For calcium-exchanged

Sodium softeners are used to treated RO influent water to remove soluble hardness (calcium, magnesium, barium, and strontium) that can form scale on RO membranes. Once known as sodium zeolite softeners, zeolites have been replaced with synthetic plastic resin beads. For sodium softeners, these resin beads are strongly acidic cation (SAC) polystyrene resin in the sodium form. The active group is benzene sulfonic acid, in the sodium, not free acid, form. Figure 8.12 shows styrene-divinylbenzene gel cation resin. Equation 8.4 shows the softening reaction for calcium exchange ... 

Figure 8.13 Sodium softening reaction for calcium exchange.

Zeolites are naturally occurring hydrous aluminum-sodium silicates in porous granule form. They are capable of exchanging their sodium base for calcium or magnesium and of expelling these alkaline earth metals for sodium by treatment with salt. Thus, they are a type of ion-exchange media. (Some zeolites act as molecular sieves by adsorption of water and polar compounds.)... 

Some zeolites have the ability to exchange sodium for calcium and thereby function as water softeners by removing Ca2+. After the zeolite has become saturated with Ca2+, it can be renewed by washing it in a concentrated NaCl solution to restore the Na+ ions. Zeolites are also used to prepare ion exchange resins, as molecular sieves and as catalysts. 

Where only univalent cations are considered, the activities calculated by Equations 9.27 and 9.28 are equivalent in value, but this is not the case if divalent ions are included. The reaction describing exchange of sodium for calcium ions,... 

Bychkova and Shvarev [16] recently prepared nanosensors (0.2-20 pm) for sodium, potassium and calcium using the precipitation method. Similarly to the previous works, the plasticized poly(vinyl chloride) included a phenoxazine chro-moionophore, a lipophilic ion exchanger and a cation-selective ionophore. The dynamic range of the very selective sensors was 5 x 10 4-0.5 M for sodium, 1 x 10 5-0.1 M for potassium and 2 x 10 4 - 0.05 M for calcium. As was demonstrated by Bakker and co-workers [45] a particle caster can be used can be used for preparation of much larger beads (011 pm). 

The modern history of ion exchange began in about 1850 when two English chemists, Thompson(4) and Way(5), studied the exchange between ammonium ions in fertilisers and calcium ions in soil. The materials responsible for the exchange were shown later to be naturally occurring alumino-silicates 6. History records very much earlier observations of the phenomenon and, for example, Aristotle(7), in 330 BC, noted that sea-water loses some of its salt when allowed to percolate through some sands. Those who claim priority for Moses(8) should note however that the process described may have been adsorption ... 

Pak, C. Y. C. Bartter, F. C. 1967. Ionic interaction with bone material, I. Evidence for isoionic calcium exchange with hydroxyapatite. Biochimica Biophysica Acta, 141, 401-409. 

FIGURE 7.16 Effect of calcium exchange for sodium in zeolite A on hydrocarbon adsorption. Replacement of four sodium ions by two calcium ions permits easy diffusion of n-alkanes into the zeolite channels. 

Cardiac glycosides increase contraction of the cardiac sarcomere by increasing the free calcium concentration in the vicinity of the contractile proteins during systole. The increase in calcium concentration is the result of a two-step process first, an increase of intracellular sodium concentration because of Na+,K+ ATPase inhibition and second, a relative reduction of calcium expulsion from the cell by the sodium-calcium exchanger (NCX in Figure 13-1) caused by the increase in intracellular sodium. The increased cytoplasmic calcium is sequestered by SERCA in the SR for later release. Other mechanisms have been proposed but are not well supported. 

Silver(I) fluoride supported on calcium fluoride has been shown to improve fluoride nuc-leophilicity for halogen exchange. Iodides give the best yields. 1-Iodooctane (1) is converted to 1-fluorooctane (2) in 80% yield.1... 

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