Langelier Saturation Index Inaccuracies

The Langelier Saturation Index is the basis of swimming pool water chemistry. A simple mathematical formula determines whether the water is in balance (neither corrosive nor scale forming). Realistically, not a measure of the success in preventing algae or bacteria, but a calculation designed specifically for the protection of the vessel, the swimming pool itself—a determination of calcium carbonate saturation. Let’s face it; we have come to depend upon the accuracy of the pool industry LSI (Langelier Saturation Index).

The water being in balance is extremely important, and the LSI is a tool we rely upon. After all, we know water needs to have a certain amount of calcium and carbonate in solution, and the level we need varies upon both temperature and pH. If the water is found to be corrosive, it will pull calcium and carbonate from the pool walls and floor, etching the surface. When the water is scale forming, everything from a cloudy water condition to calcium carbonate deposits forming about the pool.

This Photo by Unknown Author is licensed under CC BY

Many think calcium carbonate saturation is only important in plaster pools, and that’s not the case. Nowadays, liner companies utilize calcium carbonate in the production of vinyl. The amount used varies from manufacturer to manufacturer, but it has been found that corrosive water can pull calcium carbonate from vinyl when the content is greater than 7%. It is also a fact that cobalt staining is more prevalent in fiberglass pools where a corrosive saturation index persists.

Dr. Wilfred Langelier, professor emeritus of civil engineering at the University of California, Berkeley published this concept in a paper titled The analytical control of anti-corrosion water treatment in 1936 in the detailing the model was first published in 1936 in the Journal of the American Water Works Association. Most are aware of this; however, a common misconception is that we actually use Dr. Langelier’s model, which was designed to determine the Potential for calcium carbonate precipitation. In actuality, we do not.

A Mother of a Math Problem

Langelier’s formula, despite its accuracy, is extremely complex. It also was not designed to determine whether water was corrosive or scale forming. Instead, as I mentioned above, the calculation only determined the calcium carbonate precipitation potential (CCPP). Not a guarantee that a problem with calcium would occur, only a predictive measure of an increased possibility. Regardless, Langelier (already widely known for his advances in water treatment due to studies documented in his 1921 paper Coagulation of Water with Alum by Prolonged Agitation) came up with a highly complex formula. That was beyond practical for longhand calculation – the math was insane!

View the entire Journal of the American Water Works Association 1936-10, Vol 28 HERE

Due to its complexity and advances in science, the formula was simplified in 1965 by Carrier (heating, air-conditioning, and refrigeration solutions). This new version is often referred to as the ‘improved’ version and was the first to call the formula the Langelier Saturation Index. Oh yeah, I almost forgot – pHs is the name Langelier gave to his formula for predicting calcium carbonate precipitation potential.

Is Carrier’s LSI improved? Answering that question is beyond my skill set, but I can say it definitely dumbed it down. With that, of course, some suggest this revised 1965 method is too simplistic to be as accurate. However, this new calculation is more similar to the formula that we use now in the swimming pool industry. That’s right; we’re not up to what we use for swimming pools just yet. That doesn’t happen until 1974.

Change is good, right?

This was when John A. Wojtowicz of Chemcon got his hands on it. Wojtowicz pointed out that the LSI in its current version did not consider other alkaline substances that would contribute to the Total Alkalinity in a swimming pool (i.e., cyanuric acid, boric acid, etc.). So, the formula was retooled once again. See The Effect of Cyanuric Acid and
Other Interferences on Carbonate Alkalinity Measurement
in the Journal of the Swimming Pool and Spa Industry

This is where we are at now – the Wojtowicz Saturation Index? Not sure if this name will stick, but it is certainly more accurate than calling is LSI. Misnomer or not, this is the saturation index calculation that we use in the pool industry today. This is where the formula for the LSI apps we use originated. Accuracy in attribution or not, is it more accurate in predicting the potential for calcium carbonate to precipitate in an open body of water?

Crystal structure of Calcite By Materialscientist at English Wikipedia, CC BY-SA 3.0,

I thought it would be interesting to see the difference in calculation between the three using values we would likely see in a swimming pool, specifically omitting the presence of cyanuric acid, borate, or any other substances that might affect our Total Alkalinity.

Our example water test:

Water Temperature


Calcium Hardness

Total Alkalinity

Cyanuric acid



Total Dissolved Solids

81° F


300 ppm

90 ppm

0 ppm

0 ppm

0 ppm

1,000 ppm

Saturation Results:


1936 – Dr. Wilfred Langelier – pHs

1965 – Carrier – LSI

1974 – Wojtowicz – Pool Industry LSI (using a currently popular industry app)

Index Calculation







Supersaturated – Scaling possible

Scale forming and corrosive

In Balance – not scale forming, not corrosive.

A Formula never intended to be used in an open body of water exposed to atmospheric pressure 🤷‍♂️

Another factor to consider – The saturation index is said to only be accurate within certain parameters. Luckily, the chemistry we keep in swimming pools typically falls within that range. That is except for Saltwater pools.

  • pH: 6.5 to 9.5
  • Temperature: 32 to 212°F
  • Bicarbonate alkalinity: 10 to 800 ppm
  • Calcium hardness: 50 to 700 ppm (up to 900 ppm)
  • Total dissolved solids: 50 to 1000

Accuracy of the pool industry LSI

If Dr. Langelier’s pHs is the more accurate measure of determining the water’s potential of calcium carbonate precipitation and was truly only changed for ease of calculation, with the capabilities of cell phones, wouldn’t it make more sense to develop an app that could quickly perform the complex equation? Of course, Wojtowicz’s Carbonate Alkalinity would be a necessary modification in a swimming pool application.

