extended essay

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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

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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.

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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)

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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.

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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.


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.

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• 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





Average titre value 0.00364 (± 1 x 10-4

dm3)

DAY 4:

Trial number Titre value/cm3 (± 1 x 10

-4 dm

3)







dm3)

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DAY 7:


-4 dm

3)







dm3)

DAY 10:


-4 dm

3)







dm3)

DAY 13


-4 dm

3)







dm3)

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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

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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:


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.

�� = � ��

��

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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

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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.


!�.##$ ( ±

'.'

!�.##$ ) = 0.030 moles (±6.02 x 10

-5)


) (.##$ ( ±

'.'

) (.##$ ) = 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

)

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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)


C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O

-

1 : 1

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3.36 x 10-5

(±2.8 x 10

-6) moles

= 3.36 x 10

-5 (±2.8 x

10

-6) moles


On day 1,

Titre Value = volume of ascorbic acid = 0.00364 (1 x 10-4

) dm3


(±2.8 x 10

-6)

Therefore,


735./1


'.''(!+ = 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

)

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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:


) = MOLES

VOLUME


) = 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.

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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.

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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.

�� = � ��

��


!�.##$ = 0.030 moles


) (.##$ = 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


C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O

-

1 : 1




3.36 x 10-5

moles

= 3.36 x 10-5

moles


On day 1,



Therefore,


735./1


'.''(!+ = 9.23 x 10

-3 mol dm

-3

26 | P a g e

On Day 2,




'.''+($ = 7.67 x 10

-3 mol dm

-3

On Day 7,




'.''+!$ = 6.91 x 10

-3 mol dm

-3

Day 10,




'.''�)! = 6.38 x 10

-3 mol dm

-3

Day 13,




'.''��' = 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.


!�.##$ ( ±

'.'

!�.##$ ) = 0.030 moles (±6.02 x 10

-5)


) (.##$ ( ±

'.'

) (.##$ ) = 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)


C6H8O6 + 2H2O + I2 ---------------> C6H6O6 + 2I- + 2H3O

-

1 : 1




3.36 x 10-5

(±2.8 x 10

-6) moles

= 3.36 x 10

-5 (±2.8 x

10

-6) moles


On day 1,


) dm3


(±2.8 x 10

-6)

29 | P a g e

Therefore,


735./1


'.''(!+ = 9.23 x 10

-3 mol dm

-3



(.(!; ',� x 100 = 8.33 %


'.''(!+ x 100 = 2.74 %


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,


(±2.8 x 10

-6)


) dm3


'.''+($ = 7.67 x 10

-3 mol dm

-3



(.(!; ',� x 100 = 8.33 %


'.''+($ x 100 = 2.28 %


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)


(±2.8 x 10

-6)


'.''+!$ = 6.91 x 10

-3 mol dm

-3



(.(!; ',� x 100 = 8.33 %


'.''+!$ x 100 = 2.13 %


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,


(1 x 10-4

)


(±2.8 x 10

-6)


'.''�)! = 6.38 x 10

-3 mol dm

-3



(.(!; ',� x 100 = 8.33 %


'.''�)! 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


(± 6.51 x 10-4

) mol dm-3

Day 13,

Titre Value = volume of ascorbic acid = 0.00550 dm3 (1 x 10

-4)


(±2.8 x 10

-6)


'.''��' = 6.12 x 10

-3 mol dm

-3



(.(!; ',� x 100 = 8.33 %


'.''��' 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


(± 6.24 x 10-4

) mol dm-3

32 | P a g e

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