Electrode, membrane: calcium silver

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... 

Equilibrium studies of the metal conplex formation of biopolymers, e.g. polypeptides or proteins can be based on the potentio-metric determination of the free (uncomplexed) metal ion activity in solutions of different pH containing the peptide and the metal ion in different concentrations(16,17). For the measurement of the uncomplexed metal ion concentration in the presence of the metal coordinated by the bioligand, metal electrodes (e.g., silver or mercury), amalgam electrodes (e.g., zinc, cadmium, bismuth), and ion-selective membrane or ion exchange electrodes (e.g., calcium) can be used. The coordination of metal ions to donor atoms protonated at the pH of the solutions to be investigated can be followed by the pH metric equilibrium study of the deprotonation equilibria caused by this process. 

Early electrode systems. The first type of electrode used for calcium determination was an amalgam electrode which consisted of an amalgam of calcium oxalate and either lead or silver oxalate. Later, thin crystals of naturally occurring fluorite (calcium fluoride) were used as membranes for the potentiometric determination of calcium in pure aqueous solutions and in milk. Other early developments included a series of studies using membranes produced from pure paraffin containing calcium oxalate and a non-ionic detergent, and attempts to produce a glass electrode for calcium. ... 

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). 

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. 

Calcium ISEs contain a calcium-selective membrane, which encloses an inner reference solution of calcium chloride often containing saturated silver chloride (AgCl) and physiological concentrations of sodium chloride and potassium chloride (KCl) and an internal reference electrode. The reference electrode, usually of Ag/AgCl, is immersed in this inner reference solution. 

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]. 

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