If not, and today’s LSI method continues to be the route we take, do we really need an app or studies on mathematical calculations at all? I mean, the reality of it is shooting for the dead center of the ideal range on the three values for which an ideal range exists; the water will always be in LSI balance down to a temperature of 48°F and as high as 104°F anyway. The exception being a saltwater pool in which none of the calculations are apparently accurate anyway.

Specific Parameters

Even if the inaccuracy of the LSI at a Total Dissolved Solids (TDS) level above 1,000 ppm did not exist, maintaining a pH of 7.5, Carbonate Alkalinity of 90 ppm, and a Calcium Hardness of 300 ppm would keep the water balanced on today’s pool industry LSI balanced in water temp range of 63°F to well over 104 degrees (TDS of 3,000 ppm).

Have an LSI app on your phone? Try it! Set the pH at 7.5, Carbonate Alkalinity at 90 ppm, and the Calcium Hardness at 300 ppm. Now play around with different numbers for temperature and TDS. Do we really need an app? Or, would a simple dosing calculator suffice?

El mito del ácido de la piscina

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¿Puede el ácido ‘Slug’ reducir la alcalinidad total sin afectar el pH? 

La capacidad de controlar si se reduce la alcalinidad total o el pH mediante el método en el que se agrega una dosis de ácido a una piscina sin duda sería de gran valor para los operadores de piscinas. El mito del ácido de la piscina

“Para reducir la alcalinidad total drásticamente con solo un ligero efecto en el pH, simplemente tome su dosis de ácido y viértala lentamente en un punto en el extremo profundo de la piscina. De hecho, verá las burbujas que se forman alrededor de donde se agrega la “babosa”, es decir, el bicarbonato se quema y burbujea. Si desea bajar el pH sin reducir también la alcalinidad total, simplemente vierta la dosis de ácido muriático sobre la piscina “.

Todo esto sería maravilloso si fuera exacto. Aún así, al igual que el mitológico “Cloro Lock”, los cuentos folclóricos dentro de la industria de las piscinas persisten. Pero, ¿reducir la alcalinidad total sin afectar el pH?

El mito del ácido de la piscina

Cuando se agrega ácido clorhídrico (HCl) al agua, las moléculas se disocian en iones de hidrógeno (H +) e iones de cloruro (Cl-). Esto sucede, no importa cómo se agregue. Para simplificar, cuando aumenta la cantidad de iones de hidrógeno en una solución, el agua se vuelve cada vez más ácida. Recuerde, pH significa Potenz Hydrogen, que se puede interpretar del alemán original como “Concentración de hidrógeno”. Entonces, si agregar ácido al agua aumenta la concentración de iones de hidrógeno sin importar cómo lo agregue y el pH (Potenz Hydrogen) es la “Concentración de hidrógeno” … ¿Hmmm?

Todo preparado para realizar mis propios experimentos para este artículo para poder ofrecer una prueba definitiva a aquellos firmes creyentes de la eslugología ácida, me topé con una investigación que había sido realizada por la gente (Kim Skinner, Que Hales y Doug Latta) en onBalance . Su estudio, Poner en reposo el mito de la columna de ácido, involucró un experimento que describe con precisión lo que sucede, adónde va el ácido y el peligro potencial en la “babosa” y / o la “columna”. En lugar de replicar los experimentos, me acerqué a mi amigo y socio Que Hales, quien dio su bendición al compartir sus resultados.

Para llevar a cabo la investigación, Que y la pandilla agregaron polvo rojo de fenol al ácido para darle un poco de color (el ácido es claro, no lo habría visto de otra manera). El ácido es más pesado que el agua, por lo que solo tiene sentido que se hunda hasta el fondo de la piscina después de agregarlo. Luego, con una cámara subacuática, tomaron instantáneas en una cuenta jugada por jugada de lo que realmente ocurre.

una medida de la actividad de los iones de hidrógeno

Lo que se observó fue una dosis de ácido posada en el suelo de la piscina durante horas …? Considere el daño potencial que podría ocurrir con una “gota” de ácido (pH de 2.5) en un lugar, sobre el yeso, durante horas. ? Mejor aún, ¿qué pasaría si esta dosis se hubiera deslizado junto con el fondo marcando un rastro mientras se acercaba al drenaje principal? ¿Cómo afectaría esto al equipo?

rojo de fenol es el químico que usamos para probar el pH; algo fácil de recordar porque el rojo fenol comienza con pH

“El hecho simple y llano del asunto es que una determinada cantidad (o” dosis “) de ácido agregada a un volumen fijo de agua (la piscina) resultará en una reducción idéntica tanto del pH como de la alcalinidad. Siempre. No importa cómo se agrega. Esa es la regla, eso es ciencia … “- onBalance

Sí señalan en su investigación que la gota de ácido finalmente se diluyó y que si se agrega a una piscina mientras el sistema de circulación está funcionando, la dilución ocurrirá a un ritmo más rápido. Cepillar la piscina después de la dosis también acelerará la adulteración. Pero esto derrota la teoría del mito. ¿No es así? Vierta lentamente y revuelva.

progresión de un vertido ácido

La realidad es que no se puede subir ni bajar uno sin afectar al otro, con muy pocas excepciones. Si agrega ácido a la piscina para reducir el pH, también reducirá la alcalinidad total. Si agrega ácido a la piscina para reducir la alcalinidad total, también bajará el pH. Esto es cierto, no importa cómo lo agregue.

