Effect of Temperature on Vitamin C Concentration: A Titration Investigation

In the selection of my topic, I researched potential investigations into household items. I wanted to investigate not only something that would not simply satisfy the criteria for the IA, but something that is a part of my own life, even if only in a small way.

This led me to the selection of my topic. Vitamin C (Ascorbic Acid) is regarded as helping to prevent cold symptoms (whether this is true or not is another question entirely), by this logic one suffering from a cold would benefit from the consumption of a higher concentration solution of Vitamin C.

Often when I had a cold my parents would offer me a hot mug of honey and lemon, in the hope that the Vitamin C content of the lemon would aid my recovery.

And it is that relationship, between heat and Vitamin C concentration which I decided to investigate. For my experiment, I elected to test Vitamin C tablets instead to obtain more consistent results, but the premise remains of what is the effect of temperature, on Vitamin C concentration.

The tablet I selected is known as Berocca and is readily available in super markets. Using a redoxtitration, I will titrate the Vitamin C solution, with Iodine as the titre, to determine the varying concentrations of Vitamin C with temperature (see relevant equation). This investigation aims to establish a definable trend for the relationship in question.

C6H8O6 (ascorbic acid) + I2 → 2 I− + dehydroascorbic acid

Hypothesis

As I addressed in my introduction, the inspiration for my investigation stemmed from my childhood experiences with a hot mug of Lemon and Honey.

However, if my hypothesis is correct, the wisdom of such a remedy will be put into question. In an oxidation reaction, Vitamin C (Ascorbic Acid) is oxidised to form Dehydroascorbic acid:

For this reaction to occur, the Ascorbic acid must undergo a redox reaction with the air. Oxygen in the air acts as an oxidant to oxidise the Ascorbic acid and form Dehydroascorbic acid, while Ascorbic acid acts as the reductant and reduces Oxygen to form Hydrogen Dioxide, water.

This reaction, among other things is accelerated by the presence of light and heat, mainly through the weakening of bond within Ascorbic Acid because of the additional energy provided by the heat and light. Therefore, by this logic the redox reaction between Ascorbic acid and Oxygen will occur faster at higher temperatures. If this is the case, then a higher concentration of dehydroascorbic acid will be formed and therefore the existing concentration of Ascorbic acid will decrease. In conclusion, I hypothesise that at Higher temperatures, the redox reaction between Ascorbic Acid and Oxygen will occur more rapidly thus decreasing the concentration of Ascorbic Acid in solution per unit of time.

Variables

Independent Variable (Temperature of Distilled water solution)

The independent variable for this investigation is the Temperature (K) of the distilled water solution which acts as the solute for the vitamin C tablet recorded in Kelvin. To collate a sufficient range of values, and thus a reasoned trend and conclusion I will titrate 8 distinct temperatures of the Vitamin C solution. These will be room temperature on a given day and increasing in increments of 5K thereafter until all 8 temperature solutions have been titrated and concordant results achieved. A difficulty I perceive I will encounter is maintaining the temperature of a solution once it has reached the desired temperature. As I must leave the solution for 10 minutes for the redox reaction to occur, I will need to develop a method of insulating, or otherwise preserving the temperature of solutions to achieve valid results.

Dependent Variable (Ascorbic Acid Concentration)

This investigation seeks to establish a trend in the concentration of Ascorbic Acid at variable temperatures and will be measured as mol L^-1. This variable is calculated by titrating the Vitamin C solution against an Iodine solution. Anticipated difficulties which could potentially cause error are mostly making sure to measure out accurate volumes of the chosen solutions, this can be achieved easily enough if the correct equipment is used and due care is taken when taking readings as to avoid parallax error.

All possible controlled variables identified

• Time

- As the oxidation of Ascorbic Acid prior to the titration will not be performed to completion, time becomes a factor. It will be critical to achieving concordant results that the time the solution can sit after the tablet has been dissolved in the water is constant for all titrations. If this is not the case, then there can be little to no hope of three concordant results being achieved for each temperature let alone reliable and consistent raw data across the entire experiment. I will control this by timing the period using my phone or a stopwatch if one is available from when the tablet is dissolved to when it is titrated to ensure that it is constant for all titrations.

