Matthieu Capuano

Matthieu CapuanoIB Candidate Number: 000046-002219th January, 2014IB Higher Level Chemistry

The Effect of Refrigeration Time on Mass of Vitamin C in Lemon Juice

Word Count: 3457

Design Lab Report

Abstract:Free radicals are molecules that can damage cells by generating a series of oxidation processes in the cell membrane. They are naturally produced in the human body and are very reactive because they have an unpaired electron; examples of free-radicals include superoxide () and hydrogen peroxide (.Anti-oxidants can prevent free-radicals from damaging cells by releasing a loosely held hydrogen atom, known as a reducing equivalent. Many anti-oxidants are produced in the human body but others must be obtained from external sources, which are known as exogenous sources. Vitamin C (ascorbic acid) is an example of an exogenous anti-oxidant; it is found in many food sources including papaya, gooseberries, lemons and strawberries. These are grown in large quantities and sold as both fruits and juices.Vitamin C can lose a hydrogen atom (to a free-radical or another molecule) to form L-ascorbic acid or ascorbate, which is often denoted as AscH-. This anion can donate an additional H- to a free radical (R) to from an ascorbate radical (Asc-), which is much more stable than a free-radical. The ascorbate radical is more commonly known as dehydroascorbic acid. This is depicted below.

Diagram 1: Ascorbate reaction with a free radical.

This reaction can also be written as follows:

Note that the equation is not fully balanced, the first hydrogen released may be transferred to any oxidizing agent so it has not been included. If the ascorbic acid is reacted with Iodine then it turns directly to dehydroascorbic acid. The reaction is the following:

Vitamin C acid exists in foods in different concentrations. Since free radicals are present everywhere, including air, the ascorbic acid in these foods can become oxidized over time. This is one of the reasons for which fruits and juice bottles are sealed. An unopened box of lemon juice can be left for a year without spoiling but, once it has been opened, it may only last up to a week and a half. Keeping the juice refrigerated can extend the safe consumption period of the juice.Since lemon and orange juices are often purchased because they are known to be rich in vitamin C, it is useful to know how the concentration of ascorbic acid decreases over refrigerated time. That is what this lab seeks to find out.

Research Question:How does the refrigeration time (measured to the nearest minute using a digital stopwatch and increased by intervals of 24:00 hours over nine days) of lemon juice affect the measured mass (in milligrams, calculated using molar ratios and results from titrations) of Vitamin C in 10mL samples of the juice when using standard iodine titrations (iodometry)?

Hypothesis:Vitamin C can be detected through iodometric titration because it acts as an acid, it can lose a hydrogen atom (oxidation) whereas the Iodine is a moderately weak oxidizing agent (gets reduced). Over time, as it decomposes, Vitamin C is oxydized by molecules in the air to dehydroascorbic acid. When this happens, the ascorbic acid loses two hydrogen atoms and its oxidation potential, which prevent it from reacting with the iodine.Despite the lid on the bottle, air can still reach the juice and oxydize the Vitamin C in it. Since the products of oxidized Vitamin C are heavier than ascorbic acid, they sink and Vitamin C rises, making it vulnerable to oxidation. This should result in a constant rate of decrease in the concentration of ascorbic acid. This is depicted in the graph below.

Graph 1: Hypothetical graph of the change in mass of Vitamin C in Lemon Juice over time.

Note I: The mass values on the y-axis will vary depending on the brand and juice used. The scale in this graph is based on data from nutritiondata.self.com

Note II: When time = 0 the bottle has just been opened.

Variables:

Independent: The time length during which the lemon juice is refrigerated (at constant temperature). The intervals will be 0:00, 24:00, 48:00, 72:00, 96:00, 120:00, 144:00, 168:00, 192:00 and 216:00 hours, which is equivalent to a period of 9 days. This is the average time before which most lemon juices spoil. The time will be measured using a stopwatch. Though the stopwatch may measure the accuracy to seconds, it would be difficult to take the samples out and titrate them at the exact same time. For this reason, the times will be rounded to the nearest minute. This will give a slightly higher uncertainty but it will still be small relative to the overall time of the experiment, which is another reason for the long time intervals.

