Why does the pool industry and much of the online community recommend making adjustments of pH?

[Diznaster]

I'm new to pools, but not water chemistry. I have been a marine aquarium hobbyist for a few years. We care about the Alkalinity/Calcium/Magnesium balance, but just basically push the SI much higher to satisfy the needs of corals and snails to easily build themselves (making Calcium and Magnesium easy to deposit). There are some pretty solid articles from chemists in the aquarium community, and the consensus of most aquarists would say "Don't Chase pH". What that means is... Your pH is the end result of a complex water chemistry equation. pH can't tell you what to "fix", it's not an ingredient it's the end product. That said pH change is great to tell you to look for a problem in your system. Really for us aquarists, pH mostly tells how much CO2 is in the water. Because we already know (from testing) our Alk/Cal/Mag and we better not have any appreciable Borate/metals. So assuming atmospheric CO2 levels, we can calculate exactly what our pH should be. If that doesn't match our measured pH it means one of three things (in order of probability).

#1. You have more or less CO2 in your water relative to free air atmosphere equilibrium (95%),
#2. Your pH test is wrong, (4.9%),
#3 There is something really bad getting in your tank that you don't know is happening (0.1%).

If it isn't one of those things, congratulations you have officially broken chemistry. So if you have a properly calibrated pH probe (forget a color chart test that's a rough guess), it basically must be your CO2 level. Now here is the cool part, I can watch it happen in real time. My aquarium controller logs pH every minute. I can look at the chart and there is a daily cycle. As photosynthesis ramps up in the morning the available CO2 in the water drops, this causes pH to rise. At night the photosynthesis stops, but the fish are still making CO2 (breathing) and the pH drops. It looks like a sine wave every day. But it gets better! I can actually tell when people get home, or especially if the gas stove is in use. This drives atmospheric CO2 up in our house and the gas exchange drives it into the water also, again lowering pH. Keep in mind gas exchange is an equilibrium! With maximum water to air surface contact, the air and water will balance. You are never pumping an unlimited supply of carbonic acid (CO2 in water) into the water. It's not like putting in additional acid. It just equalizes with the air given enough gas exchange. If I had a party of 10 people and used the oven I would see a big pH drop in my aquarium. After the party and few hours for fresh air gas exchange my system will return to the exact same state as before, without any intervention from me. Because the air inside had returned to normal CO2 levels and the water equalized it's CO2 at that level.

I think you can see why for aquarists "adjusting pH" is dangerous. It will only be temporary and it will have negative consequences to ideal Alk/Cal/Mag levels. If you have a legitimate Alk/Cal/Mag problem, then fix that. If you have a CO2 problem, fix that. Not one of those two? It's another substance or a test error. Don't chase pH because then you are basically plugging random numbers into the Alk/Cal/Mag equation in order to look like you have the pH answer you want. Stability is our friend and nothing good happens fast in an aquarium.

Hopefully someone found that interesting, but I'd really like to explore how it might apply differently to pools.

Scenario 1: I have an algae outbreak, help! My TA/CH is OK (100/280), but my pH is high. Should I try to bring pH down before I add shock?
Answer A: Yes, the chlorine is more effective at lower pH and you should get pH adjusted first. This will obviously mess up your alkalinity, so you will need to fix that after the shock. But to actually kill the algae you need a lower pH.
Answer B: No, if you have algae it is consuming CO2, thus raising your pH somewhat artificially. Kill the algae and promote gas exchange (air injection or surface turbulence). Don't chase the pH, it will resolve itself if your TA/CH don't change.
Answer C: ?

Scenario 2: Help I can't get my pH down, my TA is 130 and my CH is 150, but my pH is 8.2. I'm adding acid and can't keep it down.
Answer A: Some pools just do that, keep adding acid.
Answer B: One of your tests is wrong, probably pH. Unless you have an algae bloom or something like a ton of Borate, that's impossible. You would need around TA 170+ and CH of 400+ to get close to that pH of 8.2. Do you have a way to verify your pH test (a reference solution)? Is there anything odd that happened recently and could be effecting tests? Some pools leach/gain carbonates into the water, and that will constantly push TA up, thus raising pH but if that were the case you would see TA rise between acid corrections.
Answer C: ?

Hopefully I didn't walk in a new room and start a battle royal. I still a have some questions about CYA and Chlorine chemistry, and I'm a totally new to trying to destroy all life in water. Aquarium or pool, I just don't see pH as anything other than a quick check that tells me I might have a problem, but I also don't trust any pH tests enough to use it for fixing any problem. I'd love to hear any ideas why this approach is wrong, and I'll be glad to learn something and admit it is wrong for pools.

 

[YippeeSkippy]

This treatise is best answered succinctly by @JoyfulNoise .

 

[Newdude]

Hey diz !! Before Matt chimes in and blinds most of us with science, i can help a little bit. PH can have some averse affects in pools that are not the case in aquariums. Too low PH for too long can wreck a vinyl liner with wrinkles, and too high can irritate eyes and be uncomfortable to swim in. Maybe the fish dont like it either but they don't complain much so its hard to tell.

 

[Oly]

Your scenario did not include a pool with an autoffill, high evaporative loss, and a TA north of 170 in your fill water.

 

[Diznaster]

Hey diz !! Before Matt chimes in and blinds most of us with science, i can help a little bit. PH can have some averse affects in pools that are not the case in aquariums. Too low PH for too long can wreck a vinyl liner with wrinkles, and too high can irritate eyes and be uncomfortable to swim in. Maybe the fish dont like it either but they don't complain much so its hard to tell.

