Calcium-selective electrodes

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Whitfield et al. [168] used a calcium-selective electrode to monitor EGTA and DCTA titrations of aqueous mixtures of calcium, magnesium, and sodium. The concentrations were selected to span the range of natural waters and the... 

One of the most important examples of a liquid-membrane ISE is the calcium-selective electrode. The salt utilized as the ion exchanger is the calcium salt of dodecylphosphoric acid, e.g. dissolved in di-( -acetylphenyl) phosphonate. The sensitivity of the electrode depends on the solubility of the ion exchanger in the test solution. The electrode response is generally nemstian down to a concentration of 10 mol dm . In the preferred pH range of 5.5-11, the selectivity of... 

Mumallah, N.A.A. and Popiel, W.J. Calcium-selective electrode measurements of calcium molybdate solubilities in water, Anal Chem., 46(13) 2055-2056, 1974. 

J. Bobacka. T. Lindfors, A. Lewenstam. and A. Ivaska, All-Solid-State Ion Sensors Using Conducting Polymers as Ion-to-Electron Transducers, Am. Lab., February 2004, 13 A. Konopka, T. Sokalski, A. Michalska, A. Lewenstam, and M. Maj-Zurawska, Factors Affecting the Potentiometric Response of All-Solid-State Solvent Polymeric Membrane Calcium-Selective Electrode for Low-Level Measurement, Anal. Chem. 2004, 76, 6410 M. Fouskaki and... 

Kramer, R. and Lagoni, H. 1969. Calcium selective electrode for the measurement of calcium ion activity in milk and milk products. Milchwissenschaft 24, 68-70 (German). 

Determination ofCa(II) in wood pulp using a calcium-selective electrode with poly(3,4-ethylenedioxythiophene) as ion-to-electron transducer... 

R3. Ross, J. W., Calcium-selective electrode with liquid ion exchanger. Science 156, 1378-1379 (1967). 

Figure 4.4. (a) FIA manifold for simultaneous determinations of pH and pCa. 5, point of injection (30 (xL) pH, flowthrough capillary glass electrode FC, flowthrough cell containing a PVC-membrane-based calcium-selective electrode and the common reference electrode, details of which are shown below (b). The carrier solution (A and B) is TRIS buffer of pH 7.4, the connecting tubes (a and b) being made as short as possible. The carrier solution supplied via line B to the tip of the reference electrode is included in order to stabilize the reference electrode junction potential for sera measurements. In (c) is shown the potenti-ometric determination of the ionized calcium content in six serum samples, bracketed by two calibration runs of aqueous calcium standards (0.5-5 mA/), all assays made in triplicate. 

The sensitivity of the analytical technique is therefore mainly dependent on the charge z of the analyte. For monovalent or divalent cations, for example, a 10-fold concentration change will yield -F59.2 mV or -1-29.6 mV change in the observed cell potential. Anionic analytes (with negative z values) will induce negative potential changes. Figure 3 shows a typical calibration curve for a calcium-selective electrode based on a liquid membrane... 

Martynova et al (S21) used a calcium-selective electrode to study the behavior of a CaS0 -NaCl-H2 0 solution at five temperatures. They concluded that the NaSO T complex must be considered as well as the aqueous molecule CaSO in order to adequately describe the system. The resulting dissociation constants for NaSO are ... 

G44. Martynova, O.i. L.G. Vasina, S.A. Pozdnyakova, V.A. Kishnevskii, "Use of the calcium-selective electrode for determining the solubility product and dissociation constant of calcium sulfate", Doklady Akad. Nauk SSSR, v217, 4, PP862-864 (1974)... 

Keil, L., Moody, G.J. and Thomas, J.D.R. (1978) An evaluation of PVC matrix membrane calcium-selective electrodes based on nitrated (octylphenyl)phosphate sensors and phosphonate mediators. Ana/. Chim. Acta, 96, 171-175. 

Rechnitz GA, Hseu TM (1961) Analytical and biochemical measurements with a new solid-membrane calcium-selective electrode. Anal Chem 41 111-115... 

Lindfors, T., F. Sundfors, L. Hofler, and R.E. Gyurcsanyi. 2011. The water uptake of plasticized polyfvinyl chloride) solid-contact calcium-selective electrodes. Electroanalysis 23 2156-2163. 

Calcium-selective electrodes are available commercially. The electrode is coupled with a reference electrode and the e.m.f of the cell is measured using a millivolt meter or an ion meter. 

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