chemistry eei 12321321.docx

Chris Poli 11.7 Chemistry

How pH affects the concentration Of vitamin C


Contents Possible areas of investigation ................................................................................................................ 3

Background Research .............................................................................................................................. 4

Research Questions ................................................................................. Error! Bookmark not defined.

Aim .......................................................................................................................................................... 5

Hypothesis .............................................................................................................................................. 5

Variables ................................................................................................................................................. 8

Independent Variable ......................................................................................................................... 8

Controlled Variables ............................................................................................................................ 8

Dependant Variable ............................................................................................................................ 8

Equipment ................................................................................................ Error! Bookmark not defined.

Procedure ................................................................................................................................................ 9

Risk Management .................................................................................... Error! Bookmark not defined.

Changes to procedure .............................................................................. Error! Bookmark not defined.

Raw Data .................................................................................................. Error! Bookmark not defined.

Abstract To investigate the effect of pH on the rate of oxidation of 0.005mol/L ascorbic acid in a solution of

iodine, HCl and starch indicator at a constant temperature and in the same light intensity. I

hypothesis the lower the pH level of the solution is, the lower the concentration of ascorbic acid will

be. To test if this hypothesis is correct, set up a redox titration of iodate into a solution of iodine,

ascorbic acid (concentration you want to find), starch indicator (to see endpoint) and the pH changer

(NaOh/ Distilled water/HCl). The hypothesis was correct, the lower the pH level of the solution is the

faster the ascorbic acid will oxidise. The result was semi successful and the hypothesis was proved to

be correct.


Background Titration is a common method used in chemistry to find an unknown concentration of an identified

analyte. A chemical reaction is set up between known concentrations of solution with an unknown

concentration of solution. This process is done by slowly adding a standard solution of titrating

reagent, or titrant, from a burette. The addition is stopped when the end-point is reached.

Vitamin C or absorbic acid is a power anti-oxidant that is essential for the human body. It works in

the human body by helping to protect the cells from damage caused by free radicals. The body also

needs Vitamin C for collagen which helps wounds to heal. Among this it also helps the immune

system to work properly in defending your body from diseases. Fruits and Vegetables are the best

source of Vitamin C with citrus fruits being at the top with the most Vitamin C. If your body does not

get enough Vitamin C (less than 10mg a day for two weeks) you may get scurvy which can cause

such symptoms as inflammation, poor wound healing, joint pains, or loss of teeth.

Oxidation is when oxygen molecules react with all different substances that they contact, can be

from metal to living tissue. With the discovery of electrons, oxidation was defined as the loss of at

least one electron when two or more substances react. Oxidation can be not such a bad thing, to

sometimes being destructive. An example of bad side of oxidation is the spoiling of fresh fruit or the

rusting of metals.

Reduction is essentially the opposite of oxidation its any reaction that involves the gaining of

electrons. A reduction reaction will always occur when an oxidation reaction does.

PH is one of the factors that can affect the rate of oxidation with absorbic acid, so it is a good

independent variable to test. The lower the pH level of the solution is, the faster the absorbic acid

should oxidise. At low pH, the H+ concentrations are high, H+ ions oxidizes a vitamin c molecule by

taking one electron from it.

Ascorbic acid, or Vitamin C, is oxidized by iodine according to the following equation: C6H8O6 + I2 =

C6H6O6 + 2HI (Thomas, 2001)


Background Research

1. Chemical structure of vitamin C?

2. What is the definition of Oxidation and Reduction?

Oxidation: A reaction in which electrons are lost to another species.

Reduction: Involves the gaining of electrons. Eg. When iron reacts with oxygen it forms a

chemical called rust, the iron is oxidized and the oxygen reduced.

3. What are some factors that will increase the rate of oxidation of Vitamin C?

Temperature of the solution can affect the rate of Oxidation. The pH levels and surface area

can also affect this.

4. What is an anti-oxidant?

Anti-oxidant are nutrients and enzymes that can help to prevent and repair damage to your

bodys tissue. This occurs by anti-oxidants slowing or preventing the effect of free radicals.

5. Why are we using redox titration?

Redox titration is a way to determine how much vitamin C there is in different things. A

common example of a redox titration is treating a solution with iodine with a reducing agent

and using starch as an indicator.

6. What colour is endpoint?

With the help of the starch indicator we can see the endpoint which in this case is a blue-

black colour.

7. Why cant we just titrate with iodine only?

Titration can be performed using just iodine solution and not iodate, but the iodate solution

is more stable and gives more accurate results.

8. What sorts of things contain vitamin C?

Oranges, fruit juice, vitamin tablets and various supplements.