It's easy to tell but the fish don't complain, the corals melt below pH 7.6ish. Everything just dies.

 

[Diznaster]

Your scenario did not include a pool with an autoffill, high evaporative loss, and a TA north of 170 in your fill water.

Yeah that sounds like a TA problem. And yes that needs fixing, but after you fix that doesn't the pH return to normal?

 

[Oly]

I am surpised to learn the pH of ocean water is 8.1

 

[Newdude]

I am surpised to learn the pH of ocean water is 8.1

I got 43 years living by one....... and i was today years old when i learned it.

 

[Diznaster]

I am surpised to learn the pH of ocean water is 8.1

Oh man this is really gonna bake your noodle. We put crushed coral skeletons (aragonite mined from land, reef safe), into a small chamber and pump compressed CO2 into it. At pH below 7.5 It melts the rocks and then we blow off the CO2 while we drip it into the tank. We can go a little lower and melt it faster, below 7.3 makes mud fast. Perfectly balanced Alk/Cal/Mag.

Edit: I'll add this is the reason for ocean acidification. As nominal atmospheric CO2 rises, so does carbonic acid in water.

 

[mgtfp]

There are certainly two aspects.

One is that you want a pH that works from three point of views: You want enough enough HOCl in your water, asking for a lower pH - but in a CYA stabilised pool that is actually less pH dependant than in an unstabilised pool. Then you don't want to damage your pool with a pH that is too low, like damaging a vinyl liner as mentioned by Newdude or dissolving the plaster in a plastered pool. And you don't want a pH that is too high resulting in calcium deposits in a SWG or on your tiles. And finally you don't want to get red eyes each time you swim. So there is certainly a need to make sure that your pH is in an acceptable range.

And then there is the pool industry. Just the other day a pool shop (not my usual shop) told me that my TA should be between 120 and 140. Had I followed that, I would increase CO2 outgassing, driving my pH up to a level where I get flakes in my SWG cell, then correcting with muriatic acid, which will also drive down the TA, which the pool shop will want me to correct again by adding alkalinity increaser (aka baking soda). Viceous cycle that is good for the pool shop, but not for me.

Some people manage to find a sweet spot with just the right chemistry where their pH seems to be stable. But that really depends on many parameters. If you for example have very high CH in your fill water, then you might not be able to operate your pool at a higher pH where your pool naturally wants to be because you'd stuff up your SWG cell or turn your pool white with calcium scaling. So you'll have to lower your pH and you will get in a cycle of adding ma and pH rising again.

But you can minimise this cycle by not listening to the standard pool industry advice, but following the TFP guidelines.

 

[Flying Tivo]

TFP started with suggested levels of TA around 100, but with time and science the suggested levels are now 50-90 with a target around 60. Thats where most pools want to balance CO2, locations will vary largely.

 

[mguzzy]

Hopefully someone found that interesting, but I'd really like to explore how it might apply differently to pools.

Diz!
Of course we found it interesting.. else no one would have responded.

So the way I see it is Aquariums differ from pools in one important aspect: In an aquarium we want to make an aquatic environment that is conducive to making things live and thrive.. whereas in a pool we want to kill everything that lives in the water. They are two ends of the spectrum. One is a recreation of a natural biome, the other is the creation of a unnatural biome. So the underlying chemistry is going to be different just from that standpoint. In an happy aquarium the environment dictates the pH. in a Happy pool, the pH dictates the environment. Because in a pool you want your chemical balance to support the sanitizing agents. An aquarium is the giving of life, a pool is the taking of life. ... But perhaps am I being too Zen in my analogy. Then lets talk physics. A ecological system, like an aquarium, will tend toward a state of stable equilibrium. That is why we find critters that coexist etc. to find a balance. A pool is a state of unstable equilibrium.. its like trying to balance a pencil on its tip.. it will stay there for a bit.. and then fall over unless you are pumping it full of chlorine.

For you physics heads out there that are looking for equations... this deep end ain't deep enough.

 

[epro05]

CSI is affected quite a bit by pH. Acquariums might not care too much about scaling or plaster etching. But plaster pools do. I adjust pH to keep CSI close to zero.

 

[Oly]

Yeah that sounds like a TA problem. And yes that needs fixing, but after you fix that doesn't the pH return to normal?

Not really ever fixed but continually managed each time enough fill water is added to the pool to raise the TA and raise the pH.

 

[JoyfulNoise]

The simple fact is this - a pool is not an aquarium and an aquarium is not a swimming pool. They are completely different water systems with different goals in mind. Go read the Sticky thread at the top called Pool Water Chemistry written by @chem geek and then come back with questions.

pH matters very little to the “effectiveness of chlorine” when cyanurates are in the water. The standard pH/HOCl S-curve only applies to water with zero CYA and pools don’t typically have that nor should they ever be run without stabilizer.

Balanced CSI is important for swimming pool surfaces and equipment and corrosion of metals is driven by pH. Since pH and TA have the largest and most immediate effect on CSI, you adjust those to get the desired CSI values. Because of the input of alkaline fill water to a pool, especially in arid climates, the CSI is always an unstable equilibrium. Therefore a pool owner needs to use adjustments to pH and reductions to TA to control water saturation balance. Very few pools will remain naturally balanced.