Phosphate anion selective electrode

In a related study (78CC934,79HCA2763) Lehn and coworkers incorporated guanidinium groups into macrocycles (e.g. 29). The resulting polyguanidium salts form stable complexes in water with phosphate and carboxylate anions. Unlike polyammonium anion receptors, these species remain protonated over a wide pH range and hold considerable potential in the development of anion-selective electrodes. 

Studies have been made towards connecting low-selectivity anion-selective electrodes to chromatographic columns, using the column to separate the ions and the ISE to detect them. For instance [30], a CWE has been reported for the detection of species such as carboxylic acids, sugar phosphates and nucleotides from mixtures which had been passed through an anion-exchange column. 

Sasaki S, Ozawa S, Citterio D et al (2004) Organic tin compounds combined with anionic additives—an ionophore system leading to a phosphate ion-selective electrode. Talanta 63 (1) 131-134... 

Phosphate was measured with the anion-selective electrode coated with a PhoE porin-lecithin membrane. When phosphate solution was added to the... 

Figure 5. Calibration curve for phosphate using PhoE porin-lecithin membrane-anion-selective electrode. The experiments were performed in 0.1 M Tricine buffer (pH 7.4), 25 C. 

Sodium and chloride may be measured using ion-selective electrodes (see Electro analytical techniques). On-line monitors exist for these ions. Sihca and phosphate may be monitored colorimetricaHy. Iron is usually monitored by analysis of filters that have had a measured amount of water flow through them. Chloride, sulfate, phosphate, and other anions may be monitored by ion chromatography using chemical suppression. On-line ion chromatography is used at many nuclear power plants. 

Ion-Selective Electrodes based on Bis-Thiourea Receptors. Bis-thiourea derivatives 14, 15, and 17, which have a good membrane solubility, sufficient lipophilicity to prevent leaching into the aqueous sample solution, and a low tendency for self-aggregation in nonpolar solvents, were incorporated into PVC matrix liquid membranes for ISEs. While membrane electrodes based on the dibutyl derivative 14 gave a phosphate response almost identical to that of a conventional anion-exchanger electrode, a membrane electrode based on the phenyl-substituted bis-thiourea 15 exhibited a slightly improved phosphate response, which seems to be the result of improved complexation of phosphate in the sensor membrane. 

The extremely low solubility of lead phosphate in water (about 6 x 10 15m) again suggests potentiometric analysis. Selig57,59 determined micro amounts of phosphate by precipitation with lead perchlorate in aqueous medium. The sample was buffered at pH 8.25-8.75 and a lead-selective electrode was used to establish the end-point. The detection limit is about 10 pg of phosphorus. Anions which form insoluble lead salts, such as molybdate, tungstate or chromate, interfere with the procedure. Similar direct potentiometric titrations of phosphate by precipitation as insoluble salts of lanthanum(III), copper(II) or cadmium(II) are suggested, the corresponding ion-selective electrodes being used to detect the end-point. 

The potentiometric response of receptor 15 for H2PC>4 is not sufficient for a phosphate-selective electrode, but the strong response and high selectivity of this electrode for the dianion SO " is striking.62 The response down to 10 6 M is a very big improvement as compared to the response of the anion exchanger ISE, for which the linear range of the SO2- response extends only down to 10 3 M. The linear range of response of the former ISE from 1.0 x 10"6 to 1.0 x 10 2 M with a slope of -28.1 1.8 mV decade"1 shows that the present electrode responds to SO and not to HSO. The electrode based on 15 is the first neutral host-based electrode with an appreciable selectivity for sulfate. 

The potentiometric behavior of electrodes based on these films was studied (Figure 8). These ISEs presented sub-Nemstian slopes for thiocyanate (from -40 to -53 mV/decade, depending on the buffer used), and had detection limits of 5xl0 7 M. The response time of the electrodes was typically less than 25 s. The selectivity pattern observed was thiocyanate > perchlorate > iodide > nitrite - salicylate bromide > chloride > bicarbonate > phosphate. This anion-selectivity behavior does not follow the Hofmeister series, with thiocyanate and nitrite being the ions that deviate the most from it. This indicates that there is a selective interaction of the immobilized porphyrin with the two anions. 

Among the anion carriers, calixpyrrole receptors have found applications as components in anion-selective membrane electrodes. The potentiometric selectivity for membranes ISEs based on calix[4]pyrrole, 1 towards a range of anions, namely fluoride, chloride, bromide and dihydrogen phosphate was found to be pH-dependent. 

Electrolyte measurements in general toxicological studies are often confined to plasma sodium (Na+), potassium (K+), and calcium (Ca++) see Chapter 1. Magnesium (Mg++) and anions such as inorganic phosphate, chloride (CF), bicarbonate (HCOj ), and lactate are measured less frequently the use of combination triple-ion (sodium/potassium/chloride) selective electrodes leads to a tendency to include the... 

Interest in the use of ion-selective electrodes in the biomedical field is a natural consequence of the electrolyte composition of bulk body and cell fluids (Table 2.1), a proportion of which is in the ionised form. In extracellular fluids, sodium is the principal cation with chloride as the major anion. In intracellular fluids, potassium is the major cation and phosphate the principal anion — except in erythrocytes where chloride predominates. Of special interest is ionised calcium, because of its importance in various physiological and biochemical processes such as bone formation, nerve conduction, cerebral function, cardiac conduction and contraction, membrane phenomena, muscle contraction and relaxation, blood coagulation, and enzyme activation [2—4]. 

Atomic spectrometric techniques such as flame atomic absorption spectrometry (FAAS), electrothermal AAS (ETAAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and ICP-MS are used for the determination of elements, particularly metals. ICP-MS is the most sensitive, typically with microgram per liter detection limits and multielement capability but it has high start-up and operating costs. UV-visible spectrophotometry is also used for the determination of metal ions and anions such as nitrate and phosphate (usually by selective deriva-tization). It is a low cost and straightforward technique, and portable (handheld) instruments are available for field deployment. Flow injection (FI) provides a highly reproducible means of manipulating solution chemistry in a contamination free environment, and is often used for sample manipulation, e.g., derivatization, dilution, preconcentration and matrix removal, in conjunction with spectrometric detection. Electroanalytical techniques, particularly voltammetry and ion-selective electrodes (ISEs), are... 

Wojciechowski K, Wroblewski W, Brzozka Z (2003) Anion buffering in the internal electrolyte resulting in extended durability of phosphate-selective electrodes. Anal Chem 75 3270-3273... 

There have been many attempts to develop selective precipitate membrane electrodes for the important sulfate and phosphate anions. Unfortunately they have all failed to date. A sulfate-sensitive solid-state membrane electrode with a pellet of 31.7% Ag2S, 31.7% PbS, 31.7% PbS04 and 5% CU2S has been described [91], but its selectivity over I, HPO4" and SO3 is low. 

Figure 15.10 Selectivity pattern of PPy(NO ) electrodes. The anions tested include (1) nitrate, (2) bromide, (3) perchlorate, (4) sahcylate, and (5) phosphate AB is the difference between the steady-state potential and the starting potential. Reprinted from Hutchins and Bachas (1995). Copyright 1995 American Chemical Society.

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