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1 | P a g e
Research question: to find how storage time affects the concentration of vitamin C in
lemons.
Background information: Vitamin C (ascorbic acid) is a water based nutrient that is essential
for plant and animal metabolisms. It is the L-enantiomer (an optical isomer) for ascorbic
acid. Vitamin C plays an essential role in protecting cells from damage and also helps in
formation of collagen which is essential for the human skin, ligaments and tendons. It is
essential for the repair of wounds and scars. Vitamin C helps in reducing the risk of cancer.
Most plants and animals have the ability to synthesize this vitamin internally. Deficiency of
this vital vitamin leads to scurvy. Those suffering from scurvy have abnormalities in their
teeth and bones. However, human beings do not have this ability hence they require
external sources like fruits, tablets and juices in order to fulfil their vitamin C requirement.
Vitamin C is not stored in the human body therefore daily intake is necessary. The
wonderful part of vitamin C is that any excess of it is not fatal.
It is pertinent that vitamin C forms an essential part of required dietary allowance. A
common man usually picks sources like fruits and vegetables from a local vendor. Lemon
forms a common source of vitamin C in an average Indian family. 16% of the world total
lemon produce comes from India. The vitamin C content of these lemons depends upon
factors like the weather of the place where the plants are grown, the soil condition and
maturity level of the tree which bears the fruit. The fertilizers used may affect the
concentration of vitamin C. High levels of Nitrogen in the soil can lower the concentration of
vitamin C.1 Vitamin C concentration decreases during the ripening period which means
young fruits tend to have the highest levels of concentration. In India, due to farmer
illiteracy and ignorance the crop production and management is poor. Therefore the quality
of lemons is low. Hence even though India has a high yield quality of the produce is still low.
Chemically, ascorbic acid, (5R)-[(1S)-1, 2-dihydroxyethyl] – 3, 4-dihydroxyfuran-2(5H)-one
(C6H8O6) is an odourless white organic molecule. It is a weak dibasic acid with acidity (pKa)
around 4.17. It has a simple structure, one that resembles a monosaccharide. A
Monosaccharide is a single unit of carbohydrate that cannot be broken into simpler
1 www.Wikipedia.com\vitaminC
2 | P a g e
carbohydrates upon hydrolysis2. It is a carboxylic acid with a vinyl double bond which
transmits electrons between the hydroxyl and the carbonyl.3 Ascorbic acid is a reducing
agent and hence helps protect fatty acids and other vitamins from oxidation. However it’s
reducing property also becomes the prime reason for the loss of vitamin C concentration
over time. In presence of oxygen it gets oxidized to form dehydroascorbic acid. .
Dehyrdoascrobic acid is the reason for the oxidative potential of the endoplasmic reticulum.
It is an antiscorbutic i.e. it is used to cure scurvy. This chemistry concept leads me to
investigate the drop in concentration of vitamin C over time.
The investigation of this concept is essential. The transport and warehousing system in this
market is a major concern. The time taken for the lemons to the reach its consumers is a lot.
There is already a significant drop in its nutritional value. In developing countries like India
people tend to migrate from rural areas to urban areas. This leads to space crunch in big
cities. In addition to that, rapid industrialization also leads to reduced farmlands in and
around big cities as the farmlands are replaced by factories. Therefore the city-dwellers are
dependent on their rural counterparts to provide for their agricultural needs. The rural
areas in India are underdeveloped with lack basic amneties like sewage and healthcare
.Therefore the public facilities like transport are also immature. It takes a long time for the
fruits to reach its consumers. After the fruits are plucked, they go through various dealings
and trading before they reach the consumers. It goes from the farms to wholesalers from
where it is distributed and sent to city wholesalers. These city wholesalers then further
distribute the product region wise and eventually it reaches the hands of the consumers
through their local vendors. This means that the fruit our local vendors sell to us claiming to
be” fresh fruits” are actually at least 10 to 15 days old. Hence the natural content of
vitamins and minerals that the fruit gives us is depleted to a very large extent. In an average
Indian family lemon is added in most of their servings of food. People tend to buy their fruit
about once a week and store their fruit for approximately 6 to 7 days. A good example is
Kodaikanal, where purchase of lemons takes place on the Sunday Market. The family then
continues to use the lemons for the rest of the week. Hence I took up this experiment in
2 Bahl, Arun, and B. S. Bahl. A Textbook of Organic Chemistry.: S.Chand and Company, 2003
3 (http://en.wikipedia.org/wiki/Ascorbic_acid
3 | P a g e
order to check the drop in concentration of vitamin C over time since following this trend
the average Indian family gets a very little amount of Vitamin C from their food. Therefore in
my experiment i chose days accordingly. I took days up to 13 days since all the lemons
bought may not have been consumed within the week. This made it necessary for me to
take readings until the 13th
day. Also if i would have taken reading for only 7 days i would
not have got accurate reading as the 1st
day and the last day would not have been very far
apart and hence the drop in concentration would not have been very large.
