Titration curves, of silica

Figure 6. Effect of ionic strength (numbers in the legend are concentrations of NaCl in mol/1 soluble is hypothetical curve without influence of electrostatic potential of surface, i.e. Z = 0) on titration curves of silica with a particle diameter of 15 nm, pKa =7, a number of dissociating groups in a particle n = ns/2. The curves were calculated from Eq. (9).

With allowance for the data of Ref. [26] Fig. 8. gives estimated curves for titration of silica with a particle size of 15 nm (182 m /g) and a dissociation constant of silanol groups pKa = 10. As seen from the figure, the account of influence of an electrostatic field of the ionized surface and a competition of hydrogen ions and sodium ions allows us to explain partly all anomalies of titration curves of silica without introducing supplementary postulates. 

The charging curves of silica shown in Figure 2.12 have basic branches similar to those obtained for metal oxides (Figure 1.1). In the acidic range, the Gg of silica tends asymptotically to zero, positive Gq is not observed, and there is no CIP. A substantial positive Gq and an intersection point for quartz are reported in [602]. A substantial positive Gq of silica (but without a clear CIP) is reported in [603,604]. An unusual course of titration curves of silica, with a high slope at a low pH, is reported in [41]. 

Figure 1.12 Photochemical titration curves of crystal violet co-entrapped in silica sol-gel matrices with different surfactants, no surfactant ( ) and in solution (x) show the impressive variations in the sensing properties for the same entrapped dye. (Reproduced from ref. 26, with permission.)... 

The pH is an important factor that can influence the ionization of the surface silica groups. As a result, C is directly dependent on the pH. Therefore, the relationship of /teof as a function of pH is governed by the behavior of the dissociation of the silanol groups. Different capillary materials result in different profiles of the electroosmotic mobility as a function of the pH (due to differences in Q. Typically a sigmoid curve behavior resembling the titration curve of the surface active groups is observed. ... 

As shown above, surface titration curves of reacted albite exhibit notable differences, particularly in the acid pH range, relative to those obtained with fresh albite (but they are very close to those obtained with silica gels). T hus, one understands that surface titrations performed on fresh silicates can only help elucidate initial dissolution mechanisms. 

Figure 47 Experimental phosphine titration curve of trimethylphosphine on silica-alumina. (From Ref. 90.)...

Silicic Acid. Figure 6 (from Ref. 18) illustrates the effect of silicic acid on the titration curve of goethite ( -FeOOH). Dissolved silica is primarily present as H4Si04 below pH = 9. (The acidity constants of H4Si04 at Z = 0.5, 25°C, are pKa = 9.5, pKag = 12.6.) The shift in the titration curve caused by silica reflects a release of protons which can be explained with the reactions... 

Binding of a second proton results in a positive surface group (=SiOH2 ). These reactions are exactly equivalent with the equations often used (equation 2). If this model is applied to the charging of silica particles it turns out that only the first protonation step is required to describe the proton titration data. The shape of the charging curve of silica using only one proton affinity constant is shown in Figure 2. 

Raw titration curves need a correction according to Equation 2.12, which is applicable only for sets of charging curves with a CIP. Therefore, the presence or absence of a CIP is essential. CIPs are usually observed for metal oxides, and sets of charging curves without a CIP are typical of silica (see Section 2.6.3) and clay minerals (see Section 2.6.6). 

The question of reproducibility of titrations has been addressed in a few publications (e.g., the results of repeated titrations are reported in [570]) but are ignored in many others. Poor reproducibility of silica titration curves at low ionic strength and good reproducibility at high ionic strength are reported in [591]. Poor reproducibility of charging curves of amorphous HFO is reported in [592]. 

The IEP (Rank Brothers, 0.001 M KNO3) shifts to low pH as the silica content increases from 0.62% (negligible shift) to 5% (shift from pH 5.5 to 3). The electrokinetic curves of particles with 1-2% silica show a hysteresis (base titration produces a higher IEP). The IEP is also sensitive to storage for 3 weeks at pH 2, but is insensitive to storage for 3 weeks at pH T [2101]. 

Paulik and co-workers (80-86) described the combination of titration analysis with the Derivatograph. By means of a continuous titrator, a curve of the evolved gas can be made, which they call the thermo gas-titrimetric (TGT) curve, or the derivative of it, the derivative thermo-gas-titrimetric (DTGT) curve. This apparatus is illustrated in Figure 8.41. The sample in the furnace is surrounded by a silica chamber that can be flushed with an inert carrier gas. Evolved gases are transported by the carrier gas to the absorbing solutions, where they are titrated with a suitable reagent. The amount of titrant used is recorded as a function of time by means of a recording buret. 

Figure 6. Adsorption of aqueous silica on goethite (a-FeOOH) and its effect on the alkalimetric titration curve and su ace charge.

Consider the titration of reactive silanols at the surface of a porous silica gel [25]. Typical titration data are shown in Fig. 7 by discrete points. One may want to approximate these data by a titration curve calculated according to the charge regulation model based on the minimal reaction set... 

Her examined the pH-tltration behavior of silicic acid in the presence of 2-hydroxypyridine 1-oxide by titrating 16 mA/ (1000 ppm SiO ) silicic acid silica from pH 10.5 to 3.0 in the presence and absence of a 43 mA/ concentration of the N-oxide. At no point did the titration curves differ, indicating that no complex had formed. In another experiment, a solution of Si(OH) containing 100 ppm as SiOj was mixed with a 200-fold excess of the above -oxidc at pH 6.15 and aged for a few hours. Tests with molybdic acid showed that the reaction rate with silica monomer was the same as a control, indicating either that no complex was formed at this pH or that it dissociated very rapidly. However, the rate of dissolution of monomer from colloidal silica particles at pH 1.4 was apparently doubled in the presence of a 20 mA/ concentration of the yV-oxide, indicating some type of interaction at low pH. -... 

The high frequency used is necessary so that changes can be observed through the glass or silica wall of the cell, via an effective capacitor having this wall as dielectric. A typical circuit and examples of titration curves are shown in figures 0.1 and 0.2. The frequency or current measurements are related to comparative conductances only, but the technique has been used in a wide variety of applications. 

A similar interaction occurs with the very stable complexes of various cations such as Cu ", Ni " or Co ". At high ammonia concentration (pH > 8). [M(NH3)6] complexes dominate in. solution and are adsorbed on various substrates [3). The UV-visible characteristic absorption of the nickel amino complex is not shifted upon adsorption on silica [4]. Adsorption of these complexes does not cause a shift in the intersection point of the proton titration curves (i.e does not cause a shift in the PZC of the oxide [5]). The negatively charged surface of the oxide acts as the counterion of the cation complex (M(NH3)e1 at the most, to the formation of ion pairs ... 

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