# [APD] Re:CO2 not dissolving

Probably it is dissolving, but that does not mean that the bubbles will disappear. The bubble may start out as 100% CO2, but if it stays around in the tank, it winds up as roughly 80% N2 and 20% O2 and just a little bit of CO2. How does this happen? It has to do with partial pressures of gasses.

Partial pressures predict whether a gas will dissolve from a gas into water or come out of solution from the water back into the gas phase. If the atmospheric pressure is 760 mm Hg---one atmosphere---then the partial pressure of nitrogen will be 80% of 760 or 608 mm Hg, and the partial pressure of oxygen will be 21% of 780, or 164 mm Hg. Since there is only 0.03% CO2 in the atmosphere, the partial pressure of CO2 will be almost nothing---around 0.2 mm Hg.

If the partial pressure of oxygen in the water is less than that in the air, then O2 will dissolve into the water from the air until the partial pressure in the water is equal to that in the air. The same is true for all other gasses.

Now, let's take a bubble of 100% CO2, just released into your tank. Since CO2 is the only gas in the bubble, and since the total pressure of the bubble is, lets say, one atmosphere, then the partial pressure of CO2 is 760 mmm Hg. Obviously, the partial pressure of CO2 in the water is much lower, so CO2 will immediately start dissolving in the water.

But, the water has nitrogen and oxygen dissolved in it, and in pretty good equilibrium with the atmosphere. Let's say that the partial pressure of N2 is 600 mm Hg, and the partial pressure of O2 is 160 mm Hg. in your tank water.

Therefore, N2 and O2 will diffuse INTO the bubble from the tank water as the CO2 is diffusing out. Remember that, initially, the partial pressures of N2 and O2 in the bubble are zero, since the bubble starts out as 100% CO2

While the bubble may get smaller because CO2 diffuses out faster than N2 and O2 diffuse in, The bubble is not going to disappear. Instead it comes into equilibrium with the partial pressures of O2, N2, and CO2 in your tank water.

So, therefore, you can not conclude that you must have been sold got 'watered down' CO2 or that your reactor is somehow sucking in gas from an outside source. You just have to remember that gasses are diffusing into those CO2 bubbles, while the CO2 is diffusing out.
--
Paul Krombholz in soggy central Mississippi
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