Soluble Reactive Phosphorus Measurements in Lake Water: Evidence for Molybdate‐enhanced Hydrolysis

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.

Review of robust measurement of phosphorus in river water: sampling, storage, fractionation and sensitivity

“…In general, the extent of interference in colorimetric P determination depends on the concentration of the interfering agent (particularly Si, As and F) in the sample matrix, the temperature of the reaction and the concentration of P. Samples from polluted rivers with high concentrations of interfering agents and from those rivers with low P concentrations are likely to be more sensitive to interference (MEWAM, 1980). A disadvantage of using the phosphomolybdenum blue method for determining orthophosphate concentrations as 'SRP' is the potential for hydrolysis of labile organic-P and condensed-P compounds and displacement of P from colloids in the <0.45 µm fraction filtrate, resulting in possible over-estimation of orthophosphate concentrations (Tarapchak, 1983;Baldwin, 1998;Denison et al, 1998). Many organic-P and condensed-P compounds may be hydrolysed in the low-pH conditions used in the colorimetric procedure and the molybdate ion can catalyse the hydrolysis of organic-P compounds (Tarapchak, 1983).…”

“…A disadvantage of using the phosphomolybdenum blue method for determining orthophosphate concentrations as 'SRP' is the potential for hydrolysis of labile organic-P and condensed-P compounds and displacement of P from colloids in the <0.45 µm fraction filtrate, resulting in possible over-estimation of orthophosphate concentrations (Tarapchak, 1983;Baldwin, 1998;Denison et al, 1998). Many organic-P and condensed-P compounds may be hydrolysed in the low-pH conditions used in the colorimetric procedure and the molybdate ion can catalyse the hydrolysis of organic-P compounds (Tarapchak, 1983). Use of ion exchange resins offers the possibility of separating inorganic orthophosphate in natural waters (Westland and Boisclair, 1974).…”

Nutrient exchange in an Antarctic macrolichen during summer snowfall–snow melt events

“…Phosphate (soluble reactive phosphorus -see Tarapchak (1983) and discussion below) was determined using the malachite green variant of the phosphomolybdate method after Van Veldhoven & Mannaerts (1987) but with the modification that the malachite green concentration in reagent C was reduced from 0n035 to 0n0075 % to lower the absorbance of blank samples. Phosphate determinations were made on meltwater samples before filtration since filtering lowered the concentrations of phosphate, and absorbance was measured using a GBC 916 UV\VIS spectrophotometer.…”

A Modified Molybdenum Blue Method for Orthophosphate Determination Suitable for Investigating Enzymatic Hydrolysis of Organic Phosphates

“…It involves the reaction of acid ammonium molybdate with P i ions to form phosphomolybdenum complexes, which are reduced to molybdenum blue by ascorbic acid. However, it is known that the assay conditions used can cause partial hydrolysis of some labile organic phosphate (P o ) and condensed P i , resulting in overestimation of P i (Lowry and Lopez 1946;Dick and Tabatabai 1977;Tarapchak 1983). Partly because of this reason, "molybdate reactive P" has been proposed to describe the P determined by the Murphy-Riley method (Ron Vaz et al 1993;Haygarth and Sharpley 2000).…”