pH and gases

[jimmyj7090]

Could someone brief me - my buddy and I are trying to discuss oxygenation, photosynthesis, carbon dioxide, and pH... how does O2 & CO2 affect pH again? O2 can increase pH... Co2 can decrease pH... how again? Thanks, man.

[SeanT]

pH article.

[Randy Holmes-Farley]

O2 has no direct effect on pH.

Adding CO2 will lower the pH because it forms carbonic acid in water:

CO2 + H2O ---> H2CO3 ---> H+ + HCO3-

Taking away CO2 will raise the pH for the same reason.

[jimmyj7090]

Well, we have a reefer and a geek trying to understand the concept of me oxygenating my organic salt solutions at work in order to raise the pH to 7.4.

[Randy Holmes-Farley]

Well, you may call it oxygenating, but I assume that the pH effect is from driving off CO2 from the solution.

What is in that solution?

[Boomer]

In short, aeration increases O2 and decreases CO2. CO2 is much more soluble that O2, so it goes into solution easier than O2 and thus can also be removed easier than O2 They are also both a function of salinity, temp and pressure. Basically CO2 is about 23 times more soluble than O2

Solubility of CO in Water

Solubility of O2 in water

[Randy Holmes-Farley]

In short, aeration increases O2 and decreases CO2.

I think that depends on whether the tank is deficient in O2 and/or CO2, or has an excess of either. For example, if you are using limewater or washing soda, the tank is deficient in CO2 and aeration brings it into the water. If you are using a CaCO3/CO2 reactor, the tank has an excess, and aeration blows off CO2.

[Boomer]

Yes, I agree and is why I said in short. And his sample is using neither soda or reactor. I have yet seen a reef tank excessive in O2

[Randy Holmes-Farley]

[jimmyj7090]

thanks for the replies.

This stemmed from a reefer and a bio lab tech trying to have a conversation.

Does the o2 blow off the co2 because the water can only hold a certain amount of total dissolved gases and the o2 is more soluble, or because of a reaction between the gases?

Sorry, I'm awfully illiterate for a reefer when it comes to chemistry.

I'll have my friend post the details of what he was doing in the lab next week if anyone is still curious.

thanks again
jk

[Boomer]

No, the O2 doesn't have anything to do with the CO2. And it is CO2 that is more soluble than O2.

I'll try to explain it in a different way. If you start to blow up a balloon, you know that the more you blow it up the greater the pressure is inside the balloon. When a gas is in water, for example, it also has a pressure just like the balloon and when a gas is in air it also has a pressure just like a balloon. If we now open the end of the balloon the air inside the balloon leaks out as it has a higher pressure.

Now if a sample of water has CO2 in it, it will have a CO2 pressure and if that CO2 pressure in the water is greater than the CO2 pressure than the air above the sample of water, the CO2 will Leak from the water to the air, just like the balloon. At some point, with no outside influence, sooner or later the CO2 pressure in the water will = the CO2 pressure in the air. We will say now that no CO2 goes from the water to the air and no air CO2 goes from the air into the water, as their pressures are equal.

When you try to "forcefully drive" something you can increase or decrease these gases somewhat, above those equal pressures and quite often these pressures are not equal. Such is the case, quite often, with water and CO2 and O2. You can forcefully drive more O2 in the water and drive off CO2 from the water. This forceful drive allows things to happen quicker than if you left them alone on their own.

Each gas has a different "chemistry", a set of properties that makes it different from another gas. Because of these properties some gases are harder to dissolve in water than other gases, yet other gases are easily dissolved in water. O2 does not like to be dissolved in water, any where near what CO2 does, so it is harder to get O2 in the water and once dissolved it is harder to get it out of the water. CO2, on the other hand, goes into water very easily, thus it can be removed much easier than O2.

Another example that takes place to a degree. When a bubble rises from an airstone the bubble will have CO2 and O2 in it. The water will also have CO2 and O2 it. If the O2 pressure inside the bubble is greater than the O2 pressure outside the bubble, the O2 will diffuse through the bubble into the water. And if the CO2 pressure is higher outside the bubble in the water the CO2 will diffuse into the bubble from the water and rise to the surface and expel the CO2 to the atmosphere. However, most of the gas exchange is done at the surface where the bubbles break, causing agitation on the surface, which is like a "driving force", which usually increases the O2 and decreases the CO2. In order to determine what is doing what and how much you would need to now the "pressure" values of the CO2 and O2 both in the water and the air above the water and their solubility.

