What Causes a High pH in a Swimming Pool?

May 8, 2019

It happens all the time. Swimming pool pH climbs, or sometimes spikes, and all sorts of problems like calcium dust and carbonate scale can occur. But what causes high pH in pools? Why does the pH sometimes climb, and other times stay relatively steady? In this article, we will discuss pH and how it shifts, and offer some remedies to correct the pH, based on each situation.

What is pH?

As a quick recap, because we have other articles about pH, pH stands for the power of Hydrogen, or more scientifically, "potenz Hydrogen" or "potential of Hydrogen", depending on what source you read. It is a negative logarithm of hydrogen concentration on a scale of zero to fourteen (0-14) with seven (7) being perfectly neutral. Every whole number has a 10x difference in concentration of hydrogen than the next number. The lower the pH, the more acidic the substance. The higher the pH, the more basic, or alkaline the substance.


High pH in Pools

So why does the pH tend to climb in swimming pools? You may notice pH almost never naturally drops over time...so there must be something going on. Well, there are several reasons–and sometimes a combination of reasons–that cause a pool's pH to rise, or even spike. So let's talk about some of them.

1. Natural pH Rise: Carbon Dioxide Loss

The chemistry of pH sounds a lot more complicated than it is. In swimming pools, the movement of pH is primarily about carbon dioxide (CO2). CO2, when dissolved in water becomes something called carbonic acid (H2CO3). See the chart below.

alkalinity in swimming pools, alkalinity equilibria, pool chemistry, pH and alkalinity, pH vs. alkalinity, carbonic acid, high pH in pool

The more carbonic acid in your water, the lower your pH will be. Injecting CO2 lowers your pH, but not total alkalinity. Acid, on the other hand, lowers both pH and total alkalinity. The opposite is also true about CO2. When COoff-gasses (from aeration, splashing or maybe a water feature that agitates water causing bubbles to escape), the amount of carbonic acid decreases; so the pH rises. So aeration itself raises the pH of water because CO2 escapes. If you want to raise the pH without adding any chemicals, just aerate the water. 

Naturally, COwants to be in about the same concentration in the water as it is in the air. So CO2 off-gasses until it is in relative equilibrium with the air above the pool. As cited in the onBalance article in the previous hyperlink, this phenomenon is known as Henry's Law. And don't worry, we had no idea what Henry's Law was either...but it makes a lot of sense as to why carbon dioxide naturally leaves, and the pH rebounds some time after putting acid in.

2. Sanitizers and their pH Impact

Every type of sanitizer has a pH impact on the pool, and the pH depends on the sanitizer used. For example, chlorine gas and trichlor are very acidic products, meaning they have a very low pH and tend to bring the pH of the pool down. Dichlor has a relatively neutral pH, as does bromine, so their pH impact is minimal, if not negligible. The popular non-stabilized chlorines calcium hypochlorite (cal hypo) and sodium hypochlorite (liquid chlorine) both have a high pH, so they tend to raise the pH of the pool. One common habit in the pool business is to add some acid to "offset" the pH rise that liquid chlorine causes. But according to renown pool chemistry expert Robert Lowry in his book Pool Chemistry for Service Pros (page 9), when liquid chlorine goes into the water, it actually won't raise the pH as much as you might think. Liquid chlorine's reactions create byproducts of both high pH (Sodium hydroxide) and low pH (Hydrochloric acid), which neutralize each other. The net pH change should be about zero.

Sanitizer is only used in a few parts-per-million, so even if there is a net pH change, it isn't always a big deal. With one exception. There is one sanitizer type that consistently raises the pH of a swimming pool: salt-generated chlorine.

saltwater pool, salt pool, salt generator, sodium hydroxide, electrolysis, high pH in pool, salt chlorineThat's right, saltwater pools are chlorine pools. Chlorine is created by electricity passing through saltwater (or brine) in the salt generator cell, and the process is called electrolysis. It creates a byproduct called sodium hydroxide (NaOH), which has a high 13+ pH. Sodium Hydroxide raises the pH of the pool. The chlorine generator reaction looks like this:

2NaCl + 2H2O → Cl2 (gas) + H2 (gas) + 2NaOH

salt + water yields chlorine gas + hydrogen gas + sodium hydroxide

In our experience, salt pools almost always have a pH that drifts up. The good news is, the pH rise is predictable and can be counteracted with a measured amount of acid or carbon dioxide feed.

3. pH and alkalinity adjustment chemicals

As mentioned in #1 above, adding carbon dioxide lowers the pH, but not the alkalinity; and when carbon dioxide leaves the water, the pH goes up, but the alkalinity stays constant. So besides pure CO2 itself, let's talk about pH adjustment chemicals that are commonly used in swimming pool treatment.

