Hydrochloric acid in pH feeder system definitely losing strength

There are two circumstances under which an acid would lose potency upon long storage:

1. The acid is a gas under standard conditions and has been dissolved in a solvent for ease of use. If the container is not well-sealed, some of the acid (which has its own vapor pressure independent of the solvent) will escape over time.
2. The acid reacts with something to which it is exposed in its storage environment. This could include the container, ambient humidity (water), air, solvent, itself in some cases, anything with which it is in contact.

Absent these conditions, acids and acid solutions do not spoil or decay.

Acids in a sealed container with no contaminants with which to react, will remain at constant "strength".

I sincerely doubt it “evaporated” away. That’s not how liquid/gas phase mixtures work. The vapor pressure of HCl vapor above a 20 Baumè (31.45%) mixture of muriatic acid is less than 0.4psi (total air pressure is about 15 psi or 1 atm). Even if the concentration did change a small amount, the equilibrium vapor pressure of the acid above the solution would fall thus slowing the “off gassing” even further. HCl/water mixtures have a very complex thermochemistry and they reach a eutectic point (azeotropic mixture) at around 15% meaning that the amount of water and acid evaporating from the surface is equal (similar to how you cannot distill alcohol/water mixtures above 97%). Muriatic acid simply does not “evaporate” away from a mixture.

More than likely what happened was some kind of mechanical failure of the dosing system either in the tank or the check valve and water backed up into the tank. It is not uncommon to see these types of failures in mechanical dosing systems. Strong acid solutions are especially damaging to the plastic and rubber components of the pumps and tank and so they often need annual preventative maintenance and replacement at a minimum.

Question of the experts here. The OP attributes the green pool to the loss of the acid pump (although he didn't report what the pH was.) Would a high pH promote algae or other aquatic growth? I though that was what chlorine prevented.
Question of the OP. Have you tested the water? Can you post results? How high did the pH go without acid additions for months?

BowserB said:

Question of the experts here. The OP attributes the green pool to the loss of the acid pump (although he didn't report what the pH was.) Would a high pH promote algae or other aquatic growth? I though that was what chlorine prevented.
Question of the OP. Have you tested the water? Can you post results? How high did the pH go without acid additions for months?

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No. pH control has nothing to do with algae. If the correct free chlorine to cyanuric acid ratio is maintained, then the amount of sanitizing (active) chlorine is not affected by pH. Active chlorine (hypochlorous acid) is only strongly affected by pH when no CYA is present.

Thanks, Matt. That's what I thought. Only had a pool since 2018 so I have to check my assumptions with you guys.

BowserB said:

Would a high pH promote algae or other aquatic growth?

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Even if there was no CYA and all of the chlorine was hypochlorite due to high pH, the water should not turn green from algae.

Even though hypochlorous acid is a better oxidizer, hypochlorite still has a significant amount of strength.

rthorntn said:

it was a while before I noticed, probably quite a few months, the pool had started to go green.

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The posts indicate that the poster was very lax in maintaining the chemistry.

rthorntn said:

Surely the acid bottle would overflow with water, why would the water stop leaking in before it overflowed

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Maybe it was pumping out and then back feeding water, which could result in a near constant level.

And what I’m saying is your theory is wrong. Acid mixtures don’t work that way. The chemistry and physics of your setup would not promote the “off-gassing” of acid vapors to any appreciable extent. Think about it - the acid is contained in a bottle that is almost completely sealed such that fresh air isn’t readily exchanged with whatever amount of vapor is in the dead space above the liquid. Off gassing, to work, would require a constant and excessive flow of fresh air into the bottle displacing whatever vapors were in there. Your setup simply doesn’t allow for that. Also, as I stated in my first response, HCl forms an azeotropic mixture at around 15% concentration. So that means, even if the liquid is evaporating due to off gassing, it’s concentration will not change. An azeotropic concentration of a mixture is one in which the amount of solvent and solute evaporate in equal amounts. So the overall concentration of the mixture no longer changes once you reach the azeotropic concentration. The example I gave was to distillation of alcohol and water - a standard thermal still can only create alcohol that is 97% pure. It will always retain 3% water because the 97/3 mixture boils in such a way that the vapor above it is also a 97/3 mixture. The only way to make 100% alcohol is to chemically treat the 97/3 mixture to remove the remaining water.

Something else is the issue, not some strange chemical magic.

JoyfulNoise said:

I sincerely doubt it “evaporated” away. That’s not how liquid/gas phase mixtures work. The vapor pressure of HCl vapor above a 20 Baumè (31.45%) mixture of muriatic acid is less than 0.4psi (total air pressure is about 15 psi or 1 atm). Even if the concentration did change a small amount, the equilibrium vapor pressure of the acid above the solution would fall thus slowing the “off gassing” even further. HCl/water mixtures have a very complex thermochemistry and they reach a eutectic point (azeotropic mixture) at around 15% meaning that the amount of water and acid evaporating from the surface is equal (similar to how you cannot distill alcohol/water mixtures above 97%). Muriatic acid simply does not “evaporate” away from a mixture.

More than likely what happened was some kind of mechanical failure of the dosing system either in the tank or the check valve and water backed up into the tank. It is not uncommon to see these types of failures in mechanical dosing systems. Strong acid solutions are especially damaging to the plastic and rubber components of the pumps and tank and so they often need annual preventative maintenance and replacement at a minimum.

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Absolutely, you won't lose all the acid by off gasing. With concentrated HCl you will lose some but it will eventually reach a stable concentration. It's actually worth diluting acid in feed systems for this reason. HCl can be at most about 37% in water and it fumes quite strongly. If you have a black tank in the sun and the solution gets hot it may off gas a little more but it won't all gas off. Dilute it one part in three (below 15%) and you're now at a vapour pressure where it doesn't fume. This means you don't lose acid to off gasing, and more importantly you don't get corrosion of any nearby equipment as the acid tanks are not gas tight, or they would collapse as you pumped the acid out. It's also less likely to degrade the dosing equipment and storage tank.

In terms of the tank overflowing if there was water being sucked back into the tank. Have you checked that the tank is properly vented. If it is in fact air tight then the pump will pump out liquid, it's a positive displacement pump, so that will create a vacuum in the tank, which might be able to suck water back through the dosing line. I've seen a few of these dosing system specs and generally speaking I would describe them as toy store quality with rolls royce pricing. Does your system have a non-return valve on the injection line ? I'm betting it doesn't.

Why doesn't it overflow ? It's only sucking back in enough water to replace the volume of the acid pumped out. The acid is then diluted and the system will pump more volume out to control the pH, and suck a similar volume back in diluting it further. A few of these cycles and you essentially have a tank of pool water. So there's a theory for you.

Now it's not a perfect theory, a well engineered peristaltic pump shouldn't allow back flow like that. But as I said most of these pool store systems are not engineered to the standard of industrial systems and I haven't seen one yet with a non-return valve on the injection point, so if there is wear in the pump head it becomes possible to have a small back flow.