Lo mismo ocurre cuando se agrega una sustancia química para aumentar el pH o el TA. Tanto el bicarbonato de sodio (bicarbonato de sodio) como el carbonato de sodio (carbonato de sodio), cuando se agregan al agua, aumentarán tanto el pH como la alcalinidad total (aunque tiene un poco más de control sobre los concomitantes de la intención real cuando se ajusta hacia arriba) .

Escala logarítmica

Mencioné que hay algunas excepciones. Puede disminuir el pH sin disminuir la alcalinidad total utilizando una inyección de dióxido de carbono (CO₂) para controlar el pH. Aún así, esto no disminuye uno sin afectar al otro. Cuando se inyecta CO₂ en el agua, se forma ácido carbónico (H₂CO₃), que reducirá el pH.

Aún así, la dosis finalmente aumentará la alcalinidad total, requiriendo que se agregue una dosis de ácido para el control de la TA en algún momento. El único método para aumentar el pH sin aumentar o disminuir la alcalinidad total es a través de la aireación, como se explica en nuestro artículo: Aumento del pH con aire

Laying to Rest the Acid Column Myth, onBalance – Kim Skinner, Que Hales y Doug Latta, Fotos y extractos compartidos con permiso: Que Hales, onBalance. onBalance Technical Research Doc de The Journal of the Swimming Pool and Spa Industry: The Addition of Muriatic Acid

Related Story: Online CPO Class

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Lowering Total Alkalinity Without Affecting pH

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Can the Acid Slug Lower Total Alkalinity Without Affecting pH?

Lowering Total Alkalinity Without Affecting pH

The ability to control whether you lower Total Alkalinity or pH by the method in which a dose of acid is added to a swimming pool would certainly be of great value to the Pool Operators.

To lower Total Alkalinity drastically with only a slight effect on pH, simply take your dose of acid and pour it slowly into one spot in the deep end of the pool. You’ll actually see the bubbles forming around where the “slug” is added – that is, the bicarb burning off and bubbling up. If you wish to lower the pH without also reducing the Total Alkalinity, simply pour the dose of muriatic acid about the pool.

This would all be wonderful if it was only accurate. Still, just like the mythological “Chlorine Lock,” folkloric tales within the swimming pool industry do persist. But, Lowering Total Alkalinity Without Affecting pH?

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When hydrochloric acid (HCl) is added to water, the molecules disassociate into both hydrogen ions (H+) and chloride ions (Cl). This happens, no matter how it is added. To keep it simple, when you increase the number of hydrogen ions in a solution, the water becomes increasingly acidic. Remember, pH stands for Potenz Hydrogen, which can be interpreted from the original German to “Hydrogen Concentration.” So, if adding acid to water increases the hydrogen ion concentration no matter how you add it and pH (Potenz Hydrogen) is the “Hydrogen Concentration”… Hmmm?

progression of an acid pour

All prepared to conduct my own experiments for this article so that I could offer definitive proof to those firm believers of Acid Slugology, I stumbled across some research that had been conducted by the folks (Kim Skinner, Que Hales, and Doug Latta) at onBalance. Their study, Laying to Rest the Acid Column Myth, involved an experiment which depicts precisely what happens, where the acid goes, and the potential danger in the “Slug” and/or “column.” Instead of replicating the experiments, I reached out to my friend and associate Que Hales, who gave his blessing in our sharing their results.

progression of an acid pour

To conduct the research, Que and the gang added phenol red powder to the acid to give it a bit of color (acid is clear, you wouldn’t have seen it otherwise). Acid is heavier than water, so it only makes sense that it would sink to the bottom of the pool after you add it. Then, with an underwater camera, they snapped shots in a play-by-play account of what actually occurs.

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What was observed was a dose of acid sitting at the floor of the pool for hours…? Consider the potential damage that could occur with a “slug” of acid (pH of 2.5) sitting in one spot, on the plaster, for hours. ? Better yet, what if this dose had crept along with the bottom etching a trail as it approached the main drain. How would this affect the equipment?

phenol red is the chemical we use to test pH; kind of easy to remember because phenol red starts with pH

progression of an acid pour

“The plain and simple fact of the matter is that a given amount (or “dose”) of acid added to a fixed volume of water (the pool) will result in an identical reduction of both pH and alkalinity. Every time. No matter how it is added. That’s the rule, that’s science…” – onBalance

progression of an acid pour

They do point out in their research that the blob of acid did eventually dilute and that if added to a swimming pool while the circulation system is running, the dilution will occur at a quicker pace. Brushing the pool following the dose will also expedite the adulteration. But, this defeats the theory in the myth. Doesn’t it? Pour it in slowly and stir ??

progression of an acid pour

The reality of it is that you can not raise or lower one without affecting the other, with very few exceptions. If you add acid to the pool to reduce the pH, you will also reduce the Total Alkalinity. If you add acid to the pool to reduce the Total Alkalinity, you will also lower the pH. This is true, no matter how you add it.

