Welcome back our discussion of key misunderstandings in pool and spa water chemistry. In our first installment, we covered a variety of issues that all interrelate in the overall water chemistry scheme. We continue examining these key facets of pool and spa water treatment here.

We'll begin with misunderstanding No. 5 — to see the first four, click here.

5. How do I prevent the pH from always going down?

If the pH (and alkalinity) of the pool water is constantly drifting down, the most likely cause is trichlor tabs being used as the main chlorinating source. Even if trichlor is not the main chlorinating source, a supplemental trichlor feeder may be causing the pH to drift low. In addition to lowering the pH and alkalinity, CYA is being added to the water constantly, which means a higher FC level will be required to control algae and bacteria. I would suggest discontinuing trichlor use and using liquid chlorine or cal hypo as the main chlorinating source. Then you can add CYA separately to 30 or 50 ppm and the CYA level will not change; as such, the target FC level will remain unchanged.

pH also decreases when the total alkalinity is too low. If the pH is drifting low and your alkalinity is 80 ppm, you should raise the target alkalinity to 90 ppm and see if the pH becomes more stable.

In short: If pH is always going down, raise the alkalinity target by 10 ppm.

6. CYA only protects chlorine, right? So why should it matter what the level is?

As mentioned above, the maximum CYA should be 50 ppm. This is mostly because the required FC level is 7.5 percent of CYA. With CYA at 50 ppm, the required FC level is 3.75 ppm (0.075 × 50 ppm = 3.75 ppm). You get almost as much protection from 30 ppm as 50 ppm CYA, so there is no advantage to having a higher level of CYA. Higher levels require higher FC. At 30 ppm, CYA the required FC is 2.25 ppm FC (0.075 × 30 ppm = 2.25 ppm).

CYA builds up quickly when using trichlor and as the CYA builds up, the requirement for FC goes up. Remember that for every 10 ppm of FC added to the pool water by trichlor, the CYA goes up by 6 ppm.

CYA buildup is actually why many pools get algae during the summer. At the beginning of the swim season, the CYA level is low — maybe 30 ppm. The chlorine consumption in most pools in summer is about 1.0 to 1.5 ppm FC per day. So if using trichlor as the main source for chlorine, in a week or maybe 10 days, the pool will use 10 ppm of chlorine and the CYA will increase by 6 ppm. The following week, CYA will increase another 6 ppm, and so on. In eight weeks, the CYA will have gone up by at least 50 ppm.

Say the pool was being maintained at 2 to 4 ppm FC in the beginning. With 2 ppm and 30 ppm CYA, you have sufficient chlorine to kill algae and bacteria. (Note: This is harder, and it takes higher level of chlorine to kill algae than bacteria.) However, after four weeks the CYA level will have gone up by about 25 ppm. Now the CYA is 55 ppm, which requires 4.125 ppm FC. If the pool was kept at 4 ppm, or superchlorinated weekly, you'll have sufficient chlorine to kill algae.

However, in one more month the CYA will have increased by another 25 ppm. Now the CYA is 80 ppm. This would require 6 ppm FC. If the pool was being kept at 4 ppm, there is insufficient FC to prevent algae growth. You now start to see algae, and wonder why it's there when you have 4 ppm FC. It's simple: The simple: 4 ppm FC is not enough chlorine to prevent algae when the CYA is 80 ppm.

7. Using liquid chlorine raises the pH of the water, right?

I have written about this many times but it is well worth repeating: Hypochlorites, sodium hypochlorite, calcium hypochlorite and lithium hypochlorite do not raise pH.

That said, it's a common misconception — I even believed it once. However, the chemistry of chlorination is quite simple and straightforward. Many people believe that liquid chlorine raises the pool water pH. It does not. Most liquid chlorine (sodium hypochlorite, 12.5 percent) has a pH of about 13; cal hypo has a pH of 11.8 and lithium hypo has a pH of 10.8. These are all very alkaline so it is logical to think that when added to water they will raise pH. And, in fact, they do.
In the case of liquid chlorine, we add it to water and it produces HOCl (hypochlorous acid – the killing form of chlorine in water) and sodium hydroxide (NaOH). The sodium hydroxide raises pH.

