Copyright © 2014 jsd

Questions and Conjectures
about Swimming Pool Chemistry
and Swimming Pool Care

John Denker

1  Introduction

A great deal of the terminology used in the pool-care business sounds like chemistry, but isn’t. It is wildly inconsistent with the terminology used in scientific chemistry.

What’s worse, many of the conventional pool-care procedures make no sense.

All too often, water-chemistry readings are easy to interpret if everything is normal, but if one of them gets out of whack it becomes difficult to interpret the others, difficult to figure out what caused the problem, and even more difficult to figure out how to fix it.

Here are some thoughts. Some of them I reckon are probably true, some are conventional wisdom [CW] that I have not yet checked sufficiently, others are mere conjectures, and some are completely open questions.

2  Some Chemistry

In the context of swimming pools, the term «chlorine» is is not the same as the term chlorine as used by chemists. When people say they are testing for «chlorine» it is hard to know what they really mean.

In particular:
  1. Among other things, you care about disinfecting power and algae control,
  2. which might or might not be provided by bleaching power
  3. which might or might not be provided by hypochlorite,
  4. which might or might not have been created by bubbling real chlorine (Cl2) through the water.

In more detail:

In this document, I use the word “hypochlorite” broadly. It is meant to include

The disinfecting power of hypochlorite depends strongly on pH. According to reference 1, neutral HOCl is more effective than the OCl ion.

Hypothesis and conjecture: Rather than adding NaOCl, which doesn’t shift the pH, one could imagine the following two-step process: Add HOCl (perhaps created from chlorine gas) in the first step, creating a low pH, and leaving the pH low for a while, long enough to provide super-effective disinfection. Then in a second step, and NaOH to raise the pH to a level suitable for swimming. Adding HOCl and NaOH at the same time would be the same as adding NaOCl, but adding them at different times creates a transient that might serve a useful purpose.

I assume somebody has tried this, but I haven’t found any documentation. [Open question.]

According to rumor, hypochlorite is less chemically stable at low pH. [Need to obtain credible data on this. Might make a good low-tech literature-search project, or perhaps lab project.]

The amount of hypchlorite needed to control algae is a strong function of temperature. Algae grows better in warm water.

Cyanuric acid (CYA) is necessary to stabilize hypchlorite against the UV in sunlight.

Too much CYA lowers the amount of hypochlorite, by reversibly reacting with it, to form dichloro(iso)cyanuric acid. see reference 1.

Shocking the pool with dichlor or trichlor is virtually guaranteed to produce a “too-high” CYA level. Specifically, if you just follow the directions, you are likely to get a rebound from a super-high bleaching power (during the shock phase) to a too-low bleaching power (afterward), because the CYA hangs around longer than the hypochlorite.

To avoid this problem, assuming the pool has a reasonable amount of CYA, shock with something that doesn’t contain stabilizer, such as plain old NaOCl. Where I shop, the “chlorinating liquid” (NaOCl) is actually cheaper (per unit bleaching power) than stabilized pool-shock powder.

AFAICT, in the pool business, the term «free chlorine» is intended to refer to NaOCl or HOCl or OCl ... or more likely, the sum of all three.

AFAICT, in the pool business, the term «residual chlorine» means the same thing as «free chlorine»

AFAICT, in the pool business, the term «total chlorine» refers to the «free chlorine» plus various chloramines. The «total chlorine» serves as a measure of total disinfecting power.

AFAICT, in the pool business, at any pH that is not ridiculously high, the term «total alkalinity» refers to the amount of CO2. (To say the same thing the other way: You could have a high alkalinity due to the presence of a strong base such as NaOH, but that would show up as a high pH.) For the next level of detail, see item 17.

The concept of «acid demand» is similar to the concept of «total alkalinity», with a slight twist:

Beware that in chemistry, the concepts of “acidity” and “alkalinity” are not parallel concepts.

Suggestion: Use the term “total alkali” instead of “total aklalinity”. It’s non-standard, but everybody will understand.

Suggestion: Speak in terms of pH, rather than “acidity”.

As a specific example: Given a buffer containing lots of CO2, adding a modest amount of acid would lower the alkali but would not have much affect on the pH.

When you have CO2 dissolved in water, only about 0.15% of it reacts to form any kind of carbonate. Most of it just remains as dissolved molecular CO2. To say the same thing the other way, aqueous H2CO3 has a strong tendency to dissociate into water plus dissolved CO2. So-called carbonated water is primarily not carbonic acid; it is mostly just dissolved molecular CO2.

