Selectivity potassium-calcium exchange

Fig. 2. The logarithm of the selectivity coefficients of a calcium-copper (a) or a calcium-potassium (b) exchange isotherm is plotted as a function of the proportion of the preferred ions adsorbed in the cell walls.

Appel, C., L. Q. Ma, R. D. Rhue, and W. Reve. 2003. Selectivities of potassium-calcium and potassium-lead exchange in two tropical soils. Soil Science Society of America Journal 67, no. 6 1707. doi 10.2136/sssaj2003.1707. 

Potassium, sodium, calcium and other positively charged ions present in the channel are exchangeable and get replaced by heavy metal ions. Heavy metals present in wastewater (chromium, mercury, lead and cadmium) are effectively adsorbed on zeolites. Clinoptilolite is a widely used zeolite for wastewater treatment due to its higher selectivity and ion exchange capability to remove heavy metal ions including strontium and cesium (Grant et al. 1987). Vaca Mier et al. (2001) studied the selectivity of zeolite for the removal of various heavy metals and observed that zeolites show higher selectivity for lead ions followed by cadmium, copper and cobalt. Table 2.2 (Bailey et al. 1999) shows the some of the reported adsorption capacities of zeolites. 

Sugar analysis by hplc has advanced greatly as a result of the development of columns specifically designed for carbohydrate separation. These columns fall into several categories. (/) Aminopropyl-bonded siHca used in reverse-phase mode with acetonitrile—water as the eluent. (2) Ion-moderated cation-exchange resins using water as the eluent. Efficiency of these columns is enhanced at elevated temperature, ca 80—90°C. Calcium is the usual counterion for carbohydrate analysis, but lead, silver, hydrogen, sodium, and potassium are used to confer specific selectivities for mono-, di-, and... 

Among cations, potassium, acetylcholine, some cationic surfactants (where the ion-exchanger ion is the / -chlorotetraphenylborate or tetra-phenylborate), calcium (long-chain alkyl esters of phosphoric acid as ion-exchanger ions), among anions, nitrate, perchlorate and tetrafluoro-borate (long-chain tetraalkylammonium cations in the membrane), etc., are determined with this type of ion-selective electrodes. 

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 apparent discrepancy could reside in the fact that if potassium ions are available at all, they will form a mica at temperatures near 100°C. Montmorillonite structures below these conditions (pressure and temperature) need not contain potassium at all. However, at the correct physical conditions the 2 1 portion of the montmorillonite must change greatly (increase of total charge on the 2 1 unit) in order to form a mica unit in a mixed layered mineral phase. Since neither Na nor Ca ions will form mica at this temperature, potassium will be selectively taken from solution. Obviously this does not occur below 100°C since cation exchange on montmorillonites shows the reverse effect, i.e., concentration of calcium ions in the interlayer sites. If potassium is not available either In coexisting solids or in solutions, the sodi-calcic montmorillonite will undoubtedly persist well above 100°C. 

Since the cation-exchange processes of the main cations (sodium, potassium, magnesium, calcium) have a significant role in the nutrient cycle of soils, the classical literature has discussed their cation-exchange processes in detail (e.g., Boyd et al. 1947 Gaines and Thomas 1953 Howery and Thomas 1965 Sposito 1981 Filep 1999). As described earlier, cation exchange is characterized by the selectivity coefficients and... 

Selection of the cations was based on several criteria. First, the raw coal contained alkaline-earth, mainly Ca and Mg, and some alkali metals on its carboxyl groups. Also, McKee (13) and Walker et al. (12) have shown that sodium, potassium and calcium are excellent catalysts for the C-O2 reaction. Thus, these cations may have a significant effect on the char burnout rate. In addition. Mg was back exchanged on the coal since it was contained on the raw coal and, as shown by Walker et al. (12), it is a poor catalyst for the C-O2 reaction. The purpose of using this alkaline-earth metal was to determine if catalysis of the heterogeneous C-O2 reaction affected the char burnout rate. This would help to elucidate whether the char burnout step was chemically or physically rate controlled. 

A novel silica-based cation exchanger is functionalized with a combination of car-boxylate and a crown ether functionalities [46]. This stationary phase is more selective toward anunonium, potassium and low-molecular weight amines. Potassium is eluted after ammonium, magnesium and calcium. 

The two most well known liquid membrane electrodes are cation-selective the potassium and calcium electrodes. For the potassium ISE, the soft plastic membrane has the neutral hydrophobic ionophore valinomycin immobilised in it. This electrode has a 10000-fold selectivity for potassium ions over sodium ions. Eor the calcium ISE, the liquid ion exchanger membrane is a water-immiscible calcium chelator. Calcium ions are... 

---