Does aeration ever stop being effective?

[mokabb]

I have been lowering my TA over the past few days. It started at ~ 150 and now is near 90 ppm. I have been using up my dry acid and starting to use muriatic aliong with aeration. My last slug of muriatic was ~ 18 hours ago, dropping the pH to ~ 7.0. Despite continuing to aerate for ~ 18 hours, the pH this time has only increased to ~ 7.3 or so. Usually, it would only take about 6 hours at the most. That is to say, the correction of pH using aeration is much slower this time despite having a lower TA (buffer).

Is it time to give up on the aeration? If need be, should further pH correction be done with Borax?

 

[dmanb2b]

Hi Mokabb...we need some more info...do you have a swg or experience rapid PH rise?

What you are experiencing is normal, as the rate of PH rise declines at lower TA levels. Hence why we recommend the lower levels when using a swg or you have a waterfall feature that provides constant aeration.

 

[mokabb]

No SWG. Converting trichlor to bleach. My TA has been between 120-150 for the past few years. I was just trying to get it down to the 60-100 range using acid and aeration. The first 40-50 ppm decrease seemed to go quicker than this last 10-20 ppm. I was asking more out of curiosity than a result of a problem. I figured that there probably is a new equilibrium reached.

 

[chem geek]

What you are experiencing is exactly as predicted. The rate of pH rise gets slower as the TA gets lower. This is the main reason you want to lower the TA in the first place. Though a lower TA has less pH buffering, TA itself is a SOURCE of rising pH and this latter effect outweighs the former. So ironically, lowering the TA makes the pH more stable IF the pH was rising previously (due to carbon dioxide outgassing).

If it is that difficult for you to raise the pH through aeration, then your TA is probably low enough to have the pH be fairly stable when the aeration is stopped.

Richard