Phosphorus

What is phosphorus?

Phosphorus (P) is a chemical element with an atomic number of 15 and an atomic mass of 31. Due to its high reactivity, phosphorus does not naturally exist as a free element. Phosphorus generally occurs as phosphate in minerals. Phosphate is typically found in the Earth’s crust at a concentration of about 1 gram per kilogram.

The two types of elemental phosphorus are white phosphorus and red phosphorus. When exposed to oxygen, white phosphorus emits a faint glow caused by oxidation (also known as chemiluminescence).

Phosphorus belongs to the pnictogen group of elements which consists of nitrogen, phosphorus, arsenic, antimony, bismuth and ununpentium (recently assigned the name moscovium). These elements are grouped because of their similar atomic structure, which lends them the ability to form stable compounds due to their tendency to form double and triple covalent bonds. Except for nitrogen (which remains a gas), the other pnictogens are solids at room temperature.

Complex organisms such as plants and animals need phosphorus because phosphates are a component of DNA, RNA, ATP and phospholipids. Phosphorus is mined for use in detergents, pesticides, nerve agents and predominantly fertilizers.

Phosphates/Orthophosphate

In orthophosphate (one of the most common phosphates), one phosphorus atom is bonded to four oxygen atoms. Orthophosphate is sometimes called “reactive phosphorus” because it bonds easily with other electron-deficient elements and compounds since the three “extra” electrons on the oxygen atoms strongly want to bond with protons alike.

Condensed Phosphates

Condensed phosphates are multiple orthophosphate molecules “condensed” together and sharing a covalent bond between adjoining phosphorus (P) and oxygen (O) atoms. This group includes metaphosphate, pyrophosphate and polyphosphate — which are often used for corrosion control in drinking water distribution systems.

Total Phosphorus/Organic Phosphorus

Total phosphorus is a sum of all the phosphorus present: orthophosphate/phosphates, condensed phosphates and organic phosphorus. Organic phosphorus is usually present in the form of phosphates contained inside or bonded to an organic compound.

Hach has several online phosphorus analyzers to address the needs of several applications and their unique requirements.

Phosphax sc is the standard for municipal Wastewater applications with its robust sampling system and outdoor enclosure.

5500sc Phosphate Analyzer is an ideal solution for power applications, providing accurate and reliable phosphate measurements in the low range where precision is crucial.

EZ Series analyzers offer you the ultimate in flexibility, with three analytical methods, a variety of measuring ranges, and multi-stream capabilities (up to 8 channels).

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 (PO43-) 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 = acidphosphorus – 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- PO43-) = 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.

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