Pool and Spa Water Chemistry
Back to the Table of
The next section describes the Advanced Pool
and Spa Water Chemistry procedures to keep clean and balanced water.
Treatment - It's Not Purely Physical
Because a swimming pool system begins as a
hole in the ground, without any water movement, it is an artificial
structure. Consequently, it lacks the three purifiers that protect
water quality in natural bodies of water:
- Aeration - This is the addition of oxygen
to the water, and results from the continual flow of water
through lakes, streams and rivers.
- Dilution of sediment - This is exactly what
it sounds like, and also results from continual water flow.
- Prevention of contaminant buildup - This
happens when water flow causes movement and dilution, while
aquatic organisms contribute to a process of biodegradation.
Since any pool lacks these purifiers, it’s
subject to rapid stagnation. In addition, it’s also generally
contaminated with bacteria, algae, and organic materials from
swimmers wastes. (Did you know that an active bather produces up to
two pints of perspiration per hour?) Add dust and dirt, and you get
plenty of ways for a pool to go bad.
The world’s fanciest physical treatment
facilities alone won’t keep a pool from getting gunked up. So let’s
look at the obvious: a swimming pool must be disinfected.
These disinfectants must remove bacteria,
algae and organic contaminants. They have to improve water clarity
and color. And they have to work together as a total system, so
everything functions well.
Enter the hero. Namely , chemicals. More
specifically, chlorine. Most pools in the U.S. use it in one form or
Why? Well, when chlorine compounds are
dissolved in water, hypochlorous acid is formed. (Remember that
name-it does the actual sanitizing.)
In most cases, the non-chlorine part of the
chlorine compound serves no other purpose than to hold the chlorine
until the product dissolves. You’ll notice we said in most cases.
Lets get more specific. There are three
categories of sanitizers:
- Chlorine gas at 100% available chlorine.
- Inorganic hypochlorites such as calcium
hypochlorite(now marketed at 65% available chlorine) and sodium
hypochlorite (liquid bleach, at 10-15% available chlorine).
- And finally, the chlorinated isocyanurates
like granular (at 56% available chlorine) and tablets (at 90%
Secondary chemical treatment with substances
that control pH and buffer the pool, is also needed for healthful
swimming. Depending on pool conditions, secondary treatment might
include decolorizers, additional algaecide, and chemicals to adjust
mineral levels (hardness) or retard evaporation.
You Use is What You Get
We can begin by saying that all three
sanitizers are compatible and effective with other chemicals n pool
water. (Let’s be fair.) Each will perform its function without
causing objectionable tastes, odors or colors in the water - if
- CHLORINE GAS… reacts with water to form
hypochlorous acid, the active sanitizing species, plus hydrogen
ion (H+) and chloride ion (Cl -). Hypochlouous acid exists in a
pH dependent equilibrium with hypochlorite ion (OCI -) in pool
Now let’s look at the nuts - and -
bolts. Chlorine’s major advantage is its relative low cost.
Many large pools and most bulk drinking and waste water
treatment systems use it for this reason.
Chlorine also has several disadvantages.
It’s a gas that must be delivered in bulky metal cylinders. It
has to be applied to the water through sophisticated metering
systems operated by trained personnel. And it’s highly
corrosive, toxic, and very acidic - due to the H+ and CI - (muriatic
acid), the byproduct of its reaction with water.
Most regulated pools are required to
install separate feeding equipment to add approximately 1.25
pounds of soda ash to neutralize the acidity of one pound of
If chlorine gas were the only chemical
available to disinfect water, it’s very likely that there
would be no home swimming pools. Probably no swimming pool
industry, either. The complexities and the hazards of this
chemical are simply beyond the capabilities of the average pool
Fortunately, more convenient methods of
chlorinating pool water have been developed. We move on.
- THE INORGANIC HYPOCHLORITES… also react
with pool water to produce hypochlorous acid, the major
disinfectant in chlorine. The hypochlorites are available as the
granular calcium hypochlorite (@65% av. Cl.) and liquid sodium
hypochlorite (NaOCI) (@ 9-15% av. Cl.)
