What Causes a High pH in a Swimming Pool?

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.

The Orenda Purge Dose

When first using Orenda products like SC-1000 and CV-600/700, the initial dose is what we call the purge dose.  So what is a purge? And why is it necessary? This article will explain.

What is a purge dose?

purge and maintenance

A purge dose of Orenda chemicals is the initial dose to overpower the targeted contaminant and also leave a residual for the future. An Orenda purge is usually one quart per ten thousand gallons (32 fl.oz./10,000 gallons). Let’s use CV-600 enzymes as an example. The CV-600 purge puts enough enzymes in circulation to not only clean the water, but also leave behind a residual. It’s kind of like chlorine in that way…you need a residual for CV-600 to keep up with the bather demand.

How long does the purge last?

The purge dose of enzymes should last for a few weeks, and SC-1000 Scale & Metal Control can last even longer. That said, these products get used up doing their job, so the water needs a maintenance dose of a few ounces per week, depending on bather load. For pools serviced weekly, once a week is fine. For commercial pools, we strongly suggest dividing the weekly maintenance dose over several days a week. Most commercial customers use a feed pump of some kind, and most residential customers just use a measuring cup.

So the process is simple: purge to start, and add the weekly maintenance dose from then on to replenish.

Why is purging necessary?

Orenda chemicals address specific problems, not every problem. SC-1000 chelates metals (including calcium, which is an alkali earth metal) to prevent stains, metal oxidation and carbonate scale. CV-600 and CV-700 enzymes break down and remove non-living organics, like bather waste. We purge because these issues stem from contaminants that are spread around in the water, and we need enough in the first dose to overcome the contaminants.

sc-1000, orenda purge, purge dose, purge a pool, dechlorinate, lower chlorine, prevent scale, chelantThink about it. Metals are in solution or suspension, and bather waste is constantly being introduced into the water. Point-of-contact systems like UV, filters, and even strainer baskets are only effective when water passes through them. But contaminants are out in the pool where people are…and that’s where Orenda products need to be too, right alongside residual chlorine.

The purge dose can overpower these contaminants and still leave behind a residual for the future. There are only some rare exceptions to this, like if metals in the tap water are absurdly high, or some other anomaly like that. Without a purge dose, the existing contaminants may not be addressed, and you never get ahead of the problem. For example, if you only chelate 70% of the metals in your water…that still leaves behind 30% of the metals in your water that will be oxidized by chlorine, and that 30% could be plenty to leave stains or change the color of your water. It’s not that SC-1000 failed, it’s that there was not enough of it to cover the demand. The same can happen with enzymes when customers just start with the weekly maintenance dose and do not purge at the start.

Weekly Maintenance

After the purge, the weekly maintenance dose replenishes what has been used up. Enzymes eventually get used up removing carbon-based waste (non-living organics), just like SC-1000 gets used up chelating metals (including calcium). Because your tap water and bather load introduce new contaminants, the weekly maintenance dose is important. Fortunately, Orenda maintenance doses are usually small and affordable. It all depends on your pool and it’s needs. If your pool has special needs, just contact us and we can help create a custom dosing program for you.

Here is a table showing purge and maintenance doses for two Orenda products.

Orenda Dosing Chart, per 10,000 gallons of water

ProductPurge DoseResidential MaintenanceCommercial Maintenance
CV-600 / CV-700 Enzymes32 fl.oz. (one quart)5 fl.oz. per week10 fl.oz. per week
SC-1000 Scale & Metal Control32 fl.oz. (one quart)3 fl.oz. per week3 fl.oz. per week

Example Situations

  1. Green or Mustard Algae: Shock the pool with chlorine and immediately follow up with 16-32 fl.oz./10,000 gallons of PR-10,000 around the perimeter of the pool. The next day, vacuum and clean up the debris (dead algae, phosphate precipitate, leaves, and whatever else may be lurking down there…) and purge with CV-600 or CV-700 enzymes. The chlorine shock kills the algae, PR-10,000 wipes out the phosphates, and the CV-600/700 enzymes remove carbon waste.
  2. Black Algae: Purge with CV-600 enzymes. The next day, thoroughly brush the algae with a wire brush, then shock with chlorine. Immediately follow up with 16-32 fl.oz./10,000 gallons of PR-10,000 in the affected area. The enzymes up front can help to soften the “shield” protecting black algae from chlorine. Brushing should remove at least part of that “shield”, enough to expose the black algae to the chlorine shock. Chlorine kills the algae, which releases its micronutrients like phosphates, and PR-10,000 wipes out the phosphates.
  3. Cloudy, dirty water: Purge with CV-600 enzymes and 8 fl.oz./10,000 gallons of PR-10,000.
  4. Carbonate scale: Purge with SC-1000 and raise the water level enough to soak the affected tile line as much as possible. Keep water circulating, and since SC-1000 can deplete chlorine levels for a day or so, manually feed chlorine as needed. Continue on maintenance of 3 fl.oz./10,000 gallons per week until the scale has softened enough to be removed.

