Silica ionic concentrations

In non-carbonated and chloride-free concrete, the concentration of hydroxyl ions (OH ) varies from 0.1 M to 0.9 M, due to the presence of both NaOH and KOH (the latter is predominant, especially in Portland cement). Other ions, e. g. Ca and S04 , are present only in very low concentrations. Addition of blast furnace slag or fly ash to Portland cement results in a moderate reduction of ionic concentration, and thus in pH. From hydroxyl ion concentrations in Table 2.1, values of pH of 13.4-13.9 can be calculated for Portland cement, and pH values of 13.0-13.5 for blended cements. Addition of condensed silica fume in higher percentages may lead to a decrease in the pH to values to below 13 [4, 10]. 

Table 2.1 Ionic concentration (in mmol/L) measured in the pore solution extracted from cement pastes, mortars and concrete made with ordinary Portland cement (OPC) and with additions of blast furnace slag (GGBS), fly ash (PFA) and silica fume (SF). n.a. = concentration not available [15]... 

The combinatimi of Eq. 1 and Eqs. (5) and (6) leads to a fuU description of the electrochemical boundary conditimis in a silica nanochannel. The double-layer interaction will influence the local ionic concentration and therefore affects the surface charge conditions. 

The worked out soi ption-photometric method of NIS determination calls preliminary sorption concentration of NIS microamounts from aqueous solutions on silica L5/40. The concentrate obtained is put in a solution with precise concentration of bromthymol-blue (BTB) anionic dye and BaCl, excess. As a result the ionic associate 1 1 is formed and is kept comparatively strongly on a surface. The BTB excess remains in an aqueous phase and it is easy to determinate it photometrically. The linear dependence of optical density of BTB solutions after soi ption on NIS concentration in an interval ITO - 2,5T0 M is observed. The indirect way of the given method is caused by the fact the calibration plot does not come from a zero point of coordinates, and NIS zero concentration corresponds to initial BTB concentration in a solution. 

Metal hydroxides (e.g., Fe, Mn, Al) can also be a problem (Rauten-bach and Albrecht, Membrane Processes, Wiley, New York, 1989). A chemical analysis of the feed solution composition along with consideration of solubility products allows one to determine the significance of precipitation. Solubility products can be affected by temperature, pH, and ionic strength. Seasonal temperature variations must be considered. Concentrations of silica need to be < 120 mg/L in the feed. 

In the calculation results (Fig. 24.1), amorphous silica, calcite (CaCCF), and sepiolite precipitate as water is removed from the system. The fluid s pH and ionic strength increase with evaporation as the water evolves toward an Na-C03 brine (Fig. 24.2). The concentrations of the components Na+, K+, Cl-, and SO4- rise monotonically (Fig. 24.2), since they are not consumed by mineral precipitation. The HCO3 and Si02(aq) concentrations increase sharply but less regularly, since they are taken up in forming the minerals. The components Ca++ and Mg++ are largely consumed by the precipitation of calcite and sepiolite. Their concentrations, after a small initial rise, decrease with evaporation. 

It has been shown elsewhere (26) that in natural waters the degree of enhancement of Mn(II) oxidation predicted on the basis of model calculations is as follows y-FeOOH > a-Fe00H > silica > alumina. It has also been shown that the rate of Mn(II) oxidation is strongly influenced by pH, y-FeOOH concentration, temperature and ionic strength. Depending on the conditions, the predicted half-life 1/2 = ln 2/ki ) f°r Mn(II) oxidation may vary from a few days to thousands of years. By way of example, at pH 8, p02 0.21 atm, 25°C in waters containing 4(iM y-FeOOH and 0.2uM Mn(II), the half-life for oxidation is about 30 days. 

The pKa of the imidazole ring is near 6 (16) so histamine would only exist as an ion in the acidic (pH = 2-3) mobile phase. One would predict no retention on a bonded phase column under this condition however, it does occur. Figure 3 is the simplest way to account for this retention. Here, the mineral acid acts as the counter-ion, as well as the buffer. All of the histamine in the mobile phase is in the ionic form and is in equilibrium with the ion-pair which is only soluble in the stationary phase chemically bonded to silica. Histamine only elutes in the ionic form and is then derivatized for detection. A sharp peak in the chromatogram with good shape and no change in retention time with variation in sample concentration indicates a working system. However, if the paired ion has some solubility in the mobile phase, peak tailing occurs. 

The first fixed-bed application of a supported ionic liquid-phase catalyst was hydroformylation of propylene, with the reactants concentrated in the gas phase (265). The catalyst was a rhodium-sulfoxantphos complex in two ionic liquids on a silica support. The supported ionic liquid phase catalysts were conveniently prepared by impregnation of a silica gel with Rh(acac)(CO) and ligands in a mixture of methanol and ionic liquids, [BMIMJPFg and [BMIM][h-C8Hi70S03], under an argon atmosphere. 

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