Effect of brewing time on mass percentage of gallic acid in green tea & black tea extract

How does the mass percentage of gallic acid, measured in g/L in tea extracts depends on the brewing time and the type of the tea extract – green tea and black tea used, determined using acid base titration with NaOH?

The ability to think about how I think, my meta-cognition has always inspired me to be an inquirer and explore scientific facts and principles that I come across. Having the habit of being awake late in the night to complete my assignments, tea and coffee has been an integral part of my life. To save time, I usually use hot water from the water heater and the tea bags to make my cup of tea. Often, I am so engrossed in my work, that the tea bag is dipped into the water for longer than usual. This has often made me realize that the taste of tea changes when the tea bags are soaked into the water for a longer time. This has made me inquire that what can be the possible reason behind this. After some research, I came to know that tea leaves contain a phytochemical – gallic acid which contributes towards the flavor and aroma of the tea we drink. The amount of gallic acid extracted depends on the brewing time; the time for which the tea leaves are soaked into the water. Recent researches about preparation of tea shows that the brewing time plays a great role in extracting the right chemicals from the tea leaves in the right amount to obtain the best flavor and aroma of that particular tea leaves and this depends on the type of tea leaves as well. Thus, can we surely claim that more the brewing time, more the amount of gallic acid extracted from tea leaves into the water? Is the extraction of gallic acid from tea leaves into the tea extracts happening in all cases in the same rate? To answer these questions, I decided to focus my Chemistry Internal Assessment on the research question stated above.

Gallic acid is a major constituent of tea leaves. On an average, gallic acid is a major nutritional ingredient of tea leaves and constitutes of around 1% of the dry mass of tea leaves. It is 3,4,5- tri hydroxy benzoic acid. It belongs to the class of hydroxy acids or phenolic acids. It has three phenolic OH groups and an aromatic carboxylic acid group. The compound is highly soluble in water due to the presence of three OH groups which can make inter molecular H bond with water. At room temperature, this is a white solid with the molecular formula – (OH)₃-C₆H₂-COOH. Two of the OH groups are at meta positions with respect to the COOH group while the other one is at para position.

The reaction between gallic acid and NaOH is an acid base reaction where the acid and NaOH reacts in the ratio 1:1. Gallic acid is a weak acid and it forms a salt 3,4,5-trihydroxy sodium benzoate on this reaction.

(OH)₃-C₆H₂-COOH (aq) + NaOH (aq) ------🡪 (OH)₃-C₆H₂-COONa (aq) + H₂O

This reaction can be used for the quantitative estimation of the amount of gallic acid in a given sample of tea leaves. As this titration is in between a weak acid and a strong base, the salt produced is a basic salt. Thus, the equivalence point lies around a pH of value greater than 7. This makes phenolphthalein as a suitable indicator for this titration.

Null hypotheses: There is no correlation between the mass of gallic acid extracted and the brewing time of the tea leaves.

Alternate hypotheses: There is no correlation between the mass of gallic acid extracted and the brewing time of the tea leaves.

Brewing time: Brewing time is the time for which the tea leaves are soaked in water. The brewing time of tea leaves is usually 2 minutes. In this investigation, the brewing time is 1.00 mins, 2.00 mins, 3.00 mins, 4.00 mins and 5.00 mins. A digital stop-watch was used to measure this time.

Type of tea leaves: To investigate that if the effect is same on all varieties of tea leaves or not, two different brands of tea leaves were used – green tea and black tea.​​​​​​​​​​​​​​

Mass percentage of gallic acid:

The tea extracts were allowed to react with a freshly prepared standard solution of NaOH. The titre value obtained was used to calculate the mass percentage of gallic acid in tea leaves.

Mass of NaOH to be added = moles of NaOH × molar mass

 \(= molar \,\,concentration × volume × molar \,\,mass = 0.10 ×\frac{100}{1000}× 40.01=0.40 g\)

• A watch glass was placed on a top pan digital mass balance.
• The reading of the balance was tared to 0.00 ± 0.01 g.
• Solid white pellets of NaOH were transferred from the reagent bottle to the watch glass using a spatula until the balance displays 0.40 ± 0.01 g as the reading.
• The exact mass of NaOH was transferred to a 100cm³ glass beaker.
• To transfer the weighed solid completely the watch glass was washed and the washings were collected in the same beaker.
• Distilled water was added in the beaker till the level of 100 cm³ using a graduated measuring cylinder.
• A glass rod was used to stir the solution and the solid was allowed to dissolve completely.

