Silica saturation concentrations

As with the calcareous tests, BSi dissolution rates depend on (1) the susceptibility of a particular shell type to dissolution and (2) the degree to which a water mass is undersaturated with respect to opaline silica. Susceptibility to dissolution is related to chemical and physical factors. For example, various trace metals lower the solubility of BSi. (See Table 11.6 for the trace metal composition of siliceous shells.) From the physical perspective, denser shells sink fester. They also tend to have thicker walls and lower surface-area-to-volume ratios, all of which contribute to slower dissolution rates. As with calcivun carbonate, the degree of saturation of seawater with respect to BSi decreases with depth. The greater the thermodynamic driving force for dissolution, the fester the dissolution rate. As shown in Table 16.1, vertical and horizontal segregation of DSi does not significantly coimter the effect of pressure in increasing the saturation concentration DSi. Thus, unlike calcite, there is no deep water that is more thermodynamically favorable for BSi preservation they are all corrosive to BSi. 

One disadvantage of all silica-based stationary phases is their instability against hydrolysis. At neutral pH and room temperature the saturation concentration of silicate in water amounts to lOOppm. Solubility increases with surface area, decreasing particle diameter drastically with pH above 7.5. This leads also to a reduction of the carbon content. Hydrolysis can be recognized during the use of columns by a loss in efficiency and/or loss of retention. Bulky silanes [32], polymer coating [33], or polymeric encapsulation [34] have been used in the preparation of bonded phases to reduce hydrolytic instability, but most of the RPs in use are prepared in the classical way, by surface silanization. Figure 2.3 schematically shows these different types of stationary phases. 

Fig. 13. Quartz ai>d amorphous silica solubility vs. temperature along the vapour saturation curve. The dashed lines show the silica concentration in water initially in equilibrium with quartz during adiabatic boiling to 100 C and subsequent cooling. The increase in aqueous silica concentrations during boiling is the consequence of steam formation. Amorphous silica saturation (shown by the dots) is attained at 188 C in the case of the 300 C aquifer water, but at 94 C in the case of the 200 C aquircr water. It was assumed that the pH of the water is not raised sufficiently during boiling to cause significant ionization of the aqueous silica. If some ionization had occurred, amorphous silica saturation would be reached at lower temperatures than those indicated in Fig. 13.

In weathering situations, saturation of fluids with SiC relative to any species of pure silica is probably only rarely achieved. In continental and shallow sea deposits, silica is precipitated in some initially amorphous form, opaline or chert when lithified or extracted by living organisms. Authigenically formed silicates are probably not in equilibrium with quartz when they are formed. As compaction increases in sediments, silica concentrations in solution are again above those of quartz saturation (15 ppm) and again it must be assumed that the diagenetic minerals formed are not in equilibrium with a silica polymorph except where amorphous silica is present. It is possible that burial depths of one or two kilometers are necessary to effectively stabilize that quartz form. It must be anticipated that the minerals formed under conditions of silica saturation near the earth s surface will be a minority of the examples found in natural rock systems. 

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. 

To deposit amorphous silica at room temperature saturation concentration of about 100 to 150 ppm dissolved silica must be reached138-140. These fluctuations are due to slight pH effects on solubility (Fig. 16). 

In a nuclear waste repository located in basalt, solution pH is controlled by interactions between groundwater and the reactive glassy portion of the Grande Ronde basalt (10). In situ measurements and experimental data for this system indicate that equilibrium or steady-state solutions are saturated with respect to silica at ambient temperatures and above. Silica saturation and the low, total-dissolved carbonate concentration indicate the pH may be controlled by the dissolution of the basalt glass (silica-rich) with subsequent buffering by the silicic acid buffer. At higher temperatures, carbonate, sulfate, and water dissociation reactions may contribute to control the final pH values. 

