For the past 16 years, I have been traveling around the United States training operators.
During that time, one thing has become abundantly clear:
There’s a lot of confusion about the relationship — and
the difference between — oxidation reduction potential and
parts per million.
Part of the problem is that many state health departments don’t regulate
ORP, but they do regulate ppm. So operators tend to focus on ppm
and forget about ORP. That’s a mistake.
When it comes to maintaining healthy pool water, ORP is actually more important
than ppm. What’s more, relying on ppm readings alone can
provide a false, and dangerous, sense of security.
To understand why, we have to start with basic water chemistry. When chlorine is
added to water, a chemical reaction occurs between chlorine and the
water, called hydrolysis. During hydrolysis, chlorine reacts
rapidly with water to form two separate chemicals: hypochlorus acid
(HOCl) and hydrochloric acid (HCl).
Of these two compounds, the important one is hypochlorus acid, or HOCl. This
compound is the “active form” of chlorine, providing
the needed oxidation, substitution and disinfection reactions.
Without getting too technical, you want significant HOCl in the
pool water for effective oxidation and
However, ppm and HOCl do not always correspond. That’s because HOCl
dissociates or separates into two components, the hydrogen ion (H+)
and the hypochlorite ion (OCl-). Like the hydrolysis reaction, the
degree of dissociation is primarily dependent on temperature and
the pH of the water. As the pH goes up (a shift to the right), the
hypochlorite concentration increases. As the pH goes down (a shift
to the left), the hypochlorus concentration is
Therefore, while your test kit may show ample amounts of chlorine at a pH 7.8,
which is within most health code standards, the HOCl may not be as
effective at killing pathogens and making the water clear. At a pH
7.8, you only have 30 percent HOCl, though the ppm of chlorine
remains the same. That’s why it can be misleading to think
that a pool with a pH of 7.8 and “free” chlorine level
of 1 to 2 ppm is a healthy swimming environment.
Most health departments don’t require an ORP minimum, but decades of
research have shown that ORP is a direct measure of the
sanitizer’s oxidation and disinfection strength in the water.
For instance, the polio virus dies in five minutes at an ORP level
of 550 millivolts (mV), but at a 650 mV ORP level, it takes only 30
seconds to kill this virus. Add another 100 mV and the kill time is
approaching one second! Kill rates have been similarly established
for all waterborne pathogens along the ORP rate-of-disinfection
scale — even for cryptosporidium, E.coli,
giardia and legionella. ORP essentially measures the
effectiveness of the sanitizer in the water. The lower the ORP, the
less HOCl in the water and the less effective the
The problem with ORP is that it can’t be measured with standard test
kits. Options for determining ORP are small, inaccurate handheld
devices or automated controllers.
So the next time you take your chlorine readings, make sure you check and
record the ORP, too. If you don’t have a device to measure
ORP, maybe it’s time to consider getting one. And remember,
ORP is the true measure of safe water.