Service techs can use three methods to measure chlorine residuals
- DPD (diethyl-p-phenylene diamine):
a chemical reagent that reacts with free, or active, chlorine. The
sample water is combined with the DPD reagent in a test vial.
Between 1 and 3 ppm (industry-recommended chlorine levels), the
sample will turn varying shades of red.
This method also can test for combined chlorine, or chloramines. To
get a sense of the combined chlorine, simply perform the initial
test for free chlorine and add a second reagent. The resulting
color will indicate a total chlorine level. Subtract the previously
ascertained free chlorine from the total chlorine level to get the
level of combined chlorine.
One note of caution when doing DPD tests for chlorine: With high
chlorine residuals (10 ppm and up), the reagent will bleach out,
and the test actually will resemble low chlorine. At this point, a
tech is likely to add even more chemicals to the pool, though the
levels are too high already.
If you suspect this phenomenon is at work in the pool you’re
testing, simply dilute the test sample by 50 percent with
sanitizer-free water and then re-test. Now, a reading of 3 ppm is
actually 6 ppm.
- OTO (ortholtolodine):
This reagent has
fallen out of favor in recent years due to its inability to
distinguish free chlorine from total chlorine, but it still has its
To test for total chlorine, add the amount of OTO as specified by
the manufacturer to the sample of water in the test cell. Cap the
test cell, and invert it to mix (do not use your finger to cover
the top of the test cell). A color change (from light yellow to a
deep orange) will result. Compare the color of the test sample to
the color comparator provided with the kit.
Another option is the FAS-DPD
method. This variation of the traditional DPD method allows users
to measure both free and combined chlorine levels as low as 0.2 ppm
— the maximum allowable level for combined chlorine,
according to most health authorities as well as APSP — and as
high as 20 ppm. By contrast, the color comparators used with the
standard DPD test generally allow readings at the low end of 0- and
0.5 ppm and at the high end of 5- or maybe 10 ppm.
In the FAS-DPD titration test, a buffered DPD indicator powder is
added to a water sample and reacts with chlorine to produce the
pink color characteristic of the standard DPD test. Ferrous
ammonium sulfate solution (FAS) then is added drop by drop until
the pink color completely and permanently disappears, signaling the
endpoint of the reaction.
To get the reading, the number of drops used to cause this color
change is multiplied by the appropriate factor for the size of the
water sample (supplied by the manufacturer).
The distinct change from a vibrant pink to no color at all
eliminates the need for color matching. This means when testing
samples with a high level of sanitizer, the user does not have to
distinguish between relatively close gradations of color or be
concerned that any color has been bleached out of the sample.
It’s also a boon to the 6- to 8 percent of the population
with red-green deficiencies in their color vision.
The second part of the FAS-DPD test determines the amount of
combined chlorine present (that is, mono-, di- or trichloramines)
by the number of drops needed to again turn the sample from bright
pink to colorless.
Note: The cost per test is a little higher and the procedure takes
a bit longer than other chlorine tests. But many techs keep it on
hand for problem pools or to use if they are “color
challenged.” Look for FAS-DPD in combination kits and as a
If the pool or spa you’re testing uses bromine rather than
chlorine, you can still test residuals using either OTO or DPD.
Follow the same procedures you would when measuring chlorine and
multiply the results by a factor of 2.25.
If you’re measuring for chlorine residuals, you’ll
likely be tracking the stabilizer levels as well.
Industry-recommended levels are between 30- and 50 ppm for cyanuric
To test CYA levels, a reagent called melamine is used. The melamine
will cause the water to become more or less cloudy (the
aforementioned turbidity test). Low CYA will produce small
particles that give the water a hazy appearance. Higher
concentrations produce far more particles and turn the water very
cloudy. The turbidity is then measured against a comparator chart
depicting the relative visibility of a dot in the test vial,
thereby indicating the corresponding CYA reading in parts per
Because chlorine only works effectively in certain pH ranges (low
7s to low 8s), it’s just as important to regularly monitor
the water’s pH level.
The reagent for testing pH is phenol red — or
phenolsulfonephthalein. This is an organic dye that comes in both
liquid and tablet form. In a small sample of water, five drops, or
one tablet, are added. The resulting colors — yellow (low),
red (middle) and purple (high) — will be accurate in a pH
range of 6.8 to 8.4.
Two other reagents also are used in pH testing: bromythol blue,
with a range of 6.0-7.4, and cresol red, with a range of 7.2-8.8.
Phenol red remains the most popular, though, because it closely
reflects the pH levels recommended for pools.
Again, be aware that a high sanitizer residual (more than 10 ppm
for chlorine, 20 ppm for bromine) can create false pH readings. At
these high levels, the sanitizer reacts with the phenol red, which
results in false colors. Many test kits include a special
neutralizing reagent that, when added prior to testing, ensures
If your tests show that the pH needs to be raised or lowered,
another test should be done to determine exactly how much adjuster
should be added. To lower pH, perform an acid demand test; to raise
it, perform a base demand test.
The number of drops required to cause a color change in the water
sample correlates to a chart that prescribes the amount of muriatic
acid (to lower pH) or soda ash (to raise it) needed to move the
Total alkalinity (TA) is the water’s ability to neutralize
acid, also known as the system’s “pH
To test for total alkalinity, two reagents are required for a water
sample. First, add the end-point indicator — that is, a
chemical that changes colors during the test. The second reagent,
titrant, is an acid used to trigger the end-point reaction.
It’s important to perform this test carefully. To properly
mix the titrant, the test vial should be gently swirled after each
drop is added.
When the titrant is added and just exceeds the water’s
ability to neutralize it (it’s an acid, remember), the
end-point indicator will suddenly change the water color. This
color change happens fast, so pay close attention. Count the number
of drops needed to generate this end-point reaction. This
determines the concentration of the water’s TA.
According to APSP’s Basic Pool & Spa Technology Manual,
Third Edition, the ideal TA level for pool water is between 80- and
120 ppm as calcium carbonate. However, the proper range will vary
depending on the type of sanitizer used.
Testing a pool for calcium hardness is particularly important if
you use calcium hypochlorite as the sanitizer, or if the source
water is naturally high in calcium. If this is the case, you should
initially test both pool and tap water, then test continually for
30 days. After that, remain current on the water’s calcium
To measure calcium hardness, use a titration method. The first step
is to add a special pH buffer to the water sample. This will raise
the pH of the sample to approximately 10, the level at which the
test is most accurate. Next, an organic dye is added that turns red
when it reacts with calcium. Finally, add the titrant: EDTA, also
known as ethylenediamine tetraacetic acid.
Add it one drop at a time. When the titrant has combined with all
the calcium in the sample, it will turn the water blue. The number
of drops required to turn the sample blue correlates to hardness in
While regularly testing chlorine and pH makes sense, certain tests,
such as the ones for metal, only need to be performed periodically,
or if the tech senses trouble and wants to do a little diagnostic
There is always going to be some sort of metal in pool water, but
if you think the levels may be high — if you see staining,
for instance — performing a few metal tests would be in
Copper and iron are the two most likely culprits. Copper can find
its way into pools via copper-based algaecides and the corrosion of
copper heat exchangers and heat sinks. Iron is usually introduced
from source water — particularly well water.
If you think there’s high metal content (as little as 1 ppm
metal can cause staining), it’s best to consider investing in
a metal kit.
Typically, kits are colorimetric tests that require two or more
reagents. Bicinchoninic acid is popular when testing for copper. It
produces shades of purple in the water sample when that metal is
present. The common reagent for detecting iron is phenatroline,
which produces shades of orange and red.