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Phosphates affect chlorine efficiency

June 26, 2017


Wistia video thumbnail - Phosphates Weaken Chlorine

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Phosphates affect chlorine efficiency, and this article will explain why. Phosphates are quite the buzz word in the pool business these days. Yet in our travels and conversations, few people actually know why it is beneficial to rid your pool of them. It's just common knowledge that it's good to address them--you know, for the benefit of water quality--or something like that.

Chlorine's two reactions in pool water

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When chlorine is introduced to water, a reaction occurs.

Cl2 + H2O → HOCl + HCl

Chlorine in water yields Hypochlorous Acid (HOCl) and Hydrochloric Acid (HCl). Hypochlorous acid (HOCl) is the "strong chlorine" that we want. It's an excellent sanitizer and a decent oxidizer too. This is the form of chlorine that kills germs, bacteria, algae, and helps makes your water safe to swim in.

But wait, that's not the only reaction that occurs. Within seconds (actually fractions of a second), a second reaction--called dissociation--happens. The dissociation reaction looks like this:

HOCl ⇌ H+ + OCl

Hypochlorous acid (strong chlorine) yields Hydrogen and Hypochlorite Ion (OCl).

This is a reversible reaction, meaning it goes back and forth, depending on conditions in the water. The key here is the hydrogen ion separates from one side of the equation to the other. The problem? Hypochlorite Ion is "a much weaker disinfectant than hypochlorous acid". So much so, that the killing speed can be over 100x slower. That's not going to get the job done in a crowded pool.

Phosphates affect chlorine's hydrogen

Okay, before the chemists reading this roll their eyes, no, chlorine itself does not contain hydrogen. This is referring to the two forms of chlorine in water, as described above: Hypochlorous Acid and Hypochlorite Ion. The point is, phosphates affect chlorine because the hydrogen in the equation (H+) is drawn away from HOCl. The result is the weaker form of chlorine.

Translation: Phosphates convert strong chlorine to weak chlorine.  While it doesn't occur all the time, according to the sources we have read and the experts we have talked to, it happens enough that it's a problem. Fortunately the dissociation reaction (HOCl ⇌ H+ + OCl) is reversible. But hey, we're pool guys, not chemists. If there's a way we can limit that dissociation from occurring, we will have stronger chlorine. Eliminating phosphates from pool water is a way to do that.

How to eliminate phosphates from your pool

Because phosphates affect chlorine, best practice is to remove them. First, understand that most pool phosphates come from two sources--but there are several other ways orthophosphates can get in your pool water.

  • Phosphates are often in the tap water by the water treatment facility. See phosphonic acid, sequest/chelants.
  • Many scale and metal control products in the pool business are phosphonic acid based, aka liquid phosphates.

When removing phosphates, it helps to know where the phosphates came from. Test your tap water for phosphates so you know what you're up against. Also, check the bottle labels of any type of stain or metal removal products you have put in the pool in the past. Unless you used our SC-1000, there's a decent chance you put in a phosphate-based product.

Orenda PR-10,000 is a strong phosphate remover


pr10000, PR-10,000, orenda phosphate remover concentrate
PR-10,000 effectively removes phosphates from pool water.


Phosphorous can be removed from water by crystallizing it, and flocking it out of solution (sinking). This reaction occurs when you introduce a phosphate remover product like PR-10,000 to the pool. If you have any questions on how to apply PR-10,000 properly without overdosing, you can contact us or use the dosing calculator on our mobile app.  WATCH:


Wistia video thumbnail - PR-10,000 Phosphate Remover in Action

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