How does Acid reduce TA?

[JohnT]

What do you mean by "carbonic acid is CO2 in water". I thought carbonic acid is in equilibrium with CO2 and H20, but not actually the same thing?

It's far more complicated than just an aqueous solution. Just trying to keep the whole topic readable. It's a complex topic on its own and I don't think equilibrium and partial pressures are worth delving into. Complex, but an effect most everyone sees in action or experiences in daily life when a soft drink is opened, when it rains or when we breathe.

 

[JamesW]

Ok.

I'm reading that carbonic acid only exists in trace amounts, in equilibrium with C02. That would mean that the carbonic acid produced when pH is lowered will quickly convert to C02 and H20, ie carbonic acid is an intermediary, yes? Is that why they're used interchangeably?

Basically, yes, that is correct.

 

[tomfrh]

Ok, it's starting to make some vague sense.


But I'm still confused why about does pH rise as CO2 floats away? Is it the corresponding reduction in carbonic acid?

 

[JoyfulNoise]

To answer the previous question as to why the consumption of bicarbonate that causes pH rise does not also lower alkalinity, you have to look deeper at the chemistry. Here are a set of some of the possible alkaline reactions (not all) that will occur in pool water between a pH pf 6 and 9 -

(most folks forget to add the water in !)

OH− + H+ → H2O

CO2 (aq) + H2O → H+ + HCO3-

H3CY → H2CY- + H+

B(OH)4− + H+ → B(OH)3 + H2O

You have water and all of the alkaline species. Total alkalinity is defined as -

TA = [HCO3-] + [H2CY-] + [B(OH)4−] + [OH−] - H+

As you can see, when there is no external addition of alkaline or acidic species, the loss of bicarbonate to the consumption of a proton (H+) forming dissolved CO2 gas conserves the TA. If both hydronium and bicarbonate decrease by the same amount (and they do based on the chemistry), then the measured TA will not change.

 

[tomfrh]

hydronium eh?

I think I better go to bed and start again tomorrow. Thanks for all your help so far.

 

[JohnT]

Ok, it's starting to make some vague sense.


But I'm still confused why about does pH rise as CO2 floats away? Is it the corresponding reduction in carbonic acid?

Yes. The easiest way to view it is to think of the graph as two-way. Say your pH is 7 so your carbonate mix is about 80% bicarbonate and 20% carbonic acid or CO₂ ​to simplify. If you force the pH to move by adding acid, those proportions will change along with the pH. But also, if you change those proportions by outgassing CO₂ , the pH will move. So if you got rid of half the CO₂ by outgassing, the new proportion will be 90% bicarbonate and 10% CO₂, so your pH has to be about 7.3. You essentially can't get off those lines, so if you change one thing the others have to change to make up for it.

If you add acid again to get the pH to 7, the mix goes back to 80/20, but the overall alkalinity is lower than it was before. With all else equal, you can't have a 90/10 bicarbonate/ CO₂ mix and have your pH at 7. If your mix is 90/10 the pH has to be 7.3 so removing carbon dioxide raises pH.

When comparing Matt's explanation to mine, he is describing the chemistry and I'm more on the engineering side where I describe the observable effects and responses.

 

[JoyfulNoise]

John brings up a very excellent point, and one I alluded to in my earlier post, and that is the pH of an aqueous solution is not independent of the other chemicals we measure. Because pool owners make separate measurements of all these parameters, it’s easy to get lulled into thinking that they can all be adjusted independently of one another. Unfortunately, that’s far too simplistic a way of looking at pool water and it is what directly leads pool owners into the various traps that occur. One good example of this is the acid/alkalinity yo-yo where pool owners get told to their alkalinity is too low and to add bicarbonate. Then a day later their pH is too high and they're told to add acid which undies all of the alkalinity they just added. Then they go on the roller coaster ride of constantly adding acid and bicarbonate until they’re so frustrated that they give up.

The pH of the pool water is affected by almost every chemical we add to it, including something as simple as salt. And all of these chemicals interact with one another to varying degrees again causing the pH to change. There are certainly simplification that can be made but if one pushes those simplification too far, you’ll wind up getting the wrong answer.

As the old saying goes - “close enough only counts in horseshoes and hand grenades...”

 

[tomfrh]

Thanks John and joyful.

And yes like many pool owners I’ve been lumbering along with a very crude understanding of the chemistry.

its confusing stuff. Sometimes it seems that pH and alkalinity are one and the same - eg when you throw in acid which reduces them both, but then the pH increases independently. Learning the underlying chemistry and equilibrium equatione is helpful.

Re. the yo-yo, I’ve heard several people here mention that pool stores try and set up a yo-yo by recommending high TA. I thought that sounded paranoid, however Im starting to believe it.

We bought our place 3 years ago, and the previous owner told me to maintan TA of at least 120-150.

I’ve dug further into the previous owners operating principle of 120-150 , and guess what - it came from a pool store boss in the early 1980s. I’ve got the note in front of me (our previous owner was an accountant and documented his conversations, lol). So it seems the pool store bosses have been onto this little trick for at least 30-40 years, and their recommendations continue to echo thru generations of pool owners.

 

[JamesW]

It seems sinister, but it’s probably mostly ignorance on the part of employees. The industry does not seem to understand the basic principles.

The reason that the pH rises as hydrogen ions are lost is that pH is basically a measure of hydrogen ion concentration.

It’s a counterintuitive relationship where the lower the hydrogen ion concentration, the higher the pH and the higher the hydrogen ion concentration, the lower the pH.

 

[JoyfulNoise]

If one uses acidic forms of chlorine (trichlor & dichlor) and fill water is low in TA and/or one gets sufficient rainfall, then running at a higher TA makes sense because you can easily deplete the primary buffer in your pool water. I don’t know that there’s much more money to be made on acid & baking soda sales above and beyond the sale of regular pool store potions, but it certainly doesn’t hurt to sell “alkalinity increases” at 4X the price of grocery store baking soda

pH is nothing more than a measure of all the alkaline and acidic species in pool water as well as their chemical interactions. The devil, as always, is in the details....

 

[JoyfulNoise]

It seems sinister, but it’s probably mostly ignorance on the part of employees. The industry does not seem to understand the basic principles.

The reason that the pH rises as hydrogen ions are lost is that pH is basically a measure of hydrogen ion concentration.

It’s a counterintuitive relationship where the lower the hydrogen ion concentration, the higher the pH and the higher the hydrogen ion concentration, the lower the pH.

Yeah, those pesky negative logarithms...just mathematicians always discovering new ways how to make the number line more complicated....

Q: Why did the chicken cross the road?

A: The answer is trivial and is left as an exercise for the reader.

 

[tomfrh]

To go back to the original question - I understand it now. The chart is helpful. Thank you all.