Dissolution hydrolytic

Commercial condensed phosphoric acids are mixtures of linear polyphosphoric acids made by the thermal process either direcdy or as a by-product of heat recovery. Wet-process acid may also be concentrated to - 70% P2O5 by evaporation. Liaear phosphoric acids are strongly hygroscopic and undergo viscosity changes and hydrolysis to less complex forms when exposed to moist air. Upon dissolution ia excess water, hydrolytic degradation to phosphoric acid occurs the hydrolysis rate is highly temperature-dependent. At 25°C, the half-life for the formation of phosphoric acid from the condensed forms is several days, whereas at 100°C the half-life is a matter of minutes. 

Hie hydrolytic depolymerization of nylon-6 was followed by gel permeation chromatography (GPC), viscometry, and gravimetry. GPC determinations were performed on a Waters 150C chromatography system using benzyl alcohol as die eluant, two Plgel 10-p.m crosslinked polystyrene columns, and a differential refractometer detector. The flow rate was 1 mL/min. The concentration of the polymer solutions was 0.5 wt% and dissolution was accomplished at 130°C. 

Hydrolytic decomposition brings disadvantages. There is continued leakage at the margins where complete dissolution can occur (Gourley Rose, 1972) and, indeed, these bases have been observed to disappear entirely (Akester, 1979 Barnes Kidd, 1979). 

This aromatic alcohol has been an effective preservative and still is used in several ophthalmic products. Over the years it has proved to be a relatively safe preservative for ophthalmic products [138] and has produced minimal effects in various tests [99,136,139]. In addition to its relatively slower rate of activity, it imposes a number of limitations on the formulation and packaging. It possesses adequate stability when stored at room temperature in an acidic solution, usually about pH 5 or below. If autoclaved for 20-30 minutes at a pH of 5, it will decompose about 30%. The hydrolytic decomposition of chlorobutanol produces hydrochloric acid (HC1), resulting in a decreasing pH as a function of time. As a result, the hydrolysis rate also decreases. Chlorobutanol is generally used at a concentration of 0.5%. Its maximum water solubility is only about 0.7% at room temperature, which may be lowered by active or excipients, and is slow to dissolve. Heat can be used to increase dissolution rate but will also cause some decomposition and loss from sublimation. Concentrations as low as 0.125% have shown antimicrobial activity under the proper conditions. 

Availability of Monomer. Under most solution conditions employed for gel formation, monomer is rapidly consumed to form a distribution of oligomeric species (12). Thereafter the only possible source of monomer is that which is produced by hydrolytic (or alcoholic) depolymerization of siloxane bonds (dissolution). According to Her (241 the dissolution of a-Si02 above pH 2 is catalyzed by OH ions which are able to increase the coordination of silicon above 4... 

In situ SAXS investigations of a variety of sol-gel-derived silicates are consistent with the above predictions. For example, silicate species formed by hydrolysis of TEOS at pH 11.5 and H20/Si = 12, conditions in which we expect monomers to be continually produced by dissolution, are dense, uniform particles with well defined interfaces as determined in SAXS experiments by the Porod slope of -4 (non-fractal) (Brinker, C. J., Hurd, A. J. and Ward, K. D., in press). By comparison, silicate polymers formed by hydrolysis at pH 2 and H20/Si = 5, conditions in which we expect reaction-limited cluster-cluster aggregation with an absence of monomer due to the hydrolytic stability of siloxane bonds, are fractal structures characterized by D - 1.9 (Porod slope — -1.9) (29-30). 

Desizing by chemical decomposition is applicable to starch-based sizes. Since starch and its hydrophilic derivatives are soluble in water, it might be assumed that a simple alkaline rinse with surfactant would be sufficient to effect removal from the fibre. As is also the case with some other size polymers, however, once the starch solution has dried to a film on the fibre surface it is much more difficult to effect rehydration and dissolution. Thus controlled chemical degradation is required to disintegrate and solubilise the size film without damaging the cellulosic fibre. Enzymatic, oxidative and hydrolytic degradation methods can be used. 

The bacteria in the intestinal tract serve as another well-known source of luminal drug degradation [61], though this is only important for the colon region as the luminal concentration of bacteria is 104 to 109-fold higher in the colon compared with the small intestine. Thus, this aspect is only relevant for drugs that reach this region, for example, due to poor permeability, slow dissolution or delivery by modified-release formulations. Hydrolytic and other reductive reactions are predominantly mediated by bacterial enzymes, and a list of the most prominent types... 

As exemplified in figure 2.10, in the attainment of new equilibrium a single reaction may proceed in opposite directions (i.e., forward and backward). In hydrolytic equilibria, the dissolution process is conventionally defined as moving in... 

It is appropriate to employ a co-solvent when the solubility of the drug is limited under a given condition. The two most common co-solvents used are acetonitrile and methanol (1). It should be noted that dissolution facilitated by a co-solvent may not always increase the degradation rate, as there are many factors involved. For further discussion of this topic see Hydrolytic section of Chapter 2. 

Smith, D. C., M. Simon, A. L. Alldredge, and F. Azam. 1992. Intense hydrolytic enzyme activity on marine aggregates and implications for rapid particle dissolution. Nature 359 139-142. 

The hydrolytic condensation of dimethyldichlorosilane is endothermic 31 KJ is absorbed per 1 mole of dimethyldichlorosilane however, the dissolution of hydrogen chloride (formed by the hydrolysis of dimethyldichlorosilane) liberates 73.4 KJ per 1 mole of HC1 in water. Consequently, the process has a positive total thermal effect and the heat should be diverted. If the heat is not diverted in time, the hydrolysate will contain more high-molecular linear products. This increases the viscosity and interferes with subsequent neutralisation by forming stable emulsions of hydrolysate and soda solution. Thus, apart from pre-cooling the components for hydrolytic condensation, the jacket of the hydrolyser is filled with salt solution to divert most of the heat. 

The type of pH modifier to make a desired mobile phase pH also has an effect on the column stability, and this is indirectly related to the peak efficiency and the retention of the analyte. As an increasing number of column volumes of the mobile phase are traversed through the column, the stability of the packing material could be comprised. Rearrangement of the packing bead leads to the loss of efficiency, dissolution of silica leads to loss in efficiency and retention, and hydrolytic decomposition of the bonded phase could impact the peak shape and retention. Different compounds, such as neutral compounds, acidic compounds, and basic compounds, could show different behaviors. 

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