On June 28th, if you recall, we discussed with pool/spa water chemistry expert Richard Falk the impact of sodium hypochlorite on pH in our piece titled “Liquid Chlorine Raises pH, or Does it?“. Richard explained that the pH rise folks typically experience in swimming pools comes from the outgassing of carbon dioxide, and not from hypochlorites. In part 2 of this 2-part series, we pose the same question to leading industry scientist Robert Lowry.
Sodium Hypochlorite (NaOCl) is one of the more popular methods of chlorinating swimming pool water in both commercial and residential applications. When NaOCl (liquid chlorine) is added to water HOCl, Na+, and OH– is formed. However, what happens after the hypochlorous acid (HOCl) is used up (either due to UV from sunlight, or as the chlorine sanitizes and disinfects) is a common topic for debate among pool operators. Chemically, the HOCl becomes HCl (hydrochloric acid) in the process – no one is arguing this. The controversy is regarding whether the amount of acid (HCl) produced is enough to counter balance the initial increase in pH the dose had generated.
To clarify, I had reached out to both Richard Falk and leading industry scientist Robert Lowry for their thoughts on the chemical reaction. Initially seeking a brief explanation in a “paragraph”, what I received in return was so much more:
So if I am using liquid chlorine and it does not raise pH, what is causing the pH to go up all the time?
According to chemical consultant, author, and pool/spa water chemistry expert Robert Lowry (AKA: The Water Coach), “The short answer is that you have the wrong or too high TA (total alkalinity). I usually recommend 90 ppm as a starting Target TA. If at this target the pH is drifting up, then change the Target to 10 ppm less or 80 ppm. And if that doesn’t work, try 70 ppm TA. If on the other hand your pH is drifting low, raise the Target TA to 100 ppm. Again, change in increments of 10 ppm until pH is stable.”
Robert Lowry went on to explain: The problem is that when the TA level is too high there is a lot of outgassing of carbon dioxide (CO2) that causes the pH to rise. You may also have a lot of splashing and aeration in the pool that accelerates such outgassing. If you were to let the TA drop to 80 ppm or even a little lower, you will find that the rate of pH rise will slow, especially if you target a pH of 7.5. If you find that a lower TA level helps, then you may need to raise the Calcium Hardness (CH) level a little bit to compensate for the saturation index to protect plaster surfaces.
Aeration of the water and turbulence will raise the pH theoretically until the amount of carbon dioxide in the water is in equilibrium with the air. This pH of equilibrium then depends on the TA level. At roughly 200 ppm TA (ignoring CYA), the pH could theoretically rise to 8.77, but in practice you will see it on go up to about 8.5 or less.
You can see that pools are way over-carbonated. We are just fortunate that the rate of outgassing is relatively slow and that even aeration has a limited effect of speeding it up. If you want to run your aeration and turbulence-causing systems all the time, then you will likely be adding acid all of the time. You might consider using a timer for aeration and turbulence causing devices so that they run when people are around to enjoy them and not when they are at work.
The double arrows indicate that things are in equilibrium and go back and forth to maintain equilibrium. So if you raise or lower any of these items, everything else shifts to keep the equilibrium.
Removing CO2 raises pH and adding CO2 lowers pH.
Rudy’s Thoughts: Something that must be considered is that in a continuous feed system, one will experience the following: NaOCl becomes HOCl, Na+, and OH–; HOCl (Hypochlorous acid) eventually becomes HCl (Hydrochloric acid) which lowers pH. However, the amount of HCl formed is only in a quantity sufficient enough to negate that from which it came from. As the chlorine level is being continuously replenished by the addition of sodium hypochlorite, the pH level remains a consistent elevated high contingent upon the continued dosing. A cyclical process where the HCl formed, at best, can merely keep up (Like Lucille Ball at the conveyor belt). So, still a net zero change to pH, but the continuous addition maintaining a constant high pH until the addition of the NaOCl (sodium hypochlorite) should cease.
Robert’s Response: You are correct that if you start with no chlorine level and you add hypochlorite then the addition will raise the pH and while it will get lowered as it is consumed or used, additional hypochlorite will raise it again so that starting from 0 ppm and getting to and maintaining 3 ppm results in an elevation of pH that is not lowered unless you explicitly adjust the pH. But that is exactly what is done. Once one achieves the FC level desired, say, 3.0 ppm, then one adjusts or lowers the pH. From that point forward, there is little change in pH from the hypochlorite itself except for the excess lye in it and that rise typically only shows up some in high bather-load pools using lots of chlorine per day. The excess lye in hypochlorite is typically 0.03%. This is very little lye. Considering that a gallon of sodium hypochlorite 12.5% is 128 fl and weighs 9.66 lbs per gallon, there is about 0.046 oz (1.4 grams) of lye. I would doubt that this amount of lye would have any effect on pH in 15,000 gallons.
So, again, if the pH is going up using continuous hypochlorite feed, the problem is high (wrong) TA, high aeration or turbulence. Start with a Target TA of 90 ppm and adjust up or down in 10 ppm increments until pH remains stable depending on whether the pH is drifting up or down.
Robert Lowry is the Author of many swimming pool industry publications. “Pool Chemistry For Service Pros” is one of his most recent. The handbook offers science based, logical, easy method of maintaining pool water chemistry using only basic chemicals – liquid chlorine or bleach, muriatic acid, bicarb, borate and air. You only have to make a few adjustments to what you are already doing to realize the benefits.
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