A possible explanation of how borates work to stabilize pH

[mas985]

Sorry to continue with this but you and Evan seem to be saying different things.

The aeration from the SWG is conceptually no different than aeration from any other source such as waterfalls, fountains, or an air compressor. Technically, it is a rather efficient form of aeration since the hydrogen gas bubbles are small and they contain no carbon dioxide in them (initially). In other words, I would not consider having an SWG turned on as being slow aeration. On the contrary, it is quite vigorous which is why most users with SWG systems find a rather strong tendency for the pH to rise.

Good, that is what I thought. So then when I add acid each week, my TA will drop a bit but it should not increase on it's own correct? After many weeks of this, assuming no alkalinity to the pool, TA should continue to drop. So if I am not adding any alkalinity, would I not see it drop over time?

My fill water does add some alkalinity and increase the PH a bit so perhaps this is what is increasing the TA. Although it would seem that the acid dose would lower TA more than the replacement water would increase it.

for your question regarding having aeration happen quickly to prevent the TA from rising the answer is no. It does not matter how quickly the aeration occurs. The physical process of having carbon dioxide leave the pool water causes the pH to rise with no change in TA. It doesn't matter how quickly or slowly this occurs.

Are you aerating to outgas the CO2 and raise your pH? If you are not then the TA will climb back up when the pH rises and the carbonic acid converts back into bicarbonates and carbonates.

This seems to be in conflict with what Evan said above that without aeration, TA will rise again. From everything you have said, it seems that once TA has been reduced it will not rise unless more alkalinity is added.

 

[chem geek]

So then when I add acid each week, my TA will drop a bit but it should not increase on it's own correct? After many weeks of this, assuming no alkalinity to the pool, TA should continue to drop. So if I am not adding any alkalinity, would I not see it drop over time?

This is correct. However, the TA drop can be quite slow so won't be measurable until many weeks. Test errors of +/- 10 ppm obscure this until the drop is large enough to be clear.

My fill water does add some alkalinity and increase the PH a bit so perhaps this is what is increasing the TA. Although it would seem that the acid dose would lower TA more than the replacement water would increase it.

Yes, adding any fill water that contains ANY TA will end up increasing the TA in your pool. Evaporation of pool water removes ONLY water from the pool with any TA components remaining behind. As for which factor dominates, it can easily go either way depending on evaporation rates vs. outgassing rates.

for your question regarding having aeration happen quickly to prevent the TA from rising the answer is no. It does not matter how quickly the aeration occurs. The physical process of having carbon dioxide leave the pool water causes the pH to rise with no change in TA. It doesn't matter how quickly or slowly this occurs.

Are you aerating to outgas the CO2 and raise your pH? If you are not then the TA will climb back up when the pH rises and the carbonic acid converts back into bicarbonates and carbonates.

This seems to be in conflict with what Evan said above that without aeration, TA will rise again. From everything you have said, it seems that once TA has been reduced it will not rise unless more alkalinity is added.

If I understand the context correctly, then yes, waterbear is wrong (or we disagree and need to figure out why). Perhaps he is referring to the TA rising with your fill water (or any other source of base) in which case he would be correct. However, if he is saying that the TA will rise when the pH rises, that is wrong IF the source of the pH rise is from outgassing of carbon dioxide. The outgassing only causes the pH to rise with no change in TA.

Richard

 

[mas985]

Ok, I think I clear now.

Thanks.

 

[Guest]

Depends on the surface of the pool. If you drop the pH you lower the MEASURABLE carbonates and bicarbonates in the water (TA) by converting them to carbonic acid. If you add an alkaline matierial you will convert some of the carbonic acid back into carbonates and bicarbonates. A plaster pool has a reactive surface and can add alkaline matierial (including carbonates!) to the water under low pH conditions and cause the TA and pH to rise again in addition to the pH rise from the outgassing of carbon dioxide. In a non reactive surface such as vinyl or fiberglass the TA would not be expected to rise. You have to take the finish of the pool and how it affects the water chemistry into account.

I stand by what I said since the pool in question has a plaster finish! Under real life condtions ALL variables have to be taken into consideration. This is why real life results don't always agree with theoretical calculations. Often there is something that is being overlooked or some assumptions are being made that might not hold true. Pool finish needs to be taken into account when dealing with any water chemistry issue.

Ockham's Razor in action! If you don't know what Ochham's Razor is it is often paraphrased as "All things being equal, the simplest solution tends to be the best one." In the case of lowering TA the simplest solution is to lower the pH and TA with acid and then aerate to raise the pH back up. It works in all types of pool surfaces! It has been demostrated time and again that just adding acid, whether by 'slug' or mixing it in slowly and just waiting is not an effective or efficient way to lower TA and keep it down and often does not work!

As Albert Einstein said,"The supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience". In other words you want to have to make as few assumptions as possible to explain something to eliminate variables that can have an influence on the results, such as the finish of the pool in this case or that pH rise is only caused by the outgassing of CO2.