Hypothesis: In this experiment our main concern is to find the effect of storage time on the
concentration of Vitamin C. Theoretically, the concentration of vitamin C in lemons or any
other fresh source of vitamin C decreases with increase in storage time which means that
the two have an inverse co-relation.
This is because when the lemons are stored for long periods of time they are exposed to
oxygen for that period of time. Hence in the presence of oxygen Vitamin C (ascorbic acid)
being a reducing agent gets oxidized to form dehydroascorbic acid.
4 | P a g e
Materials Required:
• Fresh Lemon (3 kg)
• 5 g of potassium Iodide (KI)
• 0.5 g of potassium Iodate (KIO3)
• 1 dm3 of distilled water
• 30 cm3 of sulphuric acid (3 mol dm
-3)
• Starch
• 5 Retort stands with clamps
• White tile
• 1 X 100 cm3 Measuring cylinder (±0.5 cm
3)
• 5 X 50 cm3 burettes (±0.1 cm
3)
• 2 X 250 cm3
Beakers
• 1 X 250 cm3
Graduated flask
• 1 X 5cm3 syringe (± 0.1cm
3)
• 1 X 20cm3
Pipette
• 5 X 100 cm3 Conical Flasks
• 1 Funnel
• Lemon Squeezer
• Electric kettle to boil water
• Digital Weighing scale (0.01 g)
• Digital Thermometer (0.1®C)
5 | P a g e
Procedure:
The lemons are stored at room temperature.
1. Preparing the lemon juice:
• Cut lemons in two halves and use a lemon squeezer to squeeze out the
lemon juice.
• Pour the juice into 250cm3 beaker.
• Using the filter paper and a funnel filter the lemon juice.
2. Preparing the iodine solution:
• Measure 5 grams of Potassium iodide and 0.3 grams of Potassium Iodate.
• The measured potassium iodide and potassium Iodate was mixed.
• Mix 200 cm3 of distilled water with 30 cm
3 of sulphuric acid.
• Add this to the mixture of potassium iodide and potassium Iodate to create a
solution.
• Pour this solution into a graduate flask and add another 500 cm3 of distilled
water and mix well.
• Ensure that the cap of the graduated flask is tightly closed after each usage.
The iodine solution is produced according the equation presented below.
KIO3 (aq) + 5KI (aq) + 3H2SO4 ---------------> 3I2 (aq) + 3H2O(l) + 3K2SO4 (aq)
3. Preparing the starch solution:
• Measure 2 grams of starch.
• Take 100 cm3 of boiling water.
• Mix the two and leave it to cool before use.
• Use a thermometer to ensure that the solution does not cool below 50°C.
Starch indicator is used because iodine tends to form a complex with starch
which is dark blue in colour.
6 | P a g e
4. For titration:
• Rinse the burette with lemon juice.
• Attach a burette to the clamp stand.
• Ensure that the knob of the burette is closed tightly.
• Pour the lemon juice into the burette using a funnel. The lower meniscus
should coincide with the zero mark on the burette.
• Rinse the pipette with the starch solution.
• Use a pipette carefully to transfer 20cm3 of the starch solution into a conical
flask.
• Using a syringe, add 2 cm3 of the iodine solution into the conical flask.
• Swirl gently.
• Place the conical flask on a white tile on the clamp stand.
• Open the knob of the burette and allow one drop at a time to fall out.
• Wait till end point is reached. The end point is determined when the solution
in the conical flask turns ink blue on the addition of one drop of the lemon
juice.
• Record all observations.
5. Calculation of concentration of Vitamin C (ascorbic acid) in lemons:
• In order to find the concentration of vitamin C in lemons we must first
formulate chemical equations.
• In this experiment two main equations were used:
Equation 1: This equation will enable us to find the limiting reactant in the
formation of I2 and hence the concentration of I2 used.
KIO3 (aq) + 5KI (aq) + 3H2SO4 ---------------> 3I2 (aq) + 3H2O(l) + 3K2SO4 (aq)
Equation 2: This equation is the reaction that takes place when lemon juice is
added to the starch iodine solution. This leads to the formation of
dehydroascorbic acid.
C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O
-
• As per Equation 1 we find the limiting reactant of the reaction and hence find
the number of moles of I2 used in 250cm3
of the iodine solution.
7 | P a g e
• Since we use only 2cm3 of iodine in each trial, we find the number of moles of
I2 in 2cm3 of the solution.
• Using equation 2 we know that the number of moles of ascorbic acid used is
equal to the number of moles of iodine used.
• The titre value gives us the volume of ascorbic acid required to neutralize the
starch-iodine solution.
• Using the given formula we can calculate the concentration of vitamin C in
fresh lemons.
