Silicates hydrogen dissolution

Silicon reacts with strong bases forming silicates and hberating hydrogen. Sihcon is attacked by hydrofluoric acid if there is no oxide layer over it. Flowever, since the metal has a very thin oxide film over its surface, a mixture of nitric and hydrofluoric acid is effective in dissolution of the metal. While nitric acid dissolves the oxide layer, the metal is then attacked by hydrofluoric acid. 

The mechanism of silicon etching in alkaline solutions is a process of material dissolution with a simultaneous hydrogen evolution. The main soluble product is a silicic anion Si02(0H)2 that can further be condensed to form polysilicic anions. In fact, due to the acido-basic ionization of OH radicals in a highly alkaline solution, Eq. (19) should be modified as follows ... 

Fig. 2. Plot of normalized rate vs. the activity of silicic acid for the LAWABP1 (see Table 1) glass composition at two temperatures (26 and 40 °C). Rates are all computed at steady-state conditions. Boron and Na release rates are identical at low silica activities, then decrease, and become constant at or near saturation with respect to amorphous silica (vertical dot-dashed line). Note that the B rate decreases more than the Na rate. This behaviour can be rationalized as competition between two concurrent reactions alkali-hydrogen exchange and matrix dissolution (see text). Error bars represent 2-

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

Fusion with anhydrous potassium fluoride in a platinum dish is undoubtedly the simplest, most effective and reliable method available for the complete dissolution of a wide variety of siliceous materials. The potassium fluoride cake can then be transposed in the same container to a pyrosulfate fusion with rapid and complete volatilisation of both hydrogen fluoride and silicon tetrafluoride [54]. Except for a small quantity of barium sulfate, the pyrosulfate cake will dissolve completely in dilute hydrochloric acid. The resulting pyrosulfate fusion is one of the simplest and most effective methods available for rapid, complete and dependable dissolution of nonsiliceous materials, particularly high-fired oxides. This fusion has the distinct advantage that the flux can be obtained by simply adding easily purified alkali metal sulfates to sulfuric acid, and the fusion can be carried out in either borosilicate flasks or platinum vessels with very little contamination from either reagents or containers. 

I. The dissolution rate is controlled by reaction of the unaltered silicate with aqueous hydrogen ions at the interface between the two phases ( 7, 9). 

II. The dissolution rate is controlled by interdiffusion of hydrogen or hydronium ions and species contained in lattice sites within the interior of the silicate phase. This process results in a leached layer consisting mainly of silica and alumina. Such a layer may retain the original silicate structure (W, 11) or may represent a collapsed or hydrated layer (12, 13). 

The solubility of magnesium silicate in different media is shown in Table 2.4. The dissolution mechanism of ot-chitin in N,N-dimefhylacetamide (DMAc)/ 5% LiCl can be aftribufed to the formation of a weak complex between Li ions and the carbonyl oxygens of the DMAc, which solvates the polyelectrolyte formed between the CD ions and labile proton groups (OH and NHCOCH3) of the chitin chain, disrupting the extensive intra- and intermolecular hydrogen bonds of the crystalline sheet structure of a-chitin. 

For the determination of antimony, the digestion efficiency of particulate matter, collected on Whatman 41 cellulose filters, was evaluated by means of radiotracer experiments [6], A quantitative recovery was obtained using 8 ml sulfuric acid and 4 ml hydrogen peroxide, when applied to a quarter fraction of the filter. For the dissolution of residual silicates 0.5 to 4 ml of hydrofluoric acid were added, which were later on evaporated. After digestion the solution was filtered if needed and diluted appropriately. 

Ammonium hydrogen fluoride is used for the dissolution of silicate minerals in the extraction of crude oil, for the pretreatment of aluminum prior to anodization and as a frosting agent in the glass industry. 

The pH dependence of the dissolution rates of silicates is a subject of intensive theoretical interest, based on transition-state and surface-reaction rate theories (e.g., Schott and Petit, 1987 Wollast and Chou, 1988 Stumm and Wieland, this volume). The features of the pH dependence of the silicate dissolution rates that are relevant to this section are the reported dependence of the rate in the acidic solution range (pH < 5.5) on a power of the hydrogen ion concentration, Roc[H + ]0 5 to [H + ]10, and its dependence in the alkalilne range (pH >7.5) on Roc[H + ]-°3. 

The first step in the dissolution of anhydrous silicate is an ion exchange between the alkali ions in the glass and the hydrogen ions of the surrounding water. The glass surface gets covered by a protective layer of silanol groups, while the pH of aqueous solution rises. In the second step, the alkalinity drives the depolymerization of the silicate particles [5]. 

---