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Soluble Reactive Phosphorus Measurements in Lake Water: Evidence for Molybdate-enhanced Hydrolysis¹

ABSTRACT

Measurements of ortho-phosphate (PO₄-P) concentrations in natural waters obtained by molybdenum blue methodology are based on the assumption that molybdate (Mo) complexes only with PO₄-P (in the absence of arsenate and silicate interference) to form the reductant-sensitive complex 12-molybdophosphoric acid (12-MPA). The hypothesis that Mo causes or accelerates PO₄-P release from bound sources or forms reductant-sensitive complexes with organic compounds (organic-PO₄-Mo) was tested by exposing filtered Lake Michigan water to acid, acid plus Mo simultaneously, and Mo, respectively, before measuring PO₄-P by the Chamberlain-Shapiro extraction method. Not only does Mo accelerate hydrolysis in the presence of acid, but it also either causes hydrolysis or forms organic-PO₄-Mo before samples are acidified. Although the relative amounts of 12-MPA and organic-PO₄-Mo formed in lake water cannot be assessed quantitatively, a major fraction of the blue color formed during routine analysis by this or by similar methods apparently is not the product of acid hydrolysis, but may also be due to reduction of organic-PO₄-Mo. Future developments in molybdenum blue methodology aimed at minimizing hydrolysis must consider the sequence of reagent additions and Mo, as well as acid, contact times.

Key Words: ortho-phosphorus • molybdenum blue methodology • organic compounds

¹ GLERL Contribution no. 301, Great Lakes Environmental Research Lab., National Oceanic and Atmospheric Admin., 2300 Washtenaw Ave., Ann Arbor, MI 48104.

² Research Scientist, Great Lakes Environ. Res. Lab., Ann Arbor, Mich.

Received for publication April 5, 1982.

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