How does the increase in temperature affect the potential of hydrogen (pH) of coffee?

The area of knowledge of acids and bases in relation to redox reactions in chemistry has always intrigued me, especially in the real-life application of the natural sciences.When studying acids and bases I found a connection between the ionic product of water and its effect on pH when combined with various reactions that favour either the hydroxide or hydronium ions. In this investigation, I specifically aim to investigate the effect of different temperatures at which coffee is served, and the effect that it has on the pH level found in the average cup. I have wanted to investigate the relationship between pH and determining factors such as pH since doing a class experiment on antacids. In this experiment, I aim to measure the pH level of coffee of which on average 1 billion people drink daily (Addict, 2021), additionally, globally 783.95 million people globally suffer from acid reflux and other acid-related issues. Assuming there is a crossover between the number of people suffering from acid reflux as well as the number of people who drink coffee daily 78.40% of people who have acid reflux drink coffee daily.

 

Physiologically coffee can have adverse effects on the body as it has a high caffeine concentration which acts as a stimulant to the central nervous system, heart and muscles. In the human body, coffee stimulates gastrin release and gastric acid secretion (Islamaj et al., 2021). One of the major chemical components of coffee includes caffeine. The chemical composition of caffeine (as seen in figure 1) is a nitrogenous organic compound with the chemical formula C₈H₁₀N₄O₂ . The most common sources of caffeine include coffee and tea, however, coffee has a higher concentration of caffeine as compared to tea. In creating a coffee solution caffeine is diluted by water in the reaction - C₈H₁₀N₄O_2(aq) + H₂O(l) ⇌HC₈H₁₀N₄O₂ + (aq) + OH- (aq) This reaction displays how the caffeine molecule is reduced in the forward reaction and as a result gains a hydrogen molecule while the water molecule is oxidised and loses a hydrogen atom and forms a hydroxide ion.

In accordance with the bronsten lowry theory in the above forward reaction, the caffeine molecule is classified as a base as it accepts the Hydrogen proton while water acts as an acid donating the hydrogen proton. However, as the reaction proceeds in both a forward and reverse direction, both species can act as amphiprotic species as they can be both a bronsten lowry acid and base. Caffeine while being the main reason coffee is ingested does not result in the acidity of the overall solution. When coffee beans are being roasted chlorogenic acids degrade and form quinic acids. The chemical composition of quinic acids (as seen in figure 2) has the molecular formula C₇H₁₂O₆ . when creating a coffee solution quinic acids react with water and result in C₇H₁₂O₆(s) + H₂O_(l) ⇌ C₇H₁₁O₆ - + H₃O+

In accordance with the bronsten lowry theory quinic acid reacts with water as an acid as it donates a proton while water gains a proton resulting in a bronsten lowry base. This reaction can further dissociate the H₃O ⁺ ion to form a stable water molecule and a hydrogen ion which would increase the hydrogen ion concentration in the solution/ forward reaction. Quinic acid is the main contributor to the overall acidity of a coffee solution, however, the dissociation reaction only occurs readily at a higher temperature as it is required to break the bonds between the compounds. This occurs as pH decreases with an increase in temperature due to an excess of hydrogen ions and overwhelms the mass of hydronium ions (Westlab, 2022).

 

Because of this, I hope to use this investigation to ascertain the optimum temperature to drink coffee at which has the relative pH level is the closest to neutral (hydroxide ions concentration and hydronium ions concentration are close to equal/ little difference) in order to be able to inform my own family members who suffer from acid reflux of the optimum temperature to drink their coffee at.

If the temperature of a coffee solution is increased (20°C, 25°C, 30°C, 35°C, 40°C) the pH level of coffee will decrease as it shifts from being neutral to favouring the production of hydrogen ions as a result of the increase in temperature supplying more kinetic energy for reactions between the organic compounds contained within coffee and increasing the production of hydrogen ions (C₇H₁₂O_6(s) + H₂O_(l) ⇌ C₇H₁₁O₆ - + H₃O+ ) as equilibrium shifts to the right in accordance with Le Chatelier's principle (Libretexts, 2023).