For the sake of it another example, with a FOT that is heavily loaded with fish and run poorly. The fish will give off CO2 and take up O2. As this continues the CO2 pressure in the take rises as does the CO2 concentration. If this continues the CO2 level can get deadly. The uptake of the O2 by the fish is faster than the O2 can diffuse from the ari into the water, so it is deadly and the CO2 can increase in the water faster than it can diffuse out of the water into the air =dead fish. No O2 and to mcuh CO2. If now you aggitated the water heavily the O2 in the water would increase and the CO2 would decrease=you just saved your fish :

Ok Randy, make sure you have a new chain on the saw, before you start cutting

[jimmyj7090]

boomer, thanks for the reply

I'm confused though,,,

are you saying that, assuming an elevated CO2 level, by adding O2 to the water the pressure on Co2
(overall gas concentration??) will be raised and thus drive off the Co2 because the Co2 is easier to drive off than the O2?

I think I understand the part about the levels of the gases reaching an equilibrium with the atmosphere, and the baloon analogy seems clear, but I'm not sure I'm getting what your saying about the process/interaction between the gases.

Does it come down to the overall amount of dissolved gas in the water or am I totally missing something?

If this is becoming a headache to explain don't worry about it, it's really just a curiosity question.

thanks
jk

[Boomer]

are you saying that, assuming an elevated CO2 level, by adding O2 to the water the pressure on Co2
(overall gas concentration??) will be raised and thus drive off the Co2 because the Co2 is easier to drive off than the O2?

No, the O2 has nothing to do with the CO2. There is no interaction between the gases or any reaction taking place, they are independant of each other

Does it come down to the overall amount of dissolved gas in the water or am I totally missing something?

The overall amount of each gas and "its" own pressure but yes there is also a overall amount of all the gases.

If this is becoming a headache to explain don't worry about it, it's really just a curiosity question

This is a very complex subject, maybe Randy can make it easier



Maybe it is time to do some reading


Basic Gas Pressure Laws

Getting Deeper

Really Deep

Solubility of O2 and CO2 in Water

O2

Solutions and Solubility

Seawater CO2 and O2

[jimmyj7090]

ughh??

Randy, got a laymans way of putting this? I'm having a hard time understanding what boomer is saying. The explanations seem detailed, but the examples and analogies seem very simple and I don't get how they fit together.

Is boomer saying that, in short, by adding O2 to the water there is agitation or other means of simply speeding up the process of reaching equilbrium with the atmosphere?

Is boomer saying that the pressure of each gas affects the pressure of the other gas or that the pressures do not affect each other?

I'm asking these questions here because a more complete understanding of the chemistry/physics invloved is a bit out of my grasp right now, but I would like to have a very simple working knowledge (if possible) of how the gases affect each other in a reef tank. If a layman's explanation is impossible I understand, but I just get the sense that boomer and I are not on the same page at all and I want to avoid bothering someone with questions that are over MY head.

Is there any way to simplify this somewhat? If not, I will fade away quietly.

thanks boomer and randy
jk

[Randy Holmes-Farley]

We are all saying that O2 has zero effect on pH. If you bubble pure O2 into aquarium water, even under very high pressure, it will do nothing directly to the pH.

pH is controlled (via aeration) by CO2 coming into or out of the water. Aeration with O2, just as with air, or nitrogen or helium, or anything else, may impact how CO2 moves in or out of the water, but it is the CO2 that impacts the pH.

If you are growing organisms in the water, there may well be important reasons to drive O2 into the water, and those growing organsims themselves may impact pH through their various waste products (including CO2), but that's the only effect of oxygen.

In a reef tank, you aerate to equilibrate mostly O2 and CO2, and not much else. The O2 equilibrates fairly easily because there is a lot in normal air, and not so much in water.

CO2 is much harder to equilibrate if a significant source (like a CaCO3/CO2 reactor) or sink for CO2 (like limewater) exists in the tank because seawater can hold a large amount of CO2 in various forms, but normal air holds very little.

Beyond that, I'm happy to go on and on, but I'm not sure what is unclear.

[jimmyj7090]

Thanks Randy,

that was exactly what I was looking for. I actually just put down my school reading to come back and post that the initial replies had really covered my question.

I think Boomer's great explainations simply confused me by bringing in a lot more than I was ready to digest. I think a lot of that was filling in the details behind the very basic understanding that I do have, and I got the impression that there was some entirely different process being explained.

It's tough for us soft science guys to talk with you real science guys.

Thanks much for the patience, boomer and randy.

jk

[Randy Holmes-Farley]

It's tough for us soft science guys to talk with you real science guys.




I'm glad we could be of help.