Key Misunderstandings and Problems in Pool Water Chemistry (Part II)

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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