So, yes, pH goes up initially. However, when HOCl is degraded by sunlight, degraded in the process of killing organisms or used in oxidation, the HOCl produces HCl. The amount of HCl produced is almost equal to the amount of NaOH produced when the liquid chlorine was added. So using liquid chlorine or cal hypo or lithium hypo will have a net zero effect on pH.

The advantages of using liquid chlorine or cal hypo are that they do not contribute to the CYA level and therefore do not require higher levels of FC due to CYA increases. In fact, you can add 30 or 50 ppm of CYA to the water and use liquid chlorine or cal hypo, and you'll find they will not change pH, they will not increase CYA, they will not increase alkalinity and the water balance will remain stable because nothing is being added to the water that changes water balance. If borates are used at 50 ppm, pH is 7.5 and alkalinity at 90 ppm, not much is going to change. The total alkalinity and CYA are pH buffers that keep the pH from drifting lower and the borate is a pH buffer that prevents the pH from drifting up.

8. Why is using trichlor as main source of chlorine not a good idea?

As I stated above in No. 5, I do not recommend the use of trichlor as the main chlorinating source in residential swimming pools. The biggest reason is that the increase to CYA is huge, which in turn requires more and more FC to control algae and bacteria. Second, trichlor is acidic and lowers alkalinity and pH, thus requiring frequent additions of soda ash or bicarb. We are trying to make things stable and this constant addition of CYA and lowering of pH and alkalinity makes things unstable.

Summing it all up

I have shown you a complete way to chemically take care of residential pool water that is easy and is stable. My method does not require the use of expensive chemicals and will give you great looking water with minimum effort. 

• You now know how much chlorine you need in any pool: 7.5 percent of CYA or less if using algaecide, borate or phosphate remover. Each pool will have a different FC.

• You now have a target for alkalinity: 90 ppm in most cases. This keeps pH from going up and because it is a target, you know if the water is okay based on your test results.

• You know how to raise only pH with no change in alkalinity using aeration and turbulence which makes it possible to get perfectly balanced water – pH 7.5 and alkalinity of 80 ppm in usually one day or less

• You know what to do if the pH is always going up – lower target alkalinity by 10 ppm.

• You know that using 50 ppm of borate in the water will keep pH from drifting up by acting as an algaestat.

• You know what to do if the pH is always going down, you should raise alkalinity by 10 ppm and stop using trichlor.

• You know the maximum level of CYA should be 50 ppm because higher levels require higher FC.

• You know that liquid chlorine and cal hypo do not raise pH.

• You know that you should not use trichlor as the main chlorinating source in residential pools.

• You will not need to superchlorinate regularly — only when special events happen.

• You will not need oxidizers, algaecides, stain or scale inhibitors.

To complete this discussion, let’s look at issues involving salt water chlorine generators.

SWGs will raise pH due to the chemical reaction of electrolytically making chlorine. It is a fact that the pH will go up so I always recommend a 50 to 60 ppm level of borate when using SWGs. This will slow down the rate of pH rise. You will still need acid to lower pH, and you may need sodium bicarbonate to replace lost alkalinity after acid additions. I also recommend a slightly higher CYA level of 60 to 70 ppm. The good news is that because the chlorine is being added daily and borates are being used, the required FC is 3.5 percent of CYA. So if maintaining 60 ppm CYA you would have a required FC of 2.1 and at 70 ppm CYA, a required FC of 2.5 ppm.

How much acid do I need to lower pH and alkalinity? What will the acid do to pH and alkalinity? 

These are two of the great unknowns in our industry. Also right up there: How much acid do I add to lower alkalinity to some desired level?

The good news is that there is a way to determine how much acid is required to add to lower alkalinity: 25.6 fl. oz. of muriatic acid (31.45 per cent HCl) added to 10,000 gallons of water will lower alkalinity by 10 ppm. You can use that formula to determine the correct amount of acid needed.

Suppose you have a 15,000-gallon pool and want to lower alkalinity from 140 to 90 ppm. First, subtract 90 from 140 to get 50 ppm. Next, divide 50 ppm by 10 ppm to get 5. Now divide 15,000 gallons by 10,000 gallons to get 1.5. Then multiply 25.6 fl. oz. by 1.5 and by 5 to get 192 fl. oz. of muriatic acid. And finally, 192 fl.oz. divided by 128 fl. oz. per gallon is 1.5 gallons. Therefore, adding 1.5 gallons of muriatic acid to a 15,000 gallon pool will lower alkalinity by 50 ppm or from 140 ppm to 90 ppm.