Both forms can easily be added in the
proper dosage for virtually any size pool. They’re relatively
inexpensive. However, both forms have a high pH, and may require
frequent additions of acid to maintain pool water in the proper
pH range for chlorine efficiency, equipment longevity and bather
comfort. Otherwise they tend to eat up pools… a big problem.
Calcium hypochlorite, with a pH of 10-12,
also contains 5-8% of insoluble material, which can cause cloudy
water. A by-product of this reaction is the calcium ion (Ca++),
a major component of water hardness, and a contributor to
scaling tendencies in the pool.
Sodium hypochlorite, with a pH of 12-14 is
relatively low in available chlorine concentration. Therefore,
more is required to maintain the disinfectant residual in a
pool. Because of the bulk of its liquid form and its poor
storage stability, it must be purchased frequently throughout
the pool season. Although it does not add hardness to the pool,
its high pH can contribute to scaling tendencies in hard water
Neither product provides protection
against the destructive effects of sunlight on a chlorine
residual. That’s why more frequent chemical additions and
adjustments are necessary to maintain satisfactory water
And so, friends, we move on to number
- THE POOL CHLORINATING CONCENTRATES…
provide the effectiveness of HOCI, the ease and convenience of
concentrated solids, and the benefits of stabilization, to
provide outstanding water quality with minimum effort and
expense. In case you didn’t know. Actually, granular reacts
with water to produce the same active sanitizing species,
hypochlorous acid. Therefore, it is an effective bactericide and
algaecide that will oxidize organic contaminants. Tablets react
similarly, but produce three units of hypochlorous acid.
Both the granular and tablets have a
by-product known as cyanuric acid (remember that name!) which is the
only known chemical that stabilizes a free chlorine residual without
interfering with its sanitizing effectiveness. (Don’t panic, we’ll
talk about free chlorine later. Suffice it is to say that free
chlorine is the amount of chlorine available to do its job of
sanitizing the water.)
A look at the physical and chemical properties
of the pool chlorinating concentrates will help to explain their
Both the granular and the tablets are based on
cyanuric acid, the central structure composed of alternating carbon
and nitrogen atoms.
- In the granular two atoms of chlorine are
added, giving an available chlorine of 56%. Because this is a
sodium salt, it has excellent solubility @ 26.1% and a nearly
neutral pH of 6.7. It may be added directly to the pool by hand
broadcasting. Or, it can be pre-dissolved and added as a
- The tablet contains three atoms of
chlorine, giving it 90% av. Cl. It has a relatively low pH of 2
-3. Because of its high available chlorine content, much less
needs to be added. So it, too, has a minimal impact on pH. Its
low solubility of 1.2% makes it an ideal candidate for
tabletting and use in continuous feeding systems.
We strongly recommend that this trichloro
product not be added directly to the pool in either its tabletted or
granular form. Its high available chlorine, slow solubility and
acidic pH give it the potential to bleach, etch or pit any pool
surfaces that it contacts.
Both products are free of insoluble residues,
produce a minimal impact on pH and do not contribute to water
hardness or scaling. After the chlorine has been consumed in
performing the sanitizing functions, the cyanuric acid remains
dissolved in the water to provide maximum stabilization for the free
Exactly what is chlorine residual? Good
When chlorine is added to water in a newly
filled pool, some of it is consumed in the process of destroying
algae, bacteria and other oxidizible material in the water.
The amount of chlorine consumed in this
process is referred to as the chlorine demand of the water.
Once the chlorine demand is satisfied, any
additional chlorine added is referred to as chlorine residual. Not
too complicated, eh?
Two types of chlorine residual can exist:
- Free chlorine residual-this is a chlorine
available to do its job of sanitizing the water.
- Combined chlorine residual-this is chlorine
combined with simple nitrogen compounds such as ammonia and
urea. This chlorine (or chloramine) is non-effective as a
sanitizer compared to free available chlorine.
It’s essential to maintain a free-chlorine
residual at all times to achieve sparkling clear, sanitary pool
water. This is accomplished by periodic superchlorination of the
pool water. (More on that later). Superchlorination consists of
simply adding a larger than normal dosage of chlorine to burn out
When chloramines are removed, better
efficiency of chlorine is achieved. More of the chlorine residual
can then exist as the free or active form, rather than as the less
effective combined form.
