Key Misunderstandings and Problems in Pool Water Chemistry (Part I) (2024)

Welcome to this overall look at the problems facing service techs regarding pool and spa water chemistry – presented here in two parts.

Before we get started, consider this: If we think about all of the following issues together, we can come up with an easy way to take care of pool water that makes sense. By contrast, when we try to fix one of these without considering the way it affects other chemical constituents, we often create another type of problem.

We’ll start off with a brief explanation of each problem and its solution. Then I’ll follow up with more complete answers.

(Note: My complete answers for No. 5 through No. 8 will appear in Part II.)

The Short Answers

1. How much chlorine does a pool need? Why do some pools get algae even with 2-4 ppm free chlorine?
The short answer: Each pool needs a different amount of free chlorine. You should maintain a FC level of 7.5 percent of CYA.

2. Why is the pH always going up (or down) in pools? What is the correct total alkalinity?
Water has the wrong alkalinity and maybe the wrong amount of buffer. Most pools should have an alkalinity of 90 ppm.

3. How can I balance water quickly? How do I raise pH without raising alkalinity?
The pH can be raised without changing alkalinity by aeration and turbulence. Pools can be perfectly balanced in a day by lowering alkalinity to 80 to 90 ppm with acid, and then raising pH only to 7.5 with aeration.

4. How do I prevent the pH from always going up?
This happens when water has the wrong (i.e.: too high) alkalinity. Borates can also be used to prevent pH rise.

5. How do I prevent the pH from always going down? Why is the pH always going down?
The water has the wrong (i.e.: too high) alkalinity and not enough buffering, which is controlled by alkalinity and CYA.

6. CYA only protects chlorine, right? So why should it matter what the level is?
CYA protects free chlorine from UV destruction, but it also is part of the pH buffering system that keeps the pH from being lowered. CYA also controls how much free chlorine is available for killing and oxidizing.

7. Using liquid chlorine raises the pH of the water.
Liquid chlorine does not raise pH. When added to water, liquid chlorine (which has a pH of 13) makes HOCl (hypochlorous acid – the killing form of chlorine) and NaOH (sodium hydroxide), which raises pH. But when the HOCl is degraded by UV, and when used in killing and oxidation, it creates HCl (hydrochloric acid). The amount of HCl is almost identical to the amount of NaOH. So the net effect on pH is zero (or almost zero).

8. Why is using trichlor as main source of chlorine not a good idea?
Trichlor is 55 percent CYA. Using it increases CYA, which requires a higher level of chlorine to kill bacteria and algae. Using trichlor raises CYA by 6 ppm for each 10 ppm of FC. And because it is acidic, it lowers pH and alkalinity.

If we find a solution for all of these issues, there will be no need for algaecides, superchlorination, separate oxidizers, scale inhibitors, stain removers, metal inhibitors and any other specialty chemicals. This alone will save you and your clients money. It will be better for you and your pools if you try to implement or change what you are doing on all of these issues at once rather than just picking out one or two and trying them out.

Now let’s get started with the more complete answers:

Many times I read articles that say, "Maintain this condition at this level," but they fail to tell you how often to do it or how to do it. I hope I can give you the details so you don't have to figure out what somebody else meant or what you should be doing.

The Complete Answers

1. How much chlorine does a pool need? Why do some pools get algae even with 2-4 ppm free chlorine?

I once wrote an article about this subject, which you can read in full here. It's a long article, but here's the most important part: For most residential pools, the FC level should be 7.5 percent of the CYA minimum. In addition, the CYA level needs to be no more than 50 ppm.

The reasoning: 7.5 percent of 50 ppm CYA is 3.75 ppm FC. CYA of 65 ppm would require 5 ppm FC and 100 ppm would need 7.5 ppm FC. These are obviously very high levels of FC, and the EPA and CDC say that you should not be exposed to that much chlorine. In fact, EPA says the maximum FC is 4 ppm for swimmers. Side note, the CDC’s Model Aquatic Health Code says that you can superchlorinate up to 10 ppm. In addition, CDC and EPA do not consider that chlorine is bound to CYA and that only about 3 to 7 percent of the chlorine is free in the water at any moment. So being exposed to even 10 ppm of FC with CYA in the water means that the exposure is only 3 to 7 percent of 10 or 0.3 to 0.7 ppm FC. This is really not very much exposure. If you have 2 ppm FC without CYA, you are exposed to all 2.0 ppm FC.

