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

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. [Display omitted] •Molybdenum blue chemistry for orthophosphate determination is discussed.•The choice of reductant determines the blue product(s) obtained.•Mechanisms are described for various additive and subtractive interferents.•The choice of strong mineral acid for the reaction should be considered.•Detailed recommendations are made for method optimisation.