Pillar 4: Minimal CYA

Our Fourth and final Pillar is to minimize cyanuric acid (CYA). Maintaining CYA at a manageable level can be a struggle, and we realize that. But CYA has a major impact on chlorine efficiency. It can be summarized in two words: avoid overstabilization. The action step: keep your CYA to a minimum, meaning 30 ppm or less. For commercial pools, we recommend 15 ppm or less, based on guidelines from the CDC. This article will explain what CYA is, why we use it, how it gets into our water, and how to manage it to prevent overstabilization.

action step icon

Action Step: Keep your CYA level 30 ppm or less (residential pools) and 15 ppm or less (commercial pools).


What is CYA and why do we use it?

CYA is short for Cyanuric Acid–also called conditioner, or stabilizer. CYA is a chemical used to protect chlorine from direct sunlight. Direct sunlight can break down chlorine in a matter of hours. So without protection, chlorine does not last very long in outdoor pools. Since indoor pools do not have direct UV exposure from the sun, CYA stabilization is unnecessary. And for bromine-treated water, CYA is not compatible and offers no benefit either. It’s really just for outdoor chlorine pools.

trichlor, cya, cya molecule, overstabilization, orenda, R. Baxter

Source: Swimming Pools – Chem Matters. Baxter, R. (1994)

CYA is a hexagon-shaped molecule with three places that chlorine can attach to. They are three Nitrogens, which can form weak bonds with chlorine. Without getting stuck on specifics, just know that chlorine can attach and detach as needed. We like to think of CYA as a floating raft with a big umbrella on it, with three handles. Three chlorines can grab onto the raft and be protected from sunlight, and let go of the raft when needed to sanitize or oxidize something.


Cyanuric-Acid-300x169How CYA gets into swimming pools

There are basically two ways CYA can be introduced to water:

  1. Add CYA as a granular additive, which is common for swimming pools that do not use stabilized chlorine, or
  2. Use stabilized chlorine like dichlor and trichlor.

The Benefits of Chlorine Stabilization

CYA serves an important purpose, and at low levels, it is very beneficial. It does not take much CYA to protect free chlorine from sunlight and provide adequate stabilization. As mentioned earlier, direct sunlight breaks down chlorine rapidly. It has a half-life of about 45 minutes, which means half your chlorine will be wiped out by the sun in 45 minutes. Another half will be gone after only 90 minutes. So there is no doubt that some stabilization has a benefit for the staying power of chlorine. The chart below that illustrates the benefit that low levels of stabilizer can have for chlorine’s longevity in direct sunlight:

Chlorine Staying Power, CYA, overstabilization, Orenda, cyanuric acid

As you can see, at just 10 ppm CYA, 87% of the chlorine still remains after 1 hour. This is a dramatic improvement already, compared to unstabilized chlorine (which would have only 50% after 45 minutes!). 98% of chlorine remains after 1 hour at 30 ppm CYA. Beyond that, the benefits wane and the problems take over.

Chlorine Overstabilization

The problem, however, is not stabilization, it’s overstabilization. You see, while water evaporates and chlorine gets used up, CYA does not. It just stays in your water. And if you keep adding it, CYA just accumulates.

The Mysterious Case of Disappearing CYA

If you use a non-stabilized form of chlorine, like liquid sodium hypochlorite (bleach) or cal hypo, overstabilization should not be a problem, because CYA is not constantly being introduced. Overstabilization is a problem with stabilized chlorines like dichlor and trichlor. These forms of chlorine are more than 50% cyanuric acid and increase CYA at a fairly alarming rate.

For example, in 10,000 gallons of water, just one pound of Dichlor or Trichlor raises CYA 6-7 parts per million. That adds up very quickly. A good example is trichlor tabs. Say you use one pound per week (roughly 2 tabs) in a 10,000 gallon pool. Assuming no dilution, in just 10 weeks, you will have have increased 60-70 ppm stabilizer in the water! Our fourth pillar of proactive pool care is about avoiding overstabilization by keeping CYA to a minimum. Low levels of CYA, 30ppm and below, give you the most benefit of sunlight protection while limiting the detriment to chlorine efficiency. Speaking of detriment, let’s talk about the downsides of CYA.

