Typically, one batch of medicinals is prepared as a decoction of about 9 to 18 substances. Some of these are considered as main herbs, some as ancillary herbs; within the ancillary herbs, up to three categories can be distinguished. Some ingredients are added in order to cancel out toxicity or side-effects of the main ingredients; on top of that, some medicinals require the use of other substances as catalysts. Decoction is a method of extraction by boiling, of dissolved chemicals, from herbal or plant material, which may include stems, roots, bark and rhizomes. Decoction involves first mashing, and then boiling in water to extract oils, volatile organic compounds, and other chemical substances. Decoction can be used to make tisanes, teas, coffees, tinctures and similar solutions. The process can also be applied to meats and vegetables to prepare bouillon or stock. A decoction is also the name for the resulting liquid. Although this method of extraction differs from infusion and percolation, the resultant liquids are often functionally similar. The term is used colloquially in South India to refer to black coffee prepared by the traditional method. Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations. Catalysts that speed the reaction are called positive catalysts. Substances that slow a catalyst's effect in a chemical reaction are called inhibitors. Substances that increase the activity of catalysts are called promoters, and substa
ces that deactivate catalysts are called catalytic poisons. Catalytic reactions have a lower rate-limiting free energy of activation than the corresponding uncatalyzed reaction, resulting in higher reaction rate at the same temperature. However, the mechanistic explanation of catalysis is complex. Catalysts may affect the reaction environment favorably, or bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced naturally, such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause dissociation of reagents to reactive forms, such as atomic hydrogen in catalytic hydrogenation. Kinetically, catalytic reactions are typical chemical reactions; i.e. the reaction rate depends on the frequency of contact of the reactants in the rate-determining step. Usually, the catalyst participates in this slowest step, and rates are limited by amount of catalyst and its "activity". In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. A Nanomaterial-based catalyst is an example of a heterogeneous catalyst. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solidЦliquid system or evaporate in a solidЦgas system.