Passivators concentration

The key instructions of the Third Bardo are (1) do nothing, stay calm, passive and relaxed, no matter what happens and (2) recognize where you are. If you do not recognize you will be driven by fear to make a premature and unfavorable re-entry. Only by recognizing can you maintain that state of calm, passive concentration necessary for a favorable re-entry. That is why so many recognition-points are given. If you fail on one, it is always possible, up to the very end, to succeed on another. Hence these teachings should be read carefully and remembered well. 

Figure 2.4 Streamlines of the steady cellular flow composed of an array of counter-rotating vortices (top row) and the spreading in time of a weakly diffusing passive concentration field. Time increases from top to bottom.

For more distal portions of the nephron, passive concentration of xenobiotics can occur due to the physiologic concentrating mechanism that provides a favorable gradient for the xenobiotic to undergo back-diffusion into the papillary region of the kidneys [23]. 

Passivator ions acting as oxidizers are adsorbed on the substrate and reduced easily, thus enlarging the cathode surface area. An optimum passivator solution concentration should exceed some critical value and, the higher the passivator concentration, the easier it is adsorbed, and the smaller the anodic areas on the substrate will be. This promotes an increase in anodic polarization and total passivation of the substrate. If the passivator concentration is lower than some critical value, it initiates local corrosion of the substrate. 

Figure 17.1. Polarization curves that show effect of passivator concentration on corrosion of iron. An oxidizing substance that reduces sluggishly does not induce passivity (dotted cathodic polarization curve) (schematic).

As for the mechanism of permeation through the endothelial luminal surface can be characterized by changing the background concentration of mother substances. When it is passive, concentration changes have no influence on but when a facilitated transport is involved, then raising concentration reduces the apparent PS. Facilitated transport has been identified for adenosine (Gorman et al., 1983). 

Although chromium (Cr), iron (Fe) and aluminium (Al) readily dissolve in dilute nitric acid, the concentrated acid forms a metal oxide layer that protects the metal from further oxidation, which is called passivation. Typical passivation concentrations range from 18% to 22% by weight. 

From an electrochemical viewpoint, stable pit growtli is maintained as long as tire local environment witliin tire pit keeps tire pit under active conditions. Thus, tire effective potential at tire pit base must be less anodic tlian tire passivation potential (U ) of tire metal in tire pit electrolyte. This may require tire presence of voltage-drop (IR-drop) elements. In tliis respect the most important factor appears to be tire fonnation of a salt film at tire pit base. (The salt film fonns because tire solubility limit of e.g. FeCl2 is exceeded in tire vicinity of tire dissolving surface in tlie highly Cl -concentrated electrolyte.)... 

Strong oxidising acids, for example hot concentrated sulphuric acid and nitric acid, attack finely divided boron to give boric acid H3CO3. The metallic elements behave much as expected, the metal being oxidised whilst the acid is reduced. Bulk aluminium, however, is rendered passive by both dilute and concentrated nitric acid and no action occurs the passivity is due to the formation of an impervious oxide layer. Finely divided aluminium does dissolve slowly when heated in concentrated nitric acid. 

Concentrated nitric acid renders the metal passive , i.e. chemically unreactive, due to formation of a thin oxide surface film (which can be removed by scratching or heating in hydrogen). 

Equipment should be carefiiUy and completely degreased and passivated with low concentrations of fluorine or the gaseous halogen fluoride before use. Special care should be taken that valves are completely disassembled and each part carefiiUy cleaned. 

The decomposition of aqueous hydrogen peroxide is minimized by various purification steps during manufacture, use of clean passive equipment, control of contaminants, and the addition of stabilizers. The decomposition is zero-order with respect to hydrogen peroxide concentration. 