Independent variable: Increase in temperature (20°C, 25°C, 30°C, 35°C, 40°C). The temperatures being manipulated in this experiment accommodate for people who drink iced coffee which is demonstrated by the 20 degrees celsius as well as average room temperature and higher temperatures of coffee which is the most commonly ingested. The temperature of the solution will be increased or decreased using a water bath, the water bath will also be used during the titration to ensure that no heat loss occurs and maintain the temperature of the solution. The significance of this variable being independent is that the increase in temperature also results in the increase of kinetic energy within a solution which supplies energy for various reactions increasing the yield of possible products of these reactions such as hydrogen ions or hydronium ions.

 

Dependent variable: pH of the coffee solution. The pH of the solution will be measured in two ways; first using a pH metre after heating has occurred and the second source of measurement is a neutralisation titration where I will measure the volume of a base needed to neutralise the coffee solution. The two methods of measurement are used to ensure that the data collected is reliable as well as provide further information on the concentration of hydrogen ions at different temperatures which the titration calculations can confirm.

 

Controlled variable: brand of coffee used. Changing the brand of coffee could have a possible effect on the pH level of each solution as they might increase or decrease as a result of caffeine as well as other minor reactants that may increase or decrease the pH, therefore, the temperature variable will not be the only dependent variable in the experiment decreasing the accuracy of the results.

 

Controlled variable - The volume of coffee solution titrated - 10cm³ measured using a burettes 3 then transferred to a 250ml Erlenmeyer flask. Changing the volume of the coffee solution titrated will change the concentration of hydrogen ions and hydronium ions present which heavily influences the calculations in investigating the varying pH of the coffee solution. Controlled variable: Volume of distilled water added to coffee grounds- 50cm³ measured using a 350cm measuring cylinder before being transferred to a beaker where the coffee grounds can be 3 added as heating occurs. The volume of distilled water added to the solution needs to be concentrated as it will affect the concentration of coffee and its organic compounds within the controlled mass which would alter the pH of the initial solution as well as the titrated solution. Controlled variable - Mass of coffee grounds added to distilled water- 2 grams measured using an electronic balance. The mass of coffee grounds will affect the concentration of organic compounds that are available to react with water molecules to produce hydrogen ions as well as hydronium ions which would affect the pH level measured. Controlled variable - The concentration of NaOH used in titration to neutralise the coffee solution will be 0. 500 mols dm^-3 prepared by me and tested before use. Changing the concentration of −3 the sodium hydroxide solution will alter the reaction ratio in the overall titration with the coffee solution.

Sample uncertainty calculation-

5 + 1 + 0. 01 + 0. 3 + 0. 06 + 0. 25 + 0. 05 + 0. 2/ 8 =± 0. 859%

Propagation of uncertainty -

Total uncertainty at each temperature - ± 6. 87

• Measure 50ml of water in a 50ml measuring cylinder then transfer to a 50ml beaker and place a thermometer in the liquid to gauge the temperature
• Using an electronic weighing mass measure 2 grams of coffee powder
• Begin adding 2 grams of coffee into the water while being heated and stir until it is a solution
• Heat using a water bath until the desired temperature is reached then remove from the heat source
• Use an pH metre to measure the pH level of the coffee solution
• Place back in the water bath that maintains the desired temperature to reduce heat loss.
• Use a pipette to measure a 10 cm³ aliquot of this solution into a conical flask
• Measure 100 ml of distilled water that has been heated to desired temperature using volumetric flask
• Once measured, place distilled water in a conical flask and add 10 cm³ aliquot coffee solution to create 110 cm³ coffee solution.
• Add 3 drops of phenolphthalein indicator to the coffee solution
• Titrate the coffee solution with Sodium hydroxide (NaOH) until the solution fully turns pink in colour indicating that the coffee solution has turned basic (between pH 8.2-10.0) indicating the endpoint of the reaction.
• Record the amount of NaOH used to neutralise the coffee solution to determine caffeine content.
• Repeat the 1st- 10th step for all five temperatures each including five trials

Safety considerations- Wear protective clothing including gloves, goggles and a lab coat to prevent any skin contact with substances being handled such as sodium hydroxide which is corrosive when in contact with skin.

 

Safety considerations- Wash glassware such as the burette with distilled water and allow it to dry completely before use to prevent the cross-contamination of any substances used within the experiment.