• Vessel

- For all experiments are 200cm^3 beaker will be used to minimize any affect a larger or small vessel could possibly have on the experiment

• Tablet Mass (g) / Ascorbic Acid (mg) content per tablet

- For each titration, I will use one tablet of Berocca. Prior to dissolving this tablet, I will weigh it to ensure it is within 1% of the mass of the other tablets to reduce the possibility that a tablet contains more Ascorbic Acid than another. As for the actual Ascorbic Acid content per tablet an assumption will be made that it is indeed the stated amount of 500mg per tablet, there is no other method I can perform to calculate the exact quantity of Ascorbic Acid as performing a separate titration would almost certainly yield a result less than the stated amount due to the oxidation of Ascorbic acid which occurs even at room temperature.

• Concentration of Iodine Solution (mol L^-1)

- All the Iodine solution will be taken from the same batch which I prepared to be 0.01 moldm^-3. This will reduce the possibility that there are any discrepancies between the concentration of Iodine solutions used in the titration process, thus making the experiment more likely to produce concordant results. The preparation of this solution simply entailed the dilution of 100cm^3 of 0.1 mold dm^-3 Iodine with 900cm^3 of distilled water. The 1 dm^3 of Iodine I had previously calculated to be a sufficient quantity.

Volumes of reactants and Distilled water

- The Tablet will be dissolved in a 200cm^3 conical filled to the mark with distilled water, a reading will be taken perpendicular to such a marking to avoid parallax error. Then a 5cm^3 pipette will be used to take a 5cm^3 aquilot of the Ascorbic acid solution and pipetted into a 200 cm^3 conical flask before the addition of a further 200 cm^3 of distilled water.

Improving Accuracy

As addressed in some of my variables, a method of improving accuracy is to perform the titration over a suitable range of temperatures to increase the likelihood of a valid data set. Additionally, and as mentioned also each temperature will be titrated multiple times until three concordant results within a certain quantity of each other which ended up being 0.2 cm^3, slightly larger than I had hoped for. Furthermore, it was also mentioned of the importance of taking care when measuring and taking readings of quantities of solutions, most notably steps like viewing each vessel from a perpendicular viewpoint to negate parallax error.

Additionally, when taking these readings it is important to take said reading from the bottom of the meniscus, a shallow dip in the fluid of the vessel, otherwise an additional source of error would be introduced. Finally, I will ensure that I utilize the relevant measurement devices for each measurement as to ensure the most accurate readings are taken and thus the most accurate results.

Safety considerations

Gloves and safety glasses will be worn when handling Iodine solutions as well as during all titrations, all relevant lab safety rules will be observed in full

Iodine, especially in the concentration (0.01 moldm^-3) and quantities I will be using (1dm ^3) has very little environmental effects so can be disposed of easily, the ascorbic acid solution is just a Berocca tablet dissolved in distilled water so itself poses no environmental threat. (Lenntech staff n.d.)

Method

Diagram of apparatus

Prep: to prepare a starch solution I used this method:

‘Add 1 gram of starch into 10 mL of distilled water, shake well, and pour into 100 mL of boiling, distilled water. Stir thoroughly and boil for a 1 minute. Leave to cool down.’ (Titrations.info staff, 2012)

I repeated this process a couple times as the amount I initially prepared estimated 1cm^3 per titration when in fact a 5cm^3 volume was needed to give accurate indication.