The same holds true when a chemical is added to increase pH or TA. Both sodium bicarbonate (baking soda) and sodium carbonate (soda ash), when added to water, will increase both the pH and the Total Alkalinity (though you do have a bit more control over the concomitants of the actual intent when adjusting either upward).

I did mention that there are some exceptions. You can decrease pH without lowering the Total Alkalinity using Carbon Dioxide (CO₂) injection for pH control. Still, this does not lessen one without an effect on the other. When CO₂ is injected into water, it forms carbonic acid (H₂CO₃), which will reduce the pH.

Still, the dose will ultimately increase the Total Alkalinity, requiring a dose of acid to be added for TA control at some point. The only method of increasing pH without increasing or lowering the Total Alkalinity is through aeration as discussed in our article: Raising pH with Air

Laying to Rest the Acid Column Myth, onBalance – Kim Skinner, Que Hales, and Doug Latta, Photos and excerpts shared with permission: Que Hales, onBalance. onBalance Technical Research Doc from The Journal of the Swimming Pool and Spa Industry: The Addition of Muriatic Acid


Related Story: Online CPO Class


That Chlorine Smell

Did you ever walk into an indoor pool and the second you enter, you get this smack in the face chlorine smell that literally brings tears to your eyes? ? I was chatting with a friend about a month or two back who has probably been in more natatoriums in her life (the room they keep the pool in) than I have been to swimming pools all together – FYI: that’s a lot of pools. The conversation touched upon that “chlorine smell” and the stinging eyes we associate with those types of venues. ?

I had reached out a week or two prior to that to my longtime friend and associate, Wayne Ivusich of Taylor Technologies, to see if he would be interested in putting something together for me and what he delivered, coincidentally, happened to be on this exact topic. I had first met Wayne back in ’92 (Yes, in this one the year we met plays a factor – you’ll see) back when we were both starting out in the swimming pool industry and both had full heads of hair. ? Here are Wayne’s words of wisdom as he lays down some pool chemistry:

Why do bad things happen to good treatment chemicals??? 

– Wayne Ivusich

When I was a kid (and some “friends” would say that was pre-electricity and the invention of indoor plumbing), I distinctly remember going to the local community pool and smelling the odor of chlorine as I neared the entrance. My parents would always say something like “oh they just added chlorine so the water’s safe to go in.” Of course, I ALWAYS listened to what my parents said (at least until I was 14 or 15) and happily jumped in the pool. Later, after the rashes appeared and my eyes were red, we just chalked it up to a mysterious something that caused it.

Wayne Ivusich, CPO®, CPI®, NSPF®I
Manager of Education & Technical Services
Taylor Technologies, Inc.

Fast forward too many decades and add almost 28 years in the industry, I now, of course, realize that it wasn’t too much chlorine but rather too much “combined chlorine” (aka chloramines) in the water that was causing the rashes and irritation. Amazing what a couple of decades of knowledge does for you.

So…what it boils down to is that a strong “chlorine” odor and resulting rashes and/or red, itchy eyes mean too much COMBINED chlorine.

Remember, any form of chlorine that is added to water has the exact same chemical reaction, producing hypochlorous acid (HOCl) and a hypochlorite ion (OCl) – otherwise known as “free chlorine.” Free chlorine is what you want in the water protecting bathers.

However, free chlorine will also react with organics in the water and become chloramines, which we also call “combined chlorines.” Chloramines are much weaker sanitizers and oxidizers than free chlorine. So where do the organics come from – mostly us! Our sweat and urine contribute organics to the water. So do lawn fertilizers. The chloramines are responsible for the typical “chlorine smell” we find in pool and spa waters.

This is NOT a good thing!

The problem is even worse in indoor environments with poor or non-existent fresh air exchange systems.

So…how do you tackle this problem? Breakpoint chlorination is the answer! Although it seems odd, you’re actually adding more chlorine – all at once – to eliminate the combined chlorine. To determine how much chlorine to add to achieve breakpoint, take your combined chlorine value and multiply that number by 10. The resulting number is how much in parts per million (ppm) of chlorine is added ALL AT ONCE to remove the combined chlorine reading. For example, you do your normal testing and determine that your combined chlorine level is 0.5 ppm. 0.5 x 10 = 5.0 ppm of chlorine needed to reach breakpoint. Consult dosage charts to determine the correct amount of chlorine to add based on how many gallons of water. Remember to keep pumps and filters running while doing this.

A couple of final reminders:

  1. NEVER add chlorine to a skimmer. Add chlorine in front of a return line to help it disperse throughout the water.
  2. NEVER breakpoint chlorinate with stabilized chlorine (dichlor or trichlor). Only use sodium hypochlorite, lithium hypochlorite, or calcium hypochlorite. Using dichlor or trichlor will add large amounts of stabilizer (cyanuric acid) to the water that may cause other problems.

And, finally, always listen to your parents!


In a conversation regarding the effect of water quality on competitive swimming events between Rudy Stankowitz (Aquatic Facility Training & Consultants) and Summer Sanders, the Olympian offered the following sentiment for us to share:

“The better we understand the chemistry between healthy pool water and healthy kids, the happier we will be and the faster our kids will swim!” – Summer Sanders, competitive swimmer and Olympic Gold Medalist ?, 1992 Barcelona.



Available Chlorine Content vs. Active Strength… Huh?