 

Ethical considerations- the use of coffee which is a food substance consumed by humans can be considered unethical. However, I took this into consideration and therefore only used small quantities of coffee to prepare the solution for each trial.

 

Environmental considerations- include the heating of water produces gaseous molecules, i took this into consideration in terms of my carbon footprint, however, because minimal heating was required to reach my desired temperature my carbon footprint was low.

To begin the qualitative observations I made while conducting this experiment was the colour change during titration. Initially my coffee solution being titrated was highly concentrated and therefore dark in colour because of this I was unable to observe the colour to indicate when the solution has turned basic. I therefore pipetted an aliquot solution and added a constant volume of water at the same temperature in order to maintain the desired temperature while maintaining visibility for colour change. Another qualitative observation I made was that as the titration occurred a precipitate layer of white foam began to form at the same time that the reaction had been completed, this further helped me determine when the reaction had been completed. The formation of white foam indicated to me that in the coffee solution there was a surfactant present that reduced the surface tension of the liquid solution and promoted the formation of foam as the reaction was occurring, and as the endpoint was reached the foam was formed.

 

Below are sample titration calculations that I used to determine the concentration of quinic acid neutralised at the different concentrations. Below I have used sample calculations for quinic acid at 20°C. I used the same calculation method for the averaged volume used to neutralise coffee solution at different temperatures.

 

Titration calculations; 20°C NaOH + C₈H₁₀N₄O₂ → C₈H₉N₄NaO₂ + H2O Moles of NaOH = 0.500 x 1.00 = 0.500 Molar ratio = 1 - 1

 

0.5 - x Moles of C₈H₁₀N₄O₂ = 0.500 moles Concentration of C₈H₁₀N₄O₂ = 0.500/0.01 = 50. 0 mols dm⁻³

The above line graph displays that there is a correlation between the change in temperature and the pH and concentration of caffeine in the coffee solution. The temperature change has a negative correlation with the pH of the coffee solution as the temperature increases the solution becomes more acidic in nature. Further, there is a strong positive correlation between the change in temperature and the concentration of caffeine that was neutralised in titration, this displays that as the temperature increases the solution increases in caffeine content. As my graph 12 displays the uncertainty in my experiment due to the apparatus used creates a large margin of error, despite the error bars displaying that the correlation between my data points would not be largely affected by the uncertainty. In my graph 13 there is a lower margin of error as my data points were larger and therefore the percentage error caused by uncertainty is significantly less impactful.

 

As above stated the higher the caffeine content the more quinic acid is produced to react with water in the coffee solution. I came to this conclusion using my previous knowledge I knew that the dissolution of caffeine in water is typically an exothermic process as energy is released when solvation occurs, as I do not have the standard enthalpy change for this reaction I generalised the process occuring in order to conclude that the process is exothermic. As quinic acid found in caffeine is a weak acid and partially dissociates during a reaction therefore there is a forward and reverse reaction as the process aims to achieve equilibrium, as temperature is increased throughout the experiment equilibrium shifts to the left (reactants) producing more caffeine. This is reflected in the data above as temperature increases the concentration of caffeine in the solution also increases, due to the reverse reaction (endothermic reaction) being favoured as increased temperature allows the closed system to absorb heat (Libretexts, 2023).

 

The equation C₇H₁₂O₆(s) + H₂O(l) ⇌ C₇H₁₁O₆ - + H₃O + explains further why the hydrogen ion concentration increases with caffeine content and temperature as when quinic acid reacts with water hydronium ions are produced which dissociates to form hydrogen ions. Therefore the concentration of hydrogen ions in a controlled volume increases as more quinic acid is produced to react with water. The approximate pH of quinic acid is 2.0, however as the solution was diluted heavily with water the quinic acid produced was less concentrated forming a weak acid, the equilibrium and manipulation of external factors such as temperature which shifts the direction of the reaction. As determined above more caffeine is produced at higher temperatures therefore there is more quinic acid present in the solution decreasing the pH.