RELATED: New Thinking: Chlorine/Cyanuric Acid in Balance

In my article I explained that chlorine in water is mostly bound to the CYA, and only a very small amount of chlorine (about 3 percent) is available to disinfect and oxidize. The remaining chlorine in the water (about 97 percent) is bound to CYA. It is not immediately available for disinfection and oxidation. However, as some of that 3 percent FC that is available gets used to kill and oxidize, some of the bound chlorine replaces the lost FC so that the 3 percent is always available until there is no more chlorine in the water.

As the CYA level increases; it requires a higher FC level to control bacteria and algae. Using trichlor or dichlor means that the CYA level will change (and go up) constantly. In fact, for trichlor, every 10 ppm of FC added to the water will increase CYA by 6 ppm. Many pools consume about 1 to 1.5 ppm of FC each day, so in a week or 10 days, the CYA will increase by 6 ppm or more if using trichlor. In eight weeks you have added almost 50 ppm more CYA to the pool. If you started at 50 ppm, it is now 100 ppm. This would mean that the FC requirement has gone from 3.75 to 7.5 ppm.

The main reason I do not recommend using trichlor as the main source of chlorine in residential pool water is that it changes the amount of FC required by the pool daily. The CYA is constantly being added to the water as the trichlor tab dissolves. Another problem with trichlor tabs is that they are acidic, and most have a pH of about 2.8. This low pH lowers pool water pH and alkalinity.

So there are two problems: CYA constantly going up and changing the FC requirement, and the pH and alkalinity always going down, which requires frequent additions of sodium bicarbonate or soda ash to remedy. Each of your pools will have its own minimum level of FC based on the CYA level. Forget keeping all of your pools between 2 and 4 ppm FC. This might work for some of your pools, but not all. Calculate the FC needed for each pool by multiplying CYA by 7.5 percent and use that as the minimum FC level. You can reduce that 7.5 percent if you are using an algaecide, borates or phosphate remover in the water. You should have a different target FC for each pool based on CYA level.

2. Why is the pH always going up (or down) in pools? What is the correct total alkalinity?

Total alkalinity and CYA together create of the buffering systems in pool water — they keep the pH from changing too quickly and by how much. They are better at keeping the pH from drifting low or going down. But total alkalinity is causing the pH of the water to go up. If you have high alkalinity (more than 100 ppm), it will continually raise the pH, which requires frequent acid additions to lower pH. If you don’t lower the total alkalinity, the pH will go up between visits.

The real proper total alkalinity for pools is 90 ppm. If you are experiencing the pH drifting up to high levels between weekly visits, you should lower the alkalinity by 10 ppm and use that as your new target alkalinity. So if you have a pool in which the pH in one week goes from 7.5 to 7.8 or 8, and the alkalinity is 90 ppm, you should lower alkalinity to 80 ppm and maintain that level as your new target alkalinity. If 80 ppm doesn’t work, change the target to 70 ppm. You should adjust the total alkalinity for each pool so that the weekly pH change is minimal. You may have to add a small amount of acid each weekly visit. This would be normal. But if you are having to add large amounts of acid each week, your alkalinity is way too high.

The proper target pH is 7.5. It is not 7.4 to 7.6 and it is certainly not 7.2 to 7.8. With 7.5 as a target, you will know whether it is high or low based on your water tests and what to do about it. Having targets makes it easy to know what is wrong and what to do to fix it. Having a range just makes you believe that all is okay when maybe it isn’t. I mentioned above that total alkalinity and CYA are two of the pH buffers in the pool water. They basically keep the pH from drifting low. There is a separate buffer that you can add to the pool water that keeps the pH from drifting up. It is called borate and it can be added from three different compounds. The most known product is Borax (not Boraxo, which is detergent) and the other two are sodium tetraborate pentahydrate and sodium tetraborate decahydrate.