1913 interior of hypochlorite plant

Available Chlorine Content (ACC) is perhaps the most confusing concepts to grasp. The term was created as a means of comparing the bleaching and disinfecting power of the different chlorine compounds. Simplified, ACC is nothing more than a comparison of that specific chlorine type to chlorine gas and really does not have much to do with how much chlorine is in the bucket. This measurement applies to that chlorine type as a whole, meaning that all chlorine of that type is this strength (or within that specific range of strength) compared to Chlorine gas, which is always 100 percent. Confusing? If we want to know what amount of the chlorine type is actually in the bucket you are purchasing, we will need to look at the listed Percent of Active Strength.

If we were to look at standard chlorine tablets, trichloro-s-triazinetrione (Trichlor): Available in either granule or tablet form. The compound (C3Cl3N3O3) is 90% Available Chlorine. This means that Trichlor is 90% as strong as chlorine gas. This applies to ALL Trichlor, ALL Trichlor is 90% ACC (Available Chlorine Content). The 90% listed serving as a comparison of the compounds as explained above. The product is also listed as > 99% active strength. This means that greater than 99% of what is in the bucket is actually trichloro-s-triazinetrione, the amount of the active ingredient.

Nowadays, Chlorine tablets (Trichlor) are not typically manufactured using binders to hold the product together in tablet form.  High-pressure equipment presses the compound into shape; it is strictly a lot of psi used to maintain that tablet form. Back in the day this was not always the case and binders such as xanthan gum were utilized to allow the product to maintain its shape. Still, tablets are often falsely accused due to the “gummy” build up found in erosion feeders (automatic chlorinators) following use. This, however, is due to zinc stearate which is used during production as more of a lubricant to keep the tablets from sticking to the manufacturing press; not as an actual ingredient. Think of its use as more similar to the reason one would use Pam cooking spray in a skillet.

In a quasi-similar (but not exactly the same) example, Bacardi 151 was 75.5% alcohol by volume; 151 Proof. If a bartender was to make a rum and coke utilizing one shot of Bacardi 151 and another with two shots of 151, the strength of the active ingredient (Alcohol Proof) does not change, the rum is still 151 Proof. The only change was in the amount of the Bacardi product to Coca Cola.

The product itself; a stabilized compound: contains cyanuric acid (stabilizer). This is evidenced in the formula C3Cl3N3O3 , the Cl3 (3 atoms of chlorine) displacing H(3 atoms of hydrogen) in the cyanuric acid compound C3H3N3O3. Therefore, we do add more than just chlorine nonetheless. 15 oz. of trichlor (stabilized chlorine tablets/granular) will add 10 ppm of FAC to 10,000g, but it will also increase the CYA (Cyanuric Acid) by 6 ppm and the TDS by 10 ppm. The product itself is also acidic with a pH of 3.0 and will, when used for chlorination result in lowering pH. Trichloro-s-triazinetrione has an indefinite shelf life and, in storage, will never lose any of its ACC.

1910 portable emergency hypochlorite plant

Sodium Hypochlorite (NaClO) has a Trade% of 10 to 12 (Volume % Available Chlorine). The rest is simply saltwater (the inert ingredient). A gallon of sodium hypochlorite (containing 2.34 pounds of solids) will add 12 ppm of FAC (Free Available Chlorine) to 10,000 gallons of water and will increase the TDS by 28 ppm. Sodium Hypochlorite (liquid chlorine) has a very short shelf life and drops to 8% ACC within several weeks of manufacture (household bleach from the supermarket has a 6% ACC). Sodium Hypochlorite, despite popular belief, does not increase pH; (except possibly in instances where manufactured with excess lye) as is the same with other hypochlorites (Calcium Hypochlorite, Lithium Hypochlorite, etc).

Confusing Terms for Weight %, Trade % and Available Chlorine
Weight % Available Chlorine = Trade % ÷ Specific Gravity
Trade % = Weight % Available Chlorine × Specific Gravity
Weight % NaOCl (sodium hypochlorite) = Weight % Available Chlorine × (NaOCl grams/mole)/(Cl2 grams/mole)
Weight % Available Chlorine = Weight % NaOCl × (Cl2 grams/mole) / (NaOCl grams / mole)
Weight % NaOCl = (Trade % / Specific Gravity) × (NaOCl grams/mole) / (Cl2 grams/mole)
Weight % NaOCl = (12.5/1.16) × (74.442/70.906)
Weight % NaOCl = (10.7759) × (1.0499) = 11.3136
Trade % = Weight % NaOCl × Specific Gravity × (Cl2 g/mole) / (NaOCl grams/mole)
Trade % = 11.3136 × 1.16 × (70.906/74.442)
Trade % = 11.316 × 1.16 × (.9525)
Trade % = 12.50
NaOCl grams/mole = 74.442
Cl2 grams/mole = 70.906

Liquid bleach is usually a Weight % NaOCl in the ingredients on the label and sometimes (for Clorox, for example) lists the “% Available Chlorine.” Liquid pool chlorine is most often sold by Trade %. The Trade % is technically the Volume % Available Chlorine and therefore is the only quantity that exactly matches its number with ppm in the pool water as with 1 gallon in 10,000 gallons of 12.5% chlorinating liquid produces 12.5 ppm
free chlorine.

