The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box

Abstract

The molybdenum blue reaction, used predominantly for the determination of orthophosphate in environmental waters, has been perpetually modified and re-optimised over the years, but this important reaction in analytical chemistry is usually treated as something of a 'black box' in the analytical literature. A large number of papers describe a wide variety of reaction conditions and apparently different products (as determined by UV-visible spectroscopy) but a discussion of the chemistry underlying this behaviour is often addressed superficially or not at all. This review aims to rationalise the findings of the many 'optimised' molybdenum blue methods in the literature, mainly for environmental waters, in terms of the underlying polyoxometallate chemistry and offers suggestions for the further enhancement of this time-honoured analytical reaction.

A Lab-On-Chip Phosphate Analyzer for Long-term In Situ Monitoring at Fixed Observatories: Optimization and Performance Evaluation in Estuarine and Oligotrophic Coastal Waters

“…The sensitivity and selectivity of the molybdenum blue assay is strongly affected by the Mo(VI) and acid concentrations in the reaction mixture, particularly when the reaction is carried out at room temperature (Nagul et al, 2015). The effects of these parameters were investigated using Sensor 1, by producing calibration curves (0, 1, 2 µM PO 3− 4 ) at increasing Mo(VI) concentrations (1.1-5.6 mM) while the reaction pH was fixed at set values (pH 0.9, 1.0, 1.1, and 1.3).…”

“…Although there is a vast amount of literature on the molybdenum blue chemistry (reviewed in Nagul et al, 2015), determining the optimum reaction conditions from the literature alone is a difficult task, as a wide variety of conditions and analytical protocols (batch vs. various flow methods) have been reported. In addition, the operating principles of the LOC sensor differ fundamentally from that of traditional methods in that the mixing of the analyte and reagents occurs via diffusional mixing within a microfluidic measurement cell with a large surface area to volume ratio.…”

“…While the Mo(VI) concentration and acidity of the reaction must be carefully optimized, the molybdenum blue assay tolerates a much greater range of ascorbic acid concentrations (Nagul et al, 2015). To maximize the reaction rate, a 19 mM ascorbic acid concentration was adopted.…”

Applying Different Sample Treatment Strategies for Evaluating Phosphorus Distribution in Orange Juice

“…20,21 Briefly, the method consists of preparing a mixed solution composed by ammonium molybdate (5 mmol L ) as catalyzer, according to the reaction described in the equations 1 and 2. In a polyethylene flask, it was added 1.5 mL of mixed solution, 1.0 mL of sample, 1.0 mL of ultrapure water, 1.0 mL of ascorbic acid and 0.5 mL of the catalyzer solution, in this sequence.…”

“…In this context, the spectrophotometric method of the molybdenum blue is one of the oldest, well established and widely employed for P free determination in different samples. 20,21 However, studies about P fractions have been conducted almost totally for geological matrix.…”