This method for calculating the amount of acid to lower alkalinity does not give you any information about what happens to the pH. My guess would be lower than 7.

Want an easier way? I developed a program called the Pool Acid Dose Calculator and it is available to use for free on my website. To use it, enter pool volume in gallons, current pH, current alkalinity, CYA, borate. Then enter the desired pH and the calculator will give you the exact amount of muriatic, sulfuric and dry acid to add to lower pH to that desired level. It also calculates the new alkalinity and displays it.

You can also use it to determine the amount of acid needed to change alkalinity by changing the desired pH to a lower level until the desired alkalinity is reached.

(One last plug: the Pool Acid Dose Calculator is also available for your iPhone or Android phone for a onetime purchase of $5.99. With the app, you can calculate the right dosing from the field.)

There may be some questions about lowering the 7.5 percent required for FC but it is very difficult to calculate the effect a supplement will have on the requirement. Things like borate, algaecides, phosphate removers, SWGs, ozonators and UV will all lower the FC requirement. How much, I only have guesses.

Potential Problem

There is one problem that you may face using this method. As I mentioned, the average pool in the swimming season uses about 1 ppm of FC per day due to UV degradation (which is the largest loss) and from oxidation and disinfection. Some pool may lose up to 1.5 ppm FC per day. The result is that a pool may need 7 to 10 ppm of chlorine per week. You could add enough chlorine to the pool to bring the level up to 10 ppm and hope it lasts for a week. However, the EPA and common sense say that you should not be exposed to that level of chlorine. So you have a problem: How do you make the chlorine last for a week? You may need to use a liquid chlorine dispenser, ask the pool owner to add some liquid chlorine you leave for them or perhaps try a cal hypo feeder. As I outlined above, I would not recommend supplementing with a trichlor tab feeder or dispenser.

The Good News

The good news is that if you follow a plan like this, once set up with pH, alkalinity and borate levels, your pools will only require a little liquid chlorine, a little acid, maybe some bicarb and some air to raise pH. You will spend less time and money on chemicals and have better water and happier customers.

Bob Lowry is one of the industry’s most widely published authorities on pool and spa chemistry, having written more than 200 articles and 14 books on the subject, including three training manuals for IPSSA. Lowry has been in the industry nearly 45 years, and in that time owned two chemical companies (Leisure Time Chemical and Robarb), served as a CPO instructor for 21 years and held positions at Leslie’s and DEL Ozone. Over his career, he has invented or formulated more than 110 chemical products for the pool and spa industry.



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In regards to liquid chlorine (sodium hypochlorite) and whether it raises the pH and alkalinity or not, there are several factors that affects how much it DOES RAISE the pH and alkalinity that is not mentioned in the article.

First, some high strength liquid pool chlorine products contain a much higher amount of excess lye (sodium hydroxide) than does other lower strength liquid chlorine products such as household bleach. That will make a difference in the amount of effect.

Also, pools located in hot areas of the country will need higher amounts of chlorine for treatment than pools located in cool regions. That will make a difference too.

There are situations that when using liquid chlorine in hot temperature areas, using 15% Trade liquid chlorine that contains 0.6% excess lye, can increase the pH by a net 0.3 and the alkalinity by about 10 ppm over a two-month period.

On the other hand, sometimes, a lower strength bleach is used that contains a lower amount of excess lye and a smaller amount (of liquid chlorine) is being added (due to cooler temperatures and use) would generate a much lower pH and alkalinity increase. So yes, with that type of situation, it is true that liquid chlorine or bleach only has a minor effect on raising pH and alkalinity, especially if only considering the effect of one week of liquid chlorine treatment.

A pool operator once told me about his commercial pool that had a carbon dioxide (CO2) injection system installed to control the pH (to reduce acid additions), but that the alkalinity would continually go up every month. Because he had heard the (incorrect) claim that liquid chlorine doesn’t raise the pH or alkalinity at all, he (incorrectly) assumed that it was the carbon dioxide that was increasing the alkalinity. He was happy to find out that it wasn’t the CO2 system that was raising the alkalinity level.