The credibility of the prevention of radiation enhancement is increased by the data in Table VI on one impurity at increasing concentrations. There seems to be a gradual onset of the effect, and it occurs at a concentration (0.01 mole % = 7 X lO atoms A13+/gm ZnO) that is in the range of the saturation concentrations reached by radiation-produced defects in silica gel (69) and in alumina (106). If zinc oxide is similar in this respect, one would expect that an interaction between impurity and radiation-enhancement would begin in this range of impurity concentration. Since the other observations are at least equally well established, the answer to the inconsistencies is probably not in experimental errors. 

Fig. 3.5 The experimental results of Barret, Menetrier and Bertrandie [7] superposed on the C S H system 1 W/CjS = 1,000 (there is no possibility to saturate the liquid phase withealcium hydroxide) V WIC S= 100 2 CjS + solution C = 3.95-10 mol/kg, S = 1.19-10 mol/kg 3 primary solution 5 C S processed with the solution rich in silica of concentration S =4.70 10 mol/ kg, C= 1.75 10 mol/kg 8, 9, 10 two primary solutions with different composition from the left and right side of the curve 1...

The results of the performed tests show that the reaction of equation (3) is easily promoted. However, the reaction of equation (4) occurs slower than the leaching reaction, and the silica then exists in an over-saturated concentration as shown in Figure 2(a). One may conclude that as it becomes more saturated, the silica is preferentially precipitated in a gel form, resulting in an adverse effect on the solid-liquid separation characteristics. In order to quantify this over-saturation parameter, we defined the value with the highest content of silica in solution as Max. Si02-Concentration . 

When the phases other than quartz and amorphous silica are suspended in water, the final concentration of soluble silica is not a saturation concentration, but the result of a competition between silica passing into solution as Si(OH)4 and Si(OH) readsorbing on the surface to an ever-increasing extent, blocking further dissolution. Thus a limiting concentration is reached that depends on the area of solid surface exposed per unit volume of solution. 

As for the effect of hydroxyl ion, it is not possible to see how it could catalyze the dissolution of stishovite in which silicon has already reached its maximum coordination number. No data seem to be available on the effect of pH on the dissolution rate of stishovite, but it is interesting that at pH 8.4 it dissolves about as fast as vitreous silica when compared on the basis of equal areas of surface being exposed to the solution. Furthermore, it continues to dissolve past the saturation level for vitreous or amorphous silica. The concentration of soluble silica can reach as high as 190 ppm, at which point colloidal particles are nucleated (139). It is likely that stishovite is hydrolytically unstable and would eventually decompose completely to amorphous silica. Whether or not" pH has an effect on the rate of hydrolysis is not known. 

At the upper end of the cooling-water pH range (9.0), silica solubility increases to 115 ppm (20°C) and 140 ppm (30°C). A control limit of 180 ppm would correspond to saturation levels of 1.5 and 1.3, respectively. In systems where concentration ratio is limited by silica solubility, it is recommended that the concentration ratio limit be reestablished seasonally based on amorphous silica saturation level or whenever significant temperature changes occur. 

Estriol Dip the concentrating zone of a precoated H PTLC silica gel 60 plate in a saturated ethanolic solution of Fast Dark Blue R salt, allow the solvent to evaporate, apply the sample solution and dip once [10, 59]... 

Preparation of 1 -(/3-D-arabinofuranosyl)-2-thiocytosine A solution of 2.0 g of 1 -(2, 3, 5 -0-triacetyl-/3-D-arabinofuranosyl)-2,4-dithiouracil in 100 ml of methanol is saturated with anhydrous ammonia at 0°C. The mixture, in a glass liner, is heated in a pressure bomb at 100°C for three hours. The reaction mixture is concentrated to a gum in vacuo, and most of the byproduct acetamide is removed by sublimation at 60°C/0.1 mm. The residue is chromatographed on 100 g of silica gel. Elution of the column with methylene chloride-methanol mixtures with methanol concentrationsof 2-25% gives fractions containing acetamide and a series of brown gums. The desired product is eluted with 30% methanol-methylene chloride to give a total yield of 0.386 g (30%), MP 175°-180°C (dec.). Recrystallization from methanol-iso-propanol furnishes an analytical sample, MP 180°-182°C (dec.). 

