Phosphorus Analysis. Electrochemical Methods

In 2012, Berchmans et al. reviewed the electroanalytical methods used for inorganic phosphate analysis, which include potentiometry, voltammetry, and amperometry. Although detection limits are not as low as for MS and colorimetry methods, simplicity and capability of miniaturizing the electrochemical instrumentation allow their use as field and remote sensing instrumentation. However, their low sensitivity, frequent equipment recalibrations, and drifts caused by temperature fluctuation are limiting their applications.

In 1947, Flatt and Brunisholz determined phosphate in aqueous solutions by potentiometric titration with silver nitrate when Ag₃PO₄ precipitate was formed. The silver phosphate ion selective electrode (ISE) developed by Novozamsky and van Riemsdijk was capable of measuring phosphate with a detection limit at 30 µg L^-1 as P Despite the major interference observed from chloride ions, it was one of the first direct potentiometric detection of phosphate.

In a potentiometric flow-through detector with a metallic copper electrode, Alexander et al. investigated the electrode response to phosphates (orthophosphate, pyrophosphate, and tripolyphosphate) and inorganic anions. In a similar setup with a lead (II) ISE, orthophosphate and tripolyphosphate were reproducibly measured using nonlinear calibration curves. Improved selectivity was obtained using FIA with cadmium ISE, when Cd²⁺ from the reagent stream precipitated phosphate as Cd₃(PO₄)₂. By incorporating bis(p-methylbenzyl)tin dichloride, dibenzyltin dichloride, and bis(p-chlorobenzyl)tin dichloride in plasticized polymer membranes, Glazier and Arnold achieved a high level of selectivity for dibasic phosphate.

Chen et al. developed an FIA with cobalt wire ISE for potentiometric determination of phosphate (31 µg L^-1 as P detection limit). Marco et al. measured phosphate in wastewaters and fertilizer samples using FIA with cobalt wire ISE, compensating the significant positive chloride interference by independently measuring chloride and using a chloride selectivity coefficient. A low-level detection of dibasic phosphate in wastewater samples (93 µg L^-1 as P) was achieved with a potentiometric cobalt-based screen-printing sensor fabricated by electroplating cobalt on the surface of a screen printing electrode.

The interferences of pH, dissolved oxygen, and other anions were examined. A potentiometric detector for phosphate, resistant to turbidity and color interference, was developed by Li et al. using Zn²⁺ and Cu²⁺ complexes with 2,6-bis(bis(2-pyridylmethyl)aminomethyl)-4-methylphenol as a receptor and o-mercaptophenol as an indicator.

Voltammetric measurements of orthophosphate can be based on the reduction of the ammonium phospho- molybdate complex, inhibition of the redox reaction of potassium hexacyanoferrate, or the inhibition of an enzyme-based glucose electrode. The process is inhibited by the oxygen atoms around the central phosphorus atom.

Fogg and Bsebsu proposed differential-pulse voltammetric determination of orthophosphate as molybdovanadophosphate at a glassy carbon electrode and by direct injection of phosphate into molybdate reagent. Effective blank background elimination was possible with a phosphate detection limit at 31 µg L^-1 as P. A similar analytic procedure with a glassy carbon electrode was used for the determination of phosphate, silicate, arsenate, and germanate as β-heteropolymolybdates. Recently, Barus et al. developed a phosphate sensor using square-wave voltammetry. The system does not require addition of any reagents as they are formed in situ (H⁺ and MoO₄^2-). An optimized prototype electrochemical cell requires only five minutes of complexation time and has a concentration range between 15 and 125 µg L^-1 as P.

Amperometric sensors measure the current produced by the target analyte at an electrode with a constant voltage. The reduction of ferrocene-based macrocyclic synthetic receptor and the reduction of a phosphomolybdate complex were used as indirect amperometric measurements of the orthophosphate. Simultaneous determination of phosphate and pyrophosphate by amperometric detection with immobilized enzyme reactors was proposed by Yao and Wasa. The injected sample was split into two channels, with one channel going through the immobilized pyrophosphate reactor prior to a co-immobilized nucleoside-phosphorylase-xanthine oxidase reactor to obtain orthophosphate (31 µg L^-1 as P detection limit). The second channel passed only through the latter reactor to obtain the sum of orthophosphate and pyrophosphate (62 µg L^-1 as P detection limit).

The authors also used a novel bioamperometric flow-injection system with a 16-way switching valve to simultaneously determine orthophosphate and 12 purine phosphate nucleotides. Samples were passed through two channels, acid phosphatase immobilized reactor, and a delay coil to separate the peaks corresponding to the two sample portions injected. Cheng et al. achieved lower detection limits with amperometric detection of phosphate (0.3 µM). In this FIA system, the adsorption of phosphate at the electrode was triggered by the activation of a barrel-plated nickel electrode (Ni-BPE) in alkaline media, forming a Ni(OH)₂/NiO(OH) film. Upon the addition of phosphate, the current suppression of the glucose signal at Ni-BPE was observed.

The proposed analytical system is an electrochemical sensor that can detect phosphate with good sensitivity and selectivity. A novel phosphate and bicarbonate sensing method was proposed by Honda and Morimitsu by electrochemical oxidation, with a detecting electrode obtained from amorphous RuO₂-Ta₂O₅ catalysts, formed on a titanium substrate.