Titration Process

1.  Measure 200cm^3 of distilled water into a 200cm^3 beaker
2.  Using a thermometer take the temperature of the solution waiting until no change in temperature is observed over a period of 30 seconds take the reading and convert it to Kelvin, this will be the room temperature on the given day and the first temperature to be titrated
3.  Weigh out a single Tablet of the Berocca, containing 500mg of Ascorbic acid and record the value for later reference
4.  Place the Tablet in the distilled water
5.  Cover the lid of the beaker and place it in a polystyrene container. Time a period of 10 minutes before proceeding, this allows some of the oxidation reaction between Ascorbic acid and Oxygen to occur.
6.  Take a 5 cm^3 aquilot of the dissolved solution and pipette into a 250cm^3 conical flask, then add 200 cm^3 of distilled water and 5 cm^3 of starch indicator solution.
7.  Use a burette to titrate the sample against a 0.01 moldm^-3 iodine solution, continue to add the iodine solution untill the solution takes on a slight green-blue hue, this shows that the Iodine has begun to react with the Starch solution and therefore the Ascorbic acid in solution can be considered as having been reacted to completion, take an accurate reading of the burette being careful to avoid parallax error
8.  Repeat steps 3-7 until 3 concordant results are achieved, this could take up to five attempts
9.  Now that the concentration of Ascorbic Acid in a solution at room temperature has been determined, a further 7 temperatures must be titrated similarly
10.  Set up a Bunsen burner on a heat proof mat and with a tripod directly over the Bunsen and ignite the flame, aligning it such that a blue flame is produced. Repeat step 1, then place the 200cm^3 distilled water solution onto the tripod and hold a thermometer in the water
11.  Heat the distilled water solution to temperatures of increasing increments of 5K, once the desired temperature has been reached, using a cloth, remove the beaker from the tripod and place it onto a heat proof mat
12.  Repeat steps 3-7 for each temperature (Room Temperature + 5KX, where X is the number of titrations minus 1, n-1) until three concordant results for Ascorbic Acid concentration at eight distinct temperatures has been found
13.  You should now have 3 concordant results across a range of 8 distinct temperatures increasing in increments of 5K from the room temperature on the day the first titration was performed

Elements of methodology have been included from (University of Canterbury Staff, n.d.)

