Precipitation (chemistry) Precipitation is the formation of a HYPERLINK "http://en.wikipedia.org/wiki/Solid" \o "Solid" solid in a HYPERLINK "http://en.wikipedia.org/wiki/Solution" \o "Solution" solution during a HYPERLINK "http://en.wikipedia.org/wiki/Chemical_reaction" \o "Chemical reaction" chemical reaction . When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate. Applications HYPERLINK "http://en.wikipedia.org/wiki/File:Tetratolylporphyrin.jpg" \o "Crystals of meso-tetratolylporphyrin from refluxing propionic acid precipitate on cooling" INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/6/65/Tetratolylporphyrin.jpg/180px-Tetratolylporphyrin.jpg" \* MERGEFORMATINET HYPERLINK "http://en.wikipedia.org/wiki/File:Tetratolylporphyrin.jpg" \o "Enlarge" INCLUDEPICTURE "http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" \* MERGEFORMATINET Crystals of meso-tetratolylporphyrin from refluxing HYPERLINK "http://en.wikipedia.org/wiki/Propionic_acid" \o "Propionic acid" propionic acid precipitate on cooling Precipitation reactions can be used for making HYPERLINK "http://en.wikipedia.org/wiki/Pigments" \o "Pigments" pigments , removing HYPERLINK "http://en.wikipedia.org/wiki/Salts" \o "Salts" salts from water in HYPERLINK "http://en.wikipedia.org/wiki/Water_treatment" \o "Water treatment" water treatment , and in HYPERLINK "http://en.wikipedia.org/wiki/Classical_qualitative_inorganic_analysis" \o "Classical qualitative inorganic analysis" classical qualitative inorganic analysis . Precipitation is also useful to isolate the products of a reaction during HYPERLINK "http://en.wikipedia.org/wiki/Workup_(chemistry)" \o "Workup (chemistry)" workup . Ideally, the product of the reaction is insoluble in the reaction solvent. Thus, it precipitates as it is formed, preferably HYPERLINK "http://en.wikipedia.org/wiki/Crystallization" \o "Crystallization" forming pure crystals . An example of this would be the synthesis of HYPERLINK "http://en.wikipedia.org/wiki/Porphyrin" \o "Porphyrin" porphyrins in refluxing HYPERLINK "http://en.wikipedia.org/wiki/Propionic_acid" \o "Propionic acid" propionic acid . By cooling the reaction mixture to room temperature, crystals of the porphyrin precipitate, and are collected by filtration: HYPERLINK "http://en.wikipedia.org/wiki/Precipitation_reactions" \l "cite_note-0#cite_note-0" \o ""  HYPERLINK "http://en.wikipedia.org/wiki/File:H2TPPsyn.png" \o "H2TPPsyn.png" INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/H2TPPsyn.png/400px-H2TPPsyn.png" \* MERGEFORMATINET Precipitation may also occur when an antisolvent (a solvent in which the product is insoluble) is added, drastically reducing the solubility of the desired product. Thereafter, the precipitate may easily be separated by HYPERLINK "http://en.wikipedia.org/wiki/Filtration" \o "Filtration" filtration , HYPERLINK "http://en.wikipedia.org/wiki/Decanting" \o "Decanting" decanting , or HYPERLINK "http://en.wikipedia.org/wiki/Centrifugation" \o "Centrifugation" centrifugation ). An example would be the synthesis of chromic tetraphenylporphyrin chloride: water is added to the DMF reaction solution, and the product precipitates. HYPERLINK "http://en.wikipedia.org/wiki/Precipitation_reactions" \l "cite_note-1#cite_note-1" \o ""  Precipitation is also useful in purifying products: crude bmim-Cl is taken up in HYPERLINK "http://en.wikipedia.org/wiki/Acetonitrile" \o "Acetonitrile" acetonitrile , and dropped into ethyl acetate, where it precipitates. HYPERLINK "http://en.wikipedia.org/wiki/Precipitation_reactions" \l "cite_note-2#cite_note-2" \o ""  In HYPERLINK "http://en.wikipedia.org/wiki/Metallurgy" \o "Metallurgy" metallurgy , precipitation from a HYPERLINK "http://en.wikipedia.org/wiki/Solid_solution" \o "Solid solution" solid solution is also a useful way to HYPERLINK "http://en.wikipedia.org/wiki/Precipitation_strengthening" \o "Precipitation strengthening" strengthen alloys ; this process is known as HYPERLINK "http://en.wikipedia.org/wiki/Solid_solution_strengthening" \o "Solid solution strengthening" solid solution strengthening . [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Precipitation_(chemistry)&action=edit§ion=2" \o "Edit section: Mechanism" edit ]Mechanism Precipitation can occur when an HYPERLINK "http://en.wikipedia.org/wiki/Solubility" \o "Solubility" insoluble substance is formed in the HYPERLINK "http://en.wikipedia.org/wiki/Solution_(chemistry)" \o "Solution (chemistry)" solution due to a HYPERLINK "http://en.wikipedia.org/wiki/Chemical_reaction" \o "Chemical reaction" chemical reaction or when the solution has been HYPERLINK "http://en.wikipedia.org/wiki/Supersaturated" \o "Supersaturated" supersaturated by a HYPERLINK "http://en.wikipedia.org/wiki/Compound_(chemistry)" \o "Compound (chemistry)" compound . The formation of a precipitate is a sign of a HYPERLINK "http://en.wikipedia.org/wiki/Chemical_change" \o "Chemical change" chemical change . In most situations, the solid forms ("falls") out of the solute phase, and sinks to the bottom of the solution (though it will float if it is less dense than the solvent, or form a HYPERLINK "http://en.wikipedia.org/wiki/Suspension_(chemistry)" \o "Suspension (chemistry)" suspension ). The