Phosphorus

No, the Phosphax sc cannot be used for seawater applications since this method works only up to 1000 mg/L chloride. The chloride content of seawater is typically above 19000 mg/L.

Phosphorus exists in water almost solely as phosphates, which can be dissolved, attached to particles or found in aquatic organisms (bound to organic compounds). 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 discharge or industrial waste. Some drinking water plants also add small amounts of orthophosphate or condensed phosphates during post-treatment for corrosion control. Orthophosphate is the simplest 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 form of orthophosphate (PO ₄ ^3-) or in larger molecules as condensed phosphates, inorganic polyphosphates 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) have 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 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 orthophosphate 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- PO ₄ ^3-) = reactive phosphorus

Determined using Hach Methods 8048, 8178 or 8114.

Total Phosphorus (TP) = TIP + Organic PO ₄ ^3-

                                             = o-PO ₄ ^3- + Poly PO ₄ ^3- + Organic PO ₄ ^3-

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- PO ₄ ^3- + Poly PO ₄ ^3- (condensed phosphates)

Determined using Hach Method 8180 or Hach TNT Method 8180.

Organic PO ₄ ^3- = TP-TIP

Determined by calculation only.

Condensed Phosphates (Poly PO ₄ ^3-) = acid hydrolyzable phosphorus - orthophosphate

Determined by calculation only.

Reactive phosphorous can be measured colorimetrically using several different chemistries. First, the phosphorous reacts with molybdate in an acidic solution to form a phosphomolybdate complex. For low range measurements, the phosphomolybdate complex is reduced with either an amino acid or ascorbic acid, creating a characteristic molybdenum blue species. High range measurements can be made by reacting the phosphomolybdate complex with vanadium reagent to create a yellow-colored product. Color intensity of both the blue and yellow complexes are proportional to the concentration of phosphorous in the sample.