Alright, we’ve talked about stabilization.
What is it? Also a good question.
By now, you should know that cyanuric acid
reduces chlorine consumption. The benefits of using it for
stabilization have been established and repeatedly verified since
the concept was patented by FMC Corp. That’s right patented! The
biggest problem with chlorine is that it breaks down and dissipates
very easily under the sun’s radiation. In 1958, FMC Corp.
conceived, developed and patented a way of stabilizing pool chlorine
by using cyanuric acid. Numerous field tests and evaluations have
demonstrated that the addition of a minimum of 30 parts per million
of cyanuric acid to swimming pool water prevents wasteful
destruction of the free chlorine residual by sunlight. In the
stabilization process, a portion of the chlorine residual is
temporarily bonded to the cyanuric acid molecule. Consequently, it’s
protected from the destructive effects of sunlight. The nature of
this bond is such that the chlorine demand is imposed upon the
system and continues to be released as long as a demand exists. The
role of chlorine in pool water sanitizing and the process of
stabilization can be better understood when considered as part of a
generalized pool chemical reaction. In swimming pool waters, the
free chlorine - HOCI - may be consumed in several ways:
- By destroying bacteria and algae introduced
by swimmers and by wind and rain-borne contamination.
- By reacting with reduced metals such as
Fe++ to produce the oxidized Fe+++ and chlorine ions.
- By the action of the ultraviolet energy of
sunlight, which converts free available chlorine to the inactive
- By oxidizing nitrogenous compounds such as
ammonia (NH 3) and urea introduced into the water as components
of perspiration, urine and other bodily excretions.
of Free Available Chlorine Residual
Recommended level: 1 ppm to 3 ppm of free
Ok, we’ve heard about sanitizers. They sound
good. But how are you convinced? Simple. By this side-by-side test….
Pre-conditioning the pool with 30ppm of
conditioner, and using a sanitizer to maintain that minimum,
automatically provides protection for a pool against the effects of
sunlight. The benefits of this system of pool treatment are best
illustrated in comparative tests of chemical consumption. In this
test, the effects of sunlight on pools treated once a day is
- Identical 5000 gallon pools, located side
by side to eliminate variations in climatic exposure, were
monitored over a period of several days.
- In an unstabilized pool, an inorganic
hypochlorite was added at the label directed dose of 4 ppm FAC
(84gm/3 oz.). The other pool was stabilized and treated with
stabilized granular (NaDDC*2H20) at 1ppm residual, so ½ of the
original dose of the product was added.
- 24 hours later there was no residual in the
hypochlorite pool. So, a double dose of product was added. The
stabilized pool still contained the desired 1ppm residual, so ½
of the original dose of the product was added.
- This procedure was repeated for 4 days. At
the end of the test, the hypochlorite pool, even with 16ppm (336
gm/12 oz.) of product could not hold a residual over one day.
The stabilized pool, with less than 1/3 ppm (7 gm/0.2 oz), still
maintained its residual.
These figures were obtained with no bathers
using the pools. The chlorine from the unstabilized pool was
consumed strictly by the action of sunlight.
Rate For Conditioner
Recommended maintenance level: 30 ppm minimum
During a pool season, the conditioner level in
the water may decrease from leakage, bather activity causing
splash-out or drag out, and normal maintenance operations such as
vacuuming and filter back washing.
To maximize the advantages of stabilization,
the following quantities of conditioner should be added to maintain
a 30 ppm minimum level. Because of the slightly acidic pH of
conditioner, addition of a small quantity of pH plus may be
necessary to maintain pH at the desired level.
4.2 ounces per thousand to obtain 30 ppm
1 ounce per thousand = 7 ppm
Should the conditioner concentration increase
to a regulatory agency mandated maximum level, it can be lowered by
dilution with fresh make-up water.
ONE IN A MILLION
Operating a pool is economical primarily
because of the efficiency and versatility of free available
chlorine. This disinfectant performs all the functions necessary to
maintain sanitary, high quality pool water. It does this in part per
million (ppm) concentrations - which is an extremely small quantity
in relation to the results provided. When we say small, we mean it.
