Determination of phosphorus

Abstract

Phosphorus occurs in various forms everywhere on Earth and in living organism. It has been widely used as fertilizer and pesticide for agriculture and as final products and intermediates for industry.1,2 The determination of phosphorus has been of great importance for various tasks: understanding material circulation, checking the quality of waste water, and identifying toxicity. The molybdenum blue method, which is well known for the spectrophotometric determination of phosphorus as orthophosphate, has been extensively investigated for a long time.3,4 Orthophosphate is reacted with molybdate(VI) in a highly acidic aqueous solution to form molybdophosphates, e.g. [PMo12O40]3-, both of which are redox active. Thus, the addition of reductants, such as ascorbic acid, to molybdophosphate containing solution converts molybdophosphates to intense blue-colored species, which is called as heteropoly blue. The amount of phosphorus is estimated from the absorbance of heteropoly blue species. However, such species are not fully characterized, even now, because they could exist as various types of complexes by the addition of different reductants, metals and organic solvents.3 The molybdenum blue method has been used for the determination of phosphorus in waste water as an official method in Japan, so-called Japanese Industrial Standards (JIS).5 Even a batch method enables one to determine phosphate at the sub ppb level.6 Furthermore, flow injection and sequential injection systems provide more sensitive determinations of phosphate with high reproducibility.4,7 On the other hand, phosphate sensors have also been developed to achieve simple, fast, low-cost and reproducible determination methods. Potentiometric measurements using modified molybdenum metal and Ag/AgCl/sat. KCl electrode can determine HPO4 2-in the concentration range from 1.0 x 10-5 to 0.10 M (mol/dm3) at pH 8.0-9.5 with a detection limit of 1.0 x 10-6 M in real samples: beverages and tap water.8 In addition, this modified molybdenum electrode can be used for one month. A zirconium-deposited graphene oxide modified pencil graphite electrode dipped in a phosphate-containing solution was used as a working electrode to measure voltammograms of 0.5 mM [Fe(CN)6]3-at pH 5; the obtained oxidation current magnitude decreased depending on the concentration of phosphate, leading to the determination of phosphate in a concentration range from 0.01 to 1 μM with RSD of 2.42%.9 This method can determine phosphate in human serum. A microsensor of phosphate was fabricated with a micro membrane doped with bis(dibromophenylstannyl)-methane as an ionophore set in a capillary. This microsensor gives a Nernstian response in a phosphate concentration range from 0.5 to 50 μM.10 This sensor gives a less response 5 h after its fabrication, unfortunately. Among various phosphates, adenosine triphosphate (ATP) plays a very important role as an energy storage molecule in living organisms.1 Hairpin-shaped DNA templated copper nanoparticles can specifically respond to ATP in a concentration range of 0.02-4 μM, even in the presence of adenosine, uridine triphosphate, guanosine triphosphate and cytidine triphosphate.11 The hydrolysis of adenosine triphosphate catalyzed by hydrolases produces pyrophosphate as well as phosphate. Pyrophosphate reacts with molybdates in an acidic organic water mixed solution to form redox active molybdopyrophosphates, which can be converted to molybdenum blue species with reductants. This reaction leads to the direct determination of pyrophosphate.12,13 Direct determination methods of pyrophosphate have been developed without using the molybdenum blue method. Di-(2-picolyl) amine introduced poly(phenylene)ethylene exhibit fluorescence, of which the intensity decreases in the presence of Cu2+ and increases upon the addition of pyrophosphate. The "on" and "off" property of fluorescence enables one to determine 0.5-12.0 μM pyro phosphate in blood serum and synovial fluid.14 Copperdipicolyiamine-hydroxycourmarin carbonate bound on a [Ru(bpy)3]2+-doped silica nanoparticle, which exhibits fluorescence, can recognize pyrophosphate as well as triphosphate at pH 7.3.15 Since many of pesticides contain phosphorus sensitive determinations of pesticide residue in food and various agricultural products is essential to keep us healthy. Gas chromatography, HPLC, GC-MS and LC-MS/MS have been used for the determination of pesticides. However, issues on sensitivity and resolution remain for the determination of many polar and ionic pesticides. Appropriate pretreatments are needed to determine low concentrations of pesticides. Eleven organophosphorus pesticide residues (dicrotophos, azinphos methyl, ethoprphos, quinalphos, parathion, diazinon, coumaphos, phosalone, profenofos, ethion and tribuphos), in textiles were determined at the 0.1-500 ng/mL level by HPLC-TOF/MS after optimized pretreatments based on QuEChERSb (Quick, Easy, Cheap, Effective Rugged, Safe) methods.16 Since malaoxon, which is one of the organophosphorus pesticides, inhibits the formation of thiocholine through a catalytic reaction of acetylcholinesterase, it was amperometrically detected by the electrochemical oxidation of thiocholine.17 By using a similar inhibition of acetylcholinesterase, paraoxon-ethyl, typical organophosphorus pesticide, was detected by using flow ratecontrolled 3D microfluidic paper-based analytical devices (μPAD) with a detection limit of 25.0 μg/L. This method provides the results with high reproducibility (2.63% RSD) regardless of user skills.18 This highlight introduces recent achievements of concerning the determination of phosphorus, which has been extensively investigated for several decades. The molybdenum blue method has been well established for the determination of orthophosphate, but it has less selectivity for arsenate(V). In addition, many difficulties remain in the sensitive determination of organophosphate compounds and selective and sensitive speciation methods of P(V) and P(III) containing compounds. We still have a lot of challenges concerning the determination of phosphorus.