H2CO3  H2O+CO2               
(aq)     (aq)

Secondarily, whatever carbonic acid does exist immediately ionizes (under anything resembling normal swimming-pool conditions):

H2CO3  H++HCO3               
(aq)   (aq) (aq)

In other words, carbonic acid is a moderately strong acid. Carbonated water acts like a weak acid, but that is because of equation 1, not because of equation 2. For more on this, see reference 2.

Also because of equation 1, sodium carbonate (Na2CO3) is for practical purposes a very strong base. Adding sodium carbonate is essentially the same as adding a bunch of sodium hydroxide plus some dissolved CO2. Operationally, this is a way to raise the pH and raise the total alkali at the same time.

Na2CO3 + H2O  2 NaOH+CO2      
(aq)   (aq) (aq)

Operationally, adding sodium bicarbonate (NaHCO3) raises the total alkali (i.e. CO2) just as much, but raises the pH only half as much.

The action of hypochlorite on CYA will add lots of CO2 to the water. You might think that the loss of the CYA would raise the pH, but in the short run the opposite happens, because of all the CO2. See item 20.

The action of hypochlorite on suspended organic matter will add CO2 to the water. So if you have an algae bloom and kill it using hypchlorite, expect pH and total alkalinity problems to follow. See item 20.

It is entirely possible – and indeed reasonably common – to have simultaneously too low pH (i.e. too much acidity) and too much total alkalinity. Anything that adds dissolved CO2 will move things in that direction.

I’ve seen all kinds of chemical recipes for dealing with high «total alkalinity», none of which make sense to me. They seem like the sort of myths that would be spread by someone who is trying to sell you lots of pool chemicals.

AFAICT the right way to deal with this is aeration, at the appropriate pH. The equilibrium concentration of CO2 (for water in equilibrium with the atmosphere) is very low, so all you need is some sparging to speed up the equilibration. See reference 3.

It might take a day or two, starting after you have gotten the dead algae out of the pool, and out of the filter.

The smart way to do aeration is to inject tiny bubbles deep in the water column. Note that equilibration with respect to H2O occurs very quickly, whereas equilibration with respect to CO2 occurs more slowly (because of the wildly different concentrations), which is why deep injection is advantageous. Otherwise you evaporate more water than necessary.

You can get 1/4” tubing with lots of tiny prefabricated holes, intended for irrigating small gardens.

Covering the pool is the opposite of aeration. It interferes with natural gas exchange at the surface. This creates a dilemma, because:

The “OTO” used in liquid test kits is 2-tolidine, aka o-tolidine, aka ortho-tolidine. It provides information about «free chlorine» (hypochlorite) at short times, and information about «total chlorine» (hypochlorite plus chloramines) at longer times.

Reference 4 discusses this indicator reaction (including factors that interfere with its accuracy), plus other possible indicator reactions. From the introduction:

In the o-tolidine test, each of the following would exert an effect: concentration of chlorine, oxidation potential of the chlorine or chloramine, pH and HCl concentration, concentrations of manganese and iron and the degree of oxidation of each, concentration of nitrite, concentration of the reagent and reaction time.

OTO is toxic and presumably carcinogenic. The instructions say to dispose of it “safely” and suggest pouring it down the drain. [Multiple open questions.]

Is there any reason to believe that the waste-treatment process in your locality reliably detoxifies such substances?

What are you supposed to do if there is no drain near the pool?

Would it make sense to pour it onto paper towels in a paper cup, let it dry, and then throw the whole thing in the trash? (Based on the high MP and BP, I assume it’s not very volatile.)

Is there some way for the test-kit user to detoxify the OTO chemically?

I have some “five-way test strips” that test for «total chlorine», «free chlorine», pH, total alkalinity, and cyanuric acid. I also have a liquid test kit. Under routine conditions, I alternate between the two testing schemes. In troublesome ore confusing situations, I use both.

Note that there are some things you can do with a test kit that you can’t easily do with test strips, such as titrating for acid demand.

Your local pool-supply store probably has a fancy machine for testing water chemistry. They may provide free testing, perhaps limited to one or two tests per year per customer.

If you are getting inconsistent and/or confusing test results, this provides an additional vote. Also, it probably covers more variables than you cover at home.

They will want to know the volume of your pool, so figure that out before you go. Also remember to take a water sample.

Experience shows that test strips can give results that are significantly different from liquid test kits. It would be nice to figure out why. [Open question.]

Even if both tests give the same answer, I’m not convinced that it’s the right answer. There are a lot of things that could go wrong. See reference 4.

