Univalent-cation-selective

Glass electrodes are responsive to univalent cations. The selectivity for these cations is achieved by varying the composition of a thin ion-sensitive glass membrane. 

Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

The latter statement refers to concurrent work by A. Bilewicz et al. [4] who studied the sorption of Zr, Hf, Th, and Rf on cobalt ferrocyanide surfaces. These ferrocyanides are known to be selective sorbents for heavy univalent cations such as Fr+, Cs+, and Rb+. However, some ferrocyanides such as Co ferrocyanide have been found to exhibit also particularly high affinities for tetravalent elements such as Zr4+, Hf4+, and Th4+ involving the formation of a new ferrocyanide phase between the 4+ cation and the Fe(CN)4 anion. 

Varying the composition of the glass can alter the response to various univalent cations. Eisenman carried out extensive investigations on cation selectivity as a function of glass composition. More recently, Phang and Steel reported on selectivity coefficients, pH, and light responses of several commercially available ass electrodes. 

Figure 9. Elementary cell of zeolite A containing 8 univalent cations, O, inside six-membered rings and 6 cations, O, in eight-membered rings a section is shown in which there are 16 selected directions...

Potentiometry—the measurement of electric potentials in electrochemical cells—is probably one of the oldest methods of chemical analysis still in wide use. The early, essentially qualitative, work of Luigi Galvani (1737-1798) and Count Alessandro Volta (1745-1827) had its first fruit in the work of J. Willard Gibbs (1839-1903) and Walther Nernst (1864-1941), who laid the foundations for the treatment of electrochemical equilibria and electrode potentials. The early analytical applications of potentiometry were essentially to detect the endpoints of titrations. More extensive use of direct potentiometric methods came after Haber developed the glass electrode for pH measurements in 1909. In recent years, several new classes of ion-selective sensors have been introduced, beginning with glass electrodes more or less selectively responsive to other univalent cations (Na, NH ", etc.). Now, solid-state crystalline electrodes for ions such as F , Ag", and sulfide, and liquid ion-exchange membrane electrodes responsive to many simple and complex ions—Ca , BF4", CIO "—provide the chemist with electrochemical probes responsive to a wide variety of ionic species. 

Generally, the order of selectivity among univalent cations is as follows (Helfferich, 1962) ... 

Force-area curves of valinomycin at the air-water interface in the presence of univalent cations suggest that the macrocycle undergoes a rearrangement when it complexes a cation. The observed selectivity order Rb+ K+>Cs+ is the same as that found for valinomycin in bilayer conductance and partition experiments. Conformational changes have been shown to occur on the binding of nonactin (1 R = H) to K+ and it has been confirmed that the metal ion must be stripped of its hydration shell as it enters the nonactin molecule. The loss of hydration energy (ca. 80 kcal mol ) must evidently be compensated by interactions between the cation and its eight co-ordinated... 

New chelating ion-exchange resins are able to selectively remove many heavy metals in the presence of high concentrations of univalent and divalent cations such as sodium and calcium. The heavy metals are held as weaMy acidic chelating complexes. The order of selectivity is Cu > Ni > Zn > Co > Cd > Fe + > Mn > Ca. This process is suitable for end-of-pipe polishing and for metal concentration and recovery. 

In silicate minerals, typical D values at 1200°C are of the order m /s, and typical activation energy is about 300kJ/mol. The diffusivity of cations depends on the charge of the cations. Highly charged cations (such as Si and Zr ) diffuse more slowly and have higher activation energy than univalent or divalent cations (such as Fe +-Mg + interdiffusion). Tables l-3b, l-3c, and Appendix 4 list selected diffusivities in silicate minerals. 

Figure 7.3 plots the ratio of crystal radius versus charge for selected ions. Oxyanions—sulfate, selenate, phosphate, arsenate, borate, molybdate, carbonate, and silicate—are represented by their central cations S6+, Se6+, P5+, As5+, B3+, Mo4+, C4+, and Si4+. The ions fall into three behavioral groups. Ions of high ionic potential, the alkali and alkaline earth cations and the halide anions, large univalent and divalent ions, are highly water soluble, easily weatherable, and leach readily from soils to the sea over geologic time. 

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