Phosphates, phosphites, and phosphides in environmental samples.

The common assumption that phosphorus occurs exclusively as phosphate in the environment is deserving of increased scrutiny. If a sample contained reduced phosphorus compounds (P in an oxidation state of less than +5), standard methods of phosphorus determination would incorrectly classify the compounds mostly as organic P, although significant fractions were sometimes misclassified as orthophosphates and condensed P. The disappearance of gaseous hydrogen phosphide (PH3) from samples was a function of solution composition, in that certain acids and metals enhanced removal whereas other constituents increased PH3 stability. No previously used extraction method could detect a significant portion of reduced phosphorus in representative samples by measuring PH3 evolution, particularly for highly recalcitrant iron phosphides. Despite analytical limitations, clear evidence was gathered that reduced phosphorus compounds can be leached from cast iron to water and that reduced phosphorus is also present in the scale (rust) that forms on the metal.