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What Alkalinity is Best for Pools?

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It is amazing how incorrect information gets started, and gains traction despite being wrong, and not based on science. For example, contrary to some pool literature and manuals, it has been shown that a pH of 8.0 to 8.4 is workable, and sometimes is best for some pools.

Another misconception regarding pool water maintenance involves the recommended “Ideal” alkalinity range of 80 to 120 ppm. While that may be a good range for some pool situations, it is not necessarily the best when using alkaline or pH increasing sanitizers.

It is unfortunate that pool manuals suggest that maintaining a high alkalinity range of 80 to 120 ppm is needed to keep the pH “stable.” That statement leads to a misunderstanding.

No, the pH is not stable at 7.4 or even 7.6. Instead, for many pools, depending on the sanitizer being used, the pH generally “wants” to rise to 8.2 - 8.4 when the carbonate alkalinity is 80 to 120 ppm. That is where the pH is “stable” at.

The part that is true about higher alkalinity ranges is that when adding small amounts of acid, the pH doesn’t lower significantly. Every service tech knows that when the alkalinity is high, it requires a lot more acid to lower the pH.

At these higher alkalinity levels, once acid is added to a pool and the pH is lowered to 7.2, the pH will immediately begin to rise back up rapidly due to carbon dioxide off-gassing. In fact, with an alkalinity of 100 ppm (carbonate), it is not uncommon that the 7.2 pH can typically rise to 7.6 within 24 to 36 hours depending on various factors.

As the pH rises, the speed of the rebound slows down. With 100 pm of alkalinity, a pH of 7.6 typically takes about 3 to 4 days to rise to 8.0, and up to 8.2 and perhaps 8.4 in the days following. Again, the speed of pH rise depends on various conditions.

(There are exceptions to the above situations of the pH rising quickly, and one is when using an acidic sanitizer, such as Trichlor tabs. And we will address that issue later).

Now let’s address an interesting fact about adding acid to lower the pH from 8.0 to 7.2. It only requires half as much acid to lower pH from 8.0 to 7.6 as it takes to lower the pH from 7.6 to 7.2. For example, at 100 ppm of (carbonate) alkalinity in a 20,000-gallon pool, it requires about 20 ounces of acid to lower the pH from 8.0 to 7.6. But it will require double the acid (about 40 ounces) to continue lowering the pH from 7.6 to 7.2, for a total of 60 ounces.

When adding the extra 40 ounces of acid to lower the pH from 7.6 to 7.2, about 8 ppm of additional alkalinity is consumed, and the pH only remains below 7.6 for about a day or so. Also, about 2.2 pounds of sodium bicarbonate will be needed to restore the extra lost alkalinity.

Therefore, service techs are faced with a choice. Do they add more acid to lower the pH to 7.2, or not? Other than when you are intentionally lowering a high alkalinity level, does it make sense to add the extra acid just to lower the pH past 7.6 and to 7.2 when it will be 7.6 again within a day or two? 

Of course, some will say that it is a good idea to always lower the pH to 7.2. Although for some situations it is a good idea, we suggest that it is not always a worthwhile thing to do.

Fortunately, one of the greatest developments by test kit manufacturers was producing the “Acid Demand” test feature. No longer do service techs need to guess how much acid can be added, or needs to be added, in order to lower the pH exactly to where they want it. Additionally, having that capability allows for maintaining a lower alkalinity (than the recommended Ideal parameters) without over-dosing with acid and causing the pH to go too low.

An Alternative Program

Pool manuals and literature also (incorrectly) state that an alkalinity of 60 ppm causes the pH to be unstable and erratic than when the alkalinity is higher. Again, that is incomplete and misleading. When no acid or very little acid is being added, the pH can be very stable, even at pH levels lower than 8.2 - 8.4 which occurs at higher alkalinity levels.

That can be very beneficial, and the Acid Demand testing feature can help service techs avoid over-treating with acid.
Here are some interesting facts about alkalinity. If pool water contains only 20 ppm of (carbonate) alkalinity and no additional chemicals are added (and if there are no other mitigating influences), the pH will slowly and eventually rise to a pH of about 7.8, given enough time. To have the pH stable at 7.5, the alkalinity would have to be just 10 ppm. Isn’t that amazing!

Of course, 10 ppm and 20 ppm of alkalinity is far too low and risky for swimming pools, especially if acidic sanitizers are being used. But let’s understand that the lower the TA, the slower the pH will rise, and the lower the level the pH will rise up to.

