Accessibility retarding agent

Hemicellulose is so vulnerable to acid attack that it does not constitute an accessibility retarding agent for acid hydrolysis. However, it appears that hemicellulose does protect cellulose from enzyme attacks by a shielding action. 

In the IV route, anaphylactic reactions (caused by administration of an agent to an animal previously sensitized to it or to a particularly sensitive species such as a guinea pig) may be especially severe, probably because of sudden, massive antigen-antibody reactions. When the drug is given by other routes, its access to antibody molecules is necessarily slower moreover, its further absorption can be retarded or prevented at the first sign of a serious allergic reaction. 

Phosphorus is known to be an oxidation protector and a fire retardant for carbonaceous materials [111-119]. Phosphoric acid is used as an activating agent for activated carbons. It promotes bond cleavage reactions and the formation of phosphate and fwlyphosphate bridges that expand and crosslink the carbon matrix, driving to an accessible pore structure after removal ofthe acid [118,119]. 

In the strict sense, the passive mode implies that the newly formed phase is retarding the process, that is corrosion is slowed down with time protective scale), but there are cases in which the scale is nonprotective a scale with cracks, low viscosity, or foamy texture may not hinder the access of the corrosive agent to the substrate. 

When a metal is exposed to oxygen containing solution (electrolyte or pure water) so that the whole area is uniformly accessible, without the conditions for the formation of differential areation cells, the whole area may be covered with a corrosion resistant film of oxide, thus retarding the rate of the anodic reaction. This situation can also be obtained by treating the metal with oxidizing agents, such as concentrated nitric acid. The phenomenon, called passivity , will be discussed in greater detail after the description of corrosion by electric current. 

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