The subject of test kit interferences and whackadoo results has
been discussed ad nauseam. The frequency of this topic is beneficial, because
knowing your chemistry results are accurate is pretty darn important. We don’t,
however, see a lot of the ancillary information these oddball findings bring to
light. What is initially observed as erroneous may provide detail on a
previously unknown problem. When strange results surface, we should be asking, “What else is this telling me?”
High Chlorine Levels
We know that a high chlorine level can cause the results of a DPD (N, N-diethyl-p-phenylenediamine) test to bleach. Liquid reagents will decolorize at 10 ppm of chlorine and powder reagents (tested with FAS DPD) at more than 25 ppm. You may see a flash of pink and then to clear. Sometimes it’s not easy to tell if the flash was there, no matter how many times you repeat the test.
The typical test kit sold to homeowners uses a chemical called
orthotolidine (OTO) to test for chlorine. OTO does not bleach in the presence
of high chlorine. Instead, it turns brown. When in doubt, keep a bottle of OTO
on hand to determine whether or not a chlorine level exists.
The active ingredient in DPD reagent No. 2 is a powerful
oxidizer. Adding an oxidizer to water with a high level of manganese may turn
the water in the test block pinkish-purple by oxidizing the manganese, a false
positive result for FAC. You see the same reaction in a manganese rich swimming
pool upon adding a dose of chlorine or hydrochloric acid.
Phenol red, the chemical we use to test pH, can determine a pH
level between 6.8 and 8.2. At a chlorine level of more than 15 ppm, phenol red
becomes chlorophenol red, which is only capable of determining a pH level
between 4.8 and 6.8. When this occurs, water with a pH of 6.8 or higher,
including every level in your acceptable range, will turn the solution to a
brilliant violet color.
So, if DPD reagents bleach at 10 ppm and phenol red becomes
chlorophenol red at 15 ppm, we can determine that our chlorine level is between
10 ppm and 14 ppm if our chlorine test bleached out but we can still get an
accurate pH reading. This can help us calculate the dose of thiosulphate needed
to bring the chlorine level back into range.
pH and Total Alkalinity
If we were to get a yellowish-green result on a total alkalinity
test versus the normal pink, this would indicate interference from chlorine
(one drop of sodium thiosulphate reagent neutralizing approximately 3.5 ppm of
chlorine). The thiosulphate reagent has a pH of 9.6, so adding more drops than
the directions call for will mess up your readings.
Turbulence will cause an increase in pH without affecting the
total alkalinity. With a pH that is constantly climbing and frequent additions
of muriatic acid to balance, the result is total alkalinity driven into the
toilet. The agitation could be from a water feature, a spillover or water
cannons. It may even be from the return jets as homeowners will often aim them
toward the surface because they “like
to see the water move.”
I can explain the chlorine level, pH and total alkalinity being
off a dozen ways. But the calcium hardness level can only be lowered by
replacing water, either a partial or complete drain and refill. So, if the
calcium hardness level drops and no one had drained or excessively backwashed
water from the pool, this would mean your pool leaks. Most pools have an
auto-fill, so there may not be a noticeable drop in the water level. The
calcium hardness test results could be the first indication a pool has a leak
(a drop in cyanuric acid could indicate the same).
The calcium hardness test is subject to other interferences. The
first is called fading endpoint: No matter how many drops you add, you cannot
get the color on the test to change from purple to blue. This failure to
titrate is likely caused by a high level of iron in the water, which can result
in brownish-red water and/or staining, but it could also be due to a level of
manganese, covered above.
The second calcium conundrum is clumping. While titrating, if we
see that the purple in our colored sample starts to coagulate; this indicates
the presence of magnesium hydroxide in the water. These milk-of-magnesia
meatballs won’t interfere with the test result but could lead to problems if pH
or total alkalinity spikes down the road. Magnesium hydroxide in alkaline water
is a fairly decent flocculant. With a sudden spike in pH and/or TA, you may
find a sudden appearance of an unknown white powdery cake-like precipitant
across the floor of your customer’s pool.