CHEM123: Experiment #9: Chemical and Nutritional Aspects of Vitamin C: A Project Kamil Krawczyk 50411107 A2335 Abstract: The degradation of vitamin C (ascorbic acid) by heat in pineapple juice was determined by iodometric titration of 25mL juice samples using potassium iodate (KIO 3) and was found to have decreasing levels of ascorbic acid when the samples were heated to temperatures of 70 oC and 110oC. Ascorbic acid concentrations do, in fact, decrease in response to raised temperatures, in accordance to many other scientific studies.
Introduction: Offering many health benefits, vitamin C (scientifically referred to as ascorbic acid) is a necessary component of human health (Vikram et. al 2004). Scientific experimentation has lead to the conclusion that the stability of ascorbic acid decreases with increasing temperatures; retention universally decreased with various heating methods (including, and not limited to, microwave exposure, infrared, etc.) (Vikram (Vikram et. al 2004). In this experiment, the concentration of ascorbic acid tested in pineapple juice after various levels of heat treatment has been identified with a series of iodometric titration (via potassium iodate). The purpose of the research conducted was to ascertain whether exposure to higher temperatures will degrade ascorbic acid, since it is a heat-susceptible molecule.
Experimental Section: The experimental procedure had been only slightly deviated from the initial write up to facilitate ease and more definite results. Because it is difficult to hold a temperature on a hot plate (as the initial procedure indicated that two of the three samples would be boiled to 70 oC and 110oC, rather than the initial 50 oC and 100oC as planned. These two temperatures in particular were decided because there is a great enough range between them, and they offer a high enough kinetic energy to allow the degradation of ascorbic acid. Rather than keeping it the temperature constant for 10 minutes (as again initially planned), which would have been difficult with a hot plate, the sample was taken off of the hot plate as soon as it had reached it's designated temperature. When preparing the titrant, ~2mL of EDTA EDTA was added to each flask to ensure that the presence of iron does not influence the results (since the pineapple juice had a 4% RDI of iron per 175mL, it was necessary to subdue the effects of the metal). After discovering that measuring the evaporated amount was rather difficult to calculate (and would be minor and insignificant to the results), the step was removed from the procedure altogether. Instead, to minimize error, a watch glass was placed on the beaker as it boiled (in order to minimize evaporation and ensure more accurate results). Apart from the minor changes, the experimental procedure has been followed in accordance to the Experimental Design Form (refer to page 139).
Calculations and Tabulated Results: As stated in the procedures, 25mL of pineapple juice were used per trial; after heating and reaching room temperature, the titrate was prepared with the addition of 100mL of deionized water, water, 5mL of 2M hydrochloric acid, 5mL of 0.12M potassium iodide, ~2mL of starch solution and ~2mL of EDTA (both provided in the laboratory). Mass of KIO3 used for standard solution: solution: (mass of weight boat boat + KIO 3 = 0.8915g) – (mass of weigh boat
= 0.5385g) = (mass of KIO3 = 0.3530g) This was then used to formulate a standard solution; that standard solution was diluted by taking 25mL and placing it in another volumetric flask and filled to the 250mL line with deionized water. Therefore, the concentration of the standard solution can be calculated as: (0.3530g of KIO 3 /0.250L) x (1 mol/214.0g of KIO 3) = 6.598 x 10 -3 M (in the initial solution, this number must be divided by 10 as the solution is less concentrated). (0.03530g of KIO 3 /0.250L) x (1 mol/214.0g mol/214.0g of KIO 3) = 6.598 x 10 -4 M (this is the molarity of the solution used to titrate the pineapple juice). Sample Calculation for Grams of Ascorbic Acid Acid Found in Control Sample @ Room Temperature Temperature (20 oC), First Trial: In the first trial, 34.67mL of KIO3 was used to titrate the sample. Therefore, 0.03467L KIO 3 x ( 6.598 x 10 -4 mol/L KIO3) = 2.288 x 10 -5 mol KIO3 Now we can use the 3 mol of ascorbic acid to 1 mol KIO 3 ratio to determine the moles of ascorbic acid. Therefore, 2.288 x 10 -5 mol KIO3 x (3 mol ascorbic acid/1 mol KIO 3) = 6.864 x 10 -5 mol ascorbic acid Furthermore, the conversion of moles to grams is applicable (by applying the molar mass of ascorbic acid). Therefore, 6.864 x 10 -5 mol ascorbic acid x (176.1g of ascorbic acid/mol) = 0.01209g of ascorbic acid Temperature (oC)
Volume consumed for trial #1
Volume consumed for trial #2
Grams of AA, trial #1
Grams of AA, trial #2
20 (room temp./control)
34.67mL
34.81mL
0.01209g
0.01213g
70
33.09mL
33.01mL
0.01153g
0.01151g
110
30.74mL
30.65mL
0.01072g
0.01068g
Average grams of AA, 20 oC: 0.01211g Average grams of AA, 70 oC: 0.01152g Average grams of AA, 110 oC: 0.01070g Calculation For Mass of KIO 3 Required for Standard Solution: 60mg AA/175mL pineapple pineapple juice = x mg AA/25mL pineapple juice Therefore, we solve for x and we have 8.5714mg of AA within a 25mL sample. To convert into grams of KIO 3: 8.5714mg AA x (1g AA / 1000mg AA) x (1 mol AA/ 176.1g AA) x (1 mol KIO 3 / 3 mol AA) x (1 / 0.0250L) x (0.2500L) x (214.0g KIO 3 / 1 mol KIO 3) = 0.03472 g KIO3 (Refer to calculations on the back of the Experimental Procedure sheet [page 193])
