Oxidation Is Not Sanitation: What Certified Pool Operators Need to Understand About ORP

The Olympic Pool, ORP Misinterpretation, Copper Expression, and Why Green Water Is a Symptom — Not a Diagnosis

Last week I said something that needs refinement.

Not correction.
Refinement.

Because that’s how the industry gets better.

After my ORP episode aired, I received a thoughtful critique from Terry Arko at HASA. And Terry is not guessing. He comes from chemical manufacturing — where chemistry is not theoretical, it is consequential.

He offered an alternative explanation for the 2016 Rio Olympic diving pool turning green. A copper-based event potentially driven by hydrogen peroxide introduction, sulfate presence, and pH shifts.

And here’s the important part:

His explanation is chemically valid.

But validity does not equal exclusivity.

Let’s unpack this properly — with research, not opinion.

listen to episode here ⬇️⬇️⬇️

ORP Is a Measurement of Electron Transfer Potential — Not Disinfectant Identity

Oxidation-Reduction Potential (ORP) measures the electrical potential of a solution to accept or donate electrons. It does not identify the oxidizer responsible for that potential.

This is well established in electrochemical literature.

According to ASTM D1498 and supported in water treatment research:

ORP is an aggregate indicator of oxidizing strength in solution and does not distinguish between specific oxidants.
(ASTM D1498; see also Gordon, G. et al., Journal AWWA, 1998)

Peer-reviewed electrochemical research confirms:

Hydrogen peroxide, chlorine, ozone, and even certain metal redox couples can elevate ORP values — sometimes dramatically — while having very different biological efficacy profiles.

(Huang et al., Water Research, 2008; Deborde & von Gunten, Water Research, 2008)

So when operators say:

“ORP was high. The system was fine.”

That is a category error.

ORP reports redox potential.
It does not report sanitizer concentration.
It does not report disinfectant identity.
It does not report biological kill capacity.

And hydrogen peroxide complicates this profoundly.

Similar News Story  https://time.com/4451484/why-rio-olympic-pools-turned-green-hydrogen-peroxide/

Peroxide Can Elevate ORP While Suppressing Free Chlorine

Hydrogen peroxide reacts directly with hypochlorous acid:

H₂O₂ + HOCl → O₂ + Cl⁻ + H₂O

This reaction consumes free chlorine.

(Deborde & von Gunten, Water Research, 2008)

Yet peroxide itself is an oxidizer and contributes to measured redox potential.

Meaning:

You can have elevated ORP
while free chlorine collapses
and biological control fails.

That is not hypothetical.

It is documented chemistry.

Peroxide also decomposes into oxygen radicals under certain catalytic conditions, further influencing redox measurement without providing residual disinfectant stability.

(Carey et al., Environmental Science & Technology, 2012)

So when we say “ORP lied,” what we really mean is:

We asked ORP the wrong question.

The Olympic Pool — Algae vs Copper

Now we move to the Rio pool.

The official investigation attributed the green water to sanitizer depletion and biological growth.

That conclusion was based on internal operational data: feed logs, controller history, chemical records.

None of us outside that control room have those logs.

However, Terry’s copper hypothesis centers on this:

• Copper sulfate present
• High sulfate source water
• Hydrogen peroxide introduced
• pH drop
• Copper complex shift → green coloration

This is chemically plausible.

Copper chemistry is highly responsive to pH, ligand concentration, and redox environment.

In aqueous systems, copper exists primarily as:

Hexaaquacopper(II) — blue
Tetrachlorocuprate(II) — green
Copper hydroxide — blue/teal precipitate
Copper oxide — black
Copper cyanurate complexes — purple

(Sillen & Martell, Stability Constants of Metal-Ion Complexes, 1971)

These transformations are governed by equilibrium chemistry.

Raising chloride concentration and lowering pH can shift copper from aqua complexes to chloride complexes, changing color without increasing copper mass.

This is textbook coordination chemistry.

(Cotton & Wilkinson, Advanced Inorganic Chemistry)

So yes — copper can turn water green while remaining clear.

But here is where visual analysis matters.

Copper-dominant events typically produce jewel-like optical clarity because dissolved copper complexes transmit light cleanly.

Algae events scatter light.

Photos from the Olympic pool show diffuse light behavior inconsistent with pure copper transparency.

That is observational chemistry, not opinion.

Which leads to the most mature conclusion:

Both mechanisms could have been active.

Sanitizer collapse.
Copper expression.
Peroxide interference.
Redox misinterpretation.

Complex failures are rarely single-variable.

Copper Is Not a Contaminant. It Is a Conditional Participant.

Copper does not “appear.”

It waits.

When dissolved as copper sulfate, it forms the hexaaquacopper(II) complex — brilliant blue and fully soluble.

That is stability.

But stability is conditional.

Increase carbonate concentration and copper carbonate/hydroxide phases form.

Lower pH rapidly and chloride ligands replace water molecules.

Introduce strong oxidizers like calcium hypochlorite and copper(II) can convert to copper oxide.

These are ligand exchange and redox reactions — not guesswork.

(Atkins & Jones, Chemical Principles)

Add high cyanuric acid and copper cyanurate complexes can form — poorly soluble and often purple in appearance.

(Carter & Gillham, Water Research, 1993)

None of these reactions require new copper.

They require new conditions.

That distinction changes how professionals diagnose.

Oxidation Is Not Sanitation: Certified Pool Operator 101

This is the central thesis.

Oxidation potential does not equal pathogen kill.

Chlorine efficacy depends on hypochlorous acid concentration, which is governed by pH and cyanuric acid equilibrium.

(Wojtowicz, Journal of the Swimming Pool and Spa Industry, 2004)

ORP does not account for:

• CYA buffering
• Peroxide presence
• Metal complex equilibria
• Localized pH microenvironments
• Filtration limitations

It reports electron activity.

Sanitation requires biological kill kinetics.

Those are not the same conversation.

The Field Error That Repeats Every Season

Here is the real lesson.

The Olympics was visible.

Your route pools are not.

But the same mistake happens weekly:

• Peroxide added to “clear smell”
• ORP remains elevated
• Chlorine collapses
• Biological demand increases
• Copper destabilizes
• Shock added aggressively
• Copper oxidizes to black
• Acid added rapidly
• Copper shifts green
• Customer panic

Then everyone argues about what caused it.

The cause was failure to interrogate the system.

Color is not diagnosis.

Color is a symptom.

Green water can be:

• Algae
• Copper chloride complexes
• Mixed metal-biological systems
• Suspended solids + sanitizer failure

The professional move is not to react.

It is to analyze equilibria.

Similar Article Cold Plunge Pool Maintenance: Essential Tips for Pool Professionals

What Actually Improves the Industry

Terry’s critique was valuable.

Not because it disproved algae.

But because it forced refinement.

The mature takeaway is this:

Multiple chemically valid failure modes can coexist.

Peroxide can mask chlorine loss.
ORP can mislead without context.
Copper can express under corrective chemistry.
Sanitation can fail while oxidation appears intact.

The water never lies.

But it only answers the question you actually ask.

So ask better ones.