Time substrate dissolution

These relationships are illustrated in Fig. 6 in which the variation of coating weight, crystal size, the relative surface coverage, and the rate of substrate dissolution were measured as a function of time for the phosphatation of steel in a laboratory trication phosphatation bath. The partial free metal surface ( -9) decreases as the average crystal size (di) and coating mass increase. The dissolution rate of the metal rjiss (here defined as a negative value) drops toward zero at the end of the reaction. 

Certain enzymes, proenzymes, and their substrates are present at all times in the circulation of normal individuals and perform a physiologic function in the blood. Examples of these functional plasma enzymes include lipoprotein Upase, pseudocholinesterase, and the proenzymes of blood coagulation and blood clot dissolution (Chapters 9 and 51). The majority of these enzymes are synthesized in and secreted by the liver. 

The magnitude of association between a drag compound and various cyclodextrins depends critically on the details of the fit of the substrate into the cyclodextrin cavity. As shown in Table 6, the experimental compound RS-82856 forms the strongest complexes with /3-cyclodextrin, while maximal solubility is reached with y-cyclodextrin [62], Formation of the /8-cyclodextrin complex dramatically increased the dissolution rate of the compound as well. For RS-82856 itself, 20% dissolved within 20 minutes, while more than 80% of the drug-/3-cyclodextrin complex was found to be dissolved at the same time point. 

It is therefore believed that at pH 6 and greater the corrosion process is localised and large local concentrations of ferrous iron are achieved. At pH 6 the oxidation to ferric iron is very rapid ( ) and precipitation of Fe(0H)j occurs to exhibit localised corrosion or "flash-rust" spots. At pH 5 and below a small but finite uniform dissolution of the iron substrate occurs. However, in this pH range the oxidation of the ferrous dissolution product to ferric ion is considerably slower, by almost 1000 times, and hence "flash rusting" is not observed. 

Alternate Sample Introduction — Obviously, elimination of the sample dissolution stage would greatly reduce analytical time, as it is the slowest step in the analytical scheme. Pulsed-laser vaporization using a CO2—TEA laser seems promising(63, 64). Another possibility is the introduction of a suitable prepared slurry of the sample into the nebullzer(65). Thermal vaporization studies using heated substrates such as tanta-lum(66), carbon filaments(67), or carbon rods(39) have been reported. Silvester(39) de fined the problems of vapor transport, carrier gas expansion, and solid phase chemistry associated with electrothermal sample introduction to an ICP. 

For the conversion from solid Ca-maleate to solid Ca-D-malate (Scheme 12.2), kinetic models were developed to predict kinetics of dissolution of the substrates, the enzyme-catalyzed reaction, and the precipitation of the reaction product, all of which occur at the same time [44]. 

On a polycrystal (on which most reaction rates are measured), the distribution of the various crystal planes on the surface may vaiy, partly due to time-dependent adsorption of impurities if the solution is not completely clean and partly due to differing dissolution rates of the varying crystal planes in the substrate for an anodic current. Until these unstable factors have been taken care of, the reaction rate will vaiy with time. 

To aleviate the dissolution of the enzymatic film during the glucose assay procedure, a covering membrane made by addition of 10 pi of Nafion 0.5% was used. The amperometric current increases until a constant value of 20 pA is reached on the fourth glucose assay (Fig. IB). The addition of Nafion film atop of the AQ-enzyme layer doesn t seem to denaturate the enzyme entrapped in the AQ film because the current reached a similar value as the current observed in absence of Nafion. On the other hand, the use of Nafion as covering membrane gives a thicker film that leads to mass transport limitation of the substrates in the AQ film (20). It results in a longer time to obtain the steady-state current. 

It should be mentioned that passive layers are not protective in all environments. In the presence of so-called aggressive anions, passive layers may break down locally, which leads to the formation of corrosion pits. They grow with a high local dissolution current density into the metal substrate with a serious damage of the metal within very short time. In this sense halides and some pseudo halides like SCN are effective. Chloride is of particular interest due to its presence in many environments. Pitting corrosion starts usually above a critical potential, the so-called pitting potential /i]>j. In the presence of inhibitors an upper limit, the inhibition potential Ej is observed for some metals. Both critical potentials define the potential range in which passivity may break down due to localized corrosion as indicated in Fig. 1. 

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