CYA severely weakens chlorine

There is an abundance of scientific research available on cyanuric acid and its impact on chlorine. In summary: the higher your CYA, the weaker your chlorine, and it’s almost at an exponential rate. Let’s quote renown water chemistry expert Richard Falk:

“The primary oxidizing and sanitizing compound is Hypochlorous Acid (HOCl), while Hypochlorite Ion (OCl-) and isocyanurate compounds (chlorine attached to CYA) have orders of magnitude lower oxidizing or sanitizing capability.” – Richard A. Falk

You might be astounded at how dramatic the impact of CYA is on chlorine. If you recall, the strength of chlorine is proportional to the percentage of the strong form of chlorine, Hypochlorous Acid, versus the weak form of chlorine, Hypochlorite Ion. Without any CYA, at a 7.5 pH, you have about half and half. The lower your pH, the higher the percentage of strong chlorine. Just small amounts of CYA plummet that percentage of Hypochlorous Acid. Check out the chart below that compares the normal HOCl to OCl- equilibrium without CYA, versus with CYA.

CYA and HOCl, HOCl and OCl-, CYA impact on chlorine, Richard Falk, Orenda

Source: Chlorine/CYA and Nitrogen Trichloride – Falk, Richard A.

So, there really is no comparison as you can see. CYA dramatically plummets the percentage of strong chlorine (HOCl). What do you think that does to your sanitizer’s strength? What does that do for the killing power of chlorine, as discussed in both Pillar 2 and Pillar 3? Clearly, the higher the CYA, the weaker the chlorine.

An example of this is the reduction factor for preventing algae. Again, to reference Richard Falk’s research, it takes approximately 7.5% of your CYA level in free chlorine to prevent algae. So if you have 100 ppm CYA, which is not uncommon at all, your new minimum free chlorine to prevent algae is 7.5 ppm! Can you sustain that?  It’s not practical for most pool operators, especially since the EPA maximum is 4 ppm free chlorine.

And if 7.5% of CYA is your minimum Free Chlorine, how about a maximum CYA? That ratio is 20:1, meaning 20ppm CYA for every 1ppm Free Chlorine. We didn’t make it up, we’re just sharing the information.

Or how about for disinfection? If there is an accidental fecal release (AFR) in a commercial pool, the CDC’s recommendation is a maximum of 15ppm CYA. Why? Because achieving disinfection of diseases like Cryptosporidium and Giardia requires a certain amount of contact time (CT) with chlorine. With CYA over 15ppm, disinfection becomes not only impractical, but it also becomes virtually impossible. Sure, you could do it, but the levels of free chlorine and time that would be needed are insane.

CYA Impacts the LSI Balance too

As if its impact on chlorine were not enough, there’s something else you should know. Cyanuric acid has some buffering capacity, due to its ability to accept take and release hydrogen. Remember those three nitrogen bonds that form weak connections with chlorine? Yeah, they can also form weak connections with hydrogen, which means CYA is a buffer. It’s called cyanurate alkalinity. In order to correctly calculate the LSI, the formula calls for the carbonate alkalinity, not the total alkalinity. To calculate carbonate alkalinity, we must deduct a portion of the CYA level in our water from the Total Alkalinity. Don’t worry, if you use the Orenda App, this math is all done for you. If you don’t the rule of thumb is to take 1/3 of your CYA ppm, and subtract it from your total alkalinity ppm.

Total Alkalinity – (CYA x 0.33) = Carbonate Alkalinity

If you want to be exact, the factor depends on the pH of your water, and now we’re careening down a deep rabbit hole the size of a canyon. See what happens when we start talking about CYA? There’s so much to cover because CYA can control a pool…let’s…let’s just move on now. Sorry.

So now that we know the effects of CYA on chlorine and the LSI, what can be done to reduce CYA? What if your pool is already overstabilized?

How to reduce Cyanuric Acid

how to reduce CYAIf your pool is over stabilized, let’s correct the problem. There are a few products that claim to remove CYA from water (with mixed results), and for those of you with water restrictions, reverse osmosis may be an option. But without a doubt, the safest, easiest and most affordable way to remove CYA is to drain some water and refill with fresh water. Dilution is the best solution in our opinion. This can include backwashing more, doing periodic drains or bleed offs, etc.

The math is easy too: want to cut your CYA level in half? Drain half your pool and refill it…well, it will be close anyway. Be aware that CYA also can absorb or adhere to pool finishes. We have heard of many cases where the pool has been drained and refilled, and there’s already a CYA level in it. That happens because the CYA was stuck to the walls. It’s wild.

So let’s wrap this up. The proactive strategy is to never let CYA accumulate too much in the first place. Use non-stabilized chlorine like liquid chlorine or cal hypo, and if you choose to use stabilizer, use a small amount and stop adding any more after that. Keeping CYA to a minimum is Orenda’s Fourth Pillar of Proactive Pool Care.

Here is a list of some of our sources we used for this article:

And here are some of our other related articles on the Orenda Blog: