (ºC) Over a Period of 72 hr. 

The equation of the line of best fit is y = 9.42∙10-6x + 4.63∙10-4. This is indicative of a positive correlation between the storage temperature of sunflower seed oil and its rate of oxidation.

 

Conclusion and Evaluation

This laboratory experiment has determined that the hypothesis, which states that if the
storage temperature (ºC) of sunflower oil increases, then the rate (hr-1) of oxidation will
increase is true.

Table 4 shows the mean relative rate of oxidation of sunflower seed oil. As predicted in the
hypothesis, the storage temperature and rate of oxidation of sunflower seed oil display a
directly proportional relationship. This correlation is supported by the graph in Figure 4,
which clearly shows an upward trend. It should be noted that the slope of the line determined
from this experiment’s data is significantly lower than predicted. However, because it is still
a Positive slope, the conclusion is supported by the graph. 

There is no literature value to which to compare the experimental data. Because the composition of the sunflower seed oil is not consistent in terms of the fatty acids present in the triglyceride form, there is no theoretical value for its rate of oxidation at different temperatures. For this reason, the percent error cannot be calculated. Because, in theory, increased heat should act as a catalyst for the oxidation of polyunsaturated oils as it did in the experimental trials, the results of the trials can be taken as relatively true. Again, because there is no literature value for this result, the accuracy of the solution with regard to percent The error cannot be absolutely verified.

 

Comment: Trying to relate to scientific context.

 

There are a few limitations in the execution of and materials used in the experiment:

First, although the aluminum foil around the beakers controlled the oils’ exposure to light, it did not completely control exposure to the air. Because of this, each beaker may have had a  different ratio of oxygen to oil.

 

Comment: If they were exposed to the same air (I assume they were) and the oils had the same surface area of interface with air, then this would have been constant.

 

Less oxygen in the beakers would have led to a lower rate of Oxidation and more oxygen would have led to a higher rate of oxidation. This may have been
the reason behind slight inconsistencies in some trials. Second, the places the beakers were stored over the 72 hr period had varying conditions. While the refrigerator and freezer were sealed while closed, they were opened and closed an An unknown number of times, allowing for an unknown number of brief exposures to fresh air from outside. Beakers in the water baths, however, stayed in unsealed but fairly untouched 
Environments.

 

Comment: If unsealed then surely there is constant interaction with the environment.Not sure what the student is saying here.

 

This difference in exposure to new air may have affected the results. If each exchange of air for the refrigerator increased the concentration of oxygen in the air, then the rate of oxidation of the oil may have increased.Third, because the oil was stored at different temperatures, the reaction with potassium dichromate (IV) was carried out at different temperatures.

 

Comment: This should have been standardized.Samples should have been allowed to reach same thermal equilibrium prior to measurement.

 

There was residual heat from Acidifying the potassium dichromate each time, but the reaction with 4 ºC oil and 60 ºC oil occurred at very different temperatures. A colder oil in the reaction may have slowed its
speed or reduced its efficacy. This would have reduced the amount of color change, and thus the amount of absorption and the measured rate of oxidation. 
The primary source of random error in this experiment was the uncertainty of the time of reaction. The clock used was accurate to the second, but the random error arose from the fact that there was up to .5 hr difference in the time the potassium dichromate (IV) reaction took place. This was simply due to the fact that the trials could not all be tested at once. Each datum in the final graph has a substantial % error attached to it for this reason.

Due to the lack of a literature value for the effect of storage temperature on oxidative rancidity, there is no final percent error. However, though the imprecision of the time measurement gave rise to random error, the data points in Figure 4 appear to be relatively close to the line of best fit, indicating that random error did not have a strong detrimental effect on the accuracy of the final data.

 

Comment: Reflecting on the reliability of results.

 

However, although no percent error was calculated,It is highly likely that the uncontrolled aspects of the experiment gave rise to a significant amount of systematic error. Because of this, improvements to the experiments should be made to reduce systematic error more than random error.

The above mentioned systematic error and random error can be reduced or eliminated using the following improvements:

The issue of the imprecision of time measurement can be reduced by separating the three trials for each independent variable value. This way, only five tests are conducted at a time,which would significantly decrease the absolute uncertainty of the time measurement. this would ensure a higher precision of the final conclusion. 

The error stemming from the uncontrollable interruptions in the storage environment of the samples can be addressed in a number of ways. First, each beaker can be sealed with Parafilm, limiting the exchange of air between inside the beaker and outside the beaker.

 

Comment: This will stop oxidation.

 


Second, the beakers can be placed in areas with less traffic in terms of opening and closing refrigerator doors.

 

Comment: Student not really understanding significance of errors.

 

Another source of systematic error that can be reduced is the issue surrounding the different temperatures of oil as it undergoes its reaction with potassium dichromate (IV). One way this can be improved is by controlling the temperature at which the chemicals react by having the The reaction occurs within a warm water bath.

 

Comment: Good point.

 

This would eliminate any discrepancies caused by The difference between cold oil and warm oil since all trials would be heated to the same temperature.

 

Bibliography


Beer’s law. (n.d.) Retrieved from
        http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm
Clark, J. Oxidation of alcohols. (2003). Retrieved from
        http://www.chemguide.co.uk/organicprops/alcohols/oxidation.html
Derry. L. (2008) Chemistry: For use with the ib diploma program options: Standard and
        higher levels. Port Melbourne: Pearson Education.
Fankhauser, D. B. Spectrophotometer use. (2007). Retrieved from
        http://biology.clc.uc.edu/fankhauser/Labs/Microbiology/Growth_Curve/Spectrophoto
        meter.htm
Master, F.J. (2003) Diseases of the skin. New Delhi: B Jain Pub Pvt Ltd.
Smokefoot. (Artist). (2011). representative triglyceride found in sunflower oil. [Web
        Graphic]. Retrieved from http://en.wikipedia.org/wiki/File:TriglycerideSunflower.png
Thomas, A. (2002) Fats and fatty oils. Weinheim: Wiley-VCH.

 

 

 

 

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