solid may reach the bottom of a container by means of HYPERLINK "http://en.wikipedia.org/wiki/Settling" \o "Settling" settling , HYPERLINK "http://en.wikipedia.org/wiki/Sediment" \o "Sediment" sedimentation , or HYPERLINK "http://en.wikipedia.org/wiki/Centrifugation" \o "Centrifugation" centrifugation . An important stage of the precipitation process is the onset of HYPERLINK "http://en.wikipedia.org/wiki/Nucleation" \o "Nucleation" nucleation . The creation of a hypothetical solid HYPERLINK "http://en.wiktionary.org/wiki/Particle" \o "wiktionary:Particle" particle includes the formation of an HYPERLINK "http://en.wikipedia.org/wiki/Interface_(chemistry)" \o "Interface (chemistry)" interface , which requires some HYPERLINK "http://en.wikipedia.org/wiki/Energy_(chemistry)" \o "Energy (chemistry)" energy based on the relative HYPERLINK "http://en.wikipedia.org/wiki/Surface_energy" \o "Surface energy" surface energy of the solid and the solution. If this energy is not available, and no suitable nucleation surface is available, HYPERLINK "http://en.wikipedia.org/wiki/Supersaturation" \o "Supersaturation" supersaturation occurs. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Precipitation_(chemistry)&action=edit§ion=3" \o "Edit section: Representation using chemical equations" edit ]Representation using chemical equations An example of a precipitation reaction: Aqueous HYPERLINK "http://en.wikipedia.org/wiki/Silver_nitrate" \o "Silver nitrate" silver nitrate (AgNO3) is added to a solution containing HYPERLINK "http://en.wikipedia.org/wiki/Potassium_chloride" \o "Potassium chloride" potassium chloride (KCl) and the precipitation of a white solid, HYPERLINK "http://en.wikipedia.org/wiki/Silver_chloride" \o "Silver chloride" silver chloride is observed. (Zumdahl, 2005) AgNO3 (aq) + KCl (aq) ? AgCl (s) + KNO3 (aq) The silver chloride(AgCl) has formed a solid, which is observed as a precipitate. This reaction can be written emphasizing the HYPERLINK "http://en.wikipedia.org/wiki/Dissociation" \o "Dissociation" dissociated HYPERLINK "http://en.wikipedia.org/wiki/Ions" \o "Ions" ions in a combined solution. This is known as the HYPERLINK "http://en.wikipedia.org/wiki/Ionic_equation" \o "Ionic equation" ionic equation . Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) ? AgCl (s) + K+ (aq) + NO3- (aq) A final way to represent a precipitate reaction is known as a net ionic reaction. In this case, any HYPERLINK "http://en.wikipedia.org/wiki/Spectator_ions" \o "Spectator ions" spectator ions (those which do not contribute to the reaction) are left out of the formula completely. This simplifies the above equations to the following: Ag+ (aq) + Cl- (aq) ? AgCl (s) [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Precipitation_(chemistry)&action=edit§ion=4" \o "Edit section: Cation sensitivity" edit ]Cation sensitivity Precipitate formation is useful in the detection of the type of HYPERLINK "http://en.wikipedia.org/wiki/Cation" \o "Cation" cation in HYPERLINK "http://en.wikipedia.org/wiki/Salt" \o "Salt" salt . To do this, an HYPERLINK "http://en.wikipedia.org/wiki/Alkali" \o "Alkali" alkali first reacts with the unknown salt to produce a precipitate which is the HYPERLINK "http://en.wikipedia.org/wiki/Hydroxide" \o "Hydroxide" hydroxide of the unknown salt. To identify the cation, the color of the precipitate and its solubility in excess are noted. Similar processes are often used to separate chemically similar elements, such as the HYPERLINK "http://en.wikipedia.org/wiki/Alkali_earth_metals" \o "Alkali earth metals" Alkali earth metals . [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Precipitation_(chemistry)&action=edit§ion=5" \o "Edit section: Digestion" edit ]Digestion Digestion, or precipitate ageing, happens when a freshly-formed precipitate is left, usually at a higher HYPERLINK "http://en.wikipedia.org/wiki/Temperature" \o "Temperature" temperature , in the solution from which it is precipitated. It results in cleaner and bigger particles. HYPERLINK "http://en.wikipedia.org/wiki/Precipitation_reactions" \l "cite_note-3#cite_note-3" \o ""  The physico-chemical process underlying digestion is called HYPERLINK "http://en.wikipedia.org/wiki/Ostwald_ripening" \o "Ostwald ripening" Ostwald ripening . [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Precipitation_(chemistry)&action=edit§ion=6" \o "Edit section: Coprecipitation" edit ]Coprecipitation Main article: HYPERLINK "http://en.wikipedia.org/wiki/Coprecipitation" \o "Coprecipitation" coprecipitation Coprecipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed. It is an important issue in chemical analysis, where it is often undesirable, but in some cases it can be exploited. In HYPERLINK "http://en.wikipedia.org/wiki/Gravimetric_analysis" \o "Gravimetric analysis" gravimetric analysis , it is a problem because undesired impurities often coprecipitate with the analyte, resulting in excess mass. On the other hand, in the analysis of trace elements, as is often the case in HYPERLINK "http://en.wikipedia.org/wiki/Radiochemistry" \o "Radiochemistry" radiochemistry , coprecipitation is often the only way of separating an element.
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