PH-dependent dissolution

TR Bates, JM Young, CM Wu, HA Rosenberg. pH-dependent dissolution rate of nitrofurantoin from commercial suspensions, tablets and capsules. J Pharm Sci 63 643-645, 1974. 

Shellac is the oldest known material that has been used as enteric coating material. However, as a natural material, it lacks a crucial quality criterion of more modern polymers (i.e., batch-to-batch reproducibility). Hence, the most commonly used polymers today are the synthetic methacrylate copolymers or semisynthetic derivatives of cellulose. The main structural element of these polymers is an acidic function (either phthalate or methacrylic acid), which is responsible for the pH-dependent dissolution. 

Together with acid-base reactions, where a proton transfer occurs (pH-dependent dissolution/ precipitation, sorption, complexation) redox reactions play an important role for all interaction processes in aqueous systems. Redox reactions consist of two partial reactions, oxidation and reduction, and can be characterized by oxygen or electron transfer. Many redox reactions in natural aqueous systems can actually not be described by thermodynamic equilibrium equations, since they have slow kinetics. If a redox reaction is considered as a transfer of electrons, the following general reaction can be derived ... 

Oxide dissolution in aqueous electrolytes involves transfer of metal and oxygen ions to the solution. Since O2- ions cannot be transferred into the solution, protonation must precede the ion transfer reaction, which leads to strongly pH-dependent dissolution rates [37], The cation and oxygen transfer reactions may be regarded as statistically independent with... 

For lakes which have undergone significant acidification, it has been suggested that heavy metals could be released from surface sections by pH-dependent dissolution, resulting in sub-surface maxima in sedimentary heavy metal concentrations. In two Canadian acid lakes, however, Carignan and Tessier found that downward diffusive fluxes of dissolved zinc from overlying waters into anoxic pore waters were responsible for the pronounced sub-surface sediment maxima in solid phase zinc, presumably as the insoluble sulfide. 

Ammonia is the most important basic component in the atmosphere. The concentration of gaseous NH3 and its pH-dependent dissolution in the aqueous phase are relevant. 

Dentener FJ, Carmichael GR, Zhang Y, Leheveld J, CratzenPJ (1996) Role of mineral aerosol as a reactive surface in the global atmosphere. J Geophys Res 101 22869-22889 Desboeufs KV, Losno R, Vimeux F, Cholbi S (1999) The pH-dependent dissolution of wind-transported Saharan dust. J Geophys Res 104 21287-21299... 

The overall rate of ZnO aqueous leaching might be controlled by mass transfer in the boundary layer solid-liquid (16). At this layer, a constant low pH in the aqueous phase suggests a saturation of dissolved Zr. Kakovskii et al. (16) have reported, that at high acid concentrations the rate of ZnO dissolution is controlled by diffusion of Zn away from the siuface. This local saturation appears to influence eq. 4 and suggests a pH dependent dissolution-desorption step. This step was estimated with the Avrami-Erofeev rate law applying the results of the aqueous leaching data. 

Fig. 9.18. The pH-dependent dissolution of salicylic acid (o), benzoic acid ( ), and phenobarbital ( ). (From Gibaldi M. Biopharmaceutics and Clinical Pharmacokinetics, 4th Ed. Philadelphia Lea and Febiger, 1991, with permission.)...

Effects seem insignificant, except for formulations with pH-dependent dissolution, where more rapid absorption was reported. 

An active substance, although initially released from its dosage form (and dissolved), may become unavailable for absorption due to reactimis with other medicines or food components [4]. An example is the formation of insoluble complexes of tetracycline with calcium or aluminium ions from antacids or milk products. Interaction (chelation or binding) with iron ions leads to a reduced absorption for a variety of active substances such as doxycycline, penicillamine, methyldopa and ciprofloxacin. The absorption of active substances showing pH-dependent dissolution behaviour may be influenced by medicines that influence the gastric pH, such as H2-antagonists, proton pump inhibitors and antacids. Antimycotic active substances such as ketoconazole or itraconazole dissolve better in acidic fluids. Therefore their bioavailability may be increased by the concomitant use of an acidic drink like cola, whereas the concomitant use of antacids or proton pump inhibitors is likely to reduce the bioavailability. Concomitant use of milk may increase the dissolution of acidic active substances, whereas fats from food may increase the bioavailability of lipophilic active substances like albendazole and griseofulvin. 

As I see, an essential problem in this field is that the complex aspect of adsorption from electrolyte solutions shown in Fig. 1 has not been widely accepted. I have hardly found a systematic analysis of all probable simultaneous equilibria in a given adsorption system in the relevant literature. In most cases the solution condition (e.g., pH)-dependent dissolution of the solid phase, the surface precipitation, and the speciation in the aqueous phase are omitted in the evaluations either without mentioning them or with reference to some reasoning. To demonsteate some outcomes, it is worth inspecting a simple case of the surface-charge titration of a common aluminum oxide in detail. 

Ibuprofen The average ibuprofen load, in ethanol after 1 h, was 30.4%, with a minimal amount of crystalhzed dmg on the surface. The lower loading efficiency and negligible presence on the surface were expected due to the lower solubility of ibuprofen. Although ibuprofen solubihty is pH-dependent, dissolution of the loaded drug was less pH-dependent than unloaded ibuprofen, and the release profile of loaded ibuprofen was similar to that of loaded antipyrine. 

Effect of pH. Most minerals, including carbonate minerals (Wollast 1990), exhibit a transition from pH-independent to pH-dependent dissolution (Fig. 5). The critical question is whether the pH of the transition is above or below the pH of the soil solution. The transition points for a number of minerals are shown in Table 2. If alkali feldspars and intermediate plagioclases are the most important sources of cations in weathering (Garrels and Mackenzie 1967), then the transition pH will be about 4.5, and the weathering rate will increase as What is a reasonable pH range for... 

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