CONCENTRATION (mole dm-3
) = MOLES
VOLUME
DATE COLLECTION:
DAY 1:
Trial number Titre value/dm3 (± 1 x 10
-4)
Trial number 1 0.0036
Trial number 2 0.0037
Trial number 3 0.0036
Trial number 4 0.0037
Trial number 5 0.0036
Average titre value 0.00364 (± 1 x 10-4
dm3)
DAY 4:
Trial number Titre value/cm3 (± 1 x 10
-4 dm
3)
Trial number 1 0.0043
Trial number 2 0.0044
Trial number 3 0.0044
Trial number 4 0.0044
Trial number 5 0.0044
Average titre value 0.00438 (± 1 x 10-4
dm3)
8 | P a g e
DAY 7:
Trial number Titre value/cm3 (± 1 x 10
-4 dm
3)
Trial number 1 0.0048
Trial number 2 0.0049
Trial number 3 0.0049
Trial number 4 0.0048
Trial number 5 0.0049
Average titre value 0.00486 (± 1 x 10-4
dm3)
DAY 10:
Trial number Titre value/cm3 (± 1 x 10
-4 dm
3)
Trial number 1 0.0052
Trial number 2 0.0053
Trial number 3 0.0053
Trial number 4 0.0052
Trial number 5 0.0053
Average titre value 0.00526 (± 1 x 10-4
dm3)
DAY 13
Trial number Titre value/cm3 (± 1 x 10
-4 dm
3)
Trial number 1 0.0054
Trial number 2 0.0056
Trial number 3 0.0055
Trial number 4 0.0055
Trial number 5 0.0055
Average titre value 0.00550 (± 1 x 10-4
dm3)
9 | P a g e
NOTE: The procedure mentioned above is repeated at the end of 4 days , at the end of 8 days and
then at the end of 12 days. However the iodine solution is prepared only once at the beginning of
the experiment.
Qualitative Data:
• Qu
10 | P a g e
Variables:
Table1: This table shows the independent, dependent and controlled variables.
Independent variable Dependant variable Controlled variable
Concentration of Vitamin C
in lemons.
Time
Amount of iodine - starch
solution required for the
titration.
Concentration of the iodine
solution used.
Temperature in which the
lemons are stored.
Temperature of the starch
solution.
Table 2: This tables shows the controlled variables and a suitable method to control the
variables.
Controlled variable Method to control the variables
• Concentration of the iodine
solution used.
• Temperature in which the lemons
are stored.
• Temperature of the starch solution.
• In order to control the concentration
of iodine solution we prepare a large
amount of the solution at one time so
that there are reduced fluctuations in
the concentration.
• We keep the lemons at room
temperature for intervals of 12 days.
Room temperature is around 15 to 20
°C.
• After the water comes to a complete
boil we mix starch solution in it and
let it cool for 5 minutes.
11 | P a g e
Variables that could not be controlled:
• Temperature of the surrounding could not be controlled since we left the
temperature at room temperature and hence the temperature did not stay
• The place where the lemons were grown or the soil it was grown or the fertilizer
content.
• I also did not know the duration for which the vendor had the lemons with him.
Data Processing:
Table 3: Shows the concentration of Vitamin C required to reach the end point in starch-
iodine titration for each day.
NOTE: ALL VALUES ARE IN dm3.
Trial number DAY 1 DAY 4 DAY 7 DAY 10 DAY 13
Average titre
value 0.00364 0.00438 0.00486 0.00526 0.00550
Sample calculation:
KIO3 (aq) + 5KI (aq) + 3H2SO4 ---------------> 3I2 (aq) + 3H2O(l) + 3K2SO4 (aq)
Molecular mass of KIO3 and KI.
KI = ( 39.098 + 126.90 ) = 165.998 g
KIO3 = ( 39.098 + 126.90 + 16*3 ) = 213.998g
To find the number of moles of Potassium Iodate and potassium iodide.
����������� = � ��� ���
����� �� ��
12 | P a g e
NumberofmolesforKI = �
!�.##$ = 0.030 moles
NumberofmolesforKIO3 = '.(
) (.##$ = 1.4 X 10
-3 moles
To find limiting agent ,
1.4 X 10-3
x 5 = 7 x 10-3
moles
This means that 7 x 10-3
moles of potassium iodate would have been required to react
with 0.030 moles. However only 1.4 X 10-3
moles of potassium iodate were used. Hence
potassium iodate becomes the limiting reagent.
To find the number of moles of I2 :
1 mol of KIO3 ------------- 3 mol of I2 in 0.25 dm3 of solution
1.4 x 10-3
mol of KIO3 ------------------ X mol of I2
X moles = 1.4 x 10-3
x 3 = 4.2 x 10-3
mol of I2 in 0.25 dm3 of solution
To find the number of moles of I2 in 0.002 dm3 of solution:
0.25 dm3 ------------------------------- 4.2 x 10
-3 mol of I2
0.002 dm3 ------------------------------- Z mol of I2
Z mol of I2 = '.'')*+.)* ',(
'.)�=3.36 x 10
-5
Find the number of moles of ascorbic acid:
C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O
-
1 : 1
The moles of I2 is known. Looking at the molar ration of 1:1 between ascorbic acid and
iodine we know that ,
The number of moles of I2 = number of moles of ascorbic acid
13 | P a g e
3.36 x 10-5
moles
= 3.36 x 10-5
moles
Find the concentration of ascorbic acid:
On day 1,
Titre Value = volume of ascorbic acid = 0.00364 dm3
Number of moles of ascorbic acid = 3.36 x 10-5
Therefore,
Concentration = -./01234/3516
735./1
Concentration = (.(!* ',�
'.''(!+ = 9.23 x 10
-3 mol dm
-3
Sample Calculation of Uncertainty; (for detailed calculation of uncertainty please refer to
Appendix 2)
Uncertainty for the number of moles of Potassium Iodate and potassium iodide.