Chemicals that increase pH

The two primary chemicals to increase pH are sodium carbonate, aka soda ash (Na2CO3), and sodium bicarbonate (NaHCO3). Both raise the alkalinity, but soda ash has a stronger impact on raising the pH. All things being equal, it will take more sodium bicarb to raise pH than soda ash, because the pH of bicarb is lower (8.4 pH) than soda ash (11.4-11.6 pH). Both of these products are common in the pool business.

When added too fast, or too much, sometimes soda ash clouds up the pool, but the cloudiness is not the soda ash itself, it's calcium carbonate that falls out of solution. The pH of 11.4-11.6 rapidly changes the LSI (locally where you added the soda ash) and calcium precipitates.

Chemicals that lower pH

We already talked about injecting pure CO2 into the water, and of course, the other option is to add acid. There are several types of acid. The common ones are liquid muriatic (or hydrochloric) acid, and sulfuric acid. Then there is also a dry acid called sodium bisulfate. People have their preferences, but they all lower both pH and alkalinity.

4. Over-correction of the LSI

So you probably knew about 1, 2 and 3. Most people we speak to don't think about this one: overcorrecting the LSI. In other words, when you drop the pH or alkalinity so that the LSI goes too far below 0.00, the water is going to rebound and get itself back into balance. Like if you add way too much acid, you may notice the next day that the pH is higher than it was in the first place. This can happen because your acid pour could have knocked down the alkalinity and pH, causing the LSI to go aggressive (below -0.30), causing the water to search for calcium saturation. The water etches the cement in the surface (or tile grout), which has a high pH, and your calcium hardness drifts up too.

All the water is trying to do is get back to LSI balance. Your acid pours disturbed the peace. A perfect example is on a new pool startup.

Why does the pH Spike During a Traditional Pool Startup?

Shameless plug: with the Orenda startup, the pH spike should not happen in the first place.

So you get to the pool when it's full to 'begin' the startup. Where is the pH when you arrive? Yup, it's sky high. This is because your water that filled the pool was almost certainly aggressive on the LSI, and starving for calcium saturation. So it took the most readily available calcium it could find–the still-soluble calcium hydroxide (Ca(OH)2)–in the cement of the surface. The plaster surface is still curing, and according to the National Plasterers Council, calcium hydroxide takes at least 30-60 days to cure. So when aggressive water wants calcium, it takes this easy source.

Here's the catch: calcium hydroxide has a very high pH of 12.6.

That high pH explains a lot. Not only is it what spiked your pH before you arrive on the scene for a startup, but it's also what caused the calcium to carbonate outside of the surface, and fall out of solution as plaster dust. Every speck of plaster dust you see should have carbonated (hardened) inside the cement of your surface. You can hide it all you want, but unless you stop the loss of calcium hydroxide in the first place, you're losing valuable material from your cement.

So when you arrive, the pH is sky high, but where is the LSI? The LSI is probably perfect. You see, water is an amazing thing...water wants LSI equilibrium and will stop at nothing to find it. But water has no ability to be greedy, so when it gets to perfect balance, it stops etching your surface. When you arrive at your pool, the water already found the perfect balance, at the expense of some of the calcium hydroxide in your still-curing surface.

But then, following habits and/or the traditional startup procedures out there, you probably add acid to lower the pH.

Adding a lot of acid makes the water is aggressive again, so it eats more calcium hydroxide and spikes the pH again. Perhaps the pH is even higher than the day before because the acid also reduced the alkalinity. And again on day 2. And day 3. Probably again on day 4. Eventually, you can stabilize the pH when enough calcium hardness has been pulled from your surface...you know, the "calcium drift up". Plaster dust and high pH during startup is a self-fulfilling prophecy. The water was perfectly happy when you arrived on day 1. Our habit of attacking pH without addressing what water really wanted is what causes the problem.

Every time we add acid, we are over-correcting the LSI, and therefore the pH rises again.

Low pH in pools

There are not too many cases of pools where the pH continues to stay low. Some reasons for pool pH being reduced include using acidic chlorine like Trichlor and getting a lot of rain. Rainwater tends to be acidic, and it can be even more acidic the closer you are to a major city. Another thing: rainwater contains no calcium carbonate, meaning it's not only acidic but super aggressive on the LSI. Other things that can lower pH in a pool are leaves and pine needles. This is an underestimated factor, especially during the offseason.

Climates that get a lot of rain or snow in the winter time, combined with leaves and pine needles on a mesh cover? Yep, that pH is going down in the short term, probably making the water low on the LSI, so it has to correct itself. This leads us into entirely different conversations about types of calcium dusting in pools that occur during the winter, as well as other issues like pool crystals and calcium formations. Fortunately, we have already written about those topics.

In conclusion, there are many factors that affect pH (and alkalinity), and there is always a reason for the change. If you're tired of fighting your water chemistry to stabilize pH, we hope this article helps you understand what's really going on.

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