Aluminum, silicic acid effect

Alkali metal hydroxides, dissolution rate effect, 521-523f Alkoxides, formation of uniform precipitates, 451-464 Aluminum, silicic acid effect on adsorption in food, 612/ 613 Aluminum in biological systems, 604, 605f, 606 Aluminum-modified silica sol, formation, 62, 63/ Aluminum-silicon interactions in biology,... 

Working first with Polanyi, Weissenberg, and Brill, and later as the leader of the Textile Chemistry Section, Mark successively published papers on the crystal structures of hexamethylenetetramine, pentaerythritol, zinc salts, tin, urea, tin salts, triphenylmethane, bismuth, graphite, sulfur, oxalic acid, acetaldehyde, ammonia, ethane, diborane, carbon dioxide, and some aluminum silicates. Each paper showed his and the laboratory s increasing sophistication in the technique of X-ray diffraction. Their work over the period broadened to include contributions to the theories of atomic and molecular structure and X-ray scattering theory. A number of his papers were particularly notable including his work with Polanyi on the structure of white tin ( 3, 4 ), E. Wigner on the structure of rhombic sulfur (5), and E. Pohland on the low temperature crystal structure of ammonia and carbon dioxide (6, 7). The Mark-Szilard effect, a classical component of X-ray physics, was a result of his collaboration with Leo Szilard (8). And his work with E. A. Hauser (9, 10, 11) on rubber and J. R. 

Liu, C., and Huang, P. M. (2000). Catalytic effects of hydroxyl-aluminum and silicic acid on catechol humification. In Humic Substances Versatile Components of Plants, Soil and Water, Ghabbour, E., and Davies, G., eds., Royal Society of Chemistry, Cambridge, UK, pp. 37-51. 

A membrane can be used in the so-called phase transfer catalysis as a separator between two immiscible liquids or a liquid and a gas. It serves as a well controlled contact surface. An interesting type of membrane reactor has been suggested in which a ceramic membrane is applied to regulate the contact between a gas and a liquid stream on the opposites of the membrane [De Vos, 1982 De Vos et al., 1982]. Hydrogenation of nitrobenzoic acid can be effectively performed with a porous calcium-aluminum silicate membrane reactor which essentially becomes a gas-liquid reactor. 

Heating a phenol ester in the presence of Nafion-H gave good yields of the hydroxy phenyl ketone.This Fries rearrangement gave a mixture of the ortho and para hydroxy compounds in a 1 2-2.5 ratio (Eqn. 22.19). A study of the rearrangement of phenyl benzoate over a number of solid acid catalysts indicated that Nafion-H was more effective than montmorilIonite or amorphous aluminum silicate for this reaction. " ... 

It is well known that organic complex-formers, such as simple organic acids (citric, oxalic, tartaric, salicyclic) formed by microorganisms in soils and humic or fulvic acids, solubilize mineral iron(III) and aluminum. These complex-formers not only increase the solubility of these minerals but also are able to form chelates on hydrous oxide surfaces and thus, in turn, catalyze the dissolution of oxides and aluminum silicates (Kummert and Stumm, 1980 Sigg and Stumm, 1980). The downward vertical displacement of A1 and Fe, as it is observed in the podsolization of soils, can be accounted for by considering the effect of pH and of complex formers on both solubility equilibria and dissolution rates (Schnoor and Stumm, 1985 Schnoor and Stumm, 1986). 

Table I. Effect of Silicic Acid on Inhibition of Prolyl-hydroxylase by Aluminum...

However, with the major part of the intake of aluminum being food (and antacid medication), the important question is the effect of silicic acid in water on the absorption of the aluminum in food (18). The extent of exclusion would not then be a linear function of silicic acid concentration, because a minimum level of about 100 pM Si is required for the formation of stable hydroxyaluminosilicates, in which the Si Al ratio is about 0.5 and which have minimum solubility (Figure 5). 

In an acid medium at very high H+ concentrations the value tends to 1 and the calculated solubility of aluminum silicates turns out in direct correlation with the solution acidity C +, i.e., it grows with decrease in pH. In alkaline solutions the acidity effect declines but the effect of complexation increases. In other words, with increasing alkalinity grows... 

Prolylhydroxylase, the collagen synthesis enzyme that has low activity in silicon-deficient tissue, requires iron that cycles between Fe " " and Fe " ". The apoenzyme will bind aluminum (less strongly than iron), and the enzyme is then, of course, inactive. An experiment was conducted [6] in which the apoenzyme was presented with iron first and then aluminum, aU other essential cofactors being present Activity, as measured by hydroxyproline production, was reduced by 20% of the control level. When the apoenzyme was presented with aluminum first and then iron, activity was reduced by 55% of the control levels. Silicic acid alone had no effect on the activity, which remained at the control level (Table 58.1). 

However, when the A1 first experiment was repeated in the presence of a sixfold excess of silicic acid (600 p,M) over aluminum, the inhibiting effect of aluminum was completely suppressed. Clearly, in the presence of silicic acid, aluminum is removed from competition with iron for binding in prolyUiydroxylase. 

The value of the pH for optimum stability of silicic acid depends on what impurities are present in the solution. Traces of aluminum ions and to a lesser extent, iron, thorium, and beryllium ions tend to offset the effect of fluoride ion by forming complex fluorides and thus retard polymerization in this pH range. Depending on the purity of the solution, the pH of optimum stability may range from 1 to as high as 3-3.5. In silicic acid. solutions free from aluminum impurity, as little as 1 ppm of fluorine has a marked effect on the rate of polymerization in acid solution. 

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