A mixture of 4.98 g of acetoacetic acid N-benzyl-N-methylaminoethyl ester, 2.3 g of aminocrotonic acid methyl ester, and 3 g of m-nitrobenzaldehyde was stirred for 6 hours at 100°C in an oil bath. The reaction mixture was subjected to a silica gel column chromatography (diameter 4 cm and height 25 cm) and then eluted with a 20 1 mixture of chloroform and acetone. The effluent containing the subject product was concentrated and checked by thin layer chromatography. The powdery product thus obtained was dissolved in acetone and after adjusting the solution with an ethanol solution saturated with hydrogen chloride to pH 1 -2, the solution was concentrated to provide 2 g of 2,6-dimethyl-4-(3 -nitrophenyl)-1,4-dihydropyridlne-3,5-dicarboxylic acid 3-methylester-5- -(N-benzyl-N-methylamino)ethyl ester hydrochloride. The product thus obtained was then crystallized from an acetone mixture, melting point 136°Cto 140°C (decomposed). 

This temperature is gradually raised to 95°C and the mixture kept at this temperature for 1 hour. The mixture is allowed to cool and added to 2 liters of water. The aqueous layer is extracted with ether, the ether solution washed twice with saturated sodium chloride solution, 5% Na2C03 solution, water, and then dried. The ether filtrate is concentrated with 200 grams silica-gel, and added to a five pound silica-gel column packed with 5% ether-petroleum ether. The column is eluted with 5 to 10% ether-petroleum ether and followed by TLC to give 6-fluoro-2-methylindanone. 

Purification of luciferin (Rudie etal., 1976). The luciferin fractions from the DEAE-cellulose chromatography of luciferase were combined and concentrated in a freeze-dryer. The concentrated solution was saturated with ammonium sulfate, and extracted with methyl acetate. The methyl acetate layer was dried with anhydrous sodium sulfate, concentrated to a small volume, then applied on a column of silica gel (2 x 18 cm). The luciferin adsorbed on the column was eluted with methyl acetate. Peak fractions of luciferin were combined, flash evaporated, and the residue was extracted with methanol. The methanol extract was concentrated (1 ml), then chromatographed on a column of SephadexLH-20 (2 x 80 cm) usingmethanol asthe solvent. The luciferin fractions eluted were combined and flash evaporated. The residue was... 

A solution of hex-l-yne (4.5 mmol) in THF (1ml) was added slowly to lithium bis(phenyldimethylsilyl)cuprate (Chapter 8) (5 mmol, based on CuCN) at 0°C, and the mixture was stirred for 15min at 0°C. Saturated ammonium chloride solution (1 ml) was added, and stirring was continued for 5 min at 0°C. Light petroleum was then added, and the organic layer was washed with ammonium chloride solution, and dried. Concentration and chromatographic purification on silica gel gave the vinylsilane (4.23 mmol, 94%). 

A mixture of TMSOTf (0.1 mmol, lmol%), allyltrimethylsilane (11.5 mmol) and dichioromethane (1 ml) was cooled to -78 °C, and to this was added benzaldehyde dimethylacetal (10.5 mmol) in dichioromethane (4ml). The resulting mixture was stirred for 6h at —78°C, and then poured into saturated sodium hydrogen carbonate solution (10 ml) and extracted with ether (3 x 20 ml). The combined organic extracts were washed with brine, dried and concentrated. Chromatography on silica gel (1 20 ether hexane) gave 4-pheny]-4-methoxybut-l-ene (9.2mmol, 88%). 

To (E)-stilbcnc oxide (25 mmol) in THF (35 ml) was added freshly prepared dimethylphenylsilyl lithium (25.35 mmol, 1.3 m in THF) dropwise at ambient temperature. The solution was stirred at ambient temperature for 4h, and then poured into saturated ammonium chloride solution (15ml), diluted with ether, and the separated organic layer was dried and concentrated. The crude product (97 3 (Z) (E), g.I.c.) was purified by chromatography on silica gel to give (Z)-stilbene (18.75 mmol, 75%). 

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