Causes for ph dropping in manually chlorinated pool

[Poolzzz]

There are a lot of posts about controlling rising ph but ph in my pool continually drops. I only use bleach which I understand is net neutral.

I don't use pucks so can rule that out. Rain could be acidic but it drops even with no rain and besides I have a hard cover so no rain gets in anyway. My fill water is about 7 but I manually fill and have deliberately not filled for long periods yet the ph still drops. I don't own any acid so that's not being added.

What other mechanisms exist that could be driving ph down?

 

[kimkats]

Each pool and area are so different it is really hard to tell. You say you have a hard cover on the pool. I wonder if that might be it. The water is so still that the PH drops for some reason. I really don't know how it works. Just throwing an idea out there.

How often do you run your pump? Does the water roll the top at all when it is running?

At least it is an easy fix-turn the pump on and let the return roll the top of the water.

Kim (I am going to play around with a bowl of water and see if the PH drops if I let the water sit over time)

 

[ant 83]

Do you ever shock the pool with dichlor powdered pool shock?

 

[Poolzzz]

Do you ever shock the pool with dichlor powdered pool shock?

No, never.

 

[Poolzzz]

Kim...my intex pump is astoundingly weak so, even running full bore pointing to the surface, the water remains glassy calm. There's a bubble solar cover, 4 inch air gap then the hard cover (2 inch rigid pink insulation). It's on 24/7 except when we swim. So you are correct there is very limited outgassing but my simple understanding of the process is that no out gassing would be ph neutral not cause it to drop.

 

[mknauss]

no out gassing would be ph neutral not cause it to drop

I would propose that not allowing the pool to outgas and 'breathe' makes your water acidic. The CO2 builds up and creates a mile carbolic acid. Is your TA steady?

I would suggest that every few days you remove the covers for a few hours and let the pool breathe.

Take care.

 

[domct203]

What is your TA?

Please post a complete set of recent test results.

 

[Poolzzz]

Last complete test results (2 weeks ago):

FC = 5
CC = 0 (fluctuates between 0 and 0.5)
ph = 7.5 (dropping)
TA = 80
CYA = 40
CH = 25 (old measurement, not measured recently)

These are basically the end results from the pool use over the summer because last week I drained the pool and installed 2 inch pink rigid insulation underlay. It's now officially a winter pool! LOL. So the current results from the new fill are:

FC = 4
CC= 0
ph = 7.8 (dropping)
TA = 50
CYA = 30
CH = didn't measure

The fill water was ph = 7 and TA = 20. Because the pH drops I'm just adding washing soda each time it hits 7.2. This has brought the TA up to the current 50.

Anyway, I think there are two aspects to the original question:

1) What is actually causing (ie the root cause for) the dropping pH, and
2) Regardless of the cause, how to stabilize.

#1 is kind of a"Deep End" thing, it would be nice (but not essential) to really understand the actual reason for the dropping pH.

#2 is what matters and it seems allowing outgassing (from aeration and/or cover removal) should stabilize the pH as would raising the TA further above 80. It's cold here (freezing with skiff of snow) so maybe better to raise the TA than remove the cover more (heat loss).

 

[Poolzzz]

I would propose that not allowing the pool to outgas and 'breathe' makes your water acidic. The CO2 builds up and creates a mile carbolic acid. Is your TA steady?

Possibly but I don't see a source for the C02 "build up". I would think no outgassing would just hold original levels of dissolved C02 (carbonic acid). Unless you are suggesting the C02 is being injected from another source but if so I don't know what that source would be given pool water is so heavily over carbonated relative to air.

My TA rises which each addition of washing soda / borax. I think it was around 60 in early September and now is is 80. However I've always measured *after* adding washing soda. You raise an interesting point so perhaps next time I'll measure TA *before* raising the pH to check if the TA has dropped with the pH. If it has not dropped that would support your theory of some kind of C02 buildup occurring. [EDIT]Upon reflection I'm don't think the TA test has the precision necessary for this experiment since it is 20ppm (+/- 10). [EDIT]

Regardless the reasons for the pH drop, I'm sure you suggestion would work as a solution. I've noticed over the summer extra pool use means slower pH drop. Presumably due to the cover being off more plus the extra aeration from splashing about.

 

[ant 83]

I've wondered where the co2 that out gasses comes from. I originally thought that chlorine oxidizing things forms co2, but could never find any confirmation of that theory. So that's not right I'm thinking now.

My understanding is that co2 is formed primarily in pool water from adding acid. I assume cyuranic acid qualifies, in all its various forms. Also, baking soda increases co2.

My suggestion would be to increase TA in small increments (not using baking soda) until PH stops dropping, being aware that aeration will put upward pressure on PH, being careful to not over shoot.

 

[JamesW]

CO2 comes from bicarbonate. It's in equilibrium with carbonate, bicarbonate and carbon dioxide. How much of each on a molar percentage depends on the pH.

As you lower the pH, carbonate picks up a hydrogen to become bicarbonate and bicarbonate picks up a hydrogen to become carbonic acid (carbon dioxide).

If the pH is dropping, something acidic has to be being added.

The rule that I find helpful is that if the pH keeps dropping, the TA is too low and if the pH keeps rising, the TA is too high.

 

[JoyfulNoise]

To the OP, Poolzzz,

I suggest with a vinyl pool using bleach that you keep your TA higher, above 80ppm at the very least. Also, do not add any sources of acid and try to see if you can maintain a pH in the 7.6-7.8 range. Some pools, by virtue of their construction (vinyl & fiberglass) and their input water sources (soft fill water), just need higher TA levels to support the pH of the pool water. With manual bleach chlorination, there is excess lye in it that adds a small amount to the pH & TA, but it's pretty small.

 

[Poolzzz]

...As you lower the pH, carbonate picks up a hydrogen to become bicarbonate and bicarbonate picks up a hydrogen to become carbonic acid (carbon dioxide).

So basically:

HCl (Muriatic Acid) + HCO₃​ (bicarbonate) = H₂CO₃ (carbonic acid). Carbonic acid is unstable so exists primarily as aqueous CO₂ + H₂O.

The CO₂ will outgas until the levels in the water are in equilibrium with the air which occurs around pH = 8+ (depending on other factors)

Since more CO₂ = lower pH and less CO₂ = higher pH outgassing means a rising pH and no outgassing (ie with cover) means a constant pH.

This is only one of many processes that could affect pH. Stop the outgassing and the others become visible.

If the pH is dropping, something acidic has to be being added.

Seems finding it can be tricky since it doesn't have to be directly adding an acid but could be anything that causes an overall reaction to be acidic.

...if the pH keeps dropping, the TA is too low and if the pH keeps rising, the TA is too high.

Very succinct. That probably sums it up. IMHO a useful phrase for the Pool School TA section.


Thanks everyone for the comments, the way forward seems clear even if the root cause remains a mystery.