9. Does temperature affect the rate of oxidation?

Temperature increases the rate of oxidation because the molecules are moving around

faster and having more collisions, so there are more opportunities for the reaction to occur.

Aim

To investigate the effect of pH on the rate of oxidation of 0.005 mol/L ascorbic acid in a solution of

iodine, HCl and starch indicator at constant temperature and in the same light.

Hypothesis I hypothesise the lower the pH level of the solution is, , the lower the concentration of ascorbic acid

will be.

Rationale At low pH, the H+ concentrations are high, H+ ions oxidizes a vitamin c molecule by taking one

electron from it. The concentration of the vitamin C solution will decrease with the pH of the

solution as long as other factors which may affect oxidation (such as time, light intensity and

temperature) are kept constant.


Risk Management

Hazard Risk Evaluation How to prevent

Iodine

Moderate - Skin contact- flush skin with water for 15 minutes while removing contaminated clothing and shoes. Get medical attention.

- Eye contact Flush eyes with water for 15 minutes, warm water must be used. (MSDS Solutions Center, 1991)

Hydrochloric Acid Moderate - Skin contact- flush skin with water for 15 minutes while removing contaminated clothing and shoes. Get medical attention.

- Eye contact Flush eyes with water for 15 minutes, warm water must be used. (MSDS Solutions Center, 1991)

Absorbic Acid Low - Skin Contact Wash with soap and water, get medical attention if irritation develops. (MSDS Solutions Center, 1991)

Starch Indicator Low - Skin Contact Wash with soap and water, get medical attention if irritation develops. (MSDS Solutions Center, 1991)

-

Sodium Hydroxide High - Very hazardous in case of skin contact (corrosive, irritant, permeator), of eye contact (irritant, corrosive), of ingestion, of inhalation. The amount of tissue damage depends on length of contact. Eye contact can result in corneal damage or blindness. (MSDS Solutions Center, 1991)


Equipment

- Retort Stand

- Burette

- 10ml Vitamin C Solution

- Iodine

- Iodate

- 1ml Starch Indicator

- NaOH 0.025M

- NaOH 1M

- HCL 0.025M

- HCL 1M

- Distilled Water

- Beaker 50/80/100mL

- Measuring cylinder 10/25mL

- Pipette 25/20/10mL

- Heat proof mat

- Aluminium foil


Variables

Independent Variable

The pH levels of the solution; we are changing this variable by using different acids and bases. The

Acids we used are 1M of HCl and 0.025M of HCL. For the bases we are using 1M of NaOH and

0.025M of NaOH. We are also using distilled water. We used these acids and bases to drop or

increase the pH of the absorbic acid solution. NaOH increases the pH, HCL decreases the pH and

distilled water neutralizes the pH.

Controlled Variables

- Light intensity

- Amount of solution

- Temperature

- pH of the same solution

- Time solution is left in the open

Dependant Variable

Concentration of Ascorbic Acid.


Procedure

1. Two/One day before the trialling change the pH of absorbic acid solution by adding 10mL of

absorbic acid to a flask and then putting either NaOH, if you want to increase the pH,

Distilled water if you want to have the pH neutralised and HCL if you want to lower the pH.

2. Add 10mL of Absorbic Acid solution to a flask, and then add 5mL of Potassium Iodide

(0.6mol/L), 5mL of HCL (1mol/L) and 1mL of starch indicator.

3. Set up the burette attached on the retort stand, while make sure the tap on the burette is

closed, add 50mL of 0.002mol/L potassium iodate into burette.

4. Put the solution flask with the pH changer you are trialling under the burette and turn the

tap so the iodate is slowly dripping into the solution, stop the tap when the solution in the

flask reaches endpoint (Purple colour).

5. Record titre volume and repeat titrations 5 times for each trial.

Diagram


Results

Distilled Water

Titre Initial Volume Final Volume Titre Volume

Pilot 0 4.50 4.50

1 4.50 9.30 4.80

2 9.30 14.00 4.70

3 14.00 18.40 4.40

4 18.40 23.20 4.80

5 23.20 28.10 4.40

HCl 1M


Pilot 0 3.7 3.7

1 3.7 6.8 3.1

2 6.8 10.4 3.6

3 10.4 14.1 2.7

4 14.1 17.4 3.3

5 17.4 21.5 3.1

HCl 0.025M


Pilot 0 4.5 4.5

1 4.5 8.9 4.4

2 8.9 14.0 5.1

3 14.0 18.3 4.3

4 18.3 22.2 3.9

5 22.2 26.4 4.2

NaOH 0.025M


Pilot 0 2.4 2.4

1 2.4 4.4 2

2 4.4 6.5 2.1

3 6.5 9.1 2.6

4 9.1 11.4 2.3

5 11.4 13.4 2


0

5

10

15

20

25

30

1 2 3 4 5 6

Titration of Distilled Water + Ascorbic Acid

Initial Volume

Final Volume

Titre Volume

Averages

Solution Average Titre Volume (mL)

