5 Misunderstandings about Swimming Pool pH

Of all water chemistry parameters, many experts argue that pH is the most important. Indeed, pH impacts just about everything else in water chemistry, but most swimming pool operators–both residentially and commercially–misunderstand it. We know this because we also misunderstood it, thanks to reading swimming pool operator textbooks. This article will set the record straight.

Topics covered:

5.  pH and bather comfort

4.  pH and alkalinity's buffering capacity

3.  pH "control" vs. containment

2.  pH and chlorine strength

1.  pH and carbon dioxide

5. pH and bather comfort

Why are we told to maintain 7.2 to 7.8 pH in swimming pools, and ideally 7.4 to 7.6? Well, according to the textbooks and online resources, the industry standards from NSPF (now PHTA), APSP and other acronym organizations, two reasons are cited. First, pH controls the strength of chlorine, which we will discuss later in this article.  The second reason is bather comfort. 

Let's just go right ahead and debunk this.

First of all, swimming pool water can irritate people's eyes and skin...but we are not at all convinced it is because of the pH. We do not doubt that if pH is way low or way high that it will irritate bathers, because that should be obvious. The question is whether or not bather comfort is even noticeable within the pH parameters often seen in swimming pools. Let's expand the extremes from 7.2-7.8 out to 6.2-8.8 pH, for 'worst case scenario' discussion. Hopefully your pool will not hit either of these extremes, but with overcorrections and etching plaster, these numbers are definitely possible in swimming pools.

What will 6.2 pH water feel like? How about 8.8, or even into the 9's? Fortunately we don't have to wonder, because we drink bottled water all across this range of pH, and even wider. Want to know what ~6.0 pH water feels like? You've probably tasted it and it was refreshing. What about higher pH, alkaline water? We actually pay extra for this kind of bottled water to drink. Supposedly it's better for our health. So are we really to believe that swimming in these drinking water ranges are the leading cause of skin and eye irritation?

Maybe. But we doubt it.

We have years upon years of competitive swimming experience to back up this opinion. We believe the primary cause of bather discomfort is disinfection byproducts (DBPs) like chloramines and other compounds.  In other words, combined chlorine, non-living organics and other contaminants.

These compounds are known to irritate eyes, skin, throat and lungs. Chloramines are noxious, and we want them out of your water and natatorium air. Hence, the entire reason our business exists.

pH and alkalinity's buffering capacity

Thanks to the research done over at Orenda Technologies, we know a lot more about pH and alkalinity than ever before. Most pool operators know alkalinity is important, primarily because it "buffers the pH". But alkalinity is far more interesting than simply acting like insulation against pH swings. And the reason we are bringing all this up is because there are many species of alkalinity, and pH determines the percentage of each type of alkalinity. Here's a chart to illustrate the carbonate alkalinity equilibria:

The chart above shows various forms of carbonates in water (as well as borates if you use borates in your pool. If not, ignore the orange and teal lines in the chart). On the far left, you have carbonic acid (H₂CO₃), which is just dissolved carbon dioxide: 

CO₂ + H₂O ⇌ H₂CO₃

carbon dioxide + water ⇌ carbonic acid

As the pH rises, however, a new species of alkalinity emerges, as one Hydrogen leaves. Carbonic acid converts into its conjugate base, bicarbonate alkalinity (HCO₃^-). This functions in equilibrium, so the two substances always add up to 100%. Then, once the pH gets to 8.3, there is no more carbonic acid. Another Hydrogen leaves and another new species of alkalinity presents itself. Bicarbonate converts into carbonate alkalinity (CO₃^--). Oh and if you've ever had carbonate scale in your pool, you were looking at calcium carbonate (CaCO₃), which precipitates out of solution when the LSI gets too high. A high pH, especially over 8.3, is the most powerful factor on the LSI to precipitate scale:

CO₃^{- -} + Ca⁺⁺ → CaCO₃

carbonate + calcium → calcium carbonate

Essentially, this entire spectrum of alkalinity is an equilibrium that fluctuates in real time as the pH changes in water. And as we'll discuss later in this article, bicarbonate and carbonate alkalinity play a critical role in not only stabilizing pH, but lowering it too.

And there are more types of alkalinity that are not even on the carbonate spectrum: cyanurate alkalinity (if you have CYA in your pool), and borate (if you use borates) also contribute to total alkalinity. So when doing an LSI calculation, you must deduct these from the total alkalinity in order to get corrected alkalinity, also (confusingly) called carbonate alkalinity. The bottom line is pH dictates the types of alkalinity present in water. And alkalinity slows the reduction of pH by neutralizing acid. Which brings us to our next point.

pH "control" vs. containment

Given the technology available for commercial and residential swimming pools, one could be justified for thinking pH can be controlled. But that's wishful thinking.

Sure, with enough acid and other adjustment chemicals, you can force water's pH to stay within a certain range (say, 7.4 to 7.6), but doing so fights against physics. pH is a reactionary chemistry; it moves around rapidly when you manipulate the water with chemicals, aeration, or just about anything else. It constantly fluctuates–much to the frustration of pool operators worldwide. This frustration is understandable, but it's based on a fundamental misunderstanding that pH can somehow be controlled.

In reality, pH is supposed to rise, thanks to Henry's Law of physics. It takes consistent use of acid or CO₂ injection to suppress pH and keep it within ranges well below its natural ceiling. More on this later.

And yet, for indoor swimming pools (without cyanuric acid), pH must be kept within certain ranges for optimal chlorine strength, which we will discuss next.

How to "control" pH

pH control is usually accomplished with a chemical controller that has a pH probe, and the ability to feed acid or CO₂ injection as needed. We call these sense and dispense systems. They work kind of like a pH thermostat; the probe reads a pH over a set point, and it feeds acid or CO₂ until the pH gets down to the desired set point. The challenge with this, of course, is overcorrection, especially when your acid dose is not based on the current alkalinity of the water. 