NumberofmolesforKI = �
!�.##$ ( ±
'.'
!�.##$ ) = 0.030 moles (±6.02 x 10
-5)
NumberofmolesforKIO3 = '.(
) (.##$ ( ±
'.'
) (.##$ ) = 1.4 X 10
-3 moles (± 4.67 x 10
-5)
Uncertainty For limiting reagent:
1.4 X 10-3
moles (± 4.67 x 10-5
) x 5 = 7 x 10--3
(± 4.67 x 10-5
x 5)
= 7 x 10-3
(± 2.335 x 10-4
)
14 | P a g e
Uncertainty for the number of moles of I2:
1 mol of KIO3 ------------- 3 mol of I2
1.4 x 10-3
(± 4.67 x 10-5
) mol of KIO3 ------------------ X mol of I2
X moles = 1.4 x 10-3
(± 4.67 x 10-5
) x 3 = 4.2 x 10-3
(± 4.67 x 10-5
x 3) mol of
= 4.2 x 10-3
(± 1.4 x 10-4
)
Uncertainty to find the number of moles of I2 in 0.002 dm3 of solution:
0.25 dm3 ------------------------------- 4.2 x 10
-3 (± 1.4 x 10
-4) mol of I2
0.002 ( 1 x 10-4) dm
3 ------------------------------- Z mol of I2
Relative Uncertainty for 0.002 dm3 =
( * ',+)
'.'') x 100 = 5 %
Relative uncertainty for 4.2 x 10
-3mol =
.+* ',+
+.)* ',( x 100 = 3.33 %
Therefore, (5 % + 3.33 % ) = 8.33 %
Z mol of I2 = '.'')*+.)* ',(
'.)�=3.36 x 10
-5 (± 8.33 %)
To convert to absolute uncertainty = (.(!* ',�*$.((
''
= (2.8 x
10
-6)
Number of moles of I2 3.36 x 10-5
(±2.8 x 10
-6)
Find the number of moles of ascorbic acid:
C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O
-
1 : 1
15 | P a g e
The moles of I2 is known. Looking at the molar ration of 1:1 between ascorbic acid and
iodine we know that ,
The number of moles of I2 = number of moles of ascorbic acid
3.36 x 10-5
(±2.8 x 10
-6) moles
= 3.36 x 10
-5 (±2.8 x
10
-6) moles
Find the concentration of ascorbic acid:
On day 1,
Titre Value = volume of ascorbic acid = 0.00364 (1 x 10-4
) dm3
Number of moles of ascorbic acid = 3.36 x 10-5
(±2.8 x 10
-6)
Therefore,
Concentration = -./01234/3516
735./1
Concentration = (.(!* ',�
'.''(!+ = 9.23 x 10
-3 mol dm
-3
Uncertainty for concentration of vitamin C:
Relative uncertainty for number of moles: = ().$* ',!)
(.(!; ',� x 100 = 8.33 %
Relative uncertainty for volume of ascorbic acid = = ( * ',+)
'.''(!+ x 100 = 2.74 %
Therefore total relative uncertainty for concentration of vitamin C = (8.33 + 2.74) = 11.07
Convert to absolute uncertainty = #.)(* ',(* .'<)
'' = 1.02 x 10
-3
Therefore concentration for Day 1 , 9.23 x 10-3
( ± 1.02 x 10-3
)
16 | P a g e
Summary of the calculations:
Using equation 1:
KIO3 (aq) + 5KI (aq) + 3H2SO4 ---------------> 3I2 (aq) + 3 H2O(l) + 3K2SO4 (aq)
Using this equation we find the limiting reactant and hence number of moles of iodine used.
We find that the number of moles of potassium iodide is 0.030 moles and the number of
moles of potassium iodate is 1.4 x 10-3
moles. This means that 7 x 10-3
moles (multiplied by
5) of potassium Iodate is needed to react with 0.030 moles of potassium iodide but we react
only 1.4 x 10-3
moles. Hence this makes potassium Iodate the limiting reactant.
We then find the number of moles of I2 present in 0.25 dm3 of our solution to be 4.2 x 10
-3
moles and hence the number of moles of iodine in 0.002 dm3
of the solution to be 3.36 x 10-
5 moles.
Using equation 2:
C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O
-
We know that the number of moles of ascorbic acid is equal to number of moles of iodine
used. Hence the concentration of ascorbic acid in lemons on Day 1 would be:
CONCENTRATION (mole dm-3
) = MOLES
VOLUME
CONCENTRATION (mole dm-3
) = 3.36 x 10-5
0.00364
= 9.23 x 10-3
or 0.00923 mol dm-3
The calculations for Day 4,7,10 and 13 are in Appendix 1.