A ppm is actually equivalent to: parts per million a unit of
concentration equivalent to:
1 inch in 16 miles 1 minute in 2 years 1
needle in 2000 pound haystack and 1 penny in 10,000 dollars.
To understand the economy this provides,
consider the amounts of chlorine that’d be required to operate a
pool if free available chlorine were only available at percent
concentrations. A percent is 10,000 greater than a ppm. The truth
is, there’d be very few pools if 1% or even 0.1% FAC, rather than
1 ppm, was necessary for successful pool operation. Speaking of
success, it’s time to meet a good friend of yours.
- One of the Best Salesmen
Algae is the most common fouler of pools. When
pool owners panic and run screaming to a pool pro for help, algae is
most frequently the cause. That’s why we call it one of the best
salesmen. You must deal with an algaecide. Actually, in addition to
getting rid of algae, one of the most effective uses of algaecide is
to extend the effectiveness of chlorine residual. While chlorine is
an algaecide, it’s wise to add additional quantities as a backup,
a maintenance dose.
Algae are small plants that propagate by
air-borne spores. They enter swimming pools and quickly turn the
water green. When conditions favor their growth, they can cause
black and/or green spots on pool walls. Heavy rain, intense
sunlight, and presence of nitrogenous material all contribute to
algae bloom - as the rapid growth of algae is called. Sometimes
algae bloom results in a sharp rise in pH, as the algae consume
carbon dioxide in the pool water. If algae bloom is present, super
chlorination should be used. Then an algaecide will control it, and
prevent its reoccurrence. The best insurance against algae? Maintain
a free chlorine residual in the pool at all times. An effective way
to do this is to sanitize with stabilized pool chlorinating
concentrates, and add algaecide according to the directions on the
- It Only Looks Simple
In addition to chlorine sanitizing, there are
other water factors to consider. The most common are:
- pH- a system for measuring the acidity or
alkalinity of water. Readings above 7 are alkaline: readings
below 7 are acidic.
- Total alkalinity- a measure of the
buffering capacity of pool water.
- The amount of calcium or magnesium
dissolved in pool water. High levels contribute to scale
Let’s take a closer look at these elements.
pH. Many things can change the pH of pool
water from the desired range of 7.2 - 7.8. Rain, dust and bather
wastes can all raise or lower pH and necessitate frequent testing.
Even more significant is the pH of the sanitizer used. Obviously, a
sanitizer that has a pH near the desired range will be far more
effective and convenient to use. stabilized granular, with a pH of 6
- 7, will certainly have very little effect on the pH of the pool.
Consequently, the residual it provides will be more effective with
fewer adjustments. Stabilized tablets, although low in pH, also have
a minimal effect. That’s because their high available chlorine and
stabilization means far less is needed to provide effective
sanitation. Chlorine gas, calcium hypochlorite, and liquid bleach
all require much larger amounts of pH adjustment, because their pH
is far from the desired operating range. Pool owners who insist on
using these sanitizers should be advised of the need for pH
adjusting chemicals and testing, as their pH is far from the desired
operating range. They should also be advised that pH plus and pH
minus are excellent products for raising or lowering pool water pH
to the proper range. Recommended pH range is 7.2 - 7.8 and must be
maintained within the indicated range because of its impact on
chlorine efficiency, bather comfort, corrosion and scaling. Since
total alkalinity affects the amount of pH adjusting chemical which
must be added, it should first be adjusted to the 80-125 ppm range.
TOTAL ALKALINITY. Control of pH can be
simplified by maintaining total alkalinity in the range of 80- 125.
Total alkalinity is composed of carbonates, bicarbonates, and
hydroxides, and functions as a buffer to help keep pH in the proper
* If total alkalinity is too high, you have
staining, scale and difficulty in adjusting pH.
* When total alkalinity is too low, there’s
corrosion and pH bounce.
