Lab Report Vit C Titration new

Sailing on the “C“: Titration with a Twist

Anna-Marie Finger

CHEM 1811-Section 12

September 14, 2012

Results and Discussion

In this experiment a solution containing ascorbic acid was titrated with an iodine solution in order to determine the concentration of ascorbic acid in limes, lemons and oranges (eq 1). The concentration of the ascorbic acid in the fruit samples could be determined by referring to values of a standardized iodine solution in our calculations. Finally, the central question of which fruit would be the best one to take on a 50 day long journey, where every member of the 20 sailor crew needs 75mg of ascorbic acid per day was answered with the help of calculations. Those results are shown in Table 4. and 5.

Ascorbic Acid Titration. Reaction of Ascorbic Acid or Vitamin C (1) and Iodine (2) in which 1 is oxidized and 2 is reduced (eq 1.)

1 2

Once all the ascorbic acid was oxidized and iodine that couldn’t react with any ascorbic acid molecules was left we were able to observe a change in color. This change of color is based on the formation of a purple complex between iodine and the “starch” indicator solution. However, reduced iodine is not able to react with starch and only “leftover iodine”, which indicates the complete oxidation of the ascorbic acid in the sample, indicates the end of the reaction.

Standardization of the Iodine Solution. In order to be able to determine the concentration of 1 in the fruit samples we needed to prepare a standardized iodine solution. Based on amount of ascorbic acid in our titrated solution as well as on the volume of iodine solution used to cause the color change we were able to determine the concentration of iodine in the iodine solution and the concentration of ascorbic acid in the ascorbic acid solution. We repeated this titration and collected class data so that we could use the average values of our measurements for our calculations. This approach leads to a more reliable result than just using the values of one single titration since the impact on different errors on the measurements should be minimized. Results of those calculations are shown in Table 1.

Table 1. Standardization of the Iodine Solution

Titration of the fruit samples. For determining the ascorbic acid concentration in the three different fruits we were investigating we needed to prepare sample solutions for those different fruits. By sample solutions the extracted fruit juice diluted with distilled water is meant (WHY DOESN’T THAT INFLUENCE CONCENTRATION?!) . Since our laboratory time is limited each group only titrated the fruit juice of one fruit and the data for the other fruits was gained by collecting class data shown in Table 3. and calculation the class average for each of the remaining fruits. Based on the concentration of iodine iodine we were able to determine by the standardization of the iodine solution we were able to calculate the concentration and mass and of ascorbic acid in the fruit juice (Table 2.) .

Table2. Titration of fruit juice

Table 3. Class data and calculated data based on Juice Titrations

*by displacement in water

Solving the problem. In order to solve the central question, which fruit would be the best one to take on a 50-day voyage for a 20 person sails crew on which every crew member needs 75mg ascorbic acid/day, I used the data obtained by the titration of the fruit samples and shown in Table 3. Based on these conditions we know that we would need a supply of 75g ascorbic acid for the whole journey. However this would not include the ascorbic acid need of the captain; this aspect is discussed in the final discussion section. Based on this value, the volume of the fruit juice samples, the volume of the whole fruits and the concentration of ascorbic acid in our fruit juice samples I calculated the number of fruits we would have to take to cover the 75g need, as well as the overall weight and volume the fruits we would have to take would take.

Table 4. shows how many fruits we would have to juice, as well the weight and volume the needed fruit juice would occupy on a ship if we would try to just use the juice we can press out of one fruit to cover the overall ascorbic acid need for the whole trip. Based on the ascorbic acid concentration per mL juice, sample we were able to determine by titration, as well as the volume of the fruit juice obtained by juicing the fruits I was able to determine how many grams ascorbic acid each fruit juice, obtainable through juicing the fruits, contains and how many fruits we would then have to juice in order to get out enough fruit juice to cover the overall need of 75g ascorbic acid.

Referring to those results oranges would clearly be the best fruits to take on the ship since we would need the lowest number of fruits to be able to obtain enough fruit juice to cover the ascorbic acid need of the whole crew for 50 days. Also the volume of the fruit juice we would actually have to store on the ship is the least compared to that of limes and lemons.

Table 4. Final results #1, determination of which fruit would be the best to take based on the volume of fruit juice needed

If we now disregard the amount of fruit juice we were able to contain by juicing the fruits and just consider the whole volume of the fruits, meaning that we would store whole fruits on our boat as ascorbic acid supply, we obtain slightly different results. Based on the concentration of ascorbic acid in our titrated fruit juice samples I was able to calculate the mass of ascorbic acid in each of the whole fruits (using their whole volume and not just that of the pressed fruit juice). Using this value I was able to determine the number of fruits we would have to take to cover a 75g ascorbic acid need for the entire trip, as well as the volume and weight these fruits would occupy on the ship (Table 5.)

Referring to the results of those calculations oranges would still be the best fruits to take because the number of oranges that would be needed to cover the need of ascorbic acid of the crew would be the lowest compared to limes and lemons. Additionally the volume occupied by the stored oranges, as well as the extra weight on the ship would be lowest.

