What Is the Pressure Inside a Bottle of Soda?

Bottles of soda are sealed under high pressures of CO₂, or P_CO2. This causes a substantial amount of CO₂ to dissolve into the beverage, giving the drink its fizziness. But what exactly is the pressure inside a bottle of soda?

A student of mine (Andrea Sturgis) and I worked together for several years to try to answer this question.^1,2 To do this we slightly modified a published method³ for finding the pressure in soda bottles. The analysis involves a simple application of the ideal gas law. You can watch a description of what we did below:

Video 1: Measuring the Pressure Inside a Soda Bottle⁴

This protocol is simple enough that you can carry it out at home if you have an appropriate balance.⁵ Even though the procedure it simple, it allows for a wide variety of explorations. For example, we determined P_CO2 inside bottles of various sizes: 2L, 500 mL, and 355 mL. We found that sodas generally contain pressures between 2.7 and 4.7 bar, as long as the bottles are measured prior to the expiration date stamped on the bottle.  

We further observed that bottles display higher P_CO2 at higher temperature (Figure 1). This result can be explained by noting that the escape of CO_2(aq) from solution is endothermic:

CO_2(aq) + energy ↔ CO_2(g)              Equation 1

Increasing the temperature of a soda bottle shifts the above equilibrium to the right, causing CO_2(aq) in the beverage to escape into the headspace of the bottle. This naturally has the effect of increasing the pressure. What a simple connection to Le Châtelier’s Principle!

diet_pepsi_graph.png

Figure 1: P_CO2 measured in 500 mL bottles of Diet Pepsi kept at different temperatures.²

We also noticed that beverage bottles lost P_CO2 over time – even if the bottles remained sealed! Interestingly, the loss of P_CO2 from sealed bottles was accelerated by temperature. To show this we stored 355 mL bottles of Diet Coke from the same 8-pack under different conditions of temperature: three bottles next to a heating vent, two at room temperature, and three in the refrigerator.² After two months of storage the bottles were all brought to room temperature and the P_CO2 was tested in each bottle (Figure 2). Average P_CO2 values of 3.35 bar, 3.72 bar, and 4.03 bar were measured in the bottles stored next to the heating vent, at room temperature, and in the refrigerator, respectively. This result can be explained using principles of chemical kinetics. Physical and chemical processes occur faster at higher temperature, so CO₂ escapes sealed bottles more quickly at higher temperature. We also were able to measure substantial pressure drops in 355 mL bottles of Diet Coke that were stored for two years at room temperature (P_CO2 = 0.4 bar) or in the refrigerator (P_CO2 = 1.1 bar).^1,2 But you don’t need to wait two months or two years to observe this. The loss of CO₂ from bottles can be measured by taking mass measurements of sealed bottles daily. Noticeable losses in mass can be detected in just a few days.^1,6  

pressure_vs_temp_by_storage.png

Figure 2: P_CO2 measured in 355 mL bottles of Diet Coke stored at different temperatures for two months. All pressures were measured after bringing the bottles to room temperature.³

The experiments reported here are just a sampling of the work that Andrea and I completed. I also have a few more ideas I have in mind to investigate in the future as well. Hopefully this short report will give you some inspiration to try out this experiment, and perhaps even carry out some explorations of your own. As always, I look forward to hearing from you if you learn anything interesting.

Happy experimenting!

Notes and References:

1. Kuntzleman, T.S.; Sturgis, A. The Effect of Temperature in Experiments Involving Carbonated Beverages. J. Chem. Educ. 2020, 97 (11), 4033-4038.

2. Sturgis, A. The Effect of Temperature in Experiments Using Commercially Carbonated Beverages. Undergraduate Thesis, Spring Arbor University, Spring Arbor, MI, 2018.

3. de Grys, H. Determining the Pressure inside an Unopened Carbonated Beverage. J. Chem. Educ. 2007, 84 (7), 1117-1119.

4. Tommy Technetium, Measuring the Pressure Inside a Soda Bottle, https://www.youtube.com/watch?v=AlTsaFUqZsA. Accessed December 2020.

5. A balance with a capacity of at least 500 g and precision of at least 0.01 g is recommended. At the time of this writing, balances with these specifications can be purchased on Amazon for $15-$40.

6. About 20 mg from 355 mL bottles of Diet Coke per day at room temperature – about 2 mg per day for bottles stored in the refrigerator.