anachem

• Activity

  A factor relating the concentration of a species to its activity
• For neutral species, the position of equilibrium is essentially independent of electrolyte concentration
• Electrolyte effect


  The effect of added electrolyte on equilibria, which depends on the ionic strength of the solution
• The electrolyte effect results from the electrostatic attractive and repulsive forces between the ions of an electrolyte and the ions involved in an equilibrium
• Chemists use the term 'activity' to account for the effects of electrolytes on chemical equilibria
• Debye-Huckel equation


  An equation that permits the calculation of activity coefficients of ions from their charge and average size
• Aqueous solutions often contain species that interact with one another and water to yield two or more simultaneous equilibria
• Mass-balance equations


  Equations that relate the equilibrium concentrations of various species in a solution to one another and to the analytical concentrations of the various solutes
• Charge-balance equations


  Equations that express the equality of the number of moles per liter of positive and negative charges in an electrolyte solution
• Electrolyte solutions are electrically neutral even though they may contain up to several moles per liter of charged ions
• Gravimetric methods


  Quantitative methods based on determining the mass of a pure compound to which the analyte is chemically related
• Gravimetric methods of analysis are based on mass measurements with an analytical balance, an instrument that yields highly accurate and precise data
• Precipitation gravimetry


  The analyte is separated from a solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed
• Volatilization gravimetry


  The analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition, and the mass of the gas then serves as a measure of the analyte concentration
• Electrogravimetry


  The analyte is separated by deposition on an electrode by an electrical current, and the mass of this product then provides a measure of the analyte concentration
• Gravimetric titrimetry


  The mass of a reagent of known concentration required to react completely with the analyte provides the information needed to determine the analyte concentration
• Atomic mass spectrometry


  Uses a mass spectrometer to separate the gaseous ions formed from the elements making up a sample of matter
• In precipitation gravimetry, the analyte is converted to a sparingly soluble precipitate
• Colloidal suspensions


  Suspensions with tiny particles invisible to the naked eye
• Crystalline suspension


  The temporary dispersion of colloidal particles in the liquid phase
• Supersaturated solution


  A solution that contains a higher solute concentration than a saturated solution
• Coagulation


  The process by which colloidal particles aggregate and precipitate out of the solution, which can be hastened by heating, stirring, or adding an electrolyte
• Peptization


  The process by which a coagulated colloid reverts to its original dispersed state
• Crystalline precipitates are generally more easily filtered and purified than are coagulated colloids
• Types of coprecipitation


  • Surface adsorption
  • Mixed-crystal formation
  • Occlusion
  • Mechanical entrapment
• Adsorption is often the major source of contamination in coagulated colloids but of no significance in crystalline precipitates
• Mixed-crystal formation


  A type of coprecipitation in which a contaminant ion replaces an ion in the lattice of a crystal
• Occlusion


  A type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth
• Mechanical entrapment


  A type of coprecipitation that occurs when crystals lie close together during growth
• The purity of many coagulated colloids is improved by digestion
• Reprecipitation


  A drastic but effective way to minimize the effects of adsorption, involving the precipitate being redissolved and reprecipitated
• After filtration, a gravimetric precipitate is heated until its mass becomes constant