Dependent: The mass of Vitamin C measured from 25mL samples after refrigeration. The mass will be measured in milligrams, to an accuracy of 0.1mg (as with the literature value which will come from the bottle) and will be calculated from the volumes of iodine used during the titrations. The calculations used to find this mass from the results are explained in the Data Processing section.

Controlled:VariableMethod to keep it constantPotential effect on results

Temperature of the refrigeratorThe refrigerator will be set to a specific temperature before the experiment (i.e. 2 oC). It will be kept closed at all times throughout the trials except when a sample is taken out for titration.At a higher temperature, the molecules in any liquid and in air have greater kinetic energy they move faster. Consequently, the ascorbic acid will react more readily with surrounding molecules and will be more oxidized. This would decrease the oxidation potential of the juice and the ascorbate product would not react with the iodine, falsifying the measurements. Note that this is not a major issue for small changes in temperature but the experiments should still be performed quickly and efficiently to prevent the effect of this variable.

Volumes of samplesAll samples will have a volume of 25.00mL. This will be measured using a graduated cylinder to an accuracy of 0.1mL 0.05mL.Evidently, higher volumes of lemon juice will have more ascorbic acid. Accuracy and precision in measurements of volume are particularly important because the samples are small, so a small change in volume can have a proportionately large impact on measurements. This is why the 25mL sample size was chosen: it was large enough to prevent slight missreadings in volumes from having a large impact on the results but wasnt so large that an unreasonable amount of lemon juice and iodine will be necessary.

Equipment usedThe same equipment will be used throughout the experiment. A single refrigerator and burette will be used. If it is possible, different graduated cylinders and Erlenmeyer flasks should be used for different refrigeration times. This would prevent samples with different concentrations (and thus masses) of juices from mixing. If this is not possible, as is the likely case, then the graduated cylinder should be thoroughly washed and rinsed after each trial.Different instruments (i.e. balances or thermometers in refrigerator) may have different calibrations that provide slightly conflicting results. Using the same equipment also maintains the degree of systematic error of the measurements (as opposed to increasing it with multiple instruments).

Concentration of starch1% starch solution is to be used for all trials.This is kept constant for the sake of consistency and to prevent different concentrations of starch from becoming a confounding variable.

Type of lemon juice usedThe samples of juice will come from a single bottle. The volume of the samples, number of trials and levels of measurement will require 750mL of juice in total. However, a slightly higher volume should be purchased in case some of the samples are spilled or other issues arise. Ideally, a regular 1 litre bottle will be used (which is easier to obtain than a 750mL bottle anyway). Note that lemon juice was selected over orange and other vitamin C rich juices because it has less pulp and does not generally need to be mixed before use.Numerous chemicals are usually added to juices to preserve them or simply to change their tastes. These chemicals will vary between juices and brands and may be subject to oxidation by the iodine. This would result in a higher reading of Vitamin C than what is the case. The results for the lemon juice of one brand may not apply for the juice of another.

Concentration of IodineThe stock solution should have a concentration 0.025mol dm-3 Iodine. This can be made through dilutions if necessary.Having a higher concentration of Iodine would make it difficult to accurately measure the moles of vitamin C in the juice; it would titrate within a few drops and, then, quickly turn black. On the other hand, if the concentration is lower, the titration could take a very long time. 0.025mol dm-3 should be enough to allow an efficient and accurate titration.

Sources of Iodine and starchAs with the lemon juice, the starch and Iodine should come from the same flasks, beakers or bottles.The chemical compositions of these two compounds, though theoretically identical to other iodine and starch solutions, may have changed slightly due to reactions with surrounding molecules or accidental mixtures with other particles (i.e. dust in air). This could influence the reactions and provide conflicting results.