Total alkalinity is easily measured with a
test kit, and can be adjusted with alkalinity control or acid
according to label directions. Total alkalinity, in addition to
buffering, provides some control over the corrosive or scaling
tendencies of the water. The recommended total alkalinity level
should be 80-150ppm. Total alkalinity is a measure of the buffering
capacity of pool water. If total alkalinity is too low, pH bounce
and erratic behavior are encountered. Total alkalinity can be raised
by the addition of alkalinity control. If total alkalinity is too
high, it becomes difficult to adjust pH. Also, staining and scaling
may occur. To reduce a high total alkalinity to 80ppm, the granular
addition of pH minus or muriatic acid (not more than 1 pint at a
WATER HARDNESS. Another factor in pool water
problems is hardness, a measure of the calcium and magnesium content
of the water. All water contains some natural hardness. The amount
will vary regionally, and from source to source within a region. A
certain amount is necessary in water to control its tendency to
dissolve. If too little is present in a pool, the water will attack
the materials of construction to satisfy its appetite. Hardness
treatment will increase low water hardness and prevent etching,
pitting and corrosion of surfaces and metallic components in such a
situation. Scaling occurs if too much hardness is present. This is
visible as crusty gray deposits and cloudy water, or visible as
deposits in piping. A pool’s pipes, designed to accommodate a
certain water flow at a certain pressure, will obviously not
function properly if their diameters are decreased by scale
formation on their inner surface (white rings inside the pipe). It’s
a bit like a pool’s hardening of the arteries. Scale on pool
surfaces is unsightly and unattractive. Scale in pool plumbing is
disastrous and expensive, since it interferes with the circulation
and filtration of the pool water.
YOU ARE WHAT YOU EAT: A LOOK AT CHLORINE
As you know, chlorine is very rapidly consumed
by the action of sunlight in a swimming pool. Andy chlorinated
sanitizer will produce HOCI, but up to 97% of that residual can be
lost in 2 hours! What does this mean in actual pool operation? Well,
when 4-5ppm of free available chlorine is added as calcium
hypochlorite at 6 am, it’ll be completely gone by 12 noon. Whether
or not anyone uses the pool. This same wasteful chlorine consumption
occurs with chlorine gas and sodium hypochlorite. For the bulk of
their swimming time, Mr. Or Mrs. Pool owner really have no assurance
that the pool is sanitary, or that contamination will be rapidly
eliminated as it enters the pool. What to do? The conscientious pool
owner, aware of the importance of the free available chlorine
residual, could operate his pool satisfactorily if he tested the
water at noon, and added another 4-5 ppm residual for safe afternoon
swimming. He’d then have to repeat the process at 6 pm so the
entire family could enjoy the pool in the evening. Three times a
day. Whew! Or, he could install a bulky and complex chemical feeding
system to constantly add sanitizer. He would also probably need a
separate feeder to add a pH adjusting chemical, while large
quantities of the unstabilized sanitizer are being added. Sound
unwieldy? It is. And there’s a better way. You guessed it. Our
pool owner could use a sanitizer, add the label-directed 1-1.5 ppm
of free available chlorine, and be assured that it would keep the
pool water clear and sparkling.
Water And TDS
TDS is not an abbreviation for "Tough day
on Saturday", or "Take a dip Steve". It stands for
Total Dissolved Solids. TDS are a measure of all the dissolved
chemicals in the water. Whether they’re natural components of
source water, residues of treatment chemicals, bathers’ wastes, or
wind and rain-borne atmospheric pollutants, they stay in the water
and concentrate. Eventually, TDS will cause staining, scaling,
reduced chlorine efficiency, and erratic pool behavior. All pool
water contains total dissolved solids. If a drop of TDS water could
be magnified, it might show Ca (calcium) and Na (sodium),
representing dissolved chemicals. Although a dissolved chemical is
not visible to the naked eye, it does occupy space in the water.
Take table salt, for instance. It’s visible in the shaker and
invisible in a water solution - but it reappears if the water is
boiled away. It hasn’t disappeared, it’s just been dispersed as
submicroscopic particles called ions. In ideally stabilized water,
HOCI has a direct route to reach algae, bacteria and germs that it
must destroy to provide sanitary water. The ions of the dissolved
solids are widely dispersed, and don’t hinder this action.
Unfortunately, water doesn’t remain in this ideal condition for
very long. Even the residues of sanitizers consumed in the various
categories of the California field test produced substantial amounts
of dissolved solids during one season of treatment. And these TDS
accumulations are the chemical residues from sanitizer treatment
only. Other materials add even more dissolved materials.