Raw Data

Temperature (K) (±0.5K) 294.1 Mass (g) 4.734

Titration 1 Rough 1 2 3

Initial Volume (cm^3) (±0.10cm^3) 0.00 17.10 15.00 31.80

Final Volume (cm^3) (±0.10cm^3) 17.10 33.60 31.80 48.70

Used (cm^3) (±0.10cm^3) 17.10 16.50 16.80 16.90

Average Volume Used (cm^3) (±0.10cm^3) 16.73

Temperature (K) (±0.5K) 299.2 Mass (g) 4.762

Titration 2 Rough 1 2 3

Initial Volume (cm^3) (±0.10cm^3) 0.00 16.70 33.40 30.00

Final Volume (cm^3) (±0.10cm^3) 16.70 33.40 49.90 46.80

Used (cm^3) (±0.10cm^3) 16.70 16.70 16.50 16.80

Average Volume Used (cm^3) (±0.10cm^3) 16.67

Temperature (K) (±0.5K) 304.2 Mass (g) 4.776

Titration 3 Rough 1 2 3

Initial Volume (cm^3) (±0.10cm^3) 0.00 16.15 32.70 30.00

Final Volume (cm^3) (±0.10cm^3) 16.15 32.70 49.10 46.50

Used (cm^3) (±0.10cm^3) 16.15 16.55 16.40 16.50

Average Volume Used (cm^3) (±0.10cm^3) 16.48

Temperature (K) (±0.5K) 309.2 Mass (g) 4.759

Titration 4 Rough 1 2 3

Initial Volume (cm^3) (±0.10cm^3) 0.00 16.80 33.40 29.90

Final Volume (cm^3) (±0.10cm^3) 16.80 33.40 49.90 46.60

Used (cm^3) (±0.10cm^3) 16.80 16.60 16.50 16.70

Average Volume Used (cm^3) (±0.10cm^3) 16.60

Temperature (K) (±0.5K) 314.2 Mass (g) 4.744

Titration 5 Rough 1 2 3

Initial Volume (cm^3) (±0.10cm^3) 0.00 16.50 32.80 30.00

Final Volume (cm^3) (±0.10cm^3) 16.50 32.80 49.20 46.50

Used (cm^3) (±0.10cm^3) 16.50 16.30 16.40 16.50

Average Volume Used (cm^3) (±0.10cm^3) 16.40

Temperature (K) (±0.5K) 319.2 Mass (g) 4.773

Titration 6 Rough 1 2 3

Initial Volume (cm^3) (±0.10cm^3) 0.00 16.10 32.60 30.00

Final Volume (cm^3) (±0.10cm^3) 16.10 32.60 48.70 46.70

Used (cm^3) (±0.10cm^3) 16.10 16.50 16.10 16.70

Average Volume Used (cm^3) (±0.10cm^3) 16.43

Temperature (K) (±0.5K) 324.2 Mass (g) 4.787

Titration 7 Rough 1 2 3

Initial Volume (cm^3) (±0.10cm^3) 0.00 16.30 32.70 30.00

Final Volume (cm^3) (±0.10cm^3) 16.30 32.70 49.00 46.40

Used (cm^3) (±0.10cm^3) 16.30 16.40 16.30 16.40

Average Volume Used (cm^3) (±0.10cm^3) 16.37

Temperature (K) (±0.5K) 329.2 Mass (g) 4.772

Titration 8 Rough 1 2 3

Initial Volume (cm^3) (±0.10cm^3) 0.00 16.40 8.00 24.30

Final Volume (cm^3) (±0.10cm^3) 16.40 32.70 24.30 40.50

Used (cm^3) (±0.10cm^3) 16.40 16.30 16.30 16.20

Average Volume Used (cm^3) (±0.10cm^3) 16.27

*Titration 3 not included in processed data

Uncertainties table

Measurement Average value Absolute error Percentage uncertainty

Titre volume 16.49 cm3 ±0.10 cm3 ±0.61 %

Temperature of solution 38.5 °C ±0.50 °C ±1.30 %

Volume of Aquilot 5cm^3 ±0.05cm^3 ±1.00%

Volume of distilled water 200cm^3 ±2.5cm^3 ±1.25%

Mass of Tablet 4.76g ±0.0005g 0.01%

Total - - ±4.17 %

All average values are given to the degree of accuracy respective of their various specifications

Absolute error is given to 2 significant figures

Percentage uncertainty is given to two decimal places

Calculations

Average titre values were found using the following formula:

Σ of concordant titrations at a temperature (cm^3)

Number of relevant titrations

Example calculation:

16.30 + 16.30 + 16.20

Additionally, as tablets of slightly different mass were used all average titre volumes have been calculated for volume of titre used per gram using the following formula:

Average volume of titre used (cm^3)

Mass of tablet used (g)

Example calculation:

16.27

4.772

To convert the 3.41cm^3 into cubic decimetres knowing that

1 dm^3 = 1000cm^3

Example calculation:

3.411000

The value of the titre volume per gram of tablet was then used in correspondence with the known concentration of the Iodine titrate solution (0.01mol dm^-3) to calculate the concentration of Ascorbic Acid per gram of tablet in the solution. This was done using the formula N = CV.

First, the number of moles of Iodine used per gram was calculated:

(Concentration of Iodine Solution) * (Volume of Iodine solution used per gram) = (Moles of Iodine)

Example Calculation:

0.01 * 0.003.41 = 3.41x10^-5 mol

We know from our original formula:

C6H8O6 (ascorbic acid) + I2 → 2 I− + dehydroascorbic acid

That the ratio of reactants is 1:1, where one mol of Ascorbic reacts with one mol of Iodine for the reaction to occur. Therefore, the number of moles of Ascorbic acid is equal to the calculated amount of Iodine moles. We then substitute the calculated value for the moles of Iodine in to the formula C = N/V and get our concentration of Ascorbic Acid for 1g of the tablet

(Number of mol of Ascorbic Acid)

(Volume of Ascorbic Acid solution used) / mass of tablet

Example Calculation:

3.41x10^-5 mol

5x10^-3 dm^3

Processed Data

Temperature (K) (±0.5K) Mass of Tablet (g) Average Titre Volume (cm^3) Titre used per gram of tablet (cm^3) Moles of iodine used per gram Concentration of Ascorbic Acid (moldm^-3)

