How is the conversion factor from orthophosphate (PO?) to phosphorus pentoxide (P2O5) determined?

Document ID

Document ID TE378

Version

Version 2.0

Status

Status Published

Published Date

Published Date 27/05/2017
Question
How is the conversion factor from orthophosphate (PO?) to phosphorus pentoxide (P2O5) determined?
Summary
Convert orthophosphate (PO₄) to phosphorus pentoxide (P2O5)
Answer
The Water Analysis Handbook and most other procedural publications have a conversion chart at the end of the phosphate procedure showing a conversion factor from orthophosphate (PO₄) to phosphorus pentoxide(P2O5) as 0.7473. This conversion is NOT based on the molecular weight of the two compounds. In solution phosphorus pentoxide converts to orthophosphate (and acid). The conversion of .75 is based on the rate of that reaction.