Silicic acid concentration

Also pertinent to structure retention in this step are results from experiments showing that the route yields alkyl silicates having poor structure retention when the intermediate silicic acid concentration is high and good retention when it is low. These results suggest that a low silicic acid concentration (-0.04M) aids structure retention in the first step. If so, this is easily understandable in terms of the ease with which silicic acids react with themselves. 

The fact that the alkyl silicates produced by the route fully or substantially retain the original siloxane framework also shows that the framework is at least largely retained in the second step. The results pertaining to silicic acid concentration already mentioned lead to the conclusion that a low silicic acid concentration aids this structure retention. Also aiding it, no doubt, is the ability of the alcohol to sheath and protect the silicic acid. 

A colloidal solution of silicic acid concentrated until it contains only about 7% water. It has very high absorbent properties and is used as a desiccant. 

The kinetics of sorption of arsenite and arsenate in the presence of sorbed silicic acid have been only recently examined (Waltham and Eick 2002). These authors demonstrated that the sorption of silicic acid (added 60 h before arsenic) decreased the rate and the total amount of arsenic sorbed. The amount of arsenite sorbed decreased as the surface concentration of silicic acid increased. Furthermore, the inhibition of arsenite sorbed ranged from about 4% at a pH of 6 and 0.1 mM silicic acid up to 40% at a pH of 8 and 1 mol IT1 silicic acid. In contrast, silicic acid reduced the rate of arsenate sorption which decreased by increasing pH and silicic acid concentration, but the total quantity of arsenate sorbed remained nearly constant, indicating that arsenate was able to replace silicate. 

The DSi stripped out of the Southern Ocean s surfece waters at the site of SAMW formation is converted to BSi. This BSi eventually sinks into the deep waters (CDW), where it is remineralized to DSi and driven back south to be upwelled again into the surface waters. This trapping effect is a large part of why half of the global marine sedimentary sink of BSi is located in the Southern Ocean. South of 55°S, iron limitation is so severe, as compared to the rate of upweiiing supply of DSi, that the diatoms are not able to reduce silicic acid concentrations to zero. 

When fine powders of vitreous silica, quartz, tridymite, cristobalite, coesite, and stishovite of known particle-size distribution and specific surface area are investigated for their solubility in aqueous suspensions, final concentrations at and below the level of the saturated concentration of molybdate-active silicic acid are established. Experimental evidence indicates that all final concentrations are influenced by surface adsorption of silicic acid. Thus, the true solubility, in the sense of a saturated concentration of silicic acid in dynamic equilibrium with the suspended silica modification, is obscured. Regarding this solubility, the experimental final concentration represents a more or less supersaturated state. Through adsorption, the normally slow dissolution rates of silica decrease further with increasing silicic acid concentrations. Great differences exist between the dissolution rates of the individual samples. 

The silicic acid concentrations were analyzed at different times by withdrawing 1 ml. from the solution after a high speed centrifugation of the suspended particulates. The colorimetric determination of oligomeric silicic acid by means of the blue ammonium molybdate complex, as described by Stegemann and Fitzek (16), was used for the quantitative measurements. 

However, there is experimental evidence that despite the low constant concentrations in Figure 10 the final state after several weeks does not represent a reversible equilibrium. This became apparent in a test designed to approach the final silicic acid concentration of quartz suspensions from higher concentrations. Figure 11 shows the results of three tests with quartz suspensions of 5, 15, and 45 sq. meters of total surface area, respectively. At the beginning of the tests the solvent contained 83 /utgrams SiOj/ml. Although the silicic acid concentration dropped in... 

Figure 11. Concentration patterns of adsorption tests with quartz samples of different total surface area suspended in 500 ml. of solvent. Initial silicic acid concentration is 83 iigrams Si02/ml.

There are several explanations for these deviations. As Figure 11 indicates, it takes some time to establish the adsorption equilibrium. Thus, the theoretical assumption concerning the adsorption equilibrium and resulting in Equation 3 may not be satisfied adequately. Here a higher initial silicic acid concentration would result. On the other hand, at high silicic acid concentrations the Langmuir isotherm is an idealization which will not properly represent experimental data of extensive... 

Figure 14. Concentration patterns of adsorption tests with different silica modifications. The exposed surface area is different for each sample initial silicic acid concentration is 78 ngrams Si02/ml.

Experimental evidence for this is given for quartz in Figure 11. Evaluating the concentration losses in these tests, we find that the decrease of silicic acid concentration levels off after about two or three molecular layers of acid have been adsorbed on the quartz surface. Similar results were obtained with the other crystalline modifications. 

Franck, V. M., Brzezinski, M. A., Coale, K. H., Nelson, D. M. (2000). Iron and silicic acid concentrations regulate Si uptake north and south of the Polar Frontal Zone in the Pacific Sector of the Southern Ocean. Deep-Sea Res. II47, 3315—3338. 

Despite the high silicic acid load associated with rivers (discharge-weighted average silicic acid concentrations have been estimated at 150 pM Treguer et al, 1995), there is ample evidence for biological removal in estuaries and coastal waters. Nelson and Dortch (1996) demonstrated biological removal of 80—99% of the silicic acid in the Mississippi River plume. MiUiman and Boyle (1975) reported a 25% drawdown of silicic acid within the Amazon River estuary, with further depletion in the nearshore ocean. Kimmerer (2005) demonstrated ca. 85% drawdown of silicic acid within San Francisco Bay, directly related to diatom production. In contrast, silicic acid concentrations are rarely less than 0.5 pM in much of the world s oceans ( Nelson and Dortch, 1996), despite the prevalence of diatoms and the dominance of diatom productivity in export flux. 

Since sponges and radiolarians are not great players in particle flux, the rise of the diatoms must have profoundly altered the partitioning of silicic acid between surface and deep. The familiar nutrient-type distribution may have only existed for the last 50-100 million years. The approximate 14-fold drop in silicic acid concentration also suggests that the residence time of silicon in... 

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 the TMOS concentration dependent silicification study, an assay was performed to determine the amount of silica precipitated. In order to measure the amount of silica precipitated, 10 pi of IM NaOH was added to the washed samples in clean microfuge tube and incubated at 55 C for 20 minutes. This incubation allows all precipitated silica to break down to monomeric orthosilicic acid. Silicic acid concentration was then determined via the P-molybdosilicate method by measuring the yellow molybdosilicate complex formed as described by Her. The reaction velocities were calculated to study the enzymatic effect of the peptides under consideration. 

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