What is the difference between reactive, acid hydrolyzable, and total phosphorus?

Document ID

Document ID TE365

Published Date

Published Date 02/02/2022
What is the difference between reactive, acid hydrolyzable, and total phosphorus?
Reactive, acid hydrolyzable, and total phosphorus
Phosphorus exists in water almost solely as phosphates which can be dissolved, attached to particles, or found in aquatic organisms. Phosphorus levels in natural waters such as lakes and streams are typically very low. Elevated phosphorus levels may indicate contamination from raw or treated wastewater, agricultural drainage, or industrial waste. Some drinking water plants also add small amounts of orthophosphate or condensed phosphates during treatment. Orthophosphate is the most simple form of phosphorus to measure, but total phosphorus is considered the best indicator of phosphorus levels in water because it measures all forms.

Phosphates can exist in the most simple form as orthophosphate ( PO₄³-) or in larger molecules as condensed phosphates, inorganic phosphates, or organic phosphates.

Orthophosphate is often referred to as "reactive" phosphorus because it is the only type of phosphorus that will react directly with colorimetric phosphate reagents. Orthophosphate is used by plants, bacteria, and algae and is considered a limiting nutrient in water testing.

Condensed phosphates (also called meta, pyro, or polyphosphates) are two or more orthophosphate groups that are linked together. They are strong complexing agents and are widely used in treatment systems for boiler water and are also found in many detergents. To measure condensed phosphates, the sample must be analyzed for acid hydrolyzable phosphorus and also orthophosphate:
condensed phosphates = acid hydrolyzable phosphorus - orthophosphate

Organic phosphates contain one or more orthophosphate groups that are attached to an organic molecule such as sugar. They are formed primarily by biological processes and can be found in organic matter such as plant or animal tissue, in sewage from animal or human waste and food residues, as well as in pesticides. To measure organic phosphates, the sample must be analyzed for total phosphorus and also for acid hydrolyzable phosphorus:
organic phosphates = total phosphorus - acid hydrolyzable phosphorus

Both condensed phosphates and organic phosphates are not as stable as orthophosphate and naturally break down into orthophosphates over time. Therefore an orthophosphate test will likely measure a small amount of condensed phosphates and an acid-hydrolyzable test will measure a small amount of organic phosphates.

Phosphorus Relationships are summarized below:

Orthophosphate (o- PO₄³- ) = Reactive Phosphorus
Determined using Hach Methods 8048, 8178 or 8114

Total Phosphorus (TP) = TIP + Organic PO₄³-
                                        = o-PO₄³- + Poly PO₄³- + Organic PO₄³- 
Determined using Hach Method 8190 or TNT Method 8190

TNT+ chemistries TNT843, TNT844 and TNT845 can be used with method 10209 for orthophosphate or method 10210 for total phosphorus

Total Inorganic Phosphorus (TIP/Acid Hydrolyzable) = o- PO₄³- + Poly PO₄³- (condensed phosphates)
Determined using Hach Method 8180 or Hach TNT Method 8180

Organic PO₄³- = TP-TIP 
Detemined by calculation only

Condensed Phosphates (
Poly PO₄³-) = Acid hydrolyzable phosphorus - orthophosphate
Determined by calculation only
LCK Methods for Ortho and Total can be used LCK349, LCK348 and LCK350 which meet ISO 6878-1-1986 guidelines

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