Active Transport. Maintenance of the appropriate concentrations of K" and Na" in the intra- and extracellular fluids involves active transport, ie, a process requiring energy (53). Sodium ion in the extracellular fluid (0.136—0.145 AfNa" ) diffuses passively and continuously into the intracellular fluid (<0.01 M Na" ) and must be removed. This sodium ion is pumped from the intracellular to the extracellular fluid, while K" is pumped from the extracellular (ca 0.004 M K" ) to the intracellular fluid (ca 0.14 M K" ) (53—55). The energy for these processes is provided by hydrolysis of adenosine triphosphate (ATP) and requires the enzyme Na" -K" ATPase, a membrane-bound enzyme which is widely distributed in the body. In some cells, eg, brain and kidney, 60—70 wt % of the ATP is used to maintain the required Na" -K" distribution. 

Nitric acid reacts with all metals except gold, iridium, platinum, rhodium, tantalum, titanium, and certain alloys. It reacts violentiy with sodium and potassium to produce nitrogen. Most metals are converted iato nitrates arsenic, antimony, and tin form oxides. Chrome, iron, and aluminum readily dissolve ia dilute nitric acid but with concentrated acid form a metal oxide layer that passivates the metal, ie, prevents further reaction. 

The metal dissolves readily in concentrated HCl, H PO, HI, or HCIO. Nitric acid (qv) forms a protective oxide skin on the metal and can be removed by ca 0.05 Af HF. Dissolution of Pu metal in HNO —HF mixtures is common practice in scrap-recovery plants. The metal does not dissolve readily in H2SO4 because passivation of the metal surface occurs. The reaction of water and Pu metal is slow compared to that in HCl, HI, or HCIO. ... 

Tantalum is not resistant to substances that can react with the protective oxide layer. The most aggressive chemicals are hydrofluoric acid and acidic solutions containing fluoride. Fuming sulfuric acid, concentrated sulfuric acid above 175°C, and hot concentrated aLkaU solutions destroy the oxide layer and, therefore, cause the metal to corrode. In these cases, the corrosion process occurs because the passivating oxide layer is destroyed and the underlying tantalum reacts with even mild oxidising agents present in the system. 

Most mineral acids react vigorously with thorium metal. Aqueous HCl attacks thorium metal, but dissolution is not complete. From 12 to 25% of the metal typically remains undissolved. A small amount of fluoride or fluorosiUcate is often used to assist in complete dissolution. Nitric acid passivates the surface of thorium metal, but small amounts of fluoride or fluorosiUcate assists in complete dissolution. Dilute HF, HNO, or H2SO4, or concentrated HCIO4 and H PO, slowly dissolve thorium metal, accompanied by constant hydrogen gas evolution. Thorium metal does not dissolve in alkaline hydroxide solutions. 

Passivating (anodic) inhibitors form a protective oxide film on the metal surface they are the best inhibitors because they can be used in economical concentrations and their protective films are tenacious and tend to be rapidly repaired if damaged. 

Various types of detector tubes have been devised. The NIOSH standard number S-311 employs a tube filled with 420—840 p.m (20/40 mesh) activated charcoal. A known volume of air is passed through the tube by either a handheld or vacuum pump. Carbon disulfide is used as the desorbing solvent and the solution is then analyzed by gc using a flame-ionization detector (88). Other adsorbents such as siUca gel and desorbents such as acetone have been employed. Passive (diffuse samplers) have also been developed. Passive samplers are useful for determining the time-weighted average (TWA) concentration of benzene vapor (89). Passive dosimeters allow permeation or diffusion-controlled mass transport across a membrane or adsorbent bed, ie, activated charcoal. The activated charcoal is removed, extracted with solvent, and analyzed by gc. Passive dosimeters with instant readout capabiUty have also been devised (85). 

A fresh surface of siUcon carbide is thus constantiy being exposed to the oxidizing atmosphere. Active oxidation takes place at and below approximately 30 Pa (0.23 mm Hg) oxygen pressure at 1400°C (66). Passive oxidation is determined primarily by the nature and concentration of impurities (67). 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

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