Glass dissolution process

Flintoff, J. F. HARKER, A. B. 1985. Detailed processes of surface layer formation in borosilicate waste glass dissolution. In Jantzen, C. M., Stone, J. A. Ewing, R. C. (eds) Scientific Basis for Nuclear Waste Management VIII. Materials Research Society Symposia Proceedings, 44, 147-154. 

The reaction of acids with glass may be either a leaching process or a complete dissolution process. Acids such as hydrofluoric acid attack silica glasses by dissolving the silica network. Other acids such as hydrochloric acid or nitric acid may react by dissolving certain glasses. However, the reaction mechanism is by selective extraction of alkali and the substitution of protons in a diffusion-controlled process. 

The reaction of bases with most silicate glasses produces dissolution rates when tested in 5% NaOH solution at 95°C. The mechanism also involves a complete dissolution process as that described for acid. Weaker alkaline solutions may both leach and dissolve and sometimes show greater dependence on glass composition. And, in the case of strong alkali solutions, the rate of attack doubles for each 10 K increase in temperature or each increase in pH unit. Usually higher alkali durability glasses are used for laboratory wares. 

NOJ(aqf). This includes solid ammonium nitrate, the water in which it dissolves, and the aquated ions that are the products of the dissolution process. The surroundings include the flask or beaker in which the system is held, the air above the system, and other neighboring materials. The dissolution of ammonium nitrate is spontaneous after any physical separation (such as a glass wall or a space of air) between the water and the ammonium nitrate has been removed. 

FIG. 35 Steady-state approach curves of the diffusion-limited current for the oxidation of Br to BrJ at a Pt tip (a = 2.5 /xm), as a function of distance from a glass surface ( ) and the (100) face of KBr (o), in an acetonitrile solution containing 0.05 mol dm-3 LiC104 and saturated with respect to potassium bromide. The theoretical characteristics for (a) negative feedback (----) and (b) a diffusion-controlled dissolution process (-----) are also shown. 

USP III. The formulation is placed in a cylindrical glass tube with steel screens in the bottom and the top. The mesh size of the tubes may vary. This tube is moved up and down in a larger tube that contains the dissolution fluid. The amplitude of the inner tube movements is 5-40 dips/min, and the volume of the outer tube is 300 mL. Tubes containing 100 mL and 1 L are also available. The inner tube can be moved during the dissolution process between different outer tubes, which may hold different dissolution fluids. 

As already mentioned, natural zeolites may be classified as hydrothermal or sedimentary, in reference to processes which are not ever very clearly distinguishable from each other. Basically and very schematically, the main differences involve two parameters distance between the sites of glass dissolution and the sites of crystal deposition, which are usually far in the hydrothermal process and close in sedimentary process (also said diagenesis), and temperature, which is usually higher in the former than in the latter [34],... 

The flow method ts the simplest and the most straightforward. In the flow method, the solvent fluid is supplied to a compressor by a pressure cylinder. At the desired pressure, the fluid passes into the thermostatted extractor cell that contains the solute present in appropriate matrix (e.g., multiple layer of glass wool). The fluid dissolves the solute in the extractor and, on expansion through a heated metering valve, precipitates solute into a series of collection vessels to be measured gravimetrically. The volume of the decompressed fluid is totaled by a wet or dry gas test meter. Static or equilibrium solubility measurement methods are used to eliminate the need to sample the supercritical fluid solution. A high-pressure flow cell is placed in the flow circuit to monitor the dissolution process by spectrophotometry. 

Agitate with a glass rod, if necessary, to assist the dissolution process. Now heat the solution to boiling for an additional minute. 

Problems in the accuracy of the measured Tend values arise due to the polymer s high viscosity that slows the mixing and dissolution process down. This is most prominent at temperatures close to or below the glass transition temperature (Tg) of the drug-polymer mixture as molecular mobility is reduced. The mixing and dissolution processes at temperatures close to the Tg may become so slow that equilibrium is not reached during the time frame of a DSC scan. These measurements lead to a wrong assumption of the equilibrium solubility. Therefore, solubility measurements close to or below the Tg of the mixture are not feasible with DSC measurements due to the time frame of the experiments. 

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