• A watch glass was kept on the top pan digital mass balance and the reading was tared to 0.00.
• The black tea leaves were added to the watch glass until the balance reads 2.00 ± 0.01g.
• A 100 cm³ glass beaker was taken and filled with distilled water till the mark of 100 cm³ using a graduated measuring cylinder.
• The beaker was placed on a wire gauge resting over a tripod stand with a Bunsen burner under it.
• The burner was turned on and the flame was adjusted to a blue color.
• The beaker was covered with a watch glass.
• As soon as the water begins to boil, the burner was removed and the temperature of the water was checked using a laboratory thermometer.
• The weighed sample of tea leaves were immediately added to the water in the beaker.
• The beaker was covered with watch glass and the stop-watch was started.
• As soon as the stop-watch reads 1.00 ± 0.01 mins, the beaker was removed from the wire gauge and kept on the table to allow it to cool down.
• After the beaker cooled down, the content of the beaker was filtered using a filter paper and a funnel. The filtrate was collected in a 100 cm³ conical flask and the residue was discarded.
• The filtrate was labelled as the “Black Tea extract – 1.00 mins”
• Steps 1-12 were repeated for other values of time – 2.00 ± 0.01 mins, 3.00 ± 0.01 mins, 4.00 ± 0.01 mins and 5.00 ± 0.01 mins.
• Steps 1-13 were repeated for the green tea leaves.

• The burette was washed with 0.10 moldm^-3 NaOH solution and dried.
• It was then filled with the same NaOH solution till the mark of 0.00 cm³.
• The tea extract in the conical flask was taken and two drops of the phenolphthalein solution was added to it.
• The tea extract was titrated against the NaOH solution running down the burette.
• The burette readings were recorded and noted down at the end point indicated by the appearance of permanent pink color.
• Steps 1-5 were repeated for two more times.

For brewing time = 1.00 ± 0.01 minutes

Difference in burette reading (DBR) = Final burette reading (FBR) – Initial burette reading (IBR)

= (4.60 ± 0.05) cm³ – (0.00 ± 0.05) cm³ = 4.60 ± (0.05 + 0.05) cm³ = 4.60 ± 0.10 cm³

\(\text{Mean volume of NaOH consumed} =\frac{4.60+4.50+4.50}{3}= 4.53 ± 0.10 cm3\) 

\(\text{Standard deviation} (SD) = \frac{(4.60-4.53)^2+(4.50-4.53)^2+(4.50+4.53)^2}{3}= 0.06\)

For brewing time of 1.00 ± 0.01 min in case of green tea,

C₆H₂(OH)₃(COOH) (aq) + NaOH (aq) -----🡪 C₆H₂(OH)₃(COONa) (aq) + H₂O

Gallic acid                                                         Sodium salt of gallic acid

 \(Moles \,of \,NaOH \,required \,(n) = molar \,concentration × Volume = 0.10 ×\frac{V}{1000}\)

V = Mean volume of NaOH required in cm³ (mean burette reading)

 \(Moles \,of \,gallic \,acid = \,moles \,of \,NaOH = 0.10 ×\frac{V}{1000}\)

\(Mass \,of \,gallic \,acid \,in \,100 \,cm3 \,of \,tea \,extract\, = moles × molar mass = 0.10 × \frac{V}{1000}× 170.12\)

 \(Mass \,of \,gallic \,acid \,in \,1 \,L \,of \,tea \,extract = 0.10 ×\frac{V}{1000}× 170.12 × 10 = 0.17 × V\)

 \(Mass \,percentage\, of \,gallic \,acid \,in \,tea \,extract = 0.17 × V\frac{g}{L} = 0.17 × 4.53 = 1.10 g/L\)

Absolute error in mean volume of NaOH consumed = ± 0.10 cm³

As clear from the data processing, the major source of error is the absolute error in the burette reading.

Percentage error in mass % of gallic acid in tea extract

\(\frac{absolute\ error\ in\ mean\ volume\ of\ NaOH\ consumed}{mean\ volume\ of\ NaOH\ consumed}× 100 = \frac{±0.10}{4.53}× 100 = ± 2.20\)