Graph 1: This graph shows the concentration of vitamin C using the real values obtained
over time.
17 | P a g e
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
DAY 1 DAY4 DAY 7 DAY 10 DAY 13
CO
NC
EN
TR
AT
ION
OF
VIT
AM
IN C
IN
LEM
ON
S
NUMBER OF STORAGE DAYS
CONCENTRATION OF VITAMIN C IN LEMONS V/S STORAGE
This graph was plotted using the values that I obtained in my experiment. I expected to get
a downwardly sloping (negative graph). In this graph I can clearly see a negative co-relation
between concentration of vitamin C and storage time. Hence, I can say that as storage time
increases the concentration of vitamin C decreases. There is a steep fall in the concentration
of vitamin C within the first three day. From day 4 to day 7 the rate of fall of concentration
becomes lesser. The availability of oxygen inside of the lemon could be decreasing. The
rate of fall further decreases between day 7 and day 10 after which the concentration of
vitamin C becomes rather constant.
Graph 2: This graph was plotted by extrapolating (forward forecast) the trend line of the
concentration of vitamin C v/s storage time graph.
18 | P a g e
y = -0.0003x + 0.009
-0.002
0
0.002
0.004
0.006
0.008
0.01
0.012
0 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031323334353637383940
CO
NC
EN
TR
AT
ION
OF
VIT
AM
IN C
NUMBER OF DAYS
EXTRAPOLATION OF THE TREND LINE OF THE
CONCENTRATION OF VITAMIN C
In this graph I plotted a general trend line for the values I obtained. The general trend line as
expected is a downwardly sloping graph which means that as the storage time increases
concentration of vitamin C in lemons decreases. In order to predict the time at which the
concentration of vitamin C in lemon becomes zero I extrapolated the graph forward. I
forecast the trend line forward by 25 periods after which the concentration according to the
graph turned zero. In the graph the concentration is shown to turn to become zero 36 days
after buying the lemon (assuming the day I bought the lemon to be Day 1) the concentration
of vitamin C would becomes zero. This is because we cannot determine the amount of days
it takes to reach us (the consumers) after it has been plucked.
NOTE: In order to have accurate error bars i took the average of the calculated uncertainty
for the concentration of Vitamin C at each day. I obtained an average uncertainty of ±1.28
mol dm3 for the error in number of days ( X-error bar) I averaged an uncertainty of 0.069
days by missing out by approximately 5 hours in the three day interval.
19 | P a g e
Conclusion and evaluation:
The aim of my experiment was to find the effect of storage time on the concentration of
vitamin C. Through my experiment I have found that there is a decrease in the
concentration of vitamin C with increase in storage time. This happens because as storage
time increases, the time that the lemons are exposed to oxygen increases. Ascorbic acid
being a reducing agent reacts with the oxygen in the air. Hence, ascorbic acid gets oxidized.
The highly reactive oxygen species oxidize the ascorbate ions first to form
monodehydroascobic acid and then eventually to form dehydroascorbic acid. The oxygen
present in the air gets reduced to form water.
The experiment that I conducted had a few uncontrollable variables. These variables may
have led to a drastic change in the expected concentration of Vitamin C. Most of these
reasons were those that were beyond my control due to the fact that lemons are not grown
locally in Kodaikanal. One of the many reasons maybe that I assumed that the day I bought
the lemons from the vendor to be the day they were plucked ignoring the fact that lemons
are not grown locally in Kodaikanal. The lemons therefore, would have been transported
from other nearby cities where it is grown locally. Lemons cannot possibly be grown in
Kodaikanal as the temperatures here remain low and hence do not facilitate the growing of
lemons. Hence storage time would have exceeded by approximately 10 days and in this
manner we can predict that the concentration of vitamin C would have decreased further.
Another major reason for this could have been the ignorance amongst the Indian farmers.
The farmers in India are not educated and hence tend to use large amounts of fertilizers in
their crop. Famers use large amount of nitrogen and potassium in the soil which lead to
depleted amounts of vitamin C in the soil. Large amounts of nitrogen are added to the soil in
order to make the lemon trees taller and also to make them more fruitful. Another
limitation of my experiment was that we do not know the type of soil in which the lemons
were grown and we also do not know if the lemons belonged to the same farm as well.
Different farms in India have different policies regarding addition of fertilizers and manure
which could have lead to uneven concentration of vitamin C in the lemons we used due to
various reasons. Also I do not the ripening period of the lemon. Young fruits have the
20 | P a g e
highest concentration of vitamin C and hence if the lemons were plucked immaturely or
over maturely the concentration of lemons would have varied. Also the soil the lemons
were grown in is unknown. If the soil had large amount of water in it then ascorbic then
ripening period is prolonged and hence concentration of vitamin C decreases. These were
some factors that could not be controlled by me in the laboratory. However in order to
avoid this kind of errors and uncertainty in my experiment i could have bought the lemons
from a farm directly and without wasting time carried out one trial of our experiment there
itself. This would have ensured us with more accurate and consistent readings. However this
was practically an impossible option for me because as mentioned earlier lemons are not
grown locally and going down to Madurai or Chennai would have interfered with my school
schedule.
