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  • Pilar Gonzalez

Understanding Phosphorus

Updated: Apr 14, 2021

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Confused about Phosphorus?

Good news, you’re not alone! Even trained chemists can make mistakes when talking about various phosphorus compounds and the different testing techniques used. Complex chemistry, coupled with industry jargon, can baffle even the most well-trained water treatment professional. The goal of this newsletter is to explain the different types of phosphorus testing in a way that is simple and easy to comprehend for the non-chemist water treatment professional.

3 Main Types of Phosphorus in Water

1. Orthophosphate

2. Condensed phosphate

3. Organic phosphorus

Although there are many different forms of phosphorus, orthophosphate is the only form that can be directly measured*. Therefore, all other forms of phosphorus must be converted to orthophosphate to be detected.

*This does not apply if you are using an instrument such as an ICP. With an ICP, only phosphorus (P) can be directly measured.


Orthophosphate takes the form of the phosphate ion – one phosphorus atom bonded to four oxygen atoms. Orthophosphate is sometimes referred to simply as “phosphate” or “reactive phosphorus”.

Orthophosphate, PO43-

Orthophosphate can be reported in two different ways: either as PO43- “orthophosphate” or as PO4-P “orthophosphate as phosphorus”. PO43- results combine the phosphorus and oxygen in the measurement, while PO4-P only considers the phosphorus in the measurement. Different labs may report their results in either way. To convert results reported as PO4-P to PO43-, simply multiply the PO4-P results by 3.06. You can use the formula bellow for quick conversions.

PO4-P X 3.06 = PO43-

Condensed Phosphate

Condensed phosphates are multiple orthophosphate molecules combined or “condensed” together and contain salts or metals. Some examples include pyrophosphate, metaphosphate, and polyphosphate. These compounds are often use for corrosion control in water systems.

Condensed phosphates are also known as “acid hydrolysable phosphate” because they can readily be broken down with heat and acid (hydrolyzed) to orthophosphate.




Organic Phosphorus Compounds

There is a wide class of phosphorus-based molecules called organophosphorous compounds (organic compounds containing phosphorus) having the general chemical form R-PO(OR)2, where R represents a carbon-based organic group. Organic phosphates (also referred to as organophosphates), and phosphonates are both included in this organophosphorus group.

Organic Phosphates

Organic phosphates are essentially phosphates bonded to an organic compound taking the form PO(OR)3.

General chemical structure of organic phosphates / organophosphates

Examples in this group include bio-compounds such as DNA, RNA, ATP, certain pesticides, and flame retardants.

The organic bonds in these compounds are much harder to break down than condensed phosphates. Along with heat and acid, a strong oxidizer is needed to revert these compounds to orthophosphate.


Phosphonates, along with phosphonic acids, have the general form of R-PO(OR)2. Phosphonates are effective chelating agents meaning that they can bind to metals or other dissolved compounds in water. By doing this, phosphonates can prevent the formation of insoluble precipitates (scale) and limit the amount of iron and steel corrosion.

General chemical structure of phosphonates/phosphonic acids

Some common phosphonates used in water treatment include: HEDP (1-hydroxyethylidene-1,1-diphosphonic acid), ATMP (aminotris(methylphosphonic acid)), and PBTC (2-phosphonobutane-1,2,4-tricarboxylic acid) to name a few.

Like organic phosphates, these compounds are hard to break down. The test method used relies on a UV-catalyzed oxidation to convert phosphonate to orthophosphate.

Summary of Methods

All phosphorus containing compounds must be converted to orthophosphate for measurement. To measure the compound of interest, subtract the additional components contained in the digested sample from your total reading.


Digestion? None

Sample Includes: PO43-

Condensed Phosphate

Digestion? Acid + heat

Sample Includes: Condensed Phosphate + PO43-

Organic phosphate

Digestion? Acid + heat + oxidizer

Sample Includes: Organic Phosphate + Condensed Phosphate + PO43-


Digestion? Oxidizer + UV

Sample Includes: Phosphonate + PO43-

Abbreviations and Terms

P: “Phosphorus”

(PO4)3-: “Phosphate”, “orthophosphate”, or “reactive phosphorus”

PO4-P: Phosphate measured as phosphorus

Organophosphorus: Organic compounds containing phosphorus

Orthophosphate = phosphate = reactive phosphorus

Organophosphate = organic phosphate

Condensed Phosphate = acid hydrolysable phosphate

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