Essay

IB chemistry IA on antioxidant capacity of tea with the addition of milk

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Course
International baccalaureate

Institution
International Baccalaureate

This is an IB chemistry IA on antioxidant capacity of tea with the addition of milk. The RQ is How is antioxidant capacity of black tea affected when 0, 1, 5, 10 and 15ml of milk is added to fixed total volume of 100ml tea solution, through the quantification of time interval between two consecutiv...

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Antioxidant capacity of tea with addition of milk

Research Question:
How is antioxidant capacity of black tea affected when 0, 1, 5, 10 and 15ml of milk is added to fixed total
volume of 100ml tea solution, through the quantification of time interval between two consecutive
oscillations of Briggs Rauscher reaction?

Personal Engagement:
During the summer break, I volunteered at Alzheimer’s Disease Association, where I helped the patients
who came to the centre for the treatment. Before I could be qualified as a volunteer, I went through the
training session and I learned that the imbalance between free radicals and antioxidants is one of the
causes of Alzheimer as well as many other health problems. Problems arise when antioxidants are lacking
compared to free radicals because free radicals cause oxidative damage to the cell. Free radicals are
continuously produced in human’s body through various metabolic processes such as aerobic respiration
and hence, antioxidants, also known as free radical scavengers, are required to remove them. Therefore,
the trainer said drinking tea will be good for not only the patients but also for everyone as it contains a lot
of antioxidants but it should be without milk, since milk will reduce the antioxidant capacity of the tea.

I got interested in this fact because having tea with milk added is one of the most common ways adopted
by many people. In response, this investigation aims to explore the effect of addition of milk on the
antioxidant capacity of the tea, by varying the volume of the milk added. The quantification method I
decided to use is Briggs Rauscher reaction as its oscillation interval is affected by the antioxidants.
However, through the research, it was found that Briggs Rauscher reaction usually requires 30%
hydrogen peroxide while the school lab provides maximum 6% hydrogen peroxide due to safety issues.
Hence, I had to carry out pilot trial using 6% hydrogen peroxide to verify whether the reaction still occurs.
Fortunately, 6% hydrogen peroxide turned out to be enough to carry out the oscillation reaction and this
confirmed me that I can carry out the actual trials.

Background information:
Antioxidants that are present in the tea are known as polyphenols
(Ozdal et al, “A Review”). These phenolic compounds have hydroxyl
group, OH, bonded to arenes, which are the organic compounds that
contain benzene rings (Talbot et al Christopher 346). An example of
polyphenols found in the tea is called catechin, as in figure 1. These
polyphenols are free radical scavengers, which help terminating free
Figure 1. A diagram showing radical chain reaction and this is due to their resonance structures,
molecular structure of catechin, one where electrons are delocalized around the pi bonds between singly
of polyphenols found in tea (Ozdal et
occupied p orbitals within benzene rings, as a result of !" # hybridisation,
al, “AReview”).
where one 2s and two 2p orbitals are hybridized. If this compound
reacts with free radical, an atom adjacent to the pi bond loses one
electron and becomes free radical, as $%# in figure 2. Therefore, its
electron deficient p orbital can overlap with those in resonance
Figure 2. A diagram comparing the structure, contributing to the electron delocalisation (James, “3
stability of free radical in resonance
compound compared to non
Factors”). Hence, the structure can be stabilized, preventing subsequent
resonance compound (James, “3 free radical reaction. Also, this stable radical species can be metabolized
Factors”). and excreted from the cell (“Radical Chain Reactions”).

1

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-
However, when milk is added to tea, proteins present in the milk, especially alpha and beta caseins form
complex with the polyphenols, although the mechanism is not clear yet. It is suggested that initially,
hydrogen bonds are formed between hydroxyl group, OH, of polyphenols and carboxyl group, COOH, of
amino acids, forming protein-polyphenol complex. Then hydrophobic bindings between nonpolar groups
of proteins and polyphenols are promoted, which strengthens the protein-polyphenol complexes. These
interactions are known to change the structure of polyphenols, and therefore significantly decrease their
antioxidant capacity (Ozdal et al, “A review”).

In order to verify the degree of effect of addition of milk to tea on its antioxidant capacity, Briggs
Rauscher reaction can be used. Briggs Rauscher reaction (3) is a hybrid of two other oscillating chemical
reactions (1) and (2), which are written below (“Oscillating Chemical Reactions”).

')* ( + 2%# )# + % - → %)' + 2)# + 2%# ) (1)
%)' + $%# ($))%)# → '$%($))%)# + %# ) (2)

')* ( + 2%# )# + $%# ($))%)# + % - → '$%($))%)# + 2)# + 3%# ) (3)

Reaction (2) occurs via two steps, which are reaction (4) and (5) below.

' ( + %)' + % - → '# + %# ) (4)
'# + $%# ($))%)# → '$%($))%)# + % - + ' (
(5)
Reaction (1) is dependent on the concentration of iodide ions in the solution. When ' ( is low, radical
process is dominant and when it is high, non-radical process is dominant. The product, %)', of reaction
(1) is used as a reactant of reaction (2). When radical process is dominant in reaction (1), consumption of
%)' is slower than the production. However, when non-radical process is dominant, the consumption is
faster than the production of %)'. In reaction (4), if there is an excess %)', as a result of radical process
in reaction (1), it is reduced to iodide ions by hydrogen peroxide in the solution, increasing concentration
of iodide ions for non-radical process in reaction (1), which slows down the production of %)'. Then the
consumption of %)' as well as ' ( in reaction (4) will be faster, leading to too low ' ( to continue the
non-radical process and hence, radical process starts again (“Oscillating Chemical Reactions”).

Throughout the repeating reactions above, colour changes occur from transparent to amber to blue and
back to transparent. During the radical process of reaction (1), rapid production of %)' produce '# in
reaction (4) and this turns the solution into amber. However as %)' becomes excess, it is reduced by
hydrogen peroxide into ' ( and when ' ( becomes higher than %)' , the colour changes to blue-black.
This will then turn the reaction (1) into non-radical process, where %)' will be consumed faster than it is
produced. Hence, '# in reaction (5) starts to be used up faster than it is produced in reaction (4) and
therefore, blue-black decolourises. As ' ( continues to react with %)' and its concentration falls, radical
process begins again, turning the solution into amber and the cycle repeats (“Oscillating Chemical
Reactions”)

However as mentioned, antioxidants are free radical scavengers. Therefore, if antioxidants are added to
the solution of Briggs Rauscher reaction, radical process will be inhibited and slow down. In other words,
if tea is added to Briggs Rauscher solution, its polyphenols will slow down the oscillations. Therefore,
antioxidant capacity of tea with different volumes of milk will be able to be quantified through the time
elapsed for one oscillation, which will be time interval between 2nd and 3rd blue-black appearance in this
investigation.
2

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