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.
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•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.
Water pollution is a serious environmental problem affecting millions of people, and the demand for frequent water quality monitoring is increasing. The need for analytical platforms that combine ...high sensitivity, selectivity and accuracy with low cost, portability and user friendliness remains a challenge. Microfluidic paper-based analytical devices (μPADs) are recognised as a powerful analytical platform that can satisfy these requirements.
The aim of this review is to provide a detailed overview of the μPADs that have been developed for the determination of important water quality parameters, such as nutrients, metals and organic contaminants, in a range of waters. A description of the fabrication and detection methods selected for these applications is provided, and the performance of the μPADs with respect to their precision and accuracy is critically assessed. The potential of these devices for real-life applications is also critically examined, particularly if they can determine the concentrations specified in water quality guidelines or the maximum recommended concentrations for various waters, as well as if they are suitable for field applications.
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•Microfluidic paper-based analytical devices (μPADs) for water analysis are reviewed.•The detection methods used by these devices and their pros and cons are described.•Precision and accuracy of µPAD-based water analysis is assessed critically.•Ability of µPADs to meet legislative guidelines is discussed.•Suitability of µPADs for in-field measurements is examined.
A low-cost disposable colorimetric microfluidic paper-based analytical device (μPAD) was developed for the determination of nitrite and nitrate. Nitrite is determined directly by the Griess reaction ...while nitrate is first reduced to nitrite in a hydrophilic channel of the μPAD with immobilized zinc microparticles. This μPAD is fabricated by a simple and inexpensive inkjet printing method. Under optimal conditions, the limits of detection and quantification for nitrite are 1.0 and 7.8 μM, respectively, while the corresponding values for nitrate are 19 and 48 μM, respectively. The repeatability, expressed as relative standard deviation (RSD), is less than 2.9% and 5.6% (n ≤ 8) for the determination of nitrite and nitrate, respectively. This μPAD was successfully applied to the determination of nitrate and nitrite in both synthetic and natural water samples. It is user and environmentally friendly and suitable for on-site measurement of the analytes mentioned above in environmental and drinking waters.
An inexpensive, disposable and highly selective microfluidic paper-based analytical device (μPAD) is described for the determination of ammonia (molecular ammonia and ammonium cation) in wastewaters ...which implements for the first time a gas-diffusion separation step on a paper-based platform. Its hydrophilic reagent zones were defined by printing filter paper with a hydrophobic paper sizing agent using a conventional inkjet printer. The sample was introduced into the sodium hydroxide impregnated sample zone of the μPAD. This allowed the quantitative conversion of the ammonium ion to molecular ammonia which diffused across the hydrophobic microporous Teflon membrane of the device into an adjacent hydrophilic reagent zone containing the acid–base indicator 3-nitrophenol or bromothymol blue. The change in indicator color was measured using a desktop scanner for ammonia quantification. Under optimal conditions, the μPAD is characterized by a limit of detection of 0.8 and 1.8 mg N L–1 and repeatability of 3.1 and 3.7% (n ≥ 10, 20 mg N L–1), expressed as relative standard deviation, in the case of 3-nitrophenol or bromothymol blue, respectively. This μPAD was used successfully for the determination of ammonia in sewage and soil water samples. The small dimensions, minimal reagent consumption, low cost, simplicity of operation, and possibility of using a portable scanner make the proposed μPAD suitable for on-site ammonia monitoring in contaminated environmental waters and domestic, agricultural and industrial wastewaters. The successful implementation of the gas-diffusion approach on a paper-based platform is expected to result in the development of other μPADs for volatile analytes.
This review highlights the increasing interest in polymer inclusion membranes (PIMs) in analytical chemistry as they are adapted to new and novel applications. PIMs are polymer-based liquid membranes ...and were first introduced 50 years ago as the sensing membranes in ion-selective electrodes and optodes. More recently however, PIMs have been used for other applications in analytical chemistry such as for sample separation, sample pre-concentration, electro-driven extraction, and passive sampling, and have also been incorporated into on-line and automated analysis systems.
The present review provides a general overview of the analytical chemistry applications of PIMs reported in the literature to date and illustrates their versatility for solving challenging chemical analysis problems.
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•Polymer inclusion membranes (PIMs) are a valuable tool in analytical chemistry.•Improved sensitivity and selectivity can be achieved when using PIM-based sensors.•Analyte extraction and back-extraction can be performed simultaneously using PIMs.•Sample separation and pre-concentration can be achieved through the use of PIMs.•PIM-based passive sampling simplifies the analytical process and avoids biofouling.