If the test strips and liquid test kit give different answers, it is entirely possible that both are wrong. It could be that some uncontrolled variable (e.g. iron ions, nitrate, or whatever) is affecting both tests, but affecting them differently.

For starters: I have no idea what the widely-used “test strips” actually test for. I have no idea what indicator reaction they use.

The «chlorine» test strips are evidently not based on OTO. There are separate tests for “free chlorine” and “total chlorine”. It would be nice to know what indicator reactions are being used. [Open question.]

I suspect that all sorts of non-chlorine compounds including ozone, peroxide, and various bromine compounds would show up as “chlorine”. [Checking this should be a relatively easy low-tech science project.]

I suspect that the indicator reactions are sensitive to temperature. A dry wind acting on the test strip could produce a temperature vastly lower than the actual pool temperature. So this might be a serious uncontrolled variable. [Open question.]

When reading liquid test kit, read it in transmitted light. That is to say, don’t shine a bright light on your side of the kit. Put a white background behind it, and illuminate the background. I have a piece of translucent white plastic – the lid from an old wide-mouth jar – which can be illuminated from behind. This works great.

Any professional chemist would know that the indicator chemicals are meant to be judged according to transmitted light, not reflected light, but the average homeowner doesn’t know this, and is at risk for seriously skewed and/or inconsistent readings.

Some test kit instructions hint at this, but some don’t.

Note that the reaction that produces hypochlorous acid from gaseous Cl2 is only 50% efficient:

Cl2 + HOH  HCl + HOCl

In particular, I have seen Cl2 referred to as «100% available chlorine» but that doesn’t make much sense to me.

A so-called «ppm» of «chlorine» or chloramine refers to the amount of monatomic chlorine (Cl), measured in mg/L, tied up in the hypochlorites and/or chloramines. It corresponds to a concentration of 28 micromolar for hypochlorite or monochloramine. I reckon it would be only 14 micromolar for dichloramine.

I seem to be off by a factor of 2 here. Perhaps the correct interpretation is that a ppm of «chlorine» i.e. hypochlorite refers to the amount of diatomic molecular chlorine (Cl2) needed to make that amount of hypochlorite. [Needs checking.]

CYA can be attacked by hypochlorite:


According to the US EPA,

According to rumor, some types of chlorine-resistant algae can be killed by chloramines. There are recipes [reference required] for creating chloramines for this purpose.

I reckon the real problem with chloramines is the tradeoff between disinfecting power and eye/skin irritation. With chlorine there is a good concentration window (high enough for disinfection yet low enough to avoid irritation) but for chloramines there is not. See reference 7.

Therefore I reckon that in the pool, any amount of chloramines short of the irritation threshold is not an emergency. I reckon monochloramine counts toward the disinfection requirement, equivalent to hypochlorite, on a mole-to-mole basis.

I don’t have sufficient facts to comment on dichloramine. [Open question.]

When you buy liquid «chlorine» i.e. hypochlorite from the store, beware that it loses potency over time. The concentration you get might be far lower than the concentration listed on the label.

Suggestion: At the very least, this product ought to have a sell-by date on the package. That wouldn’t solve the whole problem, but it would be a step in the right direction.

It would make an amusing low-tech science project to sample various brands and various distributors, to see how good their product is.

As a separate matter, the “most common” concentration for the muriatic acid of commerce is 34% HCl by weight; see reference 8. Most of the tables that tell you how much acid to use are based on this concentration. Recently, however, some pool-supply distributors have been selling more dilute forms. Sometimes it’s only a few percent weaker, but sometimes it’s a lot weaker. I’ve seen concentrations as low as 14%, which is less than half what it should be. They sell the adulterated stuff for the same price as the good stuff, which is quite a ripoff.

I’ve never understood the idea of trying to “burn out” chloramines by “superchlorinating” or “shocking” the pool, i.e. by adding huge amounts of hypochlorite. That seems like trading one problem for another. It seems like the sort of myth that would be spread by someone who was trying to sell you pool chemicals. Sure, the choramines cause irritation, but the amount of chlorine required for burn-out also causes irritation.

3  Other Pool-Care Issues

You really want an automatic scrubber that will remove the mud, krud, leaves, and debris from the bottom of the pool.

I’ve been happy with the Kreepy Krawly brand. It scrubs the sides as well as the bottom. It is highly effective with mud, krud, and dead algae that has settled out. It tends to pick up leaves eventually, but not very efficiently; it might take ten tries for any given leaf. Sometimes it chokes on twigs and stops working entirely, but usually it just ignores twigs, leaving them for you to pick up manually.