To evaluate the validity of the data above I calculated the Pearson's product correlation coefficient (PMCC) for the data gathered from the pH metre as well as the acid-base titration. The PMCC for the pH metre is -0.988 which validates the hypothesis that the increase in temperature would cause a decrease in the pH of the coffee solution. The PMCC for the acid-base titration is 0.863 which validates my hypothesis that the volume of NaOH that would be required to neutralise the coffee solution would increase as the acidity of the coffee solution would increase. This statistical assessment shows that the data above has a strong correlation, which shows that there is an inverse relationship between the increase in temperature and the pH of the coffee solution, further there is a positive relationship between the increase in temperature and the increase in caffeine concentration. Despite uncertainty and margin for error being slightly wide in both sets of data the strong PMCC value shows that there is still a discernible correlation between the variables, showing that uncertainty, systematic and random error had a low effect on the experimental data.

 

The experimental data and analysis conducted above validate my hypothesis that the increase in temperature (20°C, 25°C, 30°C, 35°C, 40°C) causes a decrease in the potential of hydrogen in 50 ml coffee solution. Therefore for the global population who drinks coffee daily and also suffers from acid reflux issues as well as stomach ulcers, it is unsuitable to drink coffee at higher temperatures as the caffeine concentration and by extension, the quinic acid concentration increases at higher temperatures. This is also supported by another study conducted on what a suitable temperature to drink coffee at for patients with stomach ulcers which shows that both caffeinated and decaffeinated drinks at high temperatures can increase acid production in the stomach and worsen symptoms of stomach ulcer patients (Diet for ulcer disease, 2020). Because of this, it is recommended to have iced coffee, where caffeine is not continually supplied with a kinetic energy that creates a pathway for quinic acid and hydrogen ions to be produced at faster rates. This would positively impact the average human's diet as there is less acid introduced to internal systems creating a balance that would decrease the negative response to the intake of coffee as well as prevent any acid-related illnesses from forming within the body.

One of the strengths in my experimental design is the use of two different processes that determine pH and concentration of caffeine in the stated solutions. This provided me with two sets of data points which i was able to analyse from different angles, as one of the experiments was very straightforward and gave the pH however, the addition of the second titration experiment also allowed me to ascertain the effect of temperature on caffeine content and by extension concentration of quinic acid in a given solution, this aided me in better understanding the effect of temperature as well as the relationship between caffeine content and pH. Another strength in my experimental design was that the temperature intervals remained constant and as I increased the temperature by a factor of 5. This helped me determine the critical temperature to which coffee becomes dangerously acidic for coffee drinkers. Further the lower temperature helped me accommodate iced coffee drinkers and hot coffee drinkers increasing the range of people who could benefit from the knowledge gathered in this experiment.

 

I also experienced limitations during this experiment creating both systematic and random errors, while random errors can never fully be reduced when conducting an investigation, however as I choose to have 2 sets of data and five trials for each titration I was able to obtain reliable data that has not been impacted by random error. I was able to isolate the two main systematic errors which could have had an effect on my investigation. Firstly, The colour change of the coffee solution was harder to determine as the darkness of the solution affected the visibility of the formation of the pink colour. This creates a systematic error in my work as the continual opaqueness of the colour change continually made it hard to determine when the colour change had occurred. For future experiments with a similar design I would integrate the use of a clear white tile to be placed below the conical flask to create more visibility when colour change occurs, further the person observing the colour change should do so from above where they can see the colour change more clearly.

 

Another limitation includes, the heat loss or gain as the acid-base solution was titrated could have affected the validity of the data as the pH metre was done immediately after the desired temperature was reached. However, the trials of titration occurred following this allowing for heat loss or gain. This could affect the concentration of caffeine solution, as well as the concentration of quinic acid as the heat loss would decrease the amount of hydrogen ions produced as the amount of quinic acid present would reduce. This creates a systematic error as it may have occurred for any of the trials of titration above as the surrounding environment variables could not be kept constant. For future experiments I would use a hotplate when conducting the titration as the conical flask can be placed on the hot plate as it maintains the desired temperature while titration occurs, reducing the systematic error created by heat loss.

One possible extension is measuring the effect of coffee additives such as sugar and milk on the overall pH of the coffee solution. This extension would illuminate how additives can either increase or decrease the pH of coffee making it better or worse for the consumer. Furthermore, another extension includes varying the types of coffee used in the experiment, as throughout this investigation i used nescafe coffee, as the beans, harvesting method and production vary between different coffee brands producing different types of coffee, this can determine if there are variations in the coffee type (brand) and the variation in the pH levels of solutions derived from these coffees.

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