End point of titrationMethod: The end point of the titration will have to be qualitatively evaluated. A trial run should be made before the experiment during which the colour at which the titration is complete is assessed and agreed upon. The colours seen during the titrations should look like the following:

Image 1: Titration of lemon juice.

The flask on the left is lemon juice to which no Iodine has been added. The flask in the middle is am exact, complete titration of lemon juice, this is the colour to look for. The flask on the right has excess Iodine. If possible, more than one researcher should observe the experiment and agree on the end-of-titration point. The pouring of Iodine should be performed slowly as the titration point is approached.

Effect on results: The mass of Vitamin C in the samples will be calculated from the volume and concentration of Iodine used. If too little or too much Iodine is used, this calculation will be inaccurate.

Apparatus:The following equipment will be necessary for the experiment: 1 Graduated Cylinder (if a different one is not available for all levels of measurement) 1 Litre of Lemon Juice The brand is a trivial matter but a pure juice may be better is it should have less chemicals that could interfere with the reaction. 1 Refrigerator (that can sustain a temperature of 2 oC for at least 9 days) 500mL of 0.025 mol dm-3 Iodine (500mL should be more than enough but this much should be prepared as a precaution) 100mL of 1% Starch solution 3 100mL Erlenmeyer flasks As with the graduated cylinders, it would be better not to mix samples with different refrigeration times; however, it is unlikely that researchers have access to 30 flasks so 3 can be used but must be washed and rinsed thoroughly. 1 50mL Burette 1 White tile 1 Clamp stand 1 Clamp 1 Funnel 1 Stopwatch Safety goggles Note: There are practically no safety concerns with this experiment but Iodine can stain things very easily. An apron could be worn if necessary. The titrations should be carried out on a flat, solid and stable surface near a sink and trash can where used chemicals can be disposed of. Another means of disposing of the products can be used if it is approved by a supervisor and if it is safe for both the researchers and the environment.

Procedure:The time of the day during which the experiment is performed is of little importance but should be consistent throughout the experiment. It should be a time during which researchers can come conduct three trials each day.1. All openings in the experimental area (i.e. doors, windows etc.) should be closed. Temperature and humidity regulating devices (i.e. air conditioners) should be set to a fixed value that will be used for the duration of the experiment. This is to prevent any drastic change in temperature; small changes should not be a concern, particularly since the juice will be refrigerated. The room temperature should be recorded and remeasured everyday if it is subject to change.2. Collect all the equipment and place it near the refrigerator on a solid, flat surface where all of it can be accessed easily3. The unopened bottle should be left in the fridge for a day or so. This will give the juice a temperature similar to the one it will have for the other trials. The experiment only begins once the bottle is opened so keeping it refrigerated while closed is not an issue.4. The equipment should be set up as is shown in Diagram 3 before every trial in order to maximize efficiency. Note that this includes filling the burettes with Iodine to the 0mL mark.5. On the day of the experiment take the bottle out of the refrigerator, open it and start the stopwatch.6. Take three samples of 25mL using the graduated cylinder and put them in separate 100mL Erlenmeyer flasks. Promptly replace the bottle in the refrigerator.7. Conduct the titrations as follows:a. Place five drops of starch in each flask and swirl the content.b. Place one flask directly under the burette. c. Begin letting drops of Iodine into the flask while swirling the content.d. As drops fall, the solution should turn dark green for a brief moment and then return to a light orange colour. This reversion to orange should take longer with each drop.e. Stop pouring Iodine once the solution remains a dark green. This is the point at which the titration is complete and researchers should agree on it.f. Record the volume of Iodine in the burette (the initial measurement was 0.0mL; the new one will be greater).g. Refill the burette to the 0.0mL mark, empty and clean out the flask.h. Repeat steps a-h with the other two flasks.8. Return the next day at the exact same time and repeat steps 6 and 7.9. Do this for a period of nine days (more if additional data is wanted, though the juice is likely to spoil after this period.10. This should provide all the data necessary. Pack up and clean the experimental area.

