What is the correct form of phosphorus to measure - reactive, acid hydrolyzable, or total?

Document ID

Document ID TE365

Published Date

Published Date 25/01/2018
Question
What is the correct form of phosphorus to measure - reactive, acid hydrolyzable, or total?
Summary
What is the correct form of phosphorus to measure - reactive, acid hydrolyzable, or total?
Answer
Phosphorus exists in water almost solely as phosphates, which can be dissolved, attached to particles, or found in aquatic organisms.

Phosphates can exist in simplest form as orthophosphate ( PO₄³¯) or in larger molecules as condensed 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. This type of phosphorus is used by plants, bacteria, and algae and is considered a limiting nutrient for surface waters such as lakes. Fertilizers contain orthophosphate and can contribute significant levels to waters via agricultural runoff.

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, follow a procedure for acid hydrolyzable phosphorus. This involves heating the sample with strong acid to break the condensed phosphates down into simple orthophosphate molecules. An acid hydrolyzable test will measure both condensed and reactive phosphorus:
condensed phosphates = acid hydrolyzable - 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, follow a procedure for total phosphorus. This involves heating the sample with strong acid and a strong oxidizer such as persulfate to convert the organic phosphates into orthophosphate molecules. This test will measure organic, condensed, and reactive phosphorus:
organic phosphates = total - acid hydrolyzable

**Note: 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 levels in natural waters such as lakes and streams are typically very low, less than 0.05 mg/L as P. Higher phosphorus levels reflect contributions 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 simplest phosphorus form to measure, but total phosphorus is considered the best indicator of phosphorus levels in water because it measures all three forms.

If you are required to measure and report phosphorus levels to a regulatory agency, we advise that you check with your regulator to determine whether you are required to report ortho, condensed, or total phosphorus levels.

Phosphorus Relationships:

TIP (Acid Hydrolyzable) = o- PO₄³¯ + Poly PO₄³¯ (condensed phosphates) *Determined using Hach Method 8180 or Hach TNT Method 8180

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

Organic PO₄³¯ = TP-TIP  *Detemined by calculation only.

Poly PO₄³¯ = Acid Hydrolyzable - (o-PO₄³¯)  *Determined by calculation only

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

LCK Methods for Ortho and Total can be used LCK349, LCK348 and LCK350 which meet ISO 6878-1-1986 guidelines

 

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