Table 5. Final results #2, determination of which fruit would be the best to take based on the number of fruits needed

Packing of fruits. To make those data shown above more understandable I assumed a 10x10x10cm box could be used for the packing of the fruit juice a well as for the whole fruits since it would be more efficient to use boxes which can be stapled for the transport of our ascorbic acid source than just packing the bare fruits. Furthermore, to make the example more comparable I assumed we would also fill the juice into those boxes.

Based on the total volume of fruit juice we would have to take the required number of 10x10x10cm boxes, which would have a volume of 0.001m³, could easily be determined. Those data also show that taking the fruit juice of oranges would be the best option since it would require the smallest number of boxes we would have to store on the ship (Table 6). However, since we can’t take ‘half boxes’ I had to round the needed number of boxes obtained by calculation to the next integral number. That means the boxes would take away a bit more space than is actually needed.

Table 6. Packing of fruit juice

The number of boxes we would have to take of we would want to use whole fruits as ascorbic acid supply is shown in Table 7. In this case oranges would also be the best fruits to take since the required number of boxes is the lowest in comparison to limes and lemons.

Table 7. Packing of whole fruits

Final Discussion. According to both interpretations of the data obtained by the titration oranges would clearly be most suitable to take because due to their volume-[ascorbic acid]-ratio oranges as well as their fruit juice have the best properties to serve as our vitamin C supply. That means even if oranges have a greater volume than limes and lemons and therefore occupy more space their ascorbic acid content is also bigger than that of limes and lemons. So that means if we now consider the vitamin C content relative to the volume it turns out that using oranges would be the best fruits to take because their vitamin C concentration (mass per unit volume) is the highest and therefore we need to take less oranges which outweighs the disadvantage of their big volume.

It also turned out that if we would use the fruit juice of oranges would be the most suitable to take because the smallest volume of juice would be needed to cover the need of ascorbic acid of the sails crew during the journey. This is the case because we can obtain the greatest volume of juice by juicing an orange and also because again the ascorbic acid concentration is greatest in oranges.

So how can we now decided if taking whole fruits or fruit juice would be the better option? If we would just consider the number of boxes we would have to take to store the needed amount of vitamin c for the journey taking the fruit juice would be the better option since fewer boxes would be needed. However, using the pure fruit juice as supply would most likely have other disadvantaged which would outweigh the fewer number of boxes (which isn’t so much lower than the number of boxes required for whole fruit storage). For example could we use simple cartons for the storage of whole fruits while we would have to use impregnated or maybe wooden boxes to store the fruit juice. Finally, that would mean that those boxes could be heavier and harder to produce than those for whole fruit storage and additionally juicing all those fruits would mean a lot of extra work. Also the juice could easily goo bad and the vitamin c content in the juice would decrease due to oxygen oxidation of ascorbic acid.

So as we see even if we would have to store a slightly smaller number of boxes on the ship if we would take the fruit juice, taking the whole fruits would be the favored option.

Error discussion. In my calculations I did not include the captain of the ship because the assignment was asking for the vitamin c supply for the crew but if I had in included the captain, what means that we would need 76.5g vitamin c for the whole trip instead of 75g. The number of fruits we would have to take would be slightly different. However, since the need of vitamin c for one person is not that big, the addition of the captain to my calculations would not have an crucial effect on the result and oranges would still be the best fruit to take.

Another human made mistake was that when we prepared the ascorbic acid solution for the standardization of the iodine solution we filled the Erlenmeyer flask up with a bit too much water. That means the ascorbic acid in the solution was a bit more diluted than it was meant to leading to slightly lower concentration in the ascorbic acid solution than those we used for our calculations. That means if the concentration of vitamin c in the solution titrated with iodine solution the concentration of iodine we calculated might have been inaccurate. Meaning that, if we would have used a ascorbic acid solution with the correct concentration, we would have needed a slightly higher concentration of iodine to reach the endpoint. However, since the volume of water, which was too much, wasn’t that big we can assume that the final result of our calculations is correct and that oranges would be the best fruit to take in either case.

When investigating the fruits we did not focus on the varying sizes of fruits, as well as on the thickness of their peel or their eatability. With that I mean that oranges can be easily pealed and eaten as snack what means it would be easy to use them as vitamin c source. Compared to that limes and lemons are rather hard to eat and we would have to process them otherwise to obtain their vitamin c. So that is another advantage of oranges that would support the decision to use them as vitamin c supply on the journey. However, one could also argue that oranges have a thick peel compared to limes and lemons and that the volume we measured by displacement of water is not correct since we put the whole fruit into the water and did not subtract a certain amount of volume (the peel) that would not contribute to the vitamin c content of the fruit since the sails crew wouldn’t be able to eat it. So dependent on the type of orange the peel can be very big and this fact might have a crucial effect on the calculations. However, since it is very hard to determine a the exact volume we disregarded this fact in our calculations and discussions and just assumed that the thickness of the peel would not alter our final result too much.

The last point I would like to discuss was the coloring of the fruit juice because due to the already colored juice it was hard to exactly determine the endpoint of the titration because neither of us knew how the color change would look like. That means it could be possible we were overshooting the end point and based on that calculated a slightly different concentration of vitamin c for the fruit juice. However, even if we slightly overshot the end point the concentration of the vitamin c we determined should almost be correct and should not affect our final result in so far that it would lead to another assumption about what fruit would be best to take.

DMU Timestamp: September 14, 2012 19:31