294.1 4.734 16.73 3.534713421 3.53471E-05 0.00707 ±2.95E-05

299.2 4.762 16.67 3.499930001 3.49993E-05 0.00700 ±2.92E-05

304.2 4.776 16.48 3.451284199 3.45128E-05 0.00690 ± 2.88E-05

309.2 4.759 16.60 3.488127758 3.48813E-05 0.00698 ± 2.91E-05

314.2 4.744 16.40 3.456998314 3.457E-05 0.00691 ±2.88E-05

319.2 4.773 16.43 3.442977862 3.44298E-05 0.00689 ±2.87E-05

324.2 4.787 16.37 3.418981965 3.41898E-05 0.00684 ±2.85E-05

329.2 4.772 16.27 3.4087734 3.40877E-05 0.00682 ±2.84E-05

*As previously stated the titration associated with the temperature of 304.2 Kelvin was a consistent outlier so has been excluded

Absolute Uncertainties for final values were calculated using the percentage uncertainty for the whole experiment multiplied by the respective concentration of acid

Conclusion

As I predicted, the concentration of Vitamin C present within solutions was less at higher temperatures. This is because the oxidation of Ascorbic acid to dehydroascorbic acid occurs more readily under high temperatures. As the Asocrbic acid is exposed to more heat energy, its bond weaken and provide an increasingly ideal condition for these bonds to break entirely , However, while my results were compliant with my hypothesis and reflective of a distinct linear trend, I am dissatisfied with the accuracy of data, which has led to certain errors and inaccuracies within my overall experiment most notably the requirement to eliminate a whole data point, and furthermore exemplified by the low R² value (0.9732).

Despite this, the results do comply with accepted scientific theory. In some of my preliminary research I looked at several studies investigating similar topics, most looked at the degradation of Vitamin C with time but included data which distinctly reflected a similar loss in concentration of ascorbic acid at higher temperature. One such study by B.Bunjali (2012) includes a data set which explicitly displays a similar trend to the one I encountered in my experiment:

I elected this study as a similar titre solution (0.01 moldm^-3 Iodine) and starch indicator solution were used. As can be seen in this data Vitamin C concentrations at higher temperatures were found to be considerably lower after certain periods of time. This reflects my findings that concentration of vitamin C in a higher temperature solution will be lower than that in a lower temperature solution (provided both have been prepared for the same period of time).

Operating out of a university, Bunjali had access to more precise instruments then myself as well as conducting his research over a three month period, allowing him to complete considerably more trials allowing his results to be more reliable. The concordance of my results to his, in terms of overall trend of concentrations variance with temperature is reassuring.

The concordance of my results with wider studies proves the reliability of my data in terms of the general trend established, however my low R² value and at times inconsistent results reduces the reliability of my experiment in terms of specific data acquired. Returning to my real world example of the hot lemon and honey drink, while there is evidence to suggest that a luke warm, or even cold drink would provide a greater source of vitamin C for the drinker, the extent this has an effect over shorter time periods like my experiment which looked at concentrations after just ten minutes, are not likely enough to warrant drinking such a beverage cold, even if it is slightly less rich in vitamin C.

Weaknesses

The root of my low R² value, which would suggest significant inaccuracies is no doubt stemming from an aspect, or multiple aspects of my methodology. No doubt the most consistent issue I had was the inability to titrate multiple samples simultaneously. This led to slight discrepancies within the returned values of each data point. The effect of slight differences in time taken to titrate these solutions was no doubt emphasized by my relatively short period of allowing the solution to exist at its specific temperature. While I chose a period of ten minutes, most studies, including that featured in my conclusion, used periods of multiple hours.

This was a significant error in the planning of methodology as extending the time the solution rests would not only have flattened my results due to the longer period allowing for a greater drop of vitamin c concentration, but the time taken between my titrations at a specific temperature would have been increasingly negligible. Another weakness of my experiment was the methodology I used to maintain the temperature of my solution, the insulating device I used work in most part but slight temperature drops over the period of ten minutes were observed at higher temperatures. Adding another potential source of error to my investigation.