It’s interesting to note that stabilizing
and using chloroisocyanurate produces the least amount of TDS. When
more chemicals are required, more residues result. Eventually, you
get microscopic alphabet soup. Several seasons of adding chemicals
and bathers to water that’s already exposed to a variety of
natural contaminants can create a very crowded body of water.
Obviously, as this alphabet soup gets more crowded it’s difficult
for HOCI to perform efficiently. And algae, bacteria and germs that
are not eliminated will cause problems. It’s difficult to predict
exactly how fast TDS ill accumulate, and at what concentration they’ll
cause trouble. But it’s been estimated by the NSPI chemical
treatment and process committee that TDS should be maintained at
less than 1500 ppm. Concentrations in excess of that may cause
NOTE: When problems with TDS occur, untrained
home owners and overworked service people often find a convenient
culprit in overstabilization. (There’s really no such thing.)
Cyanuric acid is easy to measure, and gives a test result that can
be interpreted individually, to explain almost every water quality
problem. A cloudy, green pool will almost certainly have one or two
ppm in excess of some arbitrary maximum. The problem pool water will
be dumped to lower the cyanuric acid content, and the problem will
WHAT IS THE PROBLEM, ANYWAY?
What isn’t usually recognized is that any
dissolved solids, chloramines, or pH buffers will be correspondingly
reduced when a pool is dumped. It’s impossible to remove only one
kind of dissolved material in water that’s all gone. Total
dissolved solids have not received the attention they deserve for
causing pool problems, because they aren’t easy to measure - and
there’s a tendency to forget they’re in the water. Additionally,
they’ve never received the kind of study and publicity that have
surrounded cyanuric products. It has been proven that cyanuric acid
causes no ill effects in pool water… but that it may, by it’s
accumulation, signal the onset of problems due to TDS in the pool.
Apparently this proof has been convincing, since the leading
proponent of the overstabilization theory has built a cyanurate
LESS IS MORE.
The fact is, there’s only one practical way
to remove dissolved solids from a pool. That’s to remove a portion
of the water in which they’re dissolved. Removal of 100 gallons of
water removes 1.7 pounds of dissolved solids from a 10,000 gallon
pool containing 2,000 ppm TDS. If the cost for control of TDS
accumulation is calculated, the partial water removal procedure
becomes the ultimate bargain in pool operation.
- The recommended rate of water removal per
week is 1-3%.
- In a 10,000 gallon pool, this represents
100 gallons per week, or 4,000 gallons in a ten month season.
- At .53 per 1000 gallons, this would cost
$2.12. Since many municipalities levy a sewage charge of 120% of
the cost of the water, an additional $2.54 is added.
- The total cost for replacing 40% of the
used water in the pool is $4.66.
If this cost seems excessive, compare it to
the initial investment in the pool, the cost of chemicals that don’t
perform efficiently, and the expense of dumping, acid washing,
refilling and balancing. You’ll find that water is the cheapest
chemical that can be added to a pool.
Since all chlorinated sanitizers react with
water to produce HOCI, chlorine consumption depends on the amount of
contamination that is present - not the brand that’s used. Enough
sanitizer must be added to meet the chlorine demand of the water
before a measurable residual can be maintained. This amount depends
on the amount of contamination present in make up water, plus
whatever is added by bather loading, rain, dust and other external
sources. One particularly troublesome type of contamination is
nitrogenous wastes from bathers bodies. Whether they are as simple
as ammonia in urine, or as complex as the components in perspiration
or saliva, they present special problems when they accumulate in
pool water. These contaminants react with HOCI to form compounds
called chloramines, or combined chlorine.
The combined chlorine reaction begins with one
unit of ammonia, combining with one unit of HOCI to form
monochloramine (NH2CI). This reacts with another unit of HOCI to
form dichloramine and finally with a third unit of HOCI to produce
trichloramine (NCI3). We’re not through yet. It takes a fourth
unit of HOCI to finally convert the original molecule of ammonia
into harmless nitrogen gas (N2), water and chloride ion (CI-) and a
fifth unit of HOCI before a free available chlorine residual can be
measured. These chloramines cause plenty of trouble in pool water.
