Different types of phosphates
Did you know there are many different types of phosphates? In the water treatment business, most of us use “phosphates” as a general term to describe them all…but sometimes that can be misleading. There are so many types of phosphates and they react differently. In water treatment, we are most concerned about phosphates weakening chlorine. Phosphates weaken chlorine through dissociation of the strong chlorine, hypochlorous acid (HOCl), leaving behind the weak chlorine, hypochlorite ion (OCl-):
HOCl ⇌ H+ + OCl−
We write articles to share ideas and little-known facts. The truth about phosphates is something that needs to be clarified, because there are a lot of opinions out there—ours included. That said, this article is meant to be informative and as objective as possible.
This article is the result of extensive–and at times, boring–research. We do not pretend for a moment that we know all this detailed information by heart…nor do we expect you to. If you want to learn more, all of this chemistry information is available online. Just check our sources that we hyperlink to. As usual, we are attempting to simplify and distill the information so it is easier to understand, and applies to you, our audience.
Let’s start by dividing all types of phosphates into two main categories: organic phosphates, and inorganic phosphates.
Organic Phosphates (Organophosphates)
Organic phosphates are esters of phosphoric acid, also known as orthophosphoric acid (H3PO4). Okay…so what are esters? An ester is what you get when an organic substance (usually a hydrocarbon or alkyl) replaces a hydrogen atom in an acid. This swap (hydrogen replaced by an organic hydrocarbon or alkyl) makes the substance change from inorganic to organic.
Those of us who are not organic chemistry experts refer to esters as fats and oils. Natural fats and essential oils (like Omega 3) are esters of fatty acids.
According to the National Institute of Environmental Health Sciences (a division of NIH), organophosphates are a key ingredient in about half of known pesticides and nerve agents. To be clear and distinguish this from the previous statement, we are now talking about organic phosphates specifically…not other esters (fats). Click here to see examples of organic phosphates. Fortunately, organic phosphates are not common in swimming pools.
Now let’s get to the more common phosphates that we deal with in the water treatment business: inorganic phosphates.
“In biological systems, phosphorus is found as a free phosphate ion in solution and is called inorganic phosphate, to distinguish it from phosphates bound in various phosphate esters. Inorganic phosphate is generally denoted Pi and at physiological (neutral) pH primarily consists of a mixture of HPO2-4 and H2PO–4 ions.” – PubChem CID 1061
We can divide inorganic phosphates into two categories: orthophosphates and condensed phosphates.
Orthophosphates are also called reactive phosphates. They are the most common in water treatment situations, as they directly contribute to the eutrophication of a body of water. Eutrophication is the enrichment of nutrients for plant growth in water, leading to problems for the ecosystem. Algae, anyone?
Orthophosphates are found naturally in the environment and in water, but are also artificially added to fertilizers. In swimming pools, orthophosphates are the most prevalent of all types of phosphates. Usually, when people in the water treatment business talk about “phosphates”, they are referring to orthophosphates.
Click here to see examples of orthophosphates. You’ll notice a substance called phosphonic acid. Phosphonic acid is commonly used in drinking water treatment as a sequestering agent for metals and minerals (like calcium). Phosphonic acid is also used in pool chemicals used for stain and scale removal/prevention. It is an effective sequest, but leaves behind orthophosphates in the water.
Condensed phosphates are types of phosphates that contain salts, metals or minerals like calcium. Within this category are pyrophosphate, metaphosphate and polyphosphate. Calcium phosphate (Ca(H2PO4)2) is a good example of a condensed phosphate. These types of phosphates are naturally occurring, but can also be synthetically combined to be used in various industries.
For the example of calcium phosphate, our bones and tooth enamel are strengthened by it. In swimming pools, calcium phosphate can harden sand and regenerative DE filter media like concrete. It can take a jackhammer to crack through it.
Types of phosphates: Conclusion
There are too many variations of phosphates to write about. It seems like any combination of P, O, H and numbers, + and – signs can be a type of phosphate. Given that we are not chemists, it gets daunting. We know. Hopefully, this article has simplified the chemistry so it is easier to understand.
As it pertains to water treatment, orthophosphates are the primary type we encounter. We recommend removing phosphates for easier water treatment, but that’s just our opinion. If you do come across organophosphates or condensed phosphates, they could give you more trouble. In pools, calcium phosphate has been known to cause severe problems in filters (hardening). In ponds and lakes (even as large as Lake Erie!), orthophosphate eutrophication can be a severe problem.