Since liquid chlorine products contain some amount of excess lye, using it will raise both the pH and alkalinity in time. Yes, in some situations, only a very minor amount, but in other situations, it can raise both more significantly.
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Rob,

There is a lot that is not quite correct in what you wrote:

"When the dissolved solution goes past the plates the Na and the Cl crack apart. The Cl immediately bonds with the H2O and makes HoCl, with one hydrogen left over. The spun off hydrogen causes the pH to rise."

First of all, when you dissolve sodium chloride (NaCl) salt in water, the sodium and chloride separate into ions and that has nothing to do with an SWG. Simply dissolving the solid salt in water separates the salt crystal into separate ions.

As far as the SWG is concerned, the sodium (Na+) is irrelevant except to provide conductivity and charge balance. It doesn't react with the SWG. It is the chloride ion (Cl-) that gives up electrons at the anode to form atomic and then molecular chlorine (Cl2) which is a gas that dissolves in water reacting with the water to form hypochlorous acid and hydrochloric acid.

Cl2 + H2O ---> HOCl + HCl
Chlorine Gas + Water ---> Hypochlorous Acid + Hydrochloric Acid

The hydrochloric acid is a strong acid that essentially completely dissociates into hydrogen ion and chloride ion while the hypochlorous acid is a weak acid that is about half dissociated at a pH of 7.5. Having hydrogen ions LOWERS the pH. pH itself is a logarithmic measure of the amount of hydrogen ions where a lower pH has more hydrogen ions (because pH is the negative logarithm). The pH is very low near the anode.

At the other SWG plate, the cathode, hydrogen ions accept electrons to form atomic and then molecular hydrogen (H2) which is a gas that does not dissolve (much) in water so remains as bubbles. Th pH is very high near the cathode. The net result of both reactions is the following:

H2O + Cl- --> OCl- + H2(g)
Water + Chloride Ion ---> Hypochlorite Ion + Hydrogen Gas

You can see that the SWG output is equivalent to adding a hypochorite source of chlorine except that the SWG also produces hydrogen gas bubbles. Also, if all the chlorine gas doesn't dissolve into the water, then the pH will rise from such outgassing since there will be less hypochlorous and hydrochloric acid produced.

Lowering the Total Alkalinity (TA) does not change the SWG chemistry. What it does is lowers the already occurring source of pH rise, namely that of carbon dioxide outgassing. However, if your SWG isn't fully dissolving chlorine gas then even a lower TA won't stop the pH from rising from chlorine gas outgassing.

As far as your aeration of the water from turning up eyeballs not balancing the pH enough to prevent borated Trichlor from continuing to lower the pH, in order to have the carbon dioxide outgassing be faster you should have increased your TA level. Just as you want a lower TA when using a hypochlorite source of chlorine or when using an SWG, you want a higher TA when you are using net acidic sources of chlorine such as Trichlor. This is why a higher TA of 100-120 ppm or more is often suggested when using Trichlor. Of course, you might not want too high a TA if you already have a high CH level since both contribute to the calcite saturation index which if too high makes calcium carbonate (calcite) scaling more likely.
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Cailley Hammel wrote:
Hi Rob,
Bob wrote about this in depth in a past issue of AQUA — you can read the story here:
aqmag.co/2htZNU0
I read the article. He seemed to be saying "Try this" . It did not look like any studies had been done. I do appreciate your time.
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When SWG's first came out, I asked "How does it work?" . The answers were typically " Great", "Wonderful". I kept at it. Finally, I got this explanation: Add NaCl to the water. When the dissolved solution goes past the plates the Na and the Cl crack apart. The Cl immediately bonds with the H2O and makes HoCl, with one hydrogen left over. The spun off hydrogen causes the pH to rise. When I was first starting, I was trained that pH was French and meant "Positive Hydrogen."
Are you telling me that if I lower the TA to 70 of less, this basic chemistry process will stop? Will the spun off hydrogen just bubble up through the surface of the water for my clients to enjoy?

PS --- Last summer, I listened to this discussion. I turned the three eyeballs on a 20,000 gal. pool as far towards the surface as they would go. We were using borated trichlor as a source of FAC. The pH still fell, even after the eyeball adjustment. At that time, I used borates to raise the pH. I must admit, I was hoping for a wondrous reaction. I don't expect that the customer will put up with an air compressor running 24/7 and a bubbler in the bottom of the pool. I wish the eyeball change had worked.
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Regarding borates, it was correctly stated in the following two articles that borates buffer the pH of the water to slow down the rate of pH rise:

aquamagazine.com/retail/the-reality-of-p...mical-treatment.html
aquamagazine.com/retail/key-misunderstan...hemistry-part-i.html

However, in this article it states that borates will prevent or keep pH from drifting up. Some people might interpret that as meaning it stops pH rise which is not the case. It slows it down.