Why? Because they are stable and persistent. The monodi and
trichloramine from this first unit of ammonia will survive and
accumulate with the chloramines formed from subsequent units of
ammonia. This is actually chlorine consumption, because HOCI
combined with ammonia forms chloramines. Chloramines have very poor
sanitizing power, so algae and bacteria can grow. In fact, they have
such poor pool sanitizing power that they would be rated at only
0-10 on a relative activity scale with HOCI rated at 10,000. Quite a
difference. It’s been estimated that chloramines could provide
germ fee water if they were present at a concentration of at least
25 - 50 ppm. But this would create additional problems in a swimming
pool, because they’re very pungent and irritating, causing eye
irritation and chlorine odors at very low concentrations.
OTO Method of Determining Chlorine Levels
For many years, the most common test for
residual chlorine has been the OTO method. It is based on the fact
that a clear, organic solution called orthotolidine (OTO) turns
yellow in the presence of free or combined chlorine. Increasingly
greater concentrations heighten color intensity until a deep orange
or red color is reached at extremely high chlorine levels. OTO test
results do not successfully differentiate between FAC or combined
OTO testing, while still available in the
U.S., is regarded by authorities as out-of-date or inferior to DPD
testing. In many public health jurisdictions, the DPD test has been
This OTO method has some
advantages that have made it popular and widely used. It also has
some major deficiencies. The fact that it cannot easily distinguish
free from combined chlorine makes it a very doubtful aid to pool
operation. Even worse, it creates a false sense of security, leading
to erroneous diagnosis of pool problems, which delays remedial
action. The pool owner could test once, twice, or three times daily
and still have no idea whether enough free available chlorine was
present to protect the quality of the water. A 1 ppm residual
measured by OTO will provide far less protection to pool than a 1
ppm residual measured by the DPD method.
DPD Methods of Determining Chlorine Levels
The DPD method uses standardized liquid or
tablet reagents containing diethyl-p phenylenediamine indicators.
The advantage of DPD testing is that it can be used to differentiate
between free and combined chlorine. DPD tablet reagents are highly
stable and easy to work with. They come in foil packets for easy
handling and have an extended shelf life. DPD reagents produce a
pink-to-red color change that is easy to read.
Separate DPD tablets or liquids are used to
test for residual chlorine in its various forms: Free available
chlorine (FAC), using DPD tablet #1 or liquid reagents #1 and #2;
total chlorine, using tablets #1 and #3 or liquids #1, #2 and #3;
and combined chlorine or chloramine, by subtracting the free reading
from the total. Other tablet numbers are normally not used in pools.
No. 2 is used for monochloramine, and #4 is for total chlorine only.
Free Available Chlorine (FAC) Test - Using DPD
- Wash the test vial with pool water. Wash
stirring rod if used.
- Fill the vial with pool water to the
graduated line on the side of the tube.
- Add one DPD tablet #1.
- Cap the top of the vial (not with fingers).
Shake well or use stirring rod to dissolve the tablet. Make sure
it is dissolved, although some residual on the bottom is normal.
- Quickly compare the resulting color with
the test-kit standards to determine the free available chlorine
Test kits with precise liquid standards,
closely graduated, are available. Some kits use clear water vials
next to the test vial to make the optical light paths similar when
judging color standards.
Total Chlorine Test Using DPD Tablets
- Follow steps 1 through 5 from the above for
the free available chlorine DPD test.
- Add one DPD tablet #3 to the FAC test
- Cap the top of the sample cell and shake or
use the rod until the tablet dissolves.
- Compare the resulting color with the
test-kit standards to determine the level of total chlorine.
Combined Chlorine Calculation
Calculate the combined chlorine (chloramine)
level by subtracting the free available chlorine level from the
total chlorine level.
Often a test sample of free available chlorine
(FAC) reads at the upper limit of the test kit's color comparator.
This sample should be regarded as possibly containing a chlorine
level that is above the test kit's ability to measure. In this case,
the dilution method of testing should be followed.
Using the dilution method, a vial is filled
with 1/2 sample water and 1/2 distilled water (or bottled water that
does not contain any ammonia or chlorine). The test is performed as
above, but the results should be doubled. Other dilutions can be
used, such as 1/3 sample water and 2/3 distilled water - in this
case, the results should be tripled.