What is the way to test for organically bound phosphorus (phosphate)?
How to test for organically bound phosphorus.
The Phosphorus for Water, Wastewater and Seawater
outlines the different phosphorous types Hach offers tests for, and how those tests work.
The applicable part to this question is below:
Condensed phosphates plus orthophosphate can be determined by acid hydrolysis using sulfuric acid, followed by the reactive phosphorus test for the appropriate range. A small amount of organically bound phosphorus will be included in this measurement. The results of the test are reported as acid-hydrolyzable phosphorus. Total phosphorus (orthophosphate, condensed and organically bound) can be determined by acid oxidation with persulfate, followed by the reactive phosphorus test. Organically bound phosphate can then be determined by subtracting the acidhydrolyzable phosphorus.
In summary, subtracting an Acid-Hydrolyzable phosphorus result (Ortho+Condensed) from a Total phosphate result (Ortho+Condensed+Organic) will give a result for Organically bound Phosphate. However some Organically bound phosphorus will be included in the Acid-Hydrolyzable phosphorus result so the recovery is expected to be low. How low is unknown and will be dependent on the sample matrix (what organically bound phosphates are in the sample).
Phosphorus, Acid Hydrolyzable Digestion-Acid Digestion Method 8180
Phosphorus, Acid Hydrolyzable-PhosVer™ 3 with Acid Hydrolysis TNT Method 8180
Phosphorus, Total Molybdovanadate Method with Acid Persulfate Digestion HR TNT Method 10127
Phosphorus, Total, Digestion-Acid Persulfate Digestion Method 8190
Phosphorus, Total-PhosVer® with Acid Persulfate Digestion TNT Method 8190
Please also see: What is the difference between reactive, acid hydrolyzable, and total phosphorus?