Water dissolution rate

Water is a competent oxidant for porous Si even at room temperature (Gupta et al. 1991). In liquid water, the oxidation of Si (Eq. 9) is often in competition with the dissolution of the product of the reaction, Si02 (Eq. 10). Dissolution is faster at larger pH values while stable in acidic solutions, above pH 7 hydrogen-terminated porous Si slowly dissolves, and at pH 10 complete dissolution of a microporous layer will occur within a few seconds. These water dissolution rates can be understood in the context of the two reactions represented by Eqs. 9 and 10. 

Beryllium fluoride is hygroscopic and highly soluble in water, although its dissolution rate is slow. FluoroberyUates can be readily prepared by crystallization or precipitation from aqueous solution. Compounds containing the BeP ion are the most readily obtained, though compounds containing other fluoroberyUate ions can also be obtained, eg, NH BeF, depending upon conditions. 

Coated fertilizers achieve controlled release by coating a soluble fertilizer core (substrate) with a water-insoluble barrier which limits the access of water to the fertilizer and thus limits its dissolution rate. 

The formation of deposits on platinised anodes can cause anode degradationThus dissolved impurities present in water which are liable to oxidation to insoluble oxides, namely Mn, Fe, Pb and Sn, can have a detrimental effect on anode life. In the case of MnOj films it has been stated that MnOj may alter the relative proportions of Cl, and O, produced and thus increase the Pt dissolution rate Fe salts may be incorporated into the TiO, oxide film and decrease the breakdown potential or form thick sludgy deposits. The latter may limit electrolyte access and iead to the development of localised acidity, at concentrations sufficient to attack the underlying substrate . 

The approximate value for the oxide coating dissolution rate in relation to current density in soils and fresh water is ... 

Chang, R. K., Price, J. C., and Whitworth, C. W., Enhancement of dissolution rate by incorporation into a water insoluble polymer, polycaprolactone. Drug Dev. Ind. Pharm., 13. 249-256, 1987. 

Investigation of the differences in crystal packing between (431) and (426) from comparison of their respective X-ray structures, revealed that (431) was more tightly packed than (442), reflected in their respective melting points of 235 and 170 °C. It was postulated that the absence of in vivo activity for (431) may be explained by the resultant reduction in water solubility and dissolution rate compared with (426). The comparatively high calculated polar surface area of (431) (122.5A ) compared with (426) (89.3 A ) was also proposed as a factor influencing the marked difference in bioavailability between the two related compounds. Compound (426) (SLV-319) is currently being developed with Bristol-Myers Squibb for the potential treatment of obesity and other metabolic disorders. Phase I trials for obesity were started in April 2004. Earlier Phase I clinical trials for the treatment of schizophrenia and psychosis, which commenced in April 2002, appear to have been abandoned. 

Addition of an alkali metal oxide as a "network modifier to the "network former causes pH sensitivity, i.e., small amounts of alkali metal induce superficial gel layer formation as a merely local chemical attack and so with limited alkali error larger amounts will result in more pronounced dissolving properties of the glass up to complete dissolution, e.g., water-glass with large amounts of sodium oxide. Simultaneous addition of an alkaline earth metal oxide, however, diminishes the dissolution rate. Substitution of lithium for sodium in pH-sensitive glass markedly reduces the alkali error. 

These mixing motions will tend to improve drug absorption for two reasons. Any factor that increases rate of dissolution will increase the rate (and possibly the extent) of absorption, especially for poorly water-soluble drugs (BCS Classes II and IV). Since rate of dissolution depends on agitation intensity, mixing movements will tend to increase dissolution rate and thereby influence absorption. As rate of absorption depends directly on membrane surface area, and since mixing increases the contact area between drug and... 

Figure 3 illustrates a situation in which this may not be true. When 250 mL of water was taken with erythromycin tablets, the extent of absorption was much greater than when the tablets were taken with only 20 mL of water. In the latter case, dissolution probably did not occur under sink conditions. Hence, the dissolution rate decreased, and it appears that not all of the erythromycin had a chance to dissolve in the GIT. Note than the dissolution was not, however, the ratedetermining step in absorption, since the time to reach the peak concentration was the same in all situations. 

Usually, an increase in Cg that would affect the dissolution rate would occur only when another process, such as membrane transport or stomach emptying, becomes the rate-limiting step in drug absorption. As a general rule pharmacists should advise patients to take their oral medications with a full glass of water to ensure that dissolution occurs under optimal conditions. 

---