Another significant weakness in my methodology included the scheduling of my titrations. I performed my first three titrations (including my third titration which I was forced to exclude) on a separate day to my final five. While I previously speculated that such a difference would have little effect on the outcome of my data, it proved otherwise. The root cause of which I deem to have been slight changes in the concentration of my Iodine titre solution, which is known to be quite unstable.

Furthermore, despite preparing my own Iodine solution after the solution provided to me proved ineffective at times, I neglected to perform a preliminary titration of the Iodine against an acid of known concentration, such a test could’ve have definitively established the concentration of my Iodine solution and have been performed on the days of my titrations which perhaps would’ve indicated why such a significant error was induced between days. This manifested itself in significantly less titrate (0.12cm^3) being required for my third titration (at 304.2K) then my fourth titration (at 309.2K) which reject the consistent trend observed elsewhere in my experiment. Indeed, this led me to question the reliability of my first two titrations. Their inclusion potentially masks a negative exponential relationship between temperature and Vitamin C concentration which can be observed in a part range graph using only the five final data points:

As can be seen in the part range graph there exists a strong trend exemplified by the greater R² value. Despite the stronger trend of such a graph, the absence of three of my data points would have left me with only five left, which would almost undermine the purpose of the entire experiment. If I had the benefit of additional lab time I would have re done the first three titrations compliant with the recommendations I have/will specify which no doubt would have benefitted the experiment.

Strengths

Despite some errors in my methodology, I did have the opportunity to prepare my own Iodine solution which I did prior to my first titrations. This benefited my experiment as I could be fairly confident in the concentration of the solution initially.

Additionally, the use of Iodine as the titre and starch as an indicator solution proved beneficial to the itraccuracy of my titrations due to the distinctive blue hue that was produce when excess iodine reacted with the starch. This made it easier to distinguish when the reaction between Iodine and Ascorbic acid in solution had ceased and the titration completed.

Improvements

I addressed some of the improvements I considered appropriate in my weaknesses section but to summarize some of my main points:

Confirming Iodine concentration

•  Instead of accepting the stated concentration of, a titration of my Iodine solution against a known concentration of acid would have concluded as to any differences between concentration of different days of the experiment

Performing all titrations on the same day

• Would have negated the extent of differences between concentrations of Iodine on different days, created more consistent results pattern which likely would have made the third data point viable as well as creating a graph across all 8 points which reflect the part-range graph of the final 5 points

Increasing the time period before titration

•  Increasing the period of time to an hour or more would have yielded more distinct results between trials at different temperatures as well as negating the effects of smaller sources of error which have a greater affect on data with smaller numerical values. A water bath could’ve been used to standardize the temperature of the ascorbic acid solution over this extended period

Extension

In my research, almost all similar experiment I encountered included took samples of different temperatures of ascorbic acid solutions at different times. While introducing another variable, it would have been interesting to explore the differing rates and trends at which ascorbic acid was oxidised over time at different temperatures in my own experiment.

Bibliography

1. B.Bundjali. (2009). Research Gate.
2. Retrieved from: https://www.researchgate.net/publication/228484005_KINETICS_OF_THE_OXIDATION_OF_VITAMIN_C Titrations.info staff. (2012). Solutions used in iodometrictitrations.
3. Retrieved from: http://www.titrations.info/iodometric-titration-solutions Metservice staff. (2018). Auckland Central weather.
4. Retrieved from: https://www.metservice.com/towns-cities/auckland/auckland-central#!/your-weather0 Oregon state staff. (n.d.). Vitamin C.
5. Retrieved from: http://chemistry.oregonstate.edu/courses/ch130/old/VITCTEXT.htm Canterbury University staff. (n.d.). Determination of Vitamin C Concentration by Titration.
6. Retrieved from: https://www.canterbury.ac.nz/media/documents/science-outreach/vitaminc_iodine.pdf Lenntech staff. (n.d.) Iodine (I) and water.
7. Retrieved from: https://www.lenntech.com/periodic/water/iodine/iodine-and-water.htm