1912  interior machinery of hypochlorite plant

Calcium Hypo (Ca(ClO)2) has a 48 to 72% ACC and a corresponding Active Strength of 48 to 72%. The inert ingredient in Calhypo is calcium chloride. Calhypo (Calcium Hypochlorite) added at a rate of 20 oz. per 10,000g will add 10 ppm of FAC, but will also increase your Calcium Hardness by 8 ppm and your Total Dissolved Solids by 15 ppm. It will take roughly three years before Calcium hypochlorite begin to lose any of its ACC, and then it does so at an extremely slow rate. Calcium Hypochlorite, as explained above with all hypochlorites, does not increase pH. The increase in pH is only temporary (except as noted above with sodium hypochlorite) because the pH of hypochlorites is high. However, when chlorine is “used up” the process is acidic, bringing the pH back down to pretty much where it started.

To reiterate, using Trichlor as an example, at 90% ACC (Available Chlorine Content) and  > 99% active strength = greater than 99% of what is in the bucket is trichloro-s-triazinetrione, it’s just that the trichloro-s-triazinetrione in the bucket is only 90% as strong as chlorine gas.

Similar Article: That Chlorine Smell ?


Special thanks to Robert Lowry, chemical consultant and pool/spa water chemistry expert, for the peer review and contributions to this article.

Confusing Terms for Weight %, Trade % and Available ChlorinePage 128 © Copyright 2018 Lowry Consulting Group, LLC All rights reserved. Duplicated with permission, Pool Chemistry for Residential Pools

Photo Credit: Historic photographs  Minnesota Department of Health, R.N. Barr Library; Librarians Melissa Rethlefsen and Marie Jones


The Duke of Pools

Duke University Aquatics

There is a lot more to consider when maintaining these collosal giants of competition and wellness than one might think. Pool Operators face a myriad of challenges above and beyond balancing the mega gallonage at a body of water whose role serves as recreational, fitness, and sports venue. Duke University’s Director of Aquatics, Abi Schaefer, joins us to discuss some of the intricacies of operating a collegiate swimming pool. Here is what Abi had to say:

Abi Schaefer
Director of Aquatics
Duke Recreation & Physical Education

Duke University is unique in many aspects of everyday life for a university, but I like to think it is tremendously unique when it come to the Aquatics world. Duke University Aquatics sits in a very rare spot when it comes to University Recreation and Aquatics. On a day to day basis we serve so many different populations and groups of people that it can become very challenging to ensure each group and population has the best possible experience, all while keeping the safety of our clients, athletes, and patrons in mind. Duke Aquatics oversees the operation and programming of three facilities; Taishoff Aquatics Pavilion, Brodie Aquatics Center, and Central Campus Pool. Each facility brings diverse challenges and opportunities alike.

Taishoff Aquatics Pavilion for example houses; the Varsity Swim and Dive teams, recreational lap swim, Masters swim team, Physical Education Courses, Club Swim team, Club Water Polo, and Duke Dive Club. On the other hand, Brodie Aquatics center supports our; learn to swim program, American Red Cross Lifeguard program, weekly Kayak Clinics, Stand Up Paddle Board programming, recreational lap swim, Physical Education Courses, and weekly Special Olympics practice. Outside of those two facilities we have seasonal outdoor facility, Central Campus Pool. This facility is used for students during the Spring and Fall semesters as a place they can go, relax, study, and take a life time out; however, during the summer months this facility is generally home to the Duke communities faculty, staff, and families. This is designed as a recreational pool that can have anything from recreational lap swim, learn to swim programming, weekly outdoor kayak clinics, birthday parties, group rentals, and just an exclusive space for the Duke Community to utilize.

All of this being said creates our biggest challenges; scheduling, spacing, timing, and safety. Everything has to run as a well-oiled machine in order to ensure that each group, club, practice, and program has access to the facilities at their scheduled time, date, and location. For example; if one facility has a maintenance issue, as all pools do from time to time, it effects more than just that specific facility because most programming has to be moved to another facility or cancelled all together. Not only does this create a scheduling nightmare, but it can take away from the overall experience we want to provide each and every patron. It truly takes a team of pool operator professionals, aquatics professionals, coaches, and key part time staff to make sure everything stays up and running. Additionally, the schedule must be put together, keeping each specific group, team, club, etc. in mind, even leaving one group out can throw off this delicate balance.

The most important aspect to remember is safety. The schedules have to be designed in such as fashion that the pools are not over loaded, that programs are not so large that they take up their space and space of another program, and that everyone is safe. For example, you would never want to put a Level One Learn to Swim Program at the same time as the Kayak Clinic. It takes a critical eye and true understanding of what each group needs to be successful to have well ran facilities and programming.

To see what University of Georgia’s Director of Aquatics, Alex Nichols, had to say on the subject in the Spotlight on UGA: University of Georgia Aquatics


Raising pH with Air

Spas, Splash pads, and Waterpark rides…

Ever wonder why the pH tends to run high in certain bodies of water no matter what you do? Always adding acid? Spas, Splash pads, swimming pools with water features, and literally everything at a waterpark. They all have one thing in common.


When water is aerated, it creates turbulence. The turbulence then causes the aqueous CO2 (carbon dioxide) to outgas. Outgassing of COfrom water results in an increase in pH. Aeration is the only means of increasing pH that will not increase the Total Alkalinity. This is both beneficial and problematic.

At Aquatic Facility Training & Consultants, we think it’s important to know the “why” behind things. To understand the reason for the increased consumption of acid at these facilities. If you can embrace the science in what we do, swimming pools become increasingly easy to care for.

