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
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.
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.
- Among other things, you care about disinfecting power and
- which might or might not be provided by bleaching power
- which might or might not be provided by hypochlorite,
- which might or might not have been created by
bubbling real chlorine (Cl2) through the water.
In more detail:
- Chloramines provide disinfecting power but not bleaching power.
So testing for bleaching power is not the same as testing for
- On the other hand, chloramines cause more eye and skin
irritation (compared to comparable levels of hypochlorite), so
testing for bleaching power and/or disinfecting power doesn’t tell
the whole story.
- Chemicals such as poly-quat
i.e. poly[oxyethylene(dimethyliminio)ethylene(dimethyliminio) ethylene
dichloride] provide algae control but neither bleaching power nor
protection against pathogens.
- HOBr provides bleaching power without chlorine.
- Ozone and/or peroxides provide bleaching power without
halogens of any kind.
In this document, I use the word “hypochlorite” broadly. It
is meant to include
- OCl− (the hypochlorite ion),
- HOCl (hydrogen hypochlorite, aka hypochlorous acid),
- NaOCl (sodium hypochlorite),
- all other salts of hypochlorous acid.
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
The amount of hypchlorite needed to control algae
is a strong function of temperature. Algae grows better in warm
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
- What is the equilibrium constant for this reaction?
- Does it depend on pH?
- Under what conditions is the reaction reversible? Are there
any conditions where the dichloro(iso)cyanuric acid gives up its
chlorine in a useful way?
- Is the chlorinated form equally effective as a sunscreen?
- Does the chlorinated form contribute at all to the
bleaching power? (I assume not.)
- Does the chlorinated form contribute at all to the disinfecting
power? (I assume not.) This matters operationally, because it
determines to what extent you need to add extra hypochlorite.
- Does the test-strip for «total chlorine» include the
Cl that is bound to CYA? This matters operationally, as follows:
Presumably if you have a “normal” amount of CYA, you can get
by with a “normal” reading on the test for «chlorine».
On the other hand, if the CYA level is too high, what should
you do? Do you need to add additional hypochlorite, or not?
- Similarly, does the liquid test for «total chlorine» include
the Cl that is bound to CYA?
- AFAICT, there is nothing you can do to lower the CYA value
except wait. Don’t add any more CYA. Switch to using unstabilized
«chlorine» and just wait. The CYA level will drift down over the
course of a few days.
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
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
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
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:
- «Acid demand» tells you how much acid you need to add to
lower the pH to a reasonable value for swimming. (You do not
need or want to destroy all the CO2 in the pool.)
- «Total alkalinity» tells you how much acid you would need to
add, hypothetically, to lower the pH to some very low value. This
includes destroying all the CO2.
Beware that in chemistry, the concepts of
“acidity” and “alkalinity” are not parallel concepts.
- Acidity depends on the amount of active, dissociated acid.
For a weak acid, this could be much lower than the titer of acid.
It is measured in terms of pH, specifically the lowness of the pH.
- Alkalinity refers to the titer of alkali. For a weak base,
this could be much higher than the amount of active, dissociated
alkali. It is absolutely not measured in terms of pH. It is
measured by titration.
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
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
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:
- There are lots of good reasons for
keeping the pool covered: to conserve water, to conserve heat, to
keep out dirt and debris, et cetera.
- On the other hand, sometimes keeping the pool covered leads to
unacceptably high concentrations of CO2, as indicated by high
total alkalinity and low pH.
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
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
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
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:
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
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,
- Chlorine is very effective at killing most potentially harmful
organisms, quickly (reference 5). (I don’t know
- Monochloramine can be used as a primary disinfectant but the
amount of time needed for treatment makes it impractical for most
utilities ... but because it is longer lasting than chlorine, it is
often used as a secondary disinfectant (reference 5).
- 4ppm of «chlorine» is comparable to 4ppm of monochloramine.
According to rumor, some types of
chlorine-resistant algae can be killed by chloramines. There are
recipes [reference required] for creating chloramines for this
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
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.
- By far the best way to achieve a low concentration of
chloramines is to persuade people to not urinate in your pool.
- If the chloramine concentration is high enough to produce a
perceptible odor, people may imagine that the pool has “too much
chlorine” ... but that is not an accurate perception. Hypochlorite
has no smell, so a pool with a chloramine odor might have too little
hypochlorite, or too much, or anywhere in between.
People are correct to associate a strong chloramine odor with eye and
skin irritation. However, chloramine is detectible by its odor at
concentrations far below that at which it produces serious
If you add enough hypochlorite to burn out the chloramines, you can
get rid of the smell, but then, ironically, you do have too
- AFAICT, for a smallish concentration of chloramines, up to half
a ppm, or maybe a whole ppm, the best and easiest thing is to just
- For higher concentrations, my limited and unscientific
experience suggests that chloramines can be removed directly via
aeration, and this is cheaper, quicker, and easier than using huge
amounts of chlorine. [Open question, needs more investigation.]
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
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
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
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
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
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
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.