any type of chlorine is added to water, it forms hypochlorous
acid (HOCl - the most powerful killing form of chlorine in water)
and hypochlorite ion (OCl-), a relatively weak form of
chlorine in water. The percentage of HOCl and OCl- is determined by
the pH of
the water. As the pH goes up, less of the chlorine is in the killing
form and more of the chlorine is in the weaker form. The combined total of
HOCl and OCl- is the measure of free available chlorine. Free available chlorine is the active, killing type of chlorine that we want in the water.
can combine with ammonia and nitrogen compounds in the water to form chloramines,
sometimes also called Combined Chlorine. By combining with ammonia and
nitrogen, a chloramine loses much of its sanitizing power. Chloramines are
60 to 80 times less effective than an uncombined free chlorine molecule.
formed any time ammonia and nitrogen are in the water. Some of the
ammonia and nitrogen compounds are introduced into the water by swimmers
and bathers in the form of perspiration, urine, saliva, sputum and
fecal matter. An active swimmer sweats one pint per hour, while the
average person sweats three pints per hour in a heated spa. Ammonia
and nitrogen compounds are also introduced into the water by rain, especially Acid Rain.
Each drop of rain has some dissolved nitrogen from our atmosphere
and from automobile emissions. Many lawn products contain both nitrogen and ammonia and phosphates, all very bad for pools, can raise your chlorine demand, and encourage algae growth.
Chloramines, or combined chlorine smell bad, they are eye and skin irritants, and they get in the way of free chlorine trying to do it's job. When a pool smells strongly of Chlorine, what smells is not free available chlorine, but Chloramines . When testing for Free and Total Chlorine with a DPD or other capable pool test kit, the level of combined chlorine molecules in your pool can be detected. The formula is quite simple. The difference between Free Chlorine reading and the Total Chlorine reading is the Combined Chlorine reading. Anything over 0.3ppm should be treated to bring the level down by removing the Chloramines from the pool.
can be removed from the water by the following three methods:
adding a mega-dose of chlorine.
Usually 3 to 6 times more chlorine than a
normal dose is added to the water, or the
level of chlorine is raised to 5 to 10 ppm
and held there for 4 hours. This is called
super-chlorination. To remove chloramines,
the ratio of chlorine to ammonia must be
at least 7.6 to 1. If this ratio is not obtained,
then more chloramines will be produced. A threshold of "breakpoint chlorination" must be reached where total oxidation takes place. This "shock" to the pool will rip apart the molecular combinations, and destroy chloramines. Swimmers
and bathers should not enter the water until
the level of chlorine has dropped to 3 ppm
adding a non-chlorine
shock to the water. The most common chemical
used for this is potassium peroxymonosulfate.
requires the addition of one pound of shock
for each 10,000 gallons of pool water. The same threshold of breakpoint oxidation must be reached when using non-chlorine shock. So, feel free to add a little extra, just to be sure.
adding ozone to the water. If an ozone generator
is installed on a pool or spa, then oxidation
of the ammonia and nitrogen compounds will
take place whenever the ozone system is operating.
The longer the system operates, the more
the ozone can destroy the ammonia and nitrogen.
Although most ozone systems operate only
when the pool or spa pump is operating, there
are 24 -hour systems available which will
continuously oxidize ammonia and nitrogen
as they enter the water.
Remember, when you smell a strong chlorine odor in a pool - and your eyes are red, it's not because there is too much [free] chlorine in the water, but too much Combined Chlorine. Free chlorine by itself does not smell, nor sting the eyes. The solution to this is to add a whole lot more chlorine, to reach breakpoint chlorination, where the molecular combination of chlorine and nitrogen [or ammonia] will be removed. At least for a while.