I’ve not been impressed by the other makes and models I’ve seen, although I have not done anything resembling a systematic survey.

You also need a manual sweeper that attaches to a long pole.

Rationale: There will always be some items that the automatic scrubber doesn’t pick up efficiently. However, these can be captured very conveniently using the manual sweeper.

Conversely, scrubbing the entire pool by hand would be unduly laborious.

You really want a skimmer, so you can selectively remove the surface layer. For a typical built-in pool, this function is performed by a weir gate. For above-ground pools, there are self-contained skimmers that you can plumb into the filtration-system intake.

Rationale: If you can skim off debris before it gets waterlogged and sinks to the bottom, you’re way ahead of the game. Tannin from leaves and bits of bark sitting on the bottom will stain the pool. The stain fades over time, but it is better to prevent the stain entirely.

Sometimes the surface layer collects a thin layer of oil. Some of this comes from sunscreen lotion. Some of it comes from natural skin oils. Some of it comes from plant material that gets blown in.

The skimmer is very effective at removing the oil, but I have to wonder, what happens after that? Does the oil gum up the sand in the filter? Does the oil get removed when the filter is backwashed? Does the oil eventually break down chemically? [Open question.]

I cobbled up a Venturi device that threads onto the return port of the filter system. I use it to suck liquid out of chemical bottles. It takes about half an hour to empty a 1-gallon bottle. This process ensures that the chemical is diluted by a factor of several hundred before it enters the pool. This is probably more important when adding acid than when adding hypochlorite, but I use it for everything because it is super-convenient.

The only drawback is that it sticks into the pool, and the automatic sweeper occasionally gets snagged on it. This should be fixable, but I haven’t quite managed to figure it out yet.

Do not allow water to get trapped anywhere. The trapped water is cut off from supplies of «chlorine», so algae can grow wild. It takes a lot more chemicals to kill algae after it is grown than to keep it from growing in the first place.

When a pool cover gets old, the plastic bubbles that provide floation start to fail. Water can get trapped in the bubbles. Also, if there is not sufficient floation, water can get trapped on top of the cover.

Water polishing: I find sand filters to be convenient, economical, and effective. The main limitation is that they are not always very efficient at removing the smallest particles.

One option is to just wait. If the pool is cloudy due to micron-sized shreds of cellulose from dead algae, if you wait long enough, they will disintegrate and/or get oxidized by the «chlorine». It is also possible that they will clump together, settle to the bottom, and get swept up. The sand filter will catch tiny particles eventually.

The sand filter is better at catching small particles when it is somewhat dirty. The dirt in the filter reduces the pore size. Often this happens naturally.

If you are in a hurry, you can speed up the process. Suppose the filter has been running for a few days with minimal back-pressure, yet the pool is less than sparkling clear. You can use the following trick: Add a tiny amount of diatomaceous earth to the filter. Ten or twenty milliliters should be enough. If you see an immediate rise in back-pressure, you added more than enough, so make a note of this and use less next time.

The downside of adding DE is that you will have to back-wash the filter sooner than you otherwise would. The upside is that the pores in the diatomaceous earth catch very small particles.

4  References

Janet Grice,
“Introduction to Pool Chemistry” [.doc file]

Stephen K. Lower,
“Carbonate equilibria in natural waters”

Ben Powell,
“Lowering Swimming Pool Alkalinity – A Step by Step Guide”

D. Tarvin, H. R. Todd, A. M. Buswell,
“The Determination Of Free Chlorine” (1935)

“How effective is monochloramine vs. chlorine as a primary disinfectant?”

“Basic Information about Disinfectants in Drinking Water:
Chloramine, Chlorine and Chlorine Dioxide”

Canada NCCEH
“Pool Chlorination and Closure Guidelines” http://ncceh.ca/en/professional_development/practice_questions/pool_closure_guidelines

Discussion of muriatic acid

Doug De La Matter,
“Swimming Pool Chemistry” (theory)

Doug De La Matter,
“Swimming Pool Chemistry : Student Activities” http://www.dougdelamatter.com/website1/science/chemistry/pool/student.pdf

“Chloramines in Drinking Water”

Aaron Askins,
“Cyanuric Acid in Commercial Swimming Pools and its Effects on Chlorine’s ‘Staying Power’ And Oxidation Reduction Potentials”

Dana William Somesla,
“Method and kit for reducing cyanuric acid levels in pool water”

John A. Wojtowicz,
“Oxidation of Cyanuric Acid with Hypochlorite”

“BiOWiSH Bio-Active Cyanuric Acid Decreaser”
Copyright © 2014 jsd