What could cause total phosphorus results of a Hach method to differ from an outside lab by about three fold?

Document ID

Document ID TE368

Version

Version 4.0

Status

Status Published

Published Date

Published Date 08/09/2018
Question
What could cause total phosphorus results of a Hach method to differ from an outside lab by about three fold?
Summary
Total phosphate results are three times higher when analyzed by an outside lab
Answer
Total phosphorus results of a Hach method may differ from results from an outside lab by about three fold if the chemical form being reported is not the same. Phosphorus readings are typically displayed in one of two chemical forms: as orthophosphate (PO4) or as phosphorus (PO4-P). A reading displayed as PO4 can be converted to PO4-P by multiplying the result by a factor of 0.3261.

Please also see What is the factor to convert from PO4-P to PO4? and Does converting a measurement from PO4-P to PO4 determine the total phosphorus concentration?
 

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