Molybdenum blue reaction for the determination of orthophosphate
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. (C) 2015 Elsevier B.V. All rights reserved.
Nagul, E. A., McKelvie, I. D., Worsfold, P., and Kolev, S. D.,The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box, Analytica Chimica Acta, 2015, 890, 60-82.
Ultrasensitive visual sensing of molybdate based on enzymatic-like etching of gold nanorods
Here, we have developed a novel approach to the visual detection of molybdate with high sensitivity and selectivity in aqueous media based on the combination of catalytic formation of iodine and iodine-mediated etching of gold nanorods. In weak acid solution, like peroxidase, molybdate can catalyze the reaction between H2O2 and I- to produce I2, a moderate oxidant, which then etches gold nanorods preferentially along the longitudinal direction in the presence of hexadecyltrimethylammonium bromide. The etching results in the longitudinal localized surface plasmon resonance extinction peak shifts to short wavelength, accompanied by a color change from blue to red. Under optimal conditions, this sensor exhibits good sensitivity with a detection limit of 1.0 nM. The approach is highlighted by its high selectivity and tolerance to interference, which enables the sensor to detect molybdate directly in real samples, such as tap water, drinking water, and seawater. In addition, perhaps the proposed sensing strategy can be also used for other targets that can selectively regulate the formation of I2 under given conditions.
Zhang, Z. Y., Chen, Z. P., and Chen, L. X.,Ultrasensitive Visual Sensing of Molybdate Based on Enzymatic-like Etching of Gold Nanorods, Langmuir, 2015, 31, 9253-9259.
Simultaneous speciation analysis of chromate, molybdate, tungstate and vanadate in welding fume alkaline extracts by HPLC-ICP-MS
A novel analytical procedure was developed for the simultaneous speciation analysis of chromate, molybdate, tungstate and vanadate by anion-exchange high performance liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC-ICP-MS). Linear gradient elution from 100% water to 100% 0.7 M NaCl was applied for chromatographic separation of metal species. In standard aqueous solution at neutral pH molybdate, tungstate and vanadate exist in several aqueous species, while chromate is present as a single CrO42- species. Consequently, only chromate can be separated from this solution in a sharp chromatographic peak. For obtaining sharp chromatographic peaks for molybdate, tungstate and vanadate, the pH of aqueous standard solutions was raised to 12. At highly alkaline conditions single CrO42, MoO42 and WO42 are present and were eluted in sharp chromatographic peaks, while VO43- species, which predominates at pH 12 was eluted in slightly broaden peak. In a mixture of aqueous standard solutions (pH 12) chromate, molybdate, tungstate and vanadate were eluted at retention times from 380 to 420 s, 320 to 370 s, 300 to 350 s and 240 to 360 s, respectively. Eluted species were simultaneously detected on-line by ICP-MS recording m/z 52, 95, 182 and 51. The developed procedure was successfully applied to the analysis of leachable concentrations of chromate, molybdate, tungstate and vanadate in alkaline extracts (2% NaOH+3% Na2CO3) of manual metal arc (MMA) welding fumes loaded on filters. Good repeatability and reproducibility of measurement (RSD+/-3.0%) for the investigated species were obtained in both aqueous standard solutions (pH 12) and in alkaline extracts of welding fumes. Low limits of detection (LODs) were found for chromate (0.02 ng Cr mL-1), molybdate (0.1 ng Mo mL-1), tungstate (0.1 ng W mL-1) and vanadate (0.2 ng V mL-1). The accuracy of analytical procedure for the determination of chromate was checked by analysis of CRM 545, Cr(VI) in welding dust loaded on a filter. Good agreement between determined and reported certified values was obtained. For molybdate, tungstate and vanadate the assessment of accuracy was performed by spiking welding fume filters. Good recoveries for all investigated species (98-101%) confirmed the accuracy of the analytical procedure.
Scancar, J., Berlinger, B., Thomassen, Y., and Milacic, R.,Simultaneous speciation analysis of chromate, molybdate, tungstate and vanadate in welding fume alkaline extracts by HPLC-ICP-MS, Talanta, 2015, 142, 164-9.