Discussion: As expected, the presence of vitamin C diminished after exposure to heat at various levels; it was found to be proportional to the temperature (ie. higher heat, higher degradation of ascorbic acid). 25mL samples were chosen because they have the appropriate amount of ascorbic acid per volume for simple testing via titration (otherwise, the results would have been less precise with less significant figures present due to the limitations of the analytical balance only reading to the fourth decimal place). The standard solution had to be diluted twice to maintain 4 digit precision. Since the value of ascorbic
acid found in 25mL of pineapple juice was less than 0.1g, that meant that the amount of KIO 3 required would be equivalent to 0.0xxx, which cannot be read to a precision of 4 digits under the usage of the analytical balance. Therefore, the value was multiplied by 10, weighed out, made into standard solution, and 25mL of that standard solution was placed in another volumetric flask and diluted to 250mL in order to maintain a precision of 4 decimal places (since the value of the initial mass of KIO 3 was known, being measured out on the balance). This provided the most precise results possible. When preparing the titrant, the various chemicals listed above were added for a very specific reason: without these present, the titration would not reach a visible endpoint. As mentioned above, EDTA EDTA was added to prevent the iron found in pineapple juice from skewing the results. Starch and KI were added to act as the indicator (since the reaction between KI, KIO 3, and ascorbic acid is the basis behind the iodometric titration employed in this experiment). HCl was added to decrease the pH level, also required for the iodometric titration to undergo properly. The titration required the KIO 3 be the titrate (located in the buret) and the solution of the pineapple juice and various other chemicals to be the titrant, located in the erlenmeyer flasks prepared during the lab.
However, However, no experiment is fr ee of error; here, a brief explanation of possible sources of error will be discussed. Unfortunately, Unfortunately, it most likely that not all the vitamin C in the foodstuff (pineapple juice) was extracted and titrated properly, due to simple human error. The most obvious (and most common) sources of error come from improper preparation of experimental equipment. For example, the rinsing of the buret may not have been ideal; despite being rinsed with water, particles from the previous experiment may not have been cleaned out. Furthermore, if the buret was not initially rinsed with water then the titrate, then the experiment's precision would have been seriously botched. Apart from common error, here are a few sources of error that may have modified the result of the experiment conducted: – When heating the pineapple juice, an obvious problem that comes hand in hand is evaporation; since it is hard to measure the volume lost due to time constraints, a watch glass was placed on top of the beaker to minimize loss due to evaporation. However, However, this is probably not the most effect method and may have modified the results.
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When observing the titrant for the endpoint, it is hard to say when the endpoint has been reached. Since it is based on colour, this may or may not be an opinion-based observation – has it reached its endpoint? Can this volume be recorded? However, due to the (again) time restraints, it is hard to sit and allow for a perfect end-point, hence why a reasonable accurate rea ding was taken (although the effect of improper reading may have been counterbalanced by the fact that a common colour was decided as the endpoint and enforced for the rest of the titrations). The addition of EDTA EDTA may or may not have helped the reading, or perhaps modified it. It is hard to determine, since no comparison data is available. The preparation of the second standard solution may have changed the concentration ever so slightly; the volumetric flask was haphazardly rinsed out due to rush. Also, the transfer was most likely not exactly 25mL – therefore, the dilution lead to a standard solution of a slightly different molarity than the initial.
Other experiments could have been performed to ensure the validity of the hypothesis; other temperature ranges could have been tested, and probably would have lead to a more accurate graphical representation of the change of presence of ascorbic acid relative to temperature. Other methods of heating could have also been used. Rather than solely relying on the hot plate, a microwave or even an oven could have been used to maintain the regularity of the results. Different volumes may have been analyzed to recreate the experiment on a grander scale, to compare to the smaller-scale results.
Conclusion: The results attained from the experiment are in clear correlation to the stated hypothesis. As ascorbic acid is exposed to higher temperatures, it degrades and its overall value decreases. From 20 oC to 70 oC, there is a difference of 5.9 x 10 -4 grams; from 70oC to 110oC, the difference is 8.2 x 10 grams, which is reasonably close to the earlier change. It seems that as temperature increases, more ascorbic acid is decreased (since the first step is produces a smaller change in mass over a smaller interval). With further experimentation, employing many more temperature ranges, a better understanding of this decrease of ascorbic acid can be understood, although the results will correlate to what has been determined by this experiment.
Bibliography:
Vikram, Vikram, W.B.; W.B.; Ramesh, M.N.; Prapulla, S.G. Int. S.G. Int. J. Food Eng . 2004, 69, 31-40.