My pool, at my house; the pH always runs on the low side. My yard has long leaf pine. Pine needles, when green, are acidic. Even if they do not fall directly into the pool, when the needles get wet, they drip. This drives the pH down. My pH, left to its own, would be a consistant 7.0

Four years ago, with the knowledge of aeration, I decided I would run an experiment. I took a single return jet (where the water comes back into the pool through the wall) and loosened the collar. I aimed the jet toward the surface so that it would cause that ripple in the water. Within three days, the pH in my 15′ x 30′ rose to 7.6. Between the turbulence caused by the jet and the acidity from the pine needles above, my pH has remained between 7.5 and 7.6 for the past four years without the need to add a chemical for adustment; up or down. Wouldn’t it be great if you could harness this power and not have to worry about a pH too low?

What about that pool you have on your service route where the pH always runs high (saltwater pools excluded)? Maybe it doesn’t have a water feature. Next time you are there, take a look at where the return jets are aimed. A lot of times home owners will aim their jets upward, because “they like to see the water move… Your real problem is not the pool. You have been fighting science and losing the battle against aeration.  Loosen the collars and aim those jets back downward – pH problem solved.

Many times, during the summer months, competition pools utilize water cannons to cool the water temps. These cannons shoot water through the air in an effort to keep the water within the USA Swimming guidelines for competition (between 25 to 28 degrees Celsius (77 to 82.4 degrees Fahrenheit)). This can be a double edged sword. The cannons are very effective at reducing water temperature. However, they do create a ?-ton of turbulence. This drives the pH upward, but remember earlier I had mentioned that aeration would not raise the Total Alkalinity? This is a problem.

So if the aeration created by the water cannons causes the pH to rise, we will need to add acid to lower it. Makes perfect sense, right? Here’s the thing: it is impossible to add acid to lower the pH without it also lowering the Total Alkalinity. But, again, the turbulence did not raise the Total Alkalinity; just the pH. That means that our continuous feed of muriatic acid necessary to counter balance the aeration of the cannons and keep the pH in check has driven our Total Alkalinity to a level too low. It is not uncommon to see that some type of sodium bicarb feed system has been added to these pools to counter the dose of acid.

An alternative at competition pools to muriatic acid is the injection of carbon dioxide (CO2). When COis added to water if forms carbonic acid (H2CO3) Carbonic acid is very effective at lowering pH. Unfortunately, injecting COto form carbonic acid (H2CO3) is the only method of lowering pH that will also increase the Total Alkalinity. Ultimately, the pool would then require the addition of muriatic acid.

I don’t want to delve deeply into the MYTH at this point in time because more information is coming in the near future. Still, we need to understand that it is impossible to add acid to lower the Total Alkalinity without lowering the pH, and vice versa. Many have said in the past, and many still believe, that the method of introduction of acid to water will enable a person to lower one without affecting the other. This is simply untrue.

pH stands for potenz Hydrogen, which translates from German to “the power of Hydrogen” Anytime acid is added to water it increases the Hydrogen ion level. If you add acid to water in order to lower the Total Alkalinity (as our CO2 method above would eventually require), the Hydrogen Ion level will increase. As pH is the measurement of the water’s demand for acid (more specifically, hydrogen activity measured in the converse), the pH would lessen; a low pH indicating a low demand. In short, if you increase the amount of Hydrogen Ions while lowering the TA, does it not make sense that you will affect a reading that measures Hydrogen ion concentration in that soultion (the power of Hydrogen (pH)) ??? The same holds true when the levels are increased (Yet another entirely different conversation), with exception to aeration. This leaves aeration the only method of altering pH with an effect something else, or does it? What would be the impact of aeration on water temperature as noted with the water cannons above?

Liquid Chlorine Raises pH, or Does it?

Sodium Hypochlorite (NaOCl) is one of the more popular methods of chlorinating swimming pool water in both commercial and residential applications. When NaOCl (liquid chlorine) is added to water HOCl, Na+, and OH– is formed. However, what happens after the hypochlorous acid (HOCl) is used up (either due to UV from sunlight, or as the chlorine sanitizes and disinfects) is a common topic for debate among pool operators. Chemically, the HOCl becomes HCl (hydrochloric acid) in the process – no one is arguing this. The controversy is regarding whether the amount of acid (HCl) produced is enough to counter balance the initial increase in pH the dose had generated.

Photo Credit: A Grande Choice Pool & Spa, Inc in Englewood, FL

In my personal experience, swimming pools that I have maintained with sodium hypochlorite have recognized a drop in pH following the initial increase, ultimately realizing a net zero change. I did not have to add muriatic acid or CO2 to balance, which would support that the amount of HCl formed was in sufficient quantity to offset the initial increase in pH. Still, I have had lengthy conversations with others in the industry, pool operators I consider experts in their own right, whose opinions and experiences were polar opposite to mine.

I theorize that the experiences my peers are facing are caused by any and all things that could contribute to a change in pH in a swimming pool: greater levels of turbulence; Total Alkalinity; fill water chemistry; etc. Anything, that is, except the addition of liquid chlorine. With an open mind, I decided to reach out to a couple of the leading chemists/scientists in the industry to get their opinions. On my behalf, I was thinking I would end up with a paragraph and had the intention of a single article. What I received was greatly appreciated and so much more:

So, does sodium hypochlorite raise pH?