There may be various other reasons that may have acted as a barrier in my experiment.
These include the nature of the titrant (here: iodine) used, parallax errors, human errors and
also some errors that are unavoidably part of titration. I also faced some personal problems
while carrying out my experiment. The errors mentioned above are discussed in detailed
below with suitable solutions to avoid this type of error.
One of the limitations caused due to iodine “is the air oxidation of acid-iodide solution”
4which means that the oxygen in the air reacts with the iodine present. This can be avoided
if Sodium carbonate is added to the conical flask. When sodium hydroxide is added to the
flask any amount of oxygen entering the flask is converted into carbon dioxide. Iodine also
happens to be a very volatile substance hence the iodine may disappear slowly from the
solution. Since we prepared a large amount of the solution on the first day of the
experiment, the iodine concentration may have gone down significantly by the last day of
the experiment. This could have been avoided by adding excess of iodine in the iodine
solution as this would give rise to I3- and hence lowers free iodine ions
5.
Parallax errors may have occurred while preparing the iodine solution and the starch
solution and also while carrying out the titration. Also I found it pretty hard to read the
exact readings from the burette. I tried to put the clamp stand on a stool so that it would be
4 http://en.wikipedia.org/wiki/Iodometry
5 http://www.titrations.info/iodometric-titration
21 | P a g e
easier for me to take the reading however I observed that there was still some parallax error
that persisted. This could only have been avoided had I taken numerous readings of one trial
in order to obtain the most appropriate average.
Human errors occurred due to various reasons. My school schedule conflicted with the time
frame in which I was to conduct the experiment due to which the lemons were stored for
about 4 to 5 hours more than they were supposed to be stored. This could not have possibly
been avoided except if I missed my classes to do so which did seem to be a likely option for
me. Hence there could have been some alteration the concentration of vitamin C.
Another likely option could have been the errors that are made during any titration. These
include errors like not rinsing the burette with distilled water before reuse or using the
same measuring cylinder and/or pipette to transfer different materials
The most common error made while carrying out a titration is being unable to find the
difference in colour change. The intensity of colour change may have varied between each
titration. This may have led to an inappropriate titre volume.
22 | P a g e
BIBLIOGRAPHY
Aggie Horticulture. Web. 24 Jan. 2010. <http://aggie-
horticulture.tamu.edu/Citrus/lemons.htm>.
"Ascorbic acid -." Wikipedia, the free encyclopedia. Web. 12 Jan. 2010.
<http://en.wikipedia.org/wiki/Ascorbic_acid>.
Ascorbic acid chemistry, metabolism, and uses : based on a symposium sponsored by the
Division of Carbohydrate Chemistry at the Second Chemical Congress of the North
American Continent (180th ACS National Meeting), Las Vegas, Nevada, August 26-27,
1980. Washington, D.C: American Chemical Society, 1982. Print.
Bahl, Arun, and B. S. Bahl. A Textbook of Organic Chemistry. New Delhi: S.Chand and
Company, 2003. Print.
Hay, Jennifer. Vitamin C everything you need to know. Allentown, Pa: People's Medical
Society, 1998. Print.
"Iodometric titration." Titration and titrimetric methods. Web. 21 Jan. 2010.
<http://www.titrations.info/iodometric-titration>.
"Iodometry: Definition from Answers.com." Answers.com: Wiki Q&A combined with free
online dictionary, thesaurus, and encyclopedias. Web. 25 Jan. 2010.
<http://www.answers.com/topic/iodometry>.
23 | P a g e
"Lemon."Web. 24 Jan. 2010. <http://www.hort.purdue.edu/newcrop/morton/lemon.html>.
Tooley, Peter. Food and Drugs. London: John Murray, 1971. Print.
"Vitamin C -." Wikipedia, the free encyclopedia. Web. 21 Jan. 2010.
<http://en.wikipedia.org/wiki/Vitamin_C>.
"Vitamin C or Ascorbic acid, What is Vitamin C or Ascorbic acid? About its Science, Chemistry
and Structure." Chemistry, Structures & 3D Molecules @ 3Dchem.com - Home. Web.
19 Jan. 2010. <http://www.3dchem.com/molecules.asp?ID=69>.
"WHFoods: Lemon/Limes." The World's Healthiest Foods. Web. 25 Jan. 2010.
<http://www.whfoods.com/genpage.php?tname=foodspice&dbid=27>.
24 | P a g e
APPENDIX 1 :
KIO3 + 5K1 + H2SO4 ------------------------------------------------> 3I2 +
Molecular mass of KIO3 and KI.
KI = ( 39.098 + 126.90 ) = 165.998 g
KIO3 = ( 39.098 + 126.90 + 16*3 ) = 213.998g
To find the number of moles of Potassium Iodate and potassium iodide.
