Divalent cation control specificity

Vaterite is thermodynamically most unstable in the three crystal structures. Vaterite, however, is expected to be used in various purposes, because it has some features such as high specific surface area, high solubility, high dispersion, and small specific gravity compared with the other two crystal systems. Spherical vaterite crystals have already been reported in the presence of divalent cations [33], a surfactant [bis(2-ethylhexyl)sodium sulfate (AOT)] [32], poly(styrene-sulfonate) [34], poly(vinylalcohol) [13], and double-hydrophilic block copolymers [31]. The control of the particle size of spherical vaterite should be important for application as pigments, fillers and dentifrice. 

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. 

If anions were measured, the indicator would be deflected in the opposite direction. A few newer instruments have been designed specifically for use with ion-selective electrodes. These have controls which enable the operator to specify the type (anion or cation) and valency (mono- or divalent) of the indicated ion, so that the pH scale can be used directly without further calibration. Often a logarithmic scale is provided as well to allow direct indication of activities and concentrations (Fig. 36). An ion meter can... 

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