Also, it should be noted that the pH buffering does not change the cumulative amount of acid that needs to be added to maintain the pH. Because the rate of rise slows down, you can add acid less frequently but would dose more each time you add acid. The cumulative amount of acid needed over a given time interval is the same as before.

To lower the amount of acid needed cumulatively, one can lower the TA level to lower the rate of carbon dioxide outgassing which is one source of rising pH. In SWG pools, one can use a higher CYA level of 80 ppm to lower chlorine loss from sunlight letting one turn down the SWG on-time which reduces aeration from hydrogen gas bubbles and outgassing of undissolved chlorine gas.

The carbonate buffer system associated with the TA measurement gets stronger at lower pH while the borate buffer system gets stronger at higher pH so they complement each other. Actual buffer strength for carbonates, borates, and Cyanuric Acid (which is also a pH buffer) is shown in the chart at the following post:

www.troublefreepool.com/threads/30200-pH-Buffer-Capacity
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As can be seen from his comments, Richard Falk has done a lot of research, and his writings are valuable and a great resource in helping to understand the complicated nature of pool water chemistry. I have learned a lot from reading his work.

In addition to what Mr. Falk explained about buffering against a rising pH, borates also buffer against the pH lowering as acid is added when the pH is 8.0 and above. That means when pool water contains 50 ppm of borates, more acid will be required to lower the pH from 8.0 or 8.2 and then down to 7.5, than when no borates are present.

So why not make the target pH a little higher, like 7.7 or 7.8 instead and add less acid? And then the TA won’t be getting lowered so much by the unneeded (extra) acid.

I believe that is why Mr. Falk suggested that similar concept in his comment below. He explained that a higher carbonate alkalinity (90+ ppm) and a lower pH increases the speed at which pH rises (meaning increased carbon dioxide out-gassing). Mr. Falk’s charts provide that specific data.

Therefore, when hypochlorites or salt systems are being used, the most effective way to reduce the amount of acid and Bicarb being added is to lower the target alkalinity and raise the target pH. The pH can be more stable with that program. The use of trichlor tabs, on the other hand, may need a higher alkalinity for good pH stability.
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What is the source for the factoid "FC needs to be 7.5 % of CYA." This is new to me. What is the deal and where does it come from. I have been a Taylor test kit (K-2005) guy for a long time.
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Hi Rob,
Bob wrote about this in depth in a past issue of AQUA — you can read the story here:
aqmag.co/2htZNU0
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The general rule for TA is that if the pH tends to rise over time the TA may be too high and should be lowered. Similarly if the pH tends to drop over time, the TA may be too low and should be raised. This is because the level of TA and pH, along with the amount of aeration in the water (e.g. waterfalls, fountains, spillovers), determines the rate of carbon dioxide outgassing and that is a source of rising pH (or compensating for lowering pH from other sources).

The experience of tens of thousands of pool owners at Trouble Free Pool shows that a TA of 90 would often be too high for those using hypochorite sources of chlorine of for SWG pools. Usually the TA is no higher than 80 for hypochlorite pools and 70 for SWG pools but if the pH tends to rise then even lower TA can be used.

In addition to a lower TA target, having a higher pH target also helps reduce the rate of pH rise from carbon dioxide outgassing. Instead of lowering the pH to 7.5 or below, try and maintain it at 7.7 to 7.8 or so. The following chart shows how much pool water is "over-carbonated" compared to carbon dioxide balance with air at various TA and pH levels (assuming 30 ppm CYA -- since it's carbonate alkalinity that really matters).

www.troublefreepool.com/~richardfalk/pool/CO2.htm

If one targets a lower TA and a higher pH but their saturation index is too low and they have a plaster pool, then this can be compensated by targeting a higher Calcium Hardness (CH) level. Note also that in areas with hard fill water and/or high evaporation rates that it may require a lot of acid to maintain a lower TA level since evaporation and refill adds the TA and CH from the fill water into the pool.

Richard Falk ("chem geek" on forums)