Dissolution probability

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. 

The quantities kc and ks in the last two equations result from a calculation of an exponential, and thus have no physical dimensions. The effective dissolution probability rate constant k eff is calculated by multiplying the above three factors, so that /c e// = kdkcks. Thus, k eff has dimension of time-1 and denotes the fraction of the total number of drug particles that can be dissolved per MCS. The mass of the tablet that will break off at any moment is given by multiplying the value of k eff by the undissolved mass of the tablet. If qd (t) is this mass, then qd (t) = [go — g (i)] d,eff and qd (t) /ip particles of the tablet with mass ip will break off, and will get separated from the larger mass. The dissolved particles now flow on their own, with the same characteristics (forward probability) as the undissolved particles. The mass qo — q (t) of the undissolved drug is then reduced by qd (t). 

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

Takeda reported a series of experiments in which he determined the amount of S02 absorbed by limestone slurries(15). He observed significant differences in the rate of S02 absorption at different slurry concentrations. Here limestone dissolution probably plays an important role in the gas absorption process. 

Specific results with hydroxypropionic and sulfopropionic acids indicate that these buffers inhibit CaCC>3 dissolution, probably because of polyacrylic acid impurities (39). These impurities are derived from the specific synthesis processes. 

This result, which agrees with the one on page 377, is valid only if the dissolution time is much shorter than the time constant X for the overall process (i.e., tdissoiution such that ln(l — x) —x for small values of x. Hence, the expression for tdissoiution here is not reahstic because complete dissolution probably requires at least five time constants (i.e., idissoiution 5A.) to achieve more than 99% of the new equihbrium or steady state, where the solid particle no longer exists. Another strategy considers that complete dissolution requires an infinite amount of time because the surface area for mass transfer becomes infinitesimally small as the pellet radius shrinks toward zero. Now, one sets R = 0 as t oo in the integrated expression for R(t) and solves for the time constant X. The final result is... 

It is probable that the solvents given in the individual reagent monographs are not suitable for all the substances with which the reagent will react. This point should be taken into account especially for quantitative work and the user should make appropriate modifications. In particular, there must be no loss of substance or reaction product by dissolution (formation of comet tails by the chromatographic zones). 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

For some non-ferrous metals (copper, lead, nickel) the attack by sulphuric acid is probably direct with the formation of sulphates. Lead sulphate is barely soluble and gives good protection. Nickel and copper sulphates are deliquescent but are gradually converted (if not leached away) into insoluble basic sulphates, e.g. Cu Cu(OH)2)3SO4, and the metals are thus protected after a period of active corrosion. For zinc and cadmium the sulphur acids probably act by dissolution of the protective basic carbonate film. This reforms, consuming metal in the process, redissolves, and so on. Zinc and cadmium sulphates are formed in polluted winter conditions whereas in the purer atmospheres of the summer the corrosion products include considerable amounts of oxide and basic carbonate. ... 

Another indication of the influence of precipitated phases on anodic behaviour may be seen in the curve for Alloy C in Fig. 4.28, where the small peak in the middle of the passive range is probably attributable to anodic dissolution of an intermetallic phase (n) and MjC carbide . ... 

Salt solutions When a zinc sheet is immersed in a solution of a salt, such as potassium chloride or potassium sulphate, corrosion usually starts at a number of points on the surface of the metal, probably where there are defects or impurities present. From these it spreads downwards in streams, if the plate is vertical. Corrosion will start at a scratch or abrasion made on the surface but it is observed that it does not necessarily occur at all such places. In the case of potassium chloride (or sodium chloride) the corrosion spreads downwards and outwards to cover a parabolic area. Evans explains this in terms of the dissolution of the protective layer of zinc oxide by zinc chloride to form a basic zinc chloride which remains in solution. 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

---