Bead sorbent Perloza MT 50 was used for selective removal of metal W, Mo, V, Ge, and Sb oxoanions. All experiments were carried out by dynamic column sorption. Sorption of tungstate and molybdate anions was successful. The sorption capacity decreased with increasing concentration of accompanying anions (chlorides, sulphates) and with increasing pH (3.5-5.5). Sorption of vanadate anion was possible but the sorption capacity was very low. Sorption of Ge(IV) and Sb(III) oxoanion was negligible.
Mistova, E., Parschova, H., and Matejka, Z., Selective sorption of metal oxoanions from dilute solution by bead cellulose sorbent, Separation Science and Technology, 2007, 42, 1231-1243.
Molybdenum concentration is usually very low in environmental samples and the sample matrix may cause serious interferences during measurement. Preconcentration and separation methods are needed to solve these problems and render more sensitive, accurate and interference-free determination. Recent developments in sample treatment — solid phase and liquid-liquid extraction as well as coprecipitation — are presented, including flow-based methodology. In addition, important extension and improvements in analytical methods for determinations of molybdenum are updated. Some examples of speciation analysis are also presented.
Pyrzynska, K., Determination of molybdenum in environmental samples, Analytica Chimica Acta, 2007, 590, 40-48.
Water molybdenum analysis
A cloud-point extraction process using micelle of the cationic surfactant CTAB to extract Mo(VI) from aqueous solutions was investigated. The detection limit of the method was 0.1 ng mL-1. The relative standard deviationand relative error for five replicate measurements of 65.0 ng mL-1 Mo(VI) were 1.1% and 1.9%. The method was applied to the determination of molybdenum(VI) in steels and tap water and well water samples.
Madrakian, T. and Ghazizadeh, F., Cloud-point preconcentration and spectrophotometric determination of trace amounts of molybdenum(VI) in steels and water samples, Journal of Hazardous Materials, 2008, 153, 695-700.
Trace molybdenum in water and biological samples
A new method has been developed for the determination of trace molybdenum based on separation and preconcentration with TiO2 nanoparticles immobilized on silica gel (immobilized TiO2 nanoparticles) prior to its determination by graphite furnace atomic absorption spectrometry (GFAAS). Molybdenum can be quantitatively retained by immobilized TiO2 nanoparticles at pH 1.0 and separated from the metal cations in the solution, then eluted completely with 0.5 mol L-1 NaOH. The detection limit of this method for Mo was 0.6 ng L-1 with an enrichment factor of 100, and the relative standard deviation (RSD) was 3.4% at the 10 ng mL-1 Mo level. The method has been applied to the determination of trace amounts of Mo in biological and water samples.
Liang, P., Li, Q., and Liu, R., Determination of trace molybdenum in biological and water samples by graphite furnace atomic absorption spectrometry after separation and preconcentration on immobilized titanium dioxide nanoparticles, Microchimica Acta, 2009, 164, 119-124.
Trace molybdenum in environmental and biological samples
A novel and sensitive spectrophotometric method for the determination of molybdenum at trace levels in environmental and biological samples is proposed. The method is based on the reaction of Mo(V) with thiocyanate (SCN-) and methyltrioctyl ammonium chloride (MTOAC) in acidic medium. The red colored complex of molybdenum is extracted with N-phenylbenzimidoyl thiourea (PBITU) in 1-pentanol for its determination by spectrophotometry. The sensitivity of the present method is higher than other conventional thiocyanate method, due to the use of MTOAC in liquid-liquid extraction. The value of molar absorptivity of the complex with respect to molybdenum is 7.6 x 104 Lmol-1 cm-1 at 470 nm. The limit of detection of the metal is 5 ng mL-1. The system obeys Beer's law between 20 and 1000 ng mL-1 with slope, intercept and correlation coefficient values of 0.81, 2.5 x 10-3 and +0.999, respectively. Most of the metal ions tested did nor interfere in the determination of molybdenum. The proposed method has been applied for the determination of the rnolybdenum in environmental and biological samples.
Shrivas, K., Agrawal, K., and Harmukh, N., Trace level determination of molybdenum in environmental and biological samples using surfactant-mediated liquid-liquid extraction, Journal of Hazardous Materials, 2009, 161, 325-329.