Chromium ionic radii

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

The chemistry of aluminium combines features in common with two other groups of elements, namely (i) divalent magnesium and calcium, and (ii) trivalent chromium and iron (Williams, 1999). It is likely that the toxic effects of aluminium are related to its interference with calcium directed processes, whereas its access to tissues is probably a function of its similarity to ferric iron (Ward and Crichton, 2001). The effective ionic radius of Al3+ in sixfold coordination (54 pm) is most like that of Fe3+ (65 pm), as is its hydrolysis behaviour in aqueous solution ... 

The ionic radius of Cr(IV), 69 pm, is smaller than that of Ti(lV), 75 pm. The unit cell and c/spacings will shrink as a result of the smaller radius of chromium. The XRD pattern for Cr02 will show identical reflections to those of rutile TiO (see Figure 8.4) but shifted to slightly higher diffraction angles. 

Another example of promotion by an added metal oxide is Cr/silica incorporating Sn(IV) ions [548,594], Like TiC>2, SnC>2 contains a tetravalent metal ion that can exist in tetrahedral coordination, and has a similar ionic radius. Indeed, SnC>2 and T1O2 are isomorphous. Mixed oxides of SnC>2 and SiC>2 are known to exhibit acidity [595-597], Figure 131 shows the result of adding SnC>2 to the Phillips catalyst. Silica was dried at 200 °C and then treated with an excess of SnCLi vapor. The support was then calcined at 500 °C to remove chloride. It was impregnated anhydrously with chromium and then activated at 500 °C in air. It was quite active in polymerization tests at 105 °C, and the MW distribution of tire polymer is shown in Figure 131. 

Such kind of emission, where the bands remain not quenched at low temperatures, was not detected in chromium activated forsterite, which was very carefully studied. Consequently its connection with other impurity may be supposed. Divalent magnesium in octahedron of the forsterite structure has ionic radius of 0.72 A, which is suitable for substimtion by especially taking into account that charge compensation is not needed in this case. Similar emission was found also in magnesium bearing sinhalite MgAlB04 (Fig. 4.177). 

Only the hydrolysis behaviour of chromium(III) will be discussed in this review. The ionic radius of the Cr " " ion given by Shannon (1976) is 0.615 A. However, this small radius is not commensurate with the relative stability of the chromium(III) hydrolysis species. The lower stability observed is likely due to the strong stabilisation of the d ion (Baes and Mesmer, 1976). To adequately describe the stability of the hydrolysis species with thermochemical models typically requires a modification to the parameterisation in the models (see Chapter 16). 

The dominant valency state of molybdenum in aqueous solution is +6, although at reduced both the +3 and +4 states can form. Hexavalent molybdenum is anionic, whereas no hydrolysis data have been reported for molybde-num(lV). Molybdenum(III) is cationic and should behave in a similar fashion to chromium(III). The ionic radius for sbc-coordinate Mo has been reported by Shannon (1976) to be 0.69 A. Stability constants for the monomeric hydrolysis species of molybdenum(III) have been reported. 

Mel chakova and Peshkova (1978) assumed that the protolysis constant of water for the conditions used was log = -14.0. This value is somewhat different from that derived in the present review for the conditions studied, that is, log =-13.88. This difference indicates that the stability constants proposed should be more positive than indicated by 0.12 log units per OH molecule in each proposed species. This suggests that the stability of molybdenum(lll) hydrolysis species would be substantially more stable than those of chromium(lll). This is considered unlikely on the basis of the corresponding ionic radii of the two ions. Molybdenum(III) has a larger ionic radius than chromium(III) (Shannon, 1976) and, as such, would likely have hydrolysis species of lesser stability. Thus, the stability constants listed by Mit kina, Mel chakova and Peshkova (1978) are not retained (but see Chapter 16). 

Cornet et al. [141] used ammonium ions with different sizes in order to check the occurrence of a critical size for transport restriction. The ionic conductivity first decreases as the ammonium size increases (from 5 to 30 A ) due to a lower mobility compared to protons. For larger coimterion sizes, the conductivity is roughly constant until 1000 A where a cutoff is observed. This result suggests a radius of 10 A for the conductive pathways, at least between two ionic domains. Despite SPI membranes being designed for fuel cell applications, these membranes can be used efficiently as the separator in electrodialysis experiments. For example, SPI membranes appear to be promising materials for separating copper or chromium ions from acidic solutions [172]. 

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