Distilled Water (pH 7)

4.68

NaOH 0.025M (pH 7) 2.23

HCl 1M (pH 1) 3.25

HCl 0.025M (pH 3) 4.40

Average Titre Volume

3.29

Graphs of Data


Titration of HCl 1M + Ascorbic Acid


Analysis

Sample Calculation To find concentration of ascorbic acid solutions with differing pH values:

1. IO3- + 5I- + 6H+ 3I2 + 3H2O

2. CHO + I -> CHO + 2I +2H

Average Titre Volume = 3.29mL

Mol= m x v

= 0.002 x 0.00329

= 6.58 x 10^-5

Using this answer find the moles of Iodide

IO3- I2

1 1

1.974 x 10^-5 x

X: 1.974 x 10^-5

To find concentration of Vitamin C:

M = Moles/V

M = 19.74 x 10^-5/3.29x 10^-3

M = 0.006mol/L

Average concentration of ascorbic acid is 6 x 10^-3 mol/L


Discussion As these results show, the lower the pH of the solution, the lower the concentration of ascorbic acid

is. The trends on the graphs show that the solutions with the lower pH did have a lower

concentration as hypothesised. The NaOH 0.025M (pH 5) has a lower pH then the distilled water as

shown in the data tables above, the average titre of NaOH 0.025M was 2.33 and the average titre of

distilled water was 4.68. This data set proves my hypothesis. The HCL 1M solution had a pH of 1

meaning it was very acidic and it had one of the lowest average titre volume out of the 5 different

solutions we used. I believe there was a mistake with recording this data as the average titre should

have been lower than the NaOH 0.025M. The HCl 0.025M (pH 3) also had a lower titre volume than

the distilled water (pH 7) proving the hypothesis further. This shows that my hypothesis was correct

as I stated, saying that the lower the pH the lower the concentration of the solution. Shown by the

average sample calculation you can see the average concentration of ascorbic acid is 0.006mol/L.

The NaOH 1M trial was excluded from the set of data as the ascorbic acid was already fully oxidised

when it was titrated, leaving all titre volumes at 0. This would have ruined the average set of data

and not been a reliable result to use. The trials that titrated and went past the end point were also

discarded as too much iodate was added to the solution; you can see this by looking at the dark

colour of the solution in the flask.

The experiment did have flaws and limitations, the main one being time and the amount of

titrations possible. The NaOH 0.025M (pH 5) titrations were not accurately measured as the average

titre is much too low compared to the other pH trials. Experiment was limited with getting the

perfect pH for each trial and having time to adjust and test the pH and make it to what we want. The

endpoint of the solutions was sometimes slightly different colours, meaning the iodate put into

the flask could be more or less than needed to actually reach the endpoint. The experiment was

somewhat rushed on time meaning the exact mL of a certain solution might have been a point off

the actual needed, this may affect results. . Time has a huge effect on vitamin c it being left out as

the ascorbic acid oxidizes from the air, meaning the ascorbic acid is turned into dehydroascorbic acid.

This means that the calculations to find the amount of ascorbic acid will not be correct.

The results of the experiment could have been more reliable if it was repeated multiple times using

an identical procedure, or an alternative is to use another substance that contains vitamin c to see if

it has the same effects. Other small issues that could have been improved are if we measured the

temperature of the solution each trial to make sure they were all the same, therefore not affecting

the results at all. Another way to improve results would be leaving the solution for a shorter time,

so the results would have worked out better, the ascorbic acid would have not oxidised as much

meaning the scrapped trial of NaOH 1M would not have been excluded, adding another data set to

the results making the experiment more accurate.

Conclusion I believe the experiment was semi-successful; there were many obvious flaws in the data and you

can see a slight trend, being the lower the pH the lower the concentration of the ascorbic acid. The

trend is though not as obvious and perfect as it should be to completely prove the hypothesis. These

results should not be considered very accurate or reliable to use for further studies.


Bibliography MSDS Solutions Center. (1991). Retrieved from MSDS: http://www.msds.com/

Thomas, M. (2001, 8 1). Vitamin C. Retrieved from MadSci: madsci.org

Acknowledgments

Benjamin Panzera Group member

chemistry eei 12321321.docx

Documents

solution of iodine

concentration of ascorbic

ascorbic acid concentration

ph level

effect of ph

moll ascorbic acid

chemistry background

starch indicator