The molybdenum blue reaction for the determination of …

 

 

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Disclaimer: This is a pre-publication version. Readers are recommended to consult the full
published version for accuracy and citation. Published in Analytica Chimica Acta, 890, 60-
82 (2015) doi: 10.1016/j.aca.2015.07.030.

The molybdenum blue reaction for the determination of

orthophosphate revisited: Opening the black box

Edward A. Nagula,b, Ian D. McKelviea,c, Paul Worsfoldc, Spas D. Koleva,b,*

aSchool of Chemistry, The University of Melbourne, Victoria 3010, Australia

bCentre for Aquatic Pollution Identification and Management (CAPIM), The University of

Melbourne, Victoria 3010, Australia

cSchool of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth

Abstract

PL48AA, UK

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 core

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.

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Keywords: Molybdenum blue reaction; orthophosphate; dissolved reactive phosphate;

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phosphomolybdate

* Corresponding author: Phone: +61 3 83447931; Fax: +61 3 93475180; Email: s.kolev@unimelb.edu.au

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1.

Introduction

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2.

Chemistry of the phosphomolybdenum blue (PMB) reaction

2.1. Reaction overview

2.2. Mo(VI) speciation and 12-molybdophosphoric acid (12-MPA) formation

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2.3. Redox chemistry of PMB

2.3.1. Reduction of 12-MPA

2.3.2. Spectral features of PMB

2.3.3. Nature of the reduced products

2.3.4. Isomerism of 12-MPA and its reduced forms

3.2. The reagent blank: isopolymolybdenum blue species

2.3.5. Organic reductants

2.3.6. Metallic reductants

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3.

PMB method optimisation

3.1. Reagent concentrations

3.3.

Product stability

3.4. Method linearity

3.5. Choice of acid

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4.

Interferences

4.1. Additive interferences

4.1.1. Arsenate

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4.1.2. Silicate

4.1.3. Organic and inorganic P hydrolysis

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4.2.

Subtractive interferences

4.2.1. Organic acids

4.2.2. Fluoride

4.2.3. Chloride (salt error)

4.3. Multifunctional interferents

4.3.1. Sulfide

4.3.2. Iron

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4.3.3. Surfactants

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5.

PMB chemistry in flow methods

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Conclusions and recommendations

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6.1. Recommended reductants

6.2. Recommended acids

6.3. Recommended optimisation procedure

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Acknowledgements

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References

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List of Abbreviations

(br): Broad absorption band

(sh): Absorption shoulder

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12-MPA: 12-Molybdophosphoric acid (H3PMo12O40)

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11-MPA: 11-Molybdophosphoric acid (H3PMo11O37)

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12-MSA: 12-Molybdosilicic acid (H4SiMo12O40)

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AA: Ascorbic acid

ANS: 1-Amino-2-naphthol-4-sulfonic acid

AsMB: Arsenomolybdenum blue

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DA: Discrete analyser

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DAPH: 2,4-Diaminophenol dihydrochloride

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DOP: Dissolved organic phosphorus MB: Molybdenum blue

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DRP: Dissolved reactive phosphorus

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ESI-MS: Electrospray ionisation mass spectrometry

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FIA: Flow injection analysis

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HQ: Hydroquinone

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HS: Hydrazine sulfate

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IVCT: Intervalence charge transfer

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LMCT: Ligand-metal charge transfer

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Metol: 4-(Methylamino)phenol sulfate

MRP: Molybdate reactive phosphorus

PMB: Phosphomolybdenum blue

rFIA: Reverse flow injection analysis

SFA: Segmented continuous flow analysis

SIA: Sequential injection analysis

SiMB: Silicomolybdenum blue

TDP: Total dissolved phosphorus

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1. Introduction

Orthophosphate is a key water quality parameter and spectrophotometric detection using the

molybdenum blue (MB) reaction is the most common means of determination [1]. It can also be

used for the spectrophotometric determination of silicate, arsenate and germanate. Strictly, this

reaction determines the ‘molybdate reactive phosphorus’ (MRP) fraction which includes other labile

phosphorus species in addition to orthophosphate [2] as discussed in Section 4.1.

The reaction involves the formation of a polyoxometallate species, a heteropoly acid, from

orthophosphate and molybdate under acidic conditions, which is then reduced to form an intensely

coloured phosphomolybdenum blue (PMB) species. This reaction was mentioned by Scheele in

1783, but is widely attributed to Berzelius (1826) [3]. It was not until 1934, however, that Keggin

proposed the structures of a range of 12-heteropoly acids [4] (Fig. 1). ‘Molybdenum blue’ refers not

to a single species, but rather to a family of reduced molybdate compounds, which may or may not

contain a heteroatom, e.g. phosphorus. Distinction between heteropoly (containing a hetero-atom)

and isopoly (containing no hetero-atom) molybdenum blue species is made in this review where

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necessary.

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Figure 1. Structure of the Keggin ion [PW12O40]3-, analogous to that of [PMo12O40]3-. The black,

grey and white spheres represent P, W and O respectively. Reproduced from Ref. [5] with

permission from The Royal Society of Chemistry

A fundamental knowledge of the inorganic chemistry of the MB reaction is important for

optimising its analytical application for the determination of phosphate. In particular, the

concentrations of the reagents can be optimised to maximise the degree of product formation and

product stability (for batch methods) and achieve good precision and accuracy.

This review systematically summarises the fundamental chemistry of the MB reaction and discusses

the optimal conditions for the selective determination of MRP using batch methods under

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equilibrium conditions. The additional requirements for non-equilibrium, flow-based methods are

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also considered.

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2. Chemistry of the molybdenum blue reaction

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2.1. Reaction overview

The MB reaction occurs in two stages; the first stage involves the formation of a Keggin ion around

the analyte anion and the second stage entails the reduction of this heteropoly acid to form a deeply

blue-coloured product. These stages can be described in the simplified forms shown in Eqs. (1) and

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(2) [2].

PO4

3- + 12MoO4

2- + 27H+  H3PO4(MoO3)12 + 12H2O

H3PMo(VI)12O40 + Reductant  [H4PMo(VI)8Mo(V)4O40]3-

(1)

(2)

All MB methods require a strong acid, a source of Mo(VI) and a reductant, normally in aqueous

solution. The concentrations of acid and molybdate are vital, not only for the formation of the

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heteropoly acid but also for controlling its reduction. It is well-known that orthophosphate (PO4

3-),

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