Electrode, membrane: calcium

The calcium membrane electrode is a vaittable Uh>I for physiological studies because calcium play s important roles in nerve conduction, bone furmati on, muscle contraction, cardiac conduction and contraction, and renal tubular function. At least some of these processes are influenced more by calcium ion activity than by calcium ion concentration activity, of course, is measured by the electrode. 

Many other cyclic and noncyclic organic carriers with remarkable ion selectivities have been used successfiilly as active hosts of various liquid membrane electrodes. These include the 14-crown-4-ether for lithium (30) 16-crown-5 derivatives for sodium bis-benzo-18-crown-6 ether for cesium the ionophore ETH 1001 [(R,R)-AA -bisd l-ethoxycarbonyl)undecyl-A,yVl-4,5-tctramcthyl-3,6-dioxaoctancdiamide] for calcium the natural macrocyclics nonactin and monensin for ammonia and sodium (31), respectively the ionophore ETH 1117 for magnesium calixarene derivatives for sodium (32) and macrocyclic thioethers for mercury and silver (33). 

Potentiometry is the measurement of the potential at an electrode or membrane electrode, so the detector response is in units of volts. The potentio-metric response tends to be slow, so potentiometry is used infrequently in analysis.47 One example is the use of a polymeric membrane impregnated with ionophores for the selective detection of potassium, sodium, ammonium, and calcium 48 In process chromatography, potentiometry may be used to monitor selected ions or pH as these values change over the course of the gradient. 

Homogeneous liquid membrane electrodes. This type, which is in limited use, is sometimes considered as a solid ion-exchange electrode as the electroactive species, e.g., calcium dioctylphosphate, after being dissolved in an ethanol-diethyl ether solution of collodion, is left to "dry and can function as an ion-selective pellet in an electrode tip. Orion37 use these electrodes with PVC-gelled membranes for Ca2+, K+, BF4 and N03. ... 

R.A. Llenado, Potentiometric response of the calcium selective membrane electrode in the presence of surfactants ,... 

Electrodes with liquid ion-exchange membranes are typified by a calcium-sensitive electrode (Figure 6.4). The membrane-liquid consists of the calcium form of a di-alkyl phosphoric acid, [(RO)2POO ] 2Ca2+, which is prepared by repeated treatment of the acid with a calcium salt. The internal solution is calcium chloride and the membrane potential, which is determined by the extent of ion-exchange reactions at the interfaces between the membrane and the internal and sample solutions, is given by... 

Liquid membrane electrodes utilize porous polymer materials, such as PVC or other plastics. An organic liquid ion exchanger immiscible with water contacts and saturates the membrane from a reservoir around the outside of the tube containing the water solution of the analyte and the silver-silver chloride wire. See Figure 14.10. Important electrodes with this design are the calcium and nitrate ion-selective electrodes. 

Figure 16.7 A Calcium-ion Polymer (Liquid) Membrane Electrode.

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... 

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... 

The theory and application of selective-ion electrodes have been extensively reviewed.143-151 One of the interesting sidelights is the fact that the internal reference electrode may be replaced by an apparent ohmic contact in many instances, as illustrated by Figure 5 Ale for the solid membrane electrode. Thus the glass electrode can be filled with mercury in place of the internal reference electrode,152 or a gold contact that is plated over with copper can be used.153 Likewise, a selective-ion electrode for calcium ion has been described that is coated on a platinum electrode 154 the contact appears to be mainly ohmic. 

Ion Exchanger Electrodes One of the most successful liquid membrane electrodes is selective toward calcium. Such an electrode relies on the ability of phosphate ions to form stable complexes with the calcium ion. It uses a liquid cation exchanger, consisting of an aliphatic diester of phosphoric acid... 

Figure 6.4. Calcium sensitive ion-exchange membrane electrode...

A calcium-sensitive heterogeneous membrane electrode has been made by evaporating an alcohol-ether solution of collodion and calcium dioctylphosphate. ... 

Figure 21-14 is a schematic of a liquid-membrane electrode for calcium. It consists of a conducting membrane that selectively binds calcium ions, an internal solution containing a fixed concentration of calcium chloride, and a silver electrode... 

Figure 21-15 Comparison of a liquid-membrane calcium ion electrode with a glass pH electrode. (Courtesy of Thermo Orion, Beverly, MA.)...

Figure 21-15 compares the structural features of a glass-membrane electrode and a commercially available liquid-membrane electrode for calcium ion. The sensitivity of the liquid-membrane electrode for calcium ion is reported to be 50 times... 

The calcium ion liquid-membrane electrode is a valuable tool for physiological investigations because this ion plays important roles in such processes as nerve conduction, bone formation, muscle contraction, cardiac expansion and contraction, renal tubular function, and perhaps hypertension. Most of these processes are influenced more by the activity than the concentration of the calcium ion activity, of course, is the parameter measured by the membrane electrode. Thus, the calcium ion electrode (and the potassium ion electrode and others) is an important tool in studying physiological processes. 

Liquid membrane electrodes were known for some years, but it is likely that the introduction of the calcium ion-selective hquid membrane electrode by Ross [9] in 1967 stimulated the intensive developmental research that resulted in the present commercial availability of hquid membrane electrodes for a significant number of cations and anions. In general the cations determinable are double-charge, while the anions are frequently single-charge. 

The reactions between cerium trichloride and oxide ions were initially studied in the pure KCl-NaCl equimolar mixture at 1000 K by the method of potentiometric titration using a calcium-stabilized zirconia membrane electrode. The titration curves clearly demonstrated the existence of the soluble cerium oxychloride CeO+ and precipitated cerium oxide ... 

Electrodes suitable for the potentiometric determination of surfactants are either specially designed liquid or solid membrane electrodes or ion-selective electrodes that in addition to being selective to a particular ion, also quantitatively respond to surfactants. For example, a nitrate ion-selective electrode responds to anionic surfactants, a calcium ion-selective electrode is sensitive to quaternary ammonium salts, and a barium ion-selective electrode can be used for assaying polyethoxylates [43], In some cases it is possible for one to perform potentiometric determination of a counter-ion, e.g. one can titrate alkylpyridinium chloride or bromide salts with silver nitrate solution using silver wire as an indicator electrode [38]. 

Electrodes with liquid ion-exchange membranes are typified by a calcium-sensitive electrode, figure 6.4. The membrane-liquid consists of the calcium... 

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