Adding and maintaining a level of 50 ppm of borate will keep the pH from rising significantly. Borate in pool water also has algaestatic ability. Algaestatic means that it helps prevent rather than kill algae. Because borate is an algaestat, you could lower the recommended FC level of 7.5 percent of CYA to 5.5 percent of CYA.

3. How can I balance water quickly? How do I raise pH without raising alkalinity?

I wrote another long paper about this that has been published previously in AQUA; I also included this information in the IPSSA Basic Training Manual 2016 Revised Edition Part 1 – Chemicals. We all know that adding acid of any type to the pool water decreases pH and alkalinity. But pH and alkalinity are not lowered equally by adding acid. The reason is that pH is measured on a logarithmic scale and alkalinity is measured on a linear scale. So they will never lower together equally. We all should know that adding soda ash will raise pH and alkalinity, and adding sodium bicarbonate will raise only alkalinity with very little effect on pH. This presents a problem – you can raise alkalinity and pH, you can raise only alkalinity and you can lower both, but there has not been a way to raise pH alone without raising alkalinity. The only chemical we use to raise pH, soda ash, also raises alkalinity. So all we have are soda ash, which raises both pH and alkalinity or bicarb, which raises only alkalinity.

If you have high pH and high alkalinity, you have a choice. You add acid to lower the pH to a desired level, which means that the alkalinity will be too high. Or you can add an amount of acid that will lower alkalinity to the desired level, but now the pH will be too low. If you add soda ash to raise the pH, it will also raise alkalinity — putting you right back where you started. If you add a dose of acid that will lower the pH to the proper level, the alkalinity will be too high. On your next visit, the pH is back up where it was last week. This is the classic yo-yo problem we face when trying to balance the water.

Here is a little known fact: If you aerate the water and create turbulence, it raises the pH with no effect on alkalinity. Aeration and turbulence causes CO2 in the water to gas off (leave the water as CO2 gas). This loss of CO2 is replaced in the water by carbonic acid in the water making more CO2. Then the lost carbonic acid is replaced by bicarbonate alkalinity, which uses hydrogen ions (H+). The loss of hydrogen ions (H+) is what raises the pH. Remember that pH is a measure of the hydrogen ion (H+) concentration in the water. So, the less H+, the higher the pH.

Now it becomes possible to lower high pH and alkalinity with acid to get the alkalinity to the desired level of 90 ppm with one addition of acid, which will cause the pH to be very low (7 or less). But now we can aerate and create turbulence and, depending on how much aeration and turbulence we generate, raise pH within hours to 7.5. We can get perfectly balanced water in the same day.

You can aerate and create turbulence by aiming return lines up so the flow of water breaks the surface, turn on spillways and turn on fountains and other water features. You could use a pressure washer, like one used to clean your car or house, by aiming it into the water for a while. You could use a Venturi injector. Or you could even use an air blower or air compressor with a manifold of PVC with holes in it. Understand also that this raising of pH is a natural process, and the pH would probably go from 6.8 to 7.4 in about two days if you do nothing. We are just speeding up the process.

4. How do I prevent the pH from always going up?

As I have said, in most pools the alkalinity is too high. As a result, the pH will continuously drift up. Another way of saying this is that the alkalinity pulls up the pH. Too much alkalinity causes pH to rise. Therefore, lowering alkalinity will keep pH from rising very fast. The proper total alkalinity is 90 ppm. And if you are experiencing the pH drifting up to high between weekly visits, you should lower the alkalinity by 10 ppm and use that as your new target alkalinity. So if you have a pool that the pH in one week goes from 7.5 to 7.8 or 8 and the alkalinity is 90 ppm, you should lower alkalinity to 80 ppm and maintain that level as your new target alkalinity.

As mentioned above, adding 50 ppm of borate to the water will keep the pH from drifting up or at least slow down the rate of rise. Borate only needs to be added once to the water unless a significant amount of water has been lost and replaced. Backwashing, leaks and winterizing all cause water loss. Also, as mentioned, borate acts as an algaestat, so it reduces the FC requirement. You may have a different alkalinity target for each pool.

In the next installment, which will print Tuesday, I'll address the rest of the water chemistry misconceptions I outlined above.