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How Much CO₂ is in a Bottle of Soda?

__Dr. David G. Haase, The Science House

I was wondering that the other day and remembered that one good way to get the carbon dioxide out of a soda is to shake it. My son demonstrated how to do this. This led to an experiment which you can try for yourself and investigate the amount of carbon dioxide dissolved in different beverages and how much can be released at different temperatures.

To do this experiment I used a few 16 ounce (454 milliliter) plastic bottles of Pepsi One (at room temperature) and an electronic scale that could hold 0.5 Kg and could be tared. Your chemistry teacher may have such a scale.

First I put a bottle of the Pepsi One on the scale and pressed the "tare" button. This sets a 'zero' on the scale so that when I measure the bottle again the scale will show how much the mass of the bottle increased or decreased.

Then I shook the bottle vigorously for a few seconds. I could see the foam produced and could feel the pressure building up as the plastic bottle expanded slightly in my hand.

Now, this is the trick, I let the foam subside and SLOWLY turned the bottlecap to release the gas pressure without spraying any liquid out of the top of the bottle. Then I tightened the bottlecap and measured the bottle on the scale. The change in mass showed how much gas was released when I opened the cap.

Then I repeated the process and measured again. I recorded the change in mass each time and found that after about five minutes of shaking the mass did not change very much. Below is a table and graph of one of my trials. For you math fans the graph looks like it decreases exponentially.

I did this experiment with three 454 milliliter bottles of Pepsi One. On the first bottle I learned how to spew Pepsi all over the room. For the other two bottles I found that each one contained about 2.2 grams of carbon dioxide under pressure dissolved in the liquid! That is enough gas to fill up a liter bottle at standard temperature and pressure!

This is not all of the carbon dioxide in the soda. There is still carbon dioxide gas dissolved in the liquid at atmospheric pressure. The amount is very small and depends on temperature. It would be interesting to try this experiment again with a cold bottle and a warm bottle. Because carbon dioxide affects the pH of the soda it might be interesting to compare the pH of the fresh out of the bottle and the "defizzed" soda.

What is the pressure that is used to put the carbon dioxide into the bottle in the first place? The bottle manufacturers design the bottle to hold about 20 atmospheres of pressure before they burst.

Some other things to experiment with:

How would results of this experiment change if you used a very cold soda?

Could you measure the pH of the soda before and after you de-fizzed it? How would you expect it to change?

Do sugary and low calorie sodas have the same amounts of CO₂ in them?

Can you invent a way to capture the gas (hint: a garbage bag) and measure its volume?

Some things we found out:

The first carbonated drinks were the naturally carbonated waters produced from mineral springs. In 1772 the famous English chemist Joseph Priestley invented a process for artificially carbonating beverages. The title of his report was "Directions for Impregnating Water with Fixed Air in order to communicate to it the peculiar Spirit and Virtues of Pyrmont Water, And other Mineral Waters of a similar nature." (Carbon Dioxide was called "fixed air" in those days.) His discovery was so well received that the Royal Society awarded him the Copley Medal. To produce the carbon dioxide Priestley dripped sulfuric acid onto chalk.

Many famous chemists, such as Robert Boyle, experimented with ways of making carbonated beverages. This was high tech stuff. The addition of CO₂ made the drink slightly acid and tastier and also served as a preservative so the drink would last longer without going bad.

The beverage field got a boost when Sir Humphry Davy and Michael Faraday first liquified carbon dioxide. Soon after, Thilorier in Paris made solid carbon dioxide - dry ice.

The cola in your bottle is pressurized to about two atmospheres of pressure and if you could get ALL of the CO₂ out of the cola it would fill a volume about four times the volume of the cola itself. Soft drinks hold more CO₂ at low temperatures than at high temperature. If you did our little experiment with cold Pepsi you should get less gas out of the bottle.

Glass bottles are better soft drink containers than plastic bottles (made of PET, polyethylene terephthalate) because carbon dioxide and air can diffuse through the plastic. Therefore, after a long time the soda would lose some of its fizz and would be oxidized by the air, reducing its shelf life.

Sources:

Manufacture and Analysis of Carbonated Beverages, Morris B. Jacobs, Chemical Publishing Co., Inc., (New York, 1959).

Chemistry and Technology of Soft Drinks and Fruit Juices, Ed. by P.R. Ashurst, Sheffield Academic Press (Sheffield, England, 1998).