����������� = � ��� ���
����� �� ��
NumberofmolesforKI = �
!�.##$ = 0.030 moles
NumberofmolesforKIO3 = '.(
) (.##$ = 1.4 X 10
-3 moles
To find limiting agent ,
1.4 X 10-3
x 5 = 7 x 10-3
moles
This means that 7 x 10-3
moles of potassium iodate would have been required to react
with 0.030 moles. However only 1.4 X 10-3
moles of potassium iodate were used. Hence
potassium iodate becomes the limiting reagent.
To find the number of moles of I2 :
1 mol of KIO3 ------------- 3 mol of I2 in 0.25 dm3 of solution
1.4 x 10-3
mol of KIO3 ------------------ X mol of I2
X moles = 1.4 x 10-3
x 3 = 4.2 x 10-3
mol of I2 in 0.25 dm3 of solution
25 | P a g e
To find the number of moles of I2 in 0.002 dm3 of solution:
0.25 dm3 ------------------------------- 4.2 x 10
-3 mol of I2
0.002 dm3 ------------------------------- Z mol of I2
Z mol of I2 = '.'')*+.)* ',(
'.)�=3.36 x 10
-5
Find the number of moles of ascorbic acid:
C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O
-
1 : 1
The moles of I2 is known. Looking at the molar ration of 1:1 between ascorbic acid and
iodine we know that ,
The number of moles of I2 = number of moles of ascorbic acid
3.36 x 10-5
moles
= 3.36 x 10-5
moles
Find the concentration of ascorbic acid:
On day 1,
Titre Value = volume of ascorbic acid = 0.00364 dm3
Number of moles of ascorbic acid = 3.36 x 10-5
Therefore,
Concentration = -./01234/3516
735./1
Concentration = (.(!* ',�
'.''(!+ = 9.23 x 10
-3 mol dm
-3
26 | P a g e
On Day 2,
Titre Value = volume of ascorbic acid = 0.00438 dm3
Number of moles of ascorbic acid = 3.36 x 10-5
Concentration = (.(!* ',�
'.''+($ = 7.67 x 10
-3 mol dm
-3
On Day 7,
Titre Value = volume of ascorbic acid = 0.00468 dm3
Number of moles of ascorbic acid = 3.36 x 10-5
Concentration = (.(!* ',�
'.''+!$ = 6.91 x 10
-3 mol dm
-3
Day 10,
Titre Value = volume of ascorbic acid = 0.00526 dm3
Number of moles of ascorbic acid = 3.36 x 10-5
Concentration = (.(!* ',�
'.''�)! = 6.38 x 10
-3 mol dm
-3
Day 13,
Titre Value = volume of ascorbic acid = 0.00550 dm3
Number of moles of ascorbic acid = 3.36 x 10-5
Concentration = (.(!* ',�
'.''��' = 6.12 x 10
-3 mol dm
-3
27 | P a g e
Appendix 2: Calculation of uncertainty:
Uncertainty for the number of moles of Potassium Iodate and potassium iodide.
NumberofmolesforKI = �
!�.##$ ( ±
'.'
!�.##$ ) = 0.030 moles (±6.02 x 10
-5)
NumberofmolesforKIO3 = '.(
) (.##$ ( ±
'.'
) (.##$ ) = 1.4 X 10
-3 moles (± 4.67 x 10
-5)
Uncertainty For limiting reagent:
1.4 X 10-3
moles (± 4.67 x 10-5
) x 5 = 7 x 10--3
(± 4.67 x 10-5
x 5)
= 7 x 10-3
(± 2.335 x 10-4
)
Uncertainty for the number of moles of I2:
1 mol of KIO3 ------------- 3 mol of I2
1.4 x 10-3
(± 4.67 x 10-5
) mol of KIO3 ------------------ X mol of I2
X moles = 1.4 x 10-3
(± 4.67 x 10-5
) x 3 = 4.2 x 10-3
(± 4.67 x 10-5
x 3) mol of
= 4.2 x 10-3
(± 1.4 x 10-4
)
28 | P a g e
Uncertainty to find the number of moles of I2 in 0.002 dm3 of solution:
0.25 dm3 ------------------------------- 4.2 x 10
-3 (± 1.4 x 10
-4) mol of I2
0.002 ( 1 x 10-4) dm
3 ------------------------------- Z mol of I2
Relative Uncertainty for 0.002 dm3 =
( * ',+)
'.'') x 100 = 5 %
Relative uncertainty for 4.2 x 10
-3mol =
.+* ',+
+.)* ',( x 100 = 3.33 %
Therefore, (5 % + 3.33 % ) = 8.33 %
Z mol of I2 = '.'')*+.)* ',(
'.)�=3.36 x 10
-5 (± 8.33 %)
To convert to absolute uncertainty = (.(!* ',�*$.((
''
= (2.8 x
10
-6)
Number of moles of I2 3.36 x 10-5
(±2.8 x 10
-6)
Find the number of moles of ascorbic acid:
C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O
-
1 : 1
The moles of I2 is known. Looking at the molar ration of 1:1 between ascorbic acid and
iodine we know that ,
The number of moles of I2 = number of moles of ascorbic acid
3.36 x 10-5
(±2.8 x 10
-6) moles
= 3.36 x 10
-5 (±2.8 x
10
-6) moles
Find the concentration of ascorbic acid:
On day 1,
Titre Value = volume of ascorbic acid = 0.00364 (1 x 10-4
) dm3
Number of moles of ascorbic acid = 3.36 x 10-5
(±2.8 x 10
-6)
29 | P a g e
Therefore,
Concentration = -./01234/3516
735./1
Concentration = (.(!* ',�
'.''(!+ = 9.23 x 10
-3 mol dm
-3
Uncertainty for concentration of vitamin C:
Relative uncertainty for number of moles: = ().$* ',!)
