Sample Preparation Guides

General Information

Occurrence – Phosphorous (P) is in group 15 just below N and a nonmetal. Its chemistry resembles that of nitrogen more than that of arsenic.  The average abundance of Phosphorous is ~1000 ppm in the Earth’s crust making it the 11^th most abundant element just behind hydrogen.  It is found in nature as phosphate. The term phosphate is used with compounds containing any number of phosphate units [PO₄^-3]   ≡ [O=P(OH)₃].  Orthophosphate is the simplest form among other phosphates. It consists of only one phosphate unit.  Orthophosphate is an essential nutrient for all life forms and naturally occurs.  Polyphosphates are formed by joining two or more orthophosphates via condensation as shown:                             

2 O=P(OH)₃   → O=P(OH)₂-O-(OH)₂P=O + H₂O

phosphoric acid→ Dipolyphosphoric acid

Polyphosphates are man-made and are commonly used in detergents and soaps. In water they will decompose to the orthophosphate.

The Federal government limits for P in streams/rivers, streams entering lakes, and lakes are 0.1, 0.05 and 0.025 ppm, respectively.  State criteria need to be consulted because some state criteria will differ from the Federal limits.²

White, red, and black phosphorus are the most important allotropic forms of phosphorus.  Elemental phosphorus is rarely encountered but if present in a sample it will decompose to the phosphate by thermal fusion or acid digestion sample preparations.

Uses - The commercial source of P is phosphate rock.  By far the major use of P is in fertilizers. P is also used in steel making, Phosphor bronze, alloys for bullets, as a cleaning agent (Na₃PO₄, also called TSP), in making fine China, fireworks, safety matches, and baking powder.  

Chemical Properties – Phosphorus (P) has an atomic number of 15 and an atomic mass of 30.97376 g/mol. ³¹P (100%) in nature occurs as the mono isotope. Typically, it is found in the +3 and +5 oxidation states. Phosphorus forms a phosphorous series of acids where the P is +3 and serves as a reducing agent. Phosphorus also forms a phosphoric acid series of acids in which the P oxidation state is +5 and can serve as an oxidizing agent. In both series the P is 4-coordinate and tetrahedrally surrounded.   These phosphate ions can condense to form isopolyacids.

Phosphorus Chemistry as Performed and Observed at IV

Typically, Inorganic Ventures uses concentrated H₃PO₄ acid as the starting material that is prepared and/or purchased to be highly pure. Custom products may use different forms of P depending upon the product. The purity is confirmed (typically 99.9998+% ) using ICP-MS and ICP-OES. The trace metallic impurities are reported on the Certificate of Analysis for each lot of product that is produced.

Sampling and Handling

Phosphorus exists in several (allotropic) forms with the most common being white, yellow, red, black, and purple Phosphorus.

White phosphorus is colorless with a waxy appearance. Density = 1.82 g/cm³, melting point 44.3°C, Boiling Point = 280 , molecular weight P₄.

Yellow phosphorus is a transparent solid that becomes coated with a thin white film when placed in water containing air. Heated above the boiling point in the absence of air it sublimes as a colorless gas depositing lustrous, transparent crystals.  At low temperatures it oxidizes slowly in air and igniting spontaneously at 60⁰ C giving a bright yellow light with lots of heat. It must be kept underwater. Handle yellow P carefully because it causes very painful, slowly healing burns. It is fatal if taken internally at doses of 0.1 grams. When heated in sealed tubes (to exclude oxygen) it changes to the red form.

Red phosphorus is a dull red powder. It does not have a definite density or melting point but is a mixture of the white and purple forms. It is only slightly poisonous, not as reactive, and needs no special precautions for its preservation (distinctive from the white variety). Two other forms are the black and purple forms of phosphorus, which are the most stable of all.

For general information on sampling and sub-sampling then please see Chapter Three of our Trace Analysis Guide.

Chemical Compatibility: H₃PO₄ is soluble in HCl, HNO₃, H₂SO₄, HF, water, and NH₄OH. P is stable with all metals and inorganic anions at low to moderate ppm levels under acidic conditions; precipitates with several metals occur in neutral media at higher concentrations.

Stability: Phosphorus has demonstrated stability in 0-1% HNO₃ / LDPE container. 1-10,000 ppm solutions chemically stable for years.

Contamination Risk – Due to the relatively high abundance of P in the Earth’s Crust it is recommended that the analyst use HEPA filtered air and a clean environment such as that found in clean rooms as described in the Inorganic Ventures literature. For more on sample contamination risks see Chapters Eight, Nine and Ten of our Trace Analysis Guide.

The Element, Oxides, Ores, and Organics

Element – The element is never found free in nature and is found as phosphate.

Oxides – Oxides are water soluble.

Ores – Ores are naturally occurring as the phosphate, except for a few rare minerals found in meteorites which can be prepared by a Na₂CO₃ fusion in Pt⁰ crucibles.¹ For more information on acid digestions of inorganic samples then please see Chapter Eleven of our Trace Analysis Guide.

Organics – Perform a dry ash mixed with Na₂CO₃ in Pt⁰ crucibles at 450 °C then increase heat to 1000 °C to fuse, or perform a H₂SO₄ / H₂O₂ acid digestion.¹

Testing Methods

Phosphorus can be difficult to measure in trace amounts due to the high ionization potential of the element, thus detection limits for ICP instruments will be relatively high. When measuring phosphorus on ICP-OES the emission lines 178.287, 177.495, and 213.618 nm are all good choices. It is important to note that potential interferences on the 177.287 nm and 213.618 nm lines may appear close to the left shoulder of the phosphorus peaks. When using those lines you may need to consider setting your background based on right-hand points only. There is also a major interference present on the 178.287 nm emission line from iodide, so it is best to avoid this line if iodide is present in your sample.

The combination of a high ionization potential with the presence of atmospheric polyatomic interferences from C, O, H, and N makes analyzing phosphorus by ICP-MS especially difficult. Phosphorus is monoisotopic at m/z 31. With no isobaric interferences on ³¹P the only doubly charged interference may come from ⁶²Ni⁺⁺. Detection limits on a single quadruple ICP-MS instrument will usually fall within the range of 5 – 10 ppb when using a no gas or He KED operating mode. Developments in triple quadrupole ICP-MS instrumentation allows for better detection limits (<1 ppb) when using H₂ or O₂ reaction gasses to mass shift ³¹P to either m/z 35 (³¹P¹H₄⁺) or m/z 47 (³¹P¹⁶O⁺). 

1. Bock, R. (1979). A Handbook of Decomposition Methods in Analytical Chemistry. Weinheim/Bergstr: Verlag Chemie GmbH.

2. Standard Methods for the Examination of Wastewater. (2012). Washington, DC: American Public Health Association.