Richard Falk (AKA: Chem Geek)

 According to pool/spa water chemistry expert Richard Falk (AKA: Chem Geek), “When any hypochlorite source of chlorine is added to a pool, the pH rises because hypochlorite is high in pH.  However, when the added chlorine is consumed over time, this is an acidic process and pH drops back down close to where it started.  The only net pH rise from the hypochlorite addition comes from the “excess lye” in the product such as chlorinating liquid that is used for greater stability of that product.”

Richard went on to explain: Nevertheless, there is the observation that pH rises in pools using hypochlorite.  The reason is that the pH rise comes from other sources, primarily from the outgassing of carbon dioxide.  Carbon dioxide outgasses from pools causing the pH to rise with no change in Total Alkalinity (TA).  The reason is that swimming pools are intentionally over-carbonated in order to (ironically) provide pH buffering and to saturate the water with calcium carbonate in order to protect plaster surfaces.  The other main source of pH rise is the plaster itself as it continues to hydrate and cure though this is most noticeable in the first months to year of a new or re-plastered surface.  Vinyl pools do not show this effect but may still have carbon dioxide outgassing.

It is for this reason that covered vinyl pools tend to be the most stable with regard to pH when using hypochlorite sources of chlorine while uncovered plaster pools with greater sources of aeration such as fountains, waterfalls, spillovers, and higher bather-load with more vigorous splashing or movement have the pH rise the most.  Pools using saltwater chlorine generators tend to rise in pH not only because their generation of hydrogen gas bubbles aerates the water, but those with short pipe runs from the generator to the returns can also outgas some undissolved chlorine gas (the rate is low, but done over an extended period of time).  Pools not using CYA can also show greater net pH rise due to some chlorine (hypochlorous acid) outgassing.  Also, there are some lesser pathways to chlorine usage that result in some pH rise, such as chlorine oxidation of nitrogenous compounds to produce nitrate instead of nitrogen gas.

If there were no carbon dioxide outgassing nor hydration/curing (or dissolving of) plaster, then with 80 ppm TA the addition of 10 ppm FC either at once or cumulatively added would have the pH rise from 7.5 to 8.12, 20 ppm FC would rise to 8.55, 40 ppm FC would rise to 8.8.  When the chlorine is then consumed/used, the pH drops down to 7.52, 7.53, 7.57, respectively.

One can minimize the amount of pH rise from carbon dioxide outgassing by operating the pool at a lower TA level and higher pH target.  The following chart shows the relative amount of over-carbonation in the pool with respect to air at various TA and pH levels (with 30 ppm CYA since TA is increased some by the CYA level as a function of pH): 

 Relative CO2 Out-Of-Equilibrium at Various Levels of pH and Alkalinity
                              30 CYA, 550 TDS, 300 CH, 80oF
<————————————— pH ———————————————>
Total Alk. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 8.0
40 10.9 8.2 6.2 4.6 3.4 2.4 1.7 1.1 0.7 0.3 0.0
50 14.5 11.1 8.5 6.4 4.8 3.6 2.6 1.8 1.2 0.7 0.4
60 18.2 14.0 10.8 8.3 6.3 4.7 3.5 2.5 1.8 1.2 0.7
70 21.8 16.9 13.1 10.1 7.7 5.9 4.4 3.3 2.3 1.6 1.1
80 25.4 19.8 15.4 11.9 9.2 7.0 5.3 4.0 2.9 2.1 1.4
90 29.1 22.7 17.7 13.7 10.6 8.2 6.2 4.7 3.5 2.5 1.8
100 32.8 25.6 20.0 15.6 12.1 9.3 7.1 5.4 4.1 3.0 2.1
110 36.4 28.5 22.3 17.4 13.5 10.5 8.0 6.1 4.6 3.4 2.5
120 40.0 31.4 24.6 19.2 15.0 11.6 8.9 6.8 5.2 3.9 2.8
130 43.6 34.3 26.9 21.0 16.4 12.7 9.8 7.6 5.8 4.3 3.2
140 47.2 37.1 29.1 22.8 17.8 13.9 10.7 8.3 6.3 4.8 3.5
150 50.8 40.0 31.4 24.6 19.3 15.0 11.6 9.0 6.9 5.2 3.9
175 59.8 47.1 37.1 29.1 22.8 17.8 13.9 10.8 8.3 6.3 4.8
200 68.8 54.2 42.7 33.6 26.4 20.7 16.1 12.5 9.7 7.4 5.6
250 86.6 68.4 54.0 42.5 33.5 26.3 20.6 16.0 12.5 9.6 7.4
300 104.4 82.5 65.2 51.4 40.5 31.9 25.0 19.6 15.2 11.8 9.1
400 139.6 110.5 87.4 69.1 54.5 43.0 33.8 26.5 20.8 16.2 12.5
NOTE: A value of 0.0 means there is an equalibrium amount of carbon dioxide in the water and in the air so there is no Carbon Dioxide outgassing.
A value of 1.0 means there is twice as much Carbon Dioxide in the water compared to the equilibrium amount.
A value of 2.0 means there is three times as much Carbon Dioxide in the water compared to the equilibrium amount.
Check back with us on July 26th (2018) for the second half of this 2-Part series when we speak with chemical consultant, author, and pool/spa water chemistry expert Robert Lowry in our cleverly titled Sequel: “Does Liquid Chlorine Raise pH?

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