In other words, how do you know the chemical controller is feeding the correct dose of acid for your pool? In most cases, the feeder simply feeds acid until the pH probe sees a better pH. But depending on your plumbing system and circulation rate, that could be–and often is–way more acid than you really need. Overuse of acid leads to etching plaster surfaces and tile grout, and the pH spiking up once again. It also leads to consuming more alkalinity than intended.

CO₂ injectors are preferred, but have different consequences. CO₂ systems will lower pH but not alkalinity...but over time, alkalinity actually rises. See our chart from earlier to see how additional CO₂ will convert into carbonic acid, then into bicarbonate alkalinity as the pH rises back up naturally.

If you are a commercial pool operator with a CO2 system, hopefully you feel relieved to know that 1) you're not alone in your frustrations chasing pH, and 2) you are not going crazy seeing alkalinity rise on its own.

pH and chlorine strength

Ah yes, chlorine strength. This is the main reason health codes demand a pH maximum (usually 7.8 in the United States, depending on the state's rules). See the chart on the left below.

Without cyanuric acid (CYA) in the water, the pH determines the strength of chlorine, as expressed by the percentage of the strong, killing form of chlorine, Hypochlorous Acid (HOCl). %HOCl is usually what we use to determine chlorine strength. Well, that and ORP. 

The lower the pH, the higher the %HOCl, the stronger and faster the chlorine. Most indoor pools do not have CYA in them, so that chart absolutely applies to natatoriums.  Outdoor pools with CYA, however, can have virtually the same strength of chlorine at 8.2 as at 7.2. See the chart above on the right side, with just 30 ppm CYA. The %HOCl is way a the bottom, and a negligible reduction occurs as the pH climbs. The FC:CYA ratio is what truly determines chlorine's strength and speed in those pools–not pH.

That being said, when pH gets above about 8.3, the weaker form of chlorine, Hypochlorite ion (OCl^-) dissociates from CYA. It breaks away, as you can see on the right chart toward the bottom corner. When it separates from CYA, it can be destroyed by sunlight. So over a pH of about 8.3, chlorine is more prone to sunlight degradation because it leaves CYA's UV protection.

pH and carbon dioxide (CO₂)

And now, for the final topic of this long, detailed article. pH is a negative logarithm of Hydrogen ions (H⁺). The lower the pH, the higher the concentration of Hydrogen, and vice versa. But for practical purposes, that definition is hard for us to visualize, especially in a pool. After all, we're not chemists. Thankfully there's a much easier way to think about pH. The amount of CO₂ dissolved in water determines the pH of the water. The more CO₂ in solution, the lower the pH, and vice versa. And since pools are over-carbonated with alkalinity, CO₂, and to a lesser degree, non-living organics (carbon chain bather waste), that CO₂ needs to equalize with the air above the swimming pool. This equalization is explained by a law of physics called Henry's Law of the Solubility of Gases:

Just like when you crack open a beer, CO₂ escapes because it is trying to equalize with the air above the beer. Eventually enough bubbles make it to the top and the beer goes flat. Voila, Henry's Law.

To reference back to our earlier point, containing pH is not only possible, but easier, more cost effective, and more practical. Here's what we mean:

pH will naturally rise over time due to the off-gassing of carbon dioxide (CO₂). This is normal, and due to Henry's Law of the Solubility of Gases. When CO₂ reaches equilibrium with the air above the pool, pH can no longer rise. So containing pH means building a foundation with the Langelier Saturation Index (LSI) as your floor, and pH_(eq) as your pH ceiling. Reducing carbonate alkalinity will lower this ceiling according to the chart below.

We alluded earlier when we were discussing alkalinity that alkalinity plays an important role not only in buffering pH, but in lowering it. This sounds counterintuitive, but it's the truth. In order to lower pH, you must have more CO₂ in the water. But acid, like muriatic (HCl) contains no CO₂. So where does it come from? Refer back to the chart earlier, and you'll see that acid simply converts carbonate and bicarbonate alkalinity down into carbonic acid, which brings down the pH. In simpler and not-so-scientific terms, acid burns through alkalinity to lower the pH. This is why they both get reduced with acid, and why CO₂ injection only lowers pH, not alkalinity. CO₂ injection bypasses this process.

Let's stop chasing pH, and embrace physics. Yes, pH is going to rise, but the chart above shows us that we can determine how high it can go. Aim to control your chemistry accordingly.

You can use the Orenda App to figure out your pH ceiling easily. Simply use the calculator and select "show" secondary readings. The pH ceiling appears in real-time as you adjust your chemistry dials.

Conclusion

To recap: pH is very important in water chemistry, but not necessarily for the reasons we have all been taught for so many years.

First, we don't buy the bather comfort argument, because in our experience it's really disinfection byproducts like chloramines that irritate swimmers. Second, pH controls the type and percentage of each form of carbonate alkalinity. Third, pH is not able to be controlled without excessive force with acid and/or carbon dioxide injection. Containment is a much more pragmatic approach. Fourth, pH absolutely controls chlorine strength (%HOCl) in pools without cyanuric acid. If your pool has any CYA, that goes out the window. And finally, the amount of CO₂ dissolved in water determines your pH, and it will naturally off-gas to equalize with the air above the pool. This means your pH is supposed to rise up until equilibrium is reached; an equilibrium point that we can control ourselves based on the water's carbonate alkalinity.

If you want to limit your pH from rising, start by reducing your alkalinity, while compensating for that lower alkalinity with higher calcium hardness to maintain LSI balance.

We know this was a lot to digest, but we hope it was helpful for you!