(.(!; ',� x 100 = 8.33 %
Relative uncertainty for volume of ascorbic acid = = ( * ',+)
'.''(!+ x 100 = 2.74 %
Therefore total relative uncertainty for concentration of vitamin C = (8.33 + 2.74) = 11.07
Convert to absolute uncertainty = #.)(* ',(* .'<)
'' = 1.02 x 10
-3
Therefore concentration for Day 1 , 9.23 x 10-3
( ± 1.02 x 10-3
)
On Day 4,
Number of moles of ascorbic acid = 3.36 x 10-5
(±2.8 x 10
-6)
Titre Value = volume of ascorbic acid = 0.00438 (1 x 10-4
) dm3
Concentration = (.(!* ',�
'.''+($ = 7.67 x 10
-3 mol dm
-3
Uncertainty for concentration of vitamin C:
Relative uncertainty for number of moles: = ().$* ',!)
(.(!; ',� x 100 = 8.33 %
Relative uncertainty for volume of ascorbic acid = = ( * ',+)
'.''+($ x 100 = 2.28 %
Therefore total relative uncertainty for concentration of vitamin C = (8.33 + 2.28) = 10.6
Convert to absolute uncertainty = <.!<* ',(* '.!
'' = 8.14 x 10
-4
Therefore concentration for Day 4, 7.67 x 10-3
(± 8.14 x 10-4
)
30 | P a g e
On Day 7,
Titre Value = volume of ascorbic acid = 0.00468 dm3 (1 x 10
-4)
Number of moles of ascorbic acid = 3.36 x 10-5
(±2.8 x 10
-6)
Concentration = (.(!* ',�
'.''+!$ = 6.91 x 10
-3 mol dm
-3
Uncertainty for concentration of vitamin C:
Relative uncertainty for number of moles: = ().$* ',!)
(.(!; ',� x 100 = 8.33 %
Relative uncertainty for volume of ascorbic acid = = ( * ',+)
'.''+!$ x 100 = 2.13 %
Therefore total relative uncertainty for concentration of vitamin C = (8.33 + 2.13) = 10.5
Convert to absolute uncertainty = !.# * ',(* '.�
'' = 7.26 x 10
-4
Therefore concentration for Day 4, = 6.91 x 10-3
(± 7.26 x 10-4
) mol dm-3
Day 10,
Titre Value = volume of ascorbic acid = 0.00526 dm3
(1 x 10-4
)
Number of moles of ascorbic acid = 3.36 x 10-5
(±2.8 x 10
-6)
Concentration = (.(!* ',�
'.''�)! = 6.38 x 10
-3 mol dm
-3
Uncertainty for concentration of vitamin C:
Relative uncertainty for number of moles: = ().$* ',!)
(.(!; ',� x 100 = 8.33 %
Relative uncertainty for volume of ascorbic acid = = ( * ',+)
'.''�)! x 100 = 1.90 %
31 | P a g e
Therefore total relative uncertainty for concentration of vitamin C = (8.33 + 1.90) = 10.2%
Convert to absolute uncertainty = !.($* ',(* '.)
'' = 6.51 x 10
-4
Therefore concentration for Day 4, = 6.38 x 10-3
(± 6.51 x 10-4
) mol dm-3
Day 13,
Titre Value = volume of ascorbic acid = 0.00550 dm3 (1 x 10
-4)
Number of moles of ascorbic acid = 3.36 x 10-5
(±2.8 x 10
-6)
Concentration = (.(!* ',�
'.''��' = 6.12 x 10
-3 mol dm
-3
Uncertainty for concentration of vitamin C:
Relative uncertainty for number of moles: = ().$* ',!)
(.(!; ',� x 100 = 8.33 %
Relative uncertainty for volume of ascorbic acid = = ( * ',+)
'.''��' x 100 = 1.82 %
Therefore total relative uncertainty for concentration of vitamin C = (8.33 + 1.82) = 10.2%
Convert to absolute uncertainty = !. )* ',(* '.)
'' = 6.24 x 10
-4
Therefore concentration for Day 4, = 6.12 x 10-3
(± 6.24 x 10-4
) mol dm-3