Pool Chemical Testing
Best Practices
Pool Chemical Tests – Best Practices
People that have experience performing pool chemistry tests are probably familiar with the scenario
where a test shows a chemical out of range and a re-test a few minutes later shows the chemical in range
(or vice-versa). While it’s likely impossible to eliminate these situations some simple rules can help reduce
their frequency.
Handling and storage
When test chemicals are received they should be marked to show the date received or the expiry date
(typically 1 year after receiving).
Reagents should be stored away from light
and kept at a constant temperature. The reagents can handle temperatures ranging from
2C to 29C (36-85F) but need to be kept at a
stable temperature. Reagents are damaged
by freezing and fluctuating from one extreme
to another. Storing reagents in a refrigerator
then using them in a hot mechanical room
will probably cause the reagent to spoil quickly, it is best to store the reagents at room
temperature (in an air conditioned space if
possible) . Never store reagents in a vehicle
because daily temperature swings will quickly spoil the reagents.
Replace reagent caps on the same bottle immediately after use. Exposure to air will shorten the lifespan of
many reagents and mixing caps will contaminate the reagents.
Refer to your test kit’s manufacturer for specific details on shelf life and handling.
High sanitizer levels affect results
High sanitizer levels (starting at 10 PPM) can start to affect test results. Very high chlorine levels cause reagents to “flash”; colour just starts to show when the reagent is added then quickly disappears. The high
sanitizer “bleaches” out the colour.
Phenol reagents (pH) will turn purple when sanitizer exceeds approximately 15 PPM. Adding one drop of
sodium thiosulphate solution N/10 (Taylor R-007, LaMotte THS-6267) to a new sample should remove this
high FAC interference. More than one drop will change the sample pH.
When reagents go bad
A spoiled or contaminated reagent will
generally produce an off-colour result and
DPD chlorine tests will not quite match
colour standards. Phenol pH tests that
have gone bad may give a false high result if FAC is greater than 5 PPM . Spoiled
DPD tests will gradually turn darker over
several minutes, making colour matching
a moving target. A good rule of thumb is
to try a fresh reagent if the results seem
strange.
Use clean equipment
A stained test tube makes it much harder to
match colours on a comparator. Rinse vials
immediately after use and shake out excess
water. Stains can be removed by soaking in
a dilute chlorine solution (approx. 10:1) or
dish soap. Residue will affect results so rinse
and soak well afterward to remove any traces of the cleaner. Eventually the tubes will
turn frosty white and need replacement.
Light tables have a polarizing glass filter that quickly builds up dirt and
stains. The filter should be wiped
clean weekly with a damp cloth. Glass
cleaner may be required if the buildup
is thick or greasy.
Clean Polarizing Filter
Dirty Polarizing Filter
Colour slides get dirty too. Clean up with soapy water and a soft brush.
These two photographs showing a slide comparator demonstrate the problem with dirty equipment. The
first photo uses clean equipment while the second does not. Both are actually 3.2 PPM Free Chlorine as
measured using CR-0870/0871 powder test. The clean equipment result shows between 3.0 and 4.0 while
the dirty equipment result appears to be between 4.0 and 5.0 PPM.
Sanitizer tests are time sensitive
Sample water contains organic material from the pool. Chlorine in a sample continues to react with these
organics and in a short time can reduce free chlorine to zero. While the free chlorine is dropping, chloramines are rising. The least possible delay between obtaining a sample and performing the test will give the
best result. If testing more than just sanitizer, do sanitizer tests first.
Take samples from one pool at a time
Taking samples from multiple pools at once introduces delays which may result in bad test results. It is also
easy to mix up the tubes and record results for the wrong pool.
Take samples from elbow depth
The official sample depth is at least 30 CM which is pretty close to elbow deep, so use that as a gauge. Organics tend to float; therefor water near the surface has a much higher organic load. The higher the organic
load the faster the sample will degrade.
Rinse tubes three times at elbow depth before taking sample. This cleans out any leftover reagents and
satisfies any chlorine demand the tubes have (yes, chlorine will react to the plastic).
NOTE: Some tests may require a cap over the tube, that cap must also be rinsed three times.
To keep surface water out of the tubes, turn them upside down, put them into the water elbow deep, then
turn then right side up to fill. Air keeps surface water out of the tubes on the way down and deep water in
the tubes keeps surface water out on the way up.
Invert vials
Go elbow deep
This table shows the difference in test results due to sampling location and delay before testing.
Delay
Elbow Deep
Surface
None
4.4 PPM
2.6 PPM
Five Minutes
3.6 PPM
1.2 PPM
Take samples from the pool tank
Due to mixing skimmer water, main drain water, and delays reaching a sampling line, the sanitizer level can
change a lot as water moves through the filter system.
This photo shows a sample from a pool tank and another
from the sensor sampling line, both were taken at the
same time yet the sample line result is 0.5 PPM lower
than pool tank result. Most pools won’t have this problem
but to be sure perform the experiment yourself during a
high use period.
Fill vial to the proper level
Water naturally curves upward at the sides of the test
tube and this is known as the meniscus. The edges of a
meniscus have very little water volume , fill the vial so
the bottom of the meniscus, not the tops of the curves,
is at the fill line.
Pool tank result
Sample line result
1 PPM
0.5 PPM
Hold bottles vertically
Reagent squeeze bottles are designed to produce a specific size of drop when held vertically. Holding the
bottle at an angle will result in a smaller drop and give inaccurate results.
Mix thoroughly
Mix thoroughly after each reagent is added. Any colour should be uniform throughout the tube. Failure to
mix thoroughly can cause incorrect results (usually a darker colour/higher reading).
Do NOT use a finger over the opening and shake as that will almost certainly contaminate the sample and
influence the result. These chemicals also have adverse health effects, do not get them on your skin.
Keep a titrating sanitizer test kit handy
Use a titrating kit if: a test is out of your test kit
range, the test equipment is in bad shape, you’re not
sure about a result , or you want to compare what
you see to the actual chlorine level. Titration tests
have a sudden change in colour to indicate when
endpoint has been reached rather than trying to
match the sample to a colour chart or slide.
As an example, the Taylor Technologies K-1515 FAS
DPD kit uses a powder added to a sample then a titrating reagent is added until the sample changes
from red to clear. The numbers of drops are multiTaylor K-1515 FAS DPD Test kit
plied by 0.2 or 0.5 depending on the sample size and
the result is free chlorine residual. This particular kit is good to 20 PPM FAC and can test for chloramines with
the same accuracy.
One tip about powder DPD tests; the instructions says to dissolve the powder before adding the titrating reagent but it is not actually necessary to wait for all the powder to dissolve. In most cases the powder simply
won’t all dissolve. A few seconds of mixing/swirling will dissolve all the powder needed for a correct test result. If the sample is not mixed enough then some colour may return after the sample has turned clear. Simply add more reagent and add the extra drops to the total—the result will still be accurate.
Copyright Patrick Meloy 2015—permission given to copy and distribute with attributions intact
"Mad scientist". Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/
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