Note: As previously mentioned, there are no major safety issues with the experiment.

Side note: It may be interesting and worthwhile to compare the rate at which the mass of Vitamin C decreased while refrigerated as opposed to how it decreases without being refrigerated. If this is to be done, the procedure above should be repeated but without placing the bottle in the refrigerator.

Diagrams 1, 2 and 3: Pouring of 25mL of lemon juice from bottle into graduated cylinder, transfer of juice from cylinder into flask and titration under burette.

100mL Graduated Cylinder (with 25mL of Lemon Juice)

Lemon Juice Bottle (1 litre)

Funnel

Erlenmeyer Flask (with the 25mL of lemon juice)

50mL Burette (with Iodine)Note: Unlike the depicted colour, the Iodine will be black

Clamp

Stand

Erlenmeyer Flask (with the 25mL of lemon juice)

Data Collection

The following are examples of the tables and graphs that could be used to record the data from this experiment and to investigate the relationship proposed in the research question.

Table 1: Volume of iodine used for each titration and average for each level of measurement.Time/ minutes/ 1 Volume of Iodine in burette/ mL/ 0.05mL

Trial 1Trial 2Trial 3

BeforeAfterBeforeAfterBeforeAfter

0(0 days)

1440(1 day)

2880(2 days)

4320(3 days)

5760(4 days)

7200(5 days)

8640(6 days)

10080(7 days)

11520(8 days)

12960(9 days)

Note: The before and after refer to the volumes of iodine in the burette before the titration started for a trial and once it was complete.

Sample calculation of time:The time was converted from days to minutes using the following equation:

For example, after three days, the time in minutes would be:

Uncertainty justifications:

Uncertainty of times:As was previously explained, an uncertainty in seconds would be unreasonably small as a titration cannot be performed at an exact time; 1 minute is a more reasonable range of time for a titration. Note that the values in the first column may seem excessively large, but they are only days converted to minutes.

Uncertainty of Volume:The uncertainty of 0.05mL was chosen based on the assumption that the burette used would provide readings down to 0.1mL. Conventionally, the uncertainty became half of that value.

Qualitative observations:Any non-numerical data about the experiment, chemicals and reactions should be noted at this point. This would include: facts about the change in colour of titrants during the titrations, any issues that arose and that were dealt with or any other information potentially relevant to the data. *Observation 1* *Observation 2* *Observation 3* Etc.

Data Processing

Table 2: Volumes of iodine used for titrations and averages for intervals of measurement.Time/ minutes/ 1Volume of Iodine used for Titration/ mL/ 0.1mL

Trial 1Trial 2Trial 3Average

0(0 days)

1440(1 day)

2880(2 days)

4320(3 days)

5760(4 days)

7200(5 days)

8640(6 days)

10080(7 days)

11520(8 days)

12960(9 days)

Sample calculation:Volume of Iodine used for individual titrations:The volume of Iodine used each trial can be calculated through a simple subtraction calculation:

Uncertainty of volume of iodine used for individual titrations:As stated directly above, the volume of iodine used for the titrations is calculated through a subtraction. The absolute uncertainty of the volume per trial will then be the sum of the absolute uncertainties of the volumes before and after titrations:

Substituting the correct values gives:

Average volume of iodine used for titrationThe average formula is the following:

Where:

In the case of this lab, this equation could be rearranged to give:

Note: The uncertainty for the average volume is the same as the volume for each individual trial since these errors were equal (all were 0.1mL)

Finding the mass of Vitamin C in the Bottle:

The next step will be to use the data in table 2 to find the mass of Vitamin C in the bottles. The following table should be created using the calculations described beneath it.

