Silicic acid dissociation

As a weak acid, silicic acid dissociates in two steps, according to the reactions with the stepwise constants given below for 25°C ... 

Using the silicic acid dissociation conslant expressions for/ff, and / 2 for reactions (7.14) and (7.16) above, we can replace the last two terms in Eq. (7.17) with terms that contain only the species H4Si04. This leads to... 

Dissociation constant of silicic acid calculated according to the a + = [(Kacx Kw)/c]1/2fbrmula for dissociation of salts formed from weak acid and strong base a+ is the activity of protons (from pH), K w is the ionization constant of water, and c is the concentration of silicate solution. 

Silicic acid is a weak acid and it dissociates in the following manner ... 

At the pH and ionic strength of seawater, the dominant dissolved species of silicon is orthosilicic acid [H4Si04(aq) or Si(OH)4(aq)]. The speciation of silicic acid is shown in Figure 5.19. At the pH of seawater, a minor amount of dissociation occurs, such that about 5% of the dissolved silicon is in the form of HjSiO faq). Dissolved organic complexes of silicon do not occur naturally. 

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. 

From the potentiometric titration data (Si02 - H+/OH") Sidorova et al.72) calculated the value of the surface potential ip0 and the dissociation constant of the surface silicic acid Ko for the reaction ... 

Fine crystalline quartz or amorphous substances (gel, quartz glass, etc.) are used as one of initial components for the synthesis [19]. Depending on temperature, pressure, pH of the medium and the presence of salts, silica can exist in solution both as simple ions or molecules, and as more complicated polymer particles. Under normal conditions silica passes into solution in monomer form, as silicic acid Si(OH)4 at large pH, silicate ions SiOj - are formed. Monosilicic acid is a very weak acid however, at increased temperature its dissociation constant increases substantially. The amount of monomer form also increases with temperature. The dissolution of Si02 is due to hydration as well as to depolymerization. 

Magnesium silicate displays low solubility in acids of up to 3 M concentration above this concentration it partially dissolves. It is dissociated in acids forming magnesium ions and silicic acid in what is referred to as "acid leaching" of silicates (Eq. (7.4)) ... 

Iead[hexa(hydrogensulfato)plumbic acid] and, 414-416 methylstannonic acid and, 414 monoalkylsilanols and, 411-412 nitrogen and, 416-417 phosphorus and, 417 physical properties of, 387 selenium dioxide and, 419-420 self-dissociation reactions of, 387-389 silicic acid derivatives and, 412-413 sulfato compounds and, 405-407... 

Weak acids such as carbonic acid (H2CO3) and silicic acid (H2Si03) are not sufficiently dissociated (strong) to protonate the weakly basic amine grouping, and therefore are not sorbed by a weak base ion exchange resin. This property is manipulated to great effect in such applications as water treatment by ion exchange. 

Where is the total concentration of acid species and A, and K2 are the first and second stepwise dissociation constants of the acids. This equation can be used to compute the buffer index of a polyprotic acid as long as successive dissociation constants differ by at least 20 times (this assures a calculation error of 5% or less). In other words, for a diprotic acid K2fK should be less than 0.05 (cf. Butler 1964). Thus, for example, Eq. (5.114) may be used to compute the buffer index due to species of carbonic acid, for which A = 10 and K2 = 10" °, or/ for species of silicic acid, for which a , = lO- and K, = 10... 

Although the solubilities of quartz, its polymorphs, and amorphous silica are fixed and pH-in-dependent in most natural waters, the dissociation of silicic acid at alkaline pH s leads to substantial increases in their solubilities above pH 9 to 10. The following calculations show how we can predict this effect. First, the solubility of any silica solid must equal the sum of the concentrations of all species of silica in solution at equilibrium. This summation is given by the mass-balance equation... 

The pK of dissociation of silanol groups is not known. But for the first dissociation of silicic acid ... 

The reaction of interest is the dissolution of amorphous silica in aqueous solution in the temperature range from 0 to 25 C, the pressure range from one to 1000 atmospheres, and the pH range from 6 to 8.5 In this pH region, dissolved silica occurs primarily as undissociated silicic acid O) above this pH range, the solubility increases due to the Increased dissociation of silicic acid. The dissolution can be written several ways, depending upon how water is indicated, as follows ... 

Reactions of this kind are by far the most important source of silicic acid in solution in natural waters because the solubility of quartz (SiO ) is very low. The hydrogen ions (H+) that are consumed by this reaction originate by dissociation of carbonic acid which forms when carbon dioxide of the atmosphere dissolves and reacts with molecules of water ... 

Weak acids and undissociated compounds such as silica and boron are poorly rejected by RO membranes at pH <8.0. For example, boron exists as a weakly dissociated boric acid, H3BO3 at pH < 8.2. Silica dissolves in water forming weakly ionised silicic acid, which in... 

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 equations for these have the form given below where Bas-j- and Kb s denote the total concentrations, B-j- and Sh, and dissociation constants, Kb and K i, of boric and silicic acids, respectively)... 

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