Divalent cation electrode

C rystalline membranes arc also available thal consist of a homogeneous mixture of silver sulfide with sulfides of copper(U), lead, or cadmiunt. Toward these divalent cations, electrodes from these materials have electrical responses similar to electrodes of the third kind (Section 23C-3). Note that these divalent sulfides, by themselves, are not conductors and thus do not exhibit ion-selectivc activity. 

Divalent Cation Electrode. The divalent cation electrode for hardness is a selective ion electrode that detects Ca and Mg + and other divalent ions in much the same way that the glass electrode detects H. Instead of developing a potential across a glass membrane, it develops a potential over an inert porous disc that is saturated with a water-immiscible, liquid ion exchanger which is selective for divalent ions. 

Fig. 7-39. Divalent cation electrode. Courtesy of Orion Research, Inc., Cambridge, Mass.

Electroanalytical chemistry is one of the areas where advantage of the unique properties of SAMs is clear, and where excellent advanced analytical strategies can be utilized, especially when coupled with more complex SAM architectures. There are a number of examples where redox reactions are used to detect biomaterials (357,358), and where guest—host chemistry has been used to exploit specific interactions (356,359). Ion-selective electrodes are an apphcation where SAMs may provide new technologies. Selectivity to divalent cations such as Cu " but not to trivalent ions such as Fe " has been demonstrated (360). 

Kimura and coworkers [17], Diamond [18], and Damien et al. [19] have described that the polymeric calix-[4]arenes have been used as ionophores in ion selective electrodes for Na (based on calixarene esters and amides) and for Na and Cs (based on p-alkylcalixarene acetates). The electrodes are stated to function as poten-tiometric sensors as well, having good selectivity for primary ion, virtually no response to divalent cations, and being stable over a wide pH range. 

For a few membrane electrodes the ion selectivity is limited, e.g., the Metrohm EA301-Me2+ electrode shows the same sensitivity for various divalent cations in fact, even non-selectivity may be accepted, e.g., where only one type of cation is present as in aqueous hydrofluoric acid where the pH can be determined with a cation-exchange resin electrode40. 

The double-layer effect in the electrode kinetics of the amalgam formation reactions was discussed [67]. The dependences on the potential of two reduction (EE) mechanisms of divalent cations at mercury electrode, and ion transfer-adsorption (lA) were compared. It was suggested that a study of temperature dependence of the course of these reactions would be helpful to differentiate these two mechanisms. 

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

In one of the most extensive studies of metal chloride catalysts,1 twenty of them supported on carbon were investigated, and a correlation was proposed between their activity and the electron affinity of the metal cation divided by the metal valence. Since the correlation consisted of two straight lines, it cannot be used predictively. However, electron affinity is necessarily a one-electron process, whereas hydrochlorination is more likely to be a two-electron process, involving the 2ir electrons of ethyne. Because many of the cations investigated are divalent, standard electrode potential was suggested1 as a more suitable parameter for correlating with activity. 

Truesdell, A. and Christ, C., Glass electrodes for calcium and other divalent cations, p. 293-322, in Eisenman, G., ed., "Glass Electrodes for Hydrogen and Other Cations," Marcel Dekker, Inc., New York, 1967. Reuter, J. and Perdue, E. Importance of heavy metal-organic matter interactions in natural waters, Geochim. Cosmochim. Acta 41, 325-244 (1977). 

A combination of chelators for divalent cations is suitable to buffer the free Ca " concentration from 0.1 -100 (iM under experimental conditions. Added Mg " and ATP as well as the pH of the medium must be considered, because they alter the equilibrium between Ca and the chelators present. The free Ca and Mg " concentrations are calculated by a computer program and controlled by Ca and Mg " specific electrodes (Fohr et al., 1993). Each Ca " buffer is prepared separately from stock solutions, with a final check of pH, pCa, or pMg. If no Ca electrode is available, the calculated total amount of Ca (as CaCy and Mg (as Mg(CH3COO)2) must be added before the pH adjustment. Buffers can be stored at -20 °C but should be thawed only once, mainly because of decomposition of ATP. 