Table 3: Moles of Iodine and Vitamin C titrated, mass of Vitamin C in samples and in bottle.Moles of Iodine (and, thus ascorbic acid) in 10mL sample/ molUncertainty of moles of Iodine and ascorbic acid/ molMass of Ascorbic Acid in Sample/ molMass of Ascorbic Acid in BottleAbs Uncert of mass of ascorbic acid in bottle

Sample Calculation:Moles of Iodine and Ascorbic Acid in 10mL samples:The concentration of the Iodine will be 0.025mol dm-3. The volume used for each titration will be obtained from the calculations previously described. With these values, the moles of Iodine used for the titrations could be calculated from:

Which is commonly abbreviated to:

More specifically, for this lab:

As was explained under the Design section, the molar ratio of ascorbic acid to Iodine in the titration reaction is 1:1. This means that whatever value is obtained for the moles of Iodine will also be the moles of Ascorbic Acid in the sample.

Percentage Uncertainty of moles of Iodine and Ascorbic acid in 10mL samples:The moles must be calculated through a multiplication of two values. It follows that the percentage uncertainty of moles is then the sum of the percentage uncertainties of the two values added:

Percentage uncertainty is calculated according to the equation:

Substituting gives:

Substituting known values gives:

Mass of Ascorbic Acid in 10mL samples and in bottle:At this point, the moles of Ascorbic acid in the sample should be known. The molar mass of ascorbic acid is 176.12g mol-1. All that will be left to do is substitute the moles into:

Absolute Uncertainty of Mass of Ascorbic Acid in bottle:The molar mass is assumed to have no error. This implies that the percentage error of moles (calculated previously) will also be the percentage error of mass. The percentage error can then be converted to absolute error using:

Matthieu Capuano

IBHL ChemistryPage 4

The following graph could be used to determine the relationship between the independent and the dependent variable.

Graph 2: The change in mass of ascorbic acid in lemon juice against the time of refrigeration of the juice

Note: Depending on the results and on the trend, a linearization may be necessary. The process for this linearization cannot be described at this point because no data is available.References

"Antioxidant." Wikipedia. Wikimedia Foundation, 22 Jan. 2013. Web. 18 Jan. 2014. "Antioxidants and Cancer Prevention." National Cancer Institute. N.p., 16 Jan. 2014. Web. 19 Jan. 2014. "Ascorbic Acid." Wikipedia. Wikimedia Foundation, 31 July 2013. Web. 19 Jan. 2014. Best, Ben. "General AntiOxidant Action." GENERAL ANTIOXIDANT ACTIONS. Ben Best, n.d. Web. 18 Jan. 2014. "Does Orange Juice Go Bad?" Does It Go Bad. N.p., n.d. Web. 18 Jan. 2014. "Free-radical Theory of Aging." Wikipedia. Wikimedia Foundation, 18 Nov. 2013. Web. 18 Jan. 2014. "Nutrition Facts." Analysis for Lemon Juice, Canned or Bottled. Self Nutrition Data, 21 Mar. 2012. Web. 17 Jan. 2014. "Super Foods for Optimal Health." Food & Recipes. WebMD, 25 Oct. 2013. Web. 18 Jan. 2014. "Vitamin C." Medline Plus, Trusted Health Information for You. U.S. National Library of Medicine, 31 Oct. 2013. Web. 17 Jan. 2014. "Vitamin C." Medline Plus, Trusted Health Information for You. U.S. National Library of Medicine, 31 Oct. 2013. Web. 17 Jan. 2014. "Vitamin C." QuickFacts. Office of Dietary Supplements, 24 June 2011. Web. 16 Jan. 2014. "Vitamin C." Wikipedia. Wikimedia Foundation, 13 Apr. 2013. Web. 19 Jan. 2014. Wandry, Mardya. "Benefits of Antioxidant for Human Body." Yahoo Contributor Network. Yahoo Health and Lifestyle Network, 23 Oct. 2009. Web. 17 Jan. 2014.