Now let’s get practical, and apply some of this chemistry. Consider maintaining an alternative alkalinity level or range for certain pools. Instead of keeping the alkalinity at 90 to 120 ppm, let’s consider lowering it to 60 to 70 ppm for pools that use bleach, calcium hypochlorite, or salt generator systems as the primary sanitizer. Those three chlorine sources also generally cause a slight increase in the pH, in addition to the natural pH rise due to carbon dioxide off-gassing.

With 60 ppm of carbonate alkalinity, the pH doesn’t want to climb to 8.4, but only to about 8.0 to 8.2 instead. Additionally, the speed of the pH rising with 60 to 70 ppm of alkalinity is slower in comparison to maintaining an alkalinity of 90 to 120 ppm. It will take about 2 to 3 days for the pH to go from 7.2 to 7.6. And then it will take about another 5 to 7 days to rise to 8.0. That is several days longer than what occurs when the alkalinity is around 100 ppm. (Again, there are additional variables and situations that affect the speed of the pH rising).

Another benefit is that at 60 ppm of alkalinity, it requires less acid (about 12 ounces) to lower the pH from 8.0 to 7.6, than with 100 ppm of alkalinity (about 20 ounces of acid). That also means that less sodium bicarbonate will be needed to restore the consumed alkalinity if needed.

Many service techs consider this pool water balancing program to be more manageable, reasonable, and productive with once-a-week chemical treatments. They also know they should keep the water LSI balanced (-0.3 to +0.5) at all times. According to the LSI, maintaining a slightly higher pH allows for maintaining a slightly lower alkalinity level. Pool water can still be LSI balanced with those adjustments.

As mentioned before, the Acid Demand test feature provides the mechanism and ability to closely monitor and adjust the pH with smaller amounts of acid, and at a lower alkalinity level.

Different Pool Conditions Benefit with Different Chemistry Programs

The effectiveness of some sanitizers, such as Ozone, UV, Bromine, and PHMB isn’t hindered by a slightly higher pH. For those sanitizers, the alternative program of maintaining a lower alkalinity of 60 to 70 ppm and a higher pH of 7.8 to 8.4 can be easier to manage. At that alkalinity and pH range, the pH can be fairly stable within that range, and that means that less acid/bicarb treatments are needed.

And as was stated in a prior email update, maintaining a higher chlorine level (2 ppm and above) allows for maintaining a higher pH of 7.8 to 8.4 with completely adequate sanitizing efficacy.

There will be some who would object to a suggested “Ideal” alkalinity range of 60 to 80 ppm. Yet, 40 years ago, the chemists at Olin Chemical stated that the “optimum” alkalinity range is 50 ppm to 100 ppm. That is a lower range than the current recommended ideal alkalinity range of 80 ppm to 120 ppm.

So why does the pool industry recommend a higher alkalinity range? Maybe it has something to do with the increased popularity of Trichlor in the 1970s-1980s. Since it is an acidic sanitizer (as opposed to previously more popular base sanitizers such as bleach and Cal hypo…), when using Trichlor, a higher alkalinity should indeed be maintained in order to buffer the pH of the water better. A benefit of trichlor is that it lowers the pH and counters the natural pH rise. A negative is that it increases the CYA and slows down the kill time of the chlorine.

In fact, in some situations a higher alkalinity than even 120 ppm may be needed to prevent the pH dropping below 7.2 when adding acidic products. This includes some non-plaster pools, such as vinyl and fiberglass surfaces, some areas of the country where a lot of “acid rain” falls in a short period of time, or even some pools that have pool covers installed.

But as has been shown for some other pool situations, a lower alkalinity level can be more effective to manage and stabilize the pH. Therefore, depending on various situations, some pools might have the pH from 7.2 to 7.6, and other pools do better with a higher pH of 7.8 to 8.2.

Also, according to the LSI, maintaining a slightly higher pH allows for maintaining a slightly lower alkalinity level. Pool water can still be in perfect balance (-0.3 to +0.5) with those changes.

Summary

Due to the above variables and situations involving all swimming pools, we suggest that the “Ideal” recommended levels be dropped, the LSI be emphasized, and allow our knowledgeable service techs to adjust the water chemistry parameters in their pools that can result in a more effective balanced and sanitized water.

Currently, the APSP is developing a new water chemistry standard for residential pools. Now is the time to modernize and correct the old material that is in need of an update. Service technicians have much to contribute to the process, rather than relying on old and occasionally incorrect information. Now is the time to create standards that can withstand scientific and reasonable scrutiny – and allow for us to work smarter rather than merely “the way it has always been done…”

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