Organic anions, concentrations precipitation

Table I. Observed Organic Anion Concentrations in Precipitation...

An inorganic or organic anion is precipitated with a solution of a cation of appropriate concentration. The compound formed should have a very low solubility. An excess of the cation is used in order to obtain complete precipitation. The mixture is filtered, centrifuged, or decanted, and the cation is determined by AAS either in the filtrate or in the precipitate. These methods are specific only if the sample solution does not contain other species that might be precipitated. 

Precipitation usually occurs when the concentration of a compound in solution exceeds the equilibrium solubility, although slow reaction kinetics may result in supersaturated solutions. For organic wastes in the deep-well environment, precipitation is not generally a significant partitioning process in certain circumstances, however, it may need to be considered. For example, pentach-lorophenol precipitates out of solution when the solution has a pH of <5,35,36 and polychlorophenols form insoluble precipitates in water high in Mg2+ and Ca2+ ions.37 Also, organic anions react with such elements as Ca2+, Fe2+, and Al3+ to form slowly soluble to nearly insoluble compounds. 

A number of anions are mentioned to cause interference. The complex formation can be eliminated completely by the presence of EDTA, oxalate, fluoride, and phosphate, the organic anions through complex formation with calcium, the inorganic anions through precipitation of calcium. Citrate and carbonate, tartrate, and borate also cause disturbances but only when present in high concentrations. 

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

Another class of indicators, known as adsorption indicators, adsorb to (or desorb from) the precipitate or colloidal particles of the silver salt of the analyte at the equivalence point. The indicator anions are attracted into the counterion layer surrounding each colloidal particle of silver salt. Thus, there is a transfer of color from the solution to the solid or from the solid to the solution at the end point. The concentration of the indicator, which is an organic compound, is not large enough to cause its precipitation as a silver salt. Thus, the color change is an adsorption and not a precipitation process. Fluorescein is a typical example of an adsorption indicator in argentometric titration. 

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