Many physiological anions, including protein, phosphate, citrate, lactate, sulfate, and oxalate, form complexes with calcium ions. Although these anions reduce the concentration of free calcium by complex formation, they do not directly interfere with the measurement of the calcium that is free. Protein deposits on the electrode may act as a divalent cation exchanger, resulting in positive interference with high concentrations of Mg. Older electrodes were sensitive to the concentration of protein in the sample. The newer electrodes use a dialysis membrane or neutral carrier to reduce or eliminate this protein effect. Investiga-... 

A divalent cation ion exchange electrode that responds to several cations is available. Its response is nearly equal for calcium and magnesium, and it.is useful for measuring water hardness. A copper and a lead electrode are also available. Anion-selective electrodes of this, type are available for nitrate, perchlorate, and chloride. They are the same in principle, except that a liquid anion exchanger is used instead of a cation exchanger. 

Liquid membranes are prepared from immiscible, liquid ion exchangers, which are retained in a porous inert. solid support. As. shown schematically in Figure 23-8. a porous, hydrophobic (that is. water-repelling), plastic disk (typical dimensions 3 X 0.15 mm) holds the organic layer between the two aqueous solutions. For divalent cation determinations, the inner tube contains an aqueous standard soittt ion of MCI, where M is the cation whose activity is to be determined. This solution is also saturated with AgCI to lorm a Ag-AgCI reference electrode with the silver lead wire. 

Ionic conduction may dominate the electrical behavior of materials with small electronic conductivity, and its study is useful in the investigation of lattice defects and decomposition mechanisms. In order to establish that conduction takes place by the motion of ions and not of electrons or holes, one can compare the transport of charge with the transport of mass plated out on electrodes in contact with the sample. In practice, this approach is not always feasible because of the very low conductivities associated with ionic motion. When ionic conductivity is suspected one usually attempts to vary the concentration of defects by introducing impurities. For example, for cation conduction in monovalent ionic compounds, addition of divalent cations should enhance the conductivity, since the vacancies produced (in order to ensure charge compensation) lead to enhanced diffusion of the monovalent cation. (The diffusion of a vacancy in one direction is equivalent to the diffusion of an ion in the opposite direction). 

Electrodes that behave in a Nernstian manner will have slopes equal to 59.16 mV per plon unit for monovalent cations, (59.16/2) mV per plon unit for divalent cations, —59.16 mV per plon unit for monovalent anions, and so on, at 25°C. [Where plon is defined exactly as is pH, as the negative logarithm to the base 10 of the ion concentration (activity) in M. So we have pF, pCa, pAg, and so on.] Many plots of concentration vs. voltage for ISEs deviate from hnearity due to activity coefficient effects, so an alternate approach is to use the MSA for calibration, especially for complex matrices. 

SO for several months. The polymeric MnOj consists of roughly spherical particles with a radius of 500 A (50,000 pm or 50 nm) and we determined that the particles are NOT electroactive at the mercury electrode. The material has a negative electrostatic charge and is precipitated as more cations are added, and divalent cations increase the rate of precipitation more than monovalent cations (12). The UV-VIS characteristics are that Beer s law is followed over the pH range used in this study (4-8) at lower pH, the increase in ionic strength causes coagulation and precipitation. Polymeric and soluble Mn02 is a possible product of chemical and bacterial Mn(II) oxidation in fresh waters such as lakes. 

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... 

The mobilities of proteins and protein complexes are affected both by the composition of the electrode buffer (concentration, pH, and presence or absence of divalent cations) and by the composition of the sample. Precise formulation of the electrode buffer is essential to obtain reproducible separations. While satisfactory separations can be obtained even with fairly high (up to 0.4 M) salt concentrations in the sample, high salt concentrations in a sample can significantly reduce the mobilities of proteins in that lane. Thus, where small differences in mobility are important, the concentrations of salt and buffer should be identical in all samples. 

---