This study contributes simultaneously to research on women board members and competitive dynamics by investigating two unresolved research questions: What is the effect of female directors on the ...firm’s competitive repertoire? Under what conditions is this effect more pronounced? Leveraging the “Awareness-Motivation-Capability” (AMC) framework, we predict that having women on the board of directors should impact the complexity, heterogeneity, and volume of the firm’s competitive moves. Relying upon a sample of U.S. pharmaceutical firms for the years 2000 to 2017, we find that adding female directors on the board positively affects the complexity and volume of a firm’s competitive moves, but negatively impacts the heterogeneity of competitive actions. In addition, the presence of a female CEO moderates these effects, leading to more complex competitive actions and increased volume. Thus, our study lends a greater understanding of how female board members influence competitive dynamics and shape the strategic direction of the firm.
The interest around polymer inclusion membranes (PIMs) has grown considerably in the past 6 years and, as a result, the number of research papers has risen exponentially. PIMs have been demonstrated ...to be useful for the selective extraction and recovery of numerous cations and anions and mimic the extraction properties of traditional solvent extraction and ion-exchange processes but have the advantage that extraction and back-extraction can be achieved in a single step. This review provides an overview of PIMs studies reported since 2005 and discusses extraction and transport studies according to the various base polymers, carriers, plasticizers and modifiers that have been used. Also discussed are the investigations of the PIM structure and stability in relation to other liquid membranes and the extension of the application of PIMs to separation problems involving analytical chemistry techniques such as flow analysis and analyte preconcentration.
► Research on polymer inclusion membranes (PIMs) since 2005 has been reviewed. ► A typical PIM consists of a base polymer and a carrier (extractant). ► In some cases a plasticizer or modifier may be added to the PIM composition. ► The effect of the composition of PIMs on their separation properties is discussed. ► An overview of PIM structural studies and analytical applications is presented.
Donnan dialysis technique was employed for the recovery of Au(III) from aqua regia solutions of electronic waste containing low concentrations of Au(III) in the presence of much higher concentrations ...of Cu(II) and Ni(II). The anion exchange pore-filled membrane was synthesized using in-situ photo-polymerization with polypropylene as the host membrane and (3-acrylamidopropyl)trimethyl ammonium chloride and (3-acryloxy-propyl)trimethoxysilane as the monomer and spacer, respectively. Photo-polymerization was carried out with ethylene glycol dimethacrylate as the cross-linker and 2,2-dimethoxy-2-phenyl acetophenone as the UV-initiator. The transport of Au(III) through this anion-exchange pore-filled membrane was studied using a two compartment cell where the receiver solution contained NaCl as the stripping reagent. Cu(II), which is the dominant metal ion in electronic waste solutions, did not interfere with the transport of Au(III) even at Au(III): Cu(II) mass ratios of up to 1:500. Gold from 100-fold diluted electronic waste solutions in aqua regia was successfully recovered (96.3%) after 4 cycles of Au(III) transport while at the same time insignificant fractions of the dominant metals in the electronic scrap (Cu and Ni) were transported. This study shows that Au(III) can be selectively recovered from diluted aqua regia solutions of electronic waste based on the proposed Donnan dialysis process.
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•An anion exchange pore filled membrane for separation of Au(III) was synthesized.•The transport of Au(III) through this membrane was optimized.•The interference in this transport of Cu(II) was studied.•Electronic scrap containing Au(III), Cu(II) and Ni(II) was dissolved in aqua regia.•Au(III) was separated from Cu(II) and Ni(II) using the newly developed membrane.
A PIM of composition 30wt% Cyphos® IL 104 and 70wt% poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) is described for the separation of Au(III) from its HCl solutions and from a solution ...obtained by the digestion of electronic scrap in aqua regia. This PIM has a high stability in acidic solutions such that no dilution of the digested aqua regia is necessary. Au(III) is back-extracted from the PIM using 0.50molL−1 Na2SO3. The stoichiometry of the extracted Au(III) ion-pair and the mechanism of back-extraction are discussed.
Transport studies using the proposed PIM with an electronic scrap solution in aqua regia as the feed solution and 0.50molL−1 Na2SO3 solution as the receiving solution showed complete transport of Au(III) from the feed to the receiving solution even in the presence of a number of other metal ions at much higher concentrations than that of Au(III), including Cu(II) which was present at a concentration more than 1500-fold higher than that of Au(III).
The proposed PIM-based technology is an attractive alternative to conventional solvent extraction methods since it eliminates the use of diluents which are often toxic, volatile and flammable. It successfully overcomes back-extraction issues associated with the use of other PIM-based systems and therefore it is expected to simplify and improve the efficiency of gold recovery from electronic scrap and jewellery.
•A polymer inclusion membrane (PIM) containing 30wt% Cyphos® IL 104 is prepared.•Poly(vinylidene fluoride-co-hexafluoropropylene) is used as the base-polymer.•Au(III) is separated from an undiluted aqua regia digest of electronic scrap.•Transport of other metal ions into the receiving solution is insignificant.•The PIM shows excellent stability when exposed to undiluted aqua regia.
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