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• Gravimetric methods


  Quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related

  
• Gravimetric methods of analysis


  • 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


  1. The analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition

  2. The mass of the gas then serves as a measure of the analyte concentration
• Electrogravimetry


  1. The analyte is separated by deposition on an electrode by an electrical current

  2. 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


  1. Uses a mass spectrometer to separate the gaseous ions formed from the elements making up a sample of matter

  2. The concentration of the resulting ions is then determined by measuring the electrical current produced when they fall on the surface of an ion detector

  
• Chemistry is at least as old as recorded history, but what we recognize as experimental chemistry did not emerge until the end of the 16th century
• Phlogiston Theory


  Claimed that when a metal burns or rusts it gives off a "substance" called phlogiston
• When it was observed that iron gained mass when it rusted, the phlogistonists simply postulated that phlogiston had negative mass

  
• Antoine Laurent Lavoisier


  • Dealt the death blow to the phlogiston theory by performing quantitative experiments with mercury and air in a closed system
  • Correctly explained combustion, and demonstrated that air was a mixture of nitrogen and oxygen
  • His genius, his instinctive recognition of the law of conservation of mass, and his use of the balance, made him the forefather of the quantitative era in chemistry

  
• You can disprove a theory by finding even a single observation that disagrees with the predictions of the theory
• Each time new experiments are observed to agree with the predictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to disagree, we have to abandon or modify the theory
• Classical analysis


  Gravimetry and titrimetry
• Gravimetry


  The determination of an element or species through the measurement of the mass of a well-characterized insoluble product of a definite chemical reaction involving that element or species

  
• Gravimetric methods are quantitative methods that are 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
• Volatilization gravimetry


  • The analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition
  • 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
  • The mass of this product then provides a measure of 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
  • The concentration of the resulting ions is then determined by measuring the electrical current produced when they fall on the surface of an ion detector

  
• Determination of Ca in water

  1. An excess of oxalic acid, H₂C₂O₄, is added to an aqueous solution of the sample

  2. Ammonia is then added, which neutralizes the acid and causes essentially all of the calcium in the sample to precipitate as calcium oxalate

  3. The CaC₂O₄ precipitate is filtered using a weighed filtering crucible, then dried and ignited

  4. This process converts the precipitate entirely to calcium oxide

  5. After cooling, the crucible and precipitate are weighed, and the mass of calcium oxide is determined by subtracting the known mass of the crucible

  6. The calcium content of the sample is then computed

  
• Ideal precipitating agent


  • Should react specifically or at least selectively with the analyte
  • Easily filtered and washed free of contaminants
  • Of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing
  • Unreactive with constituents of the atmosphere
  • Of known chemical composition after it is dried or, if necessary, ignited
• Colloidal suspension


  • Invisible to the naked eye (10ˉ⁷ to 10ˉ⁴ cm in diameter)
  • Show no tendency to settle from solution
  • Difficult to filter
• Crystalline suspension


  • Tend to settle spontaneously
  • Easily filtered
• Factors that determine the particle size of precipitates


  • Precipitate solubility
  • Temperature
  • Reactant concentrations
  • The rate at which reactants are mixed
• Relative supersaturation


  Q = the concentration of the solute at any instant, and S = its equilibrium solubility
• Generally, precipitation reactions are slow so that, even when a precipitating reagent is added drop by drop to a solution of an analyte, some supersaturation is likely
• Experimental evidence indicates that the particle size of a precipitate varies inversely with the average relative supersaturation during the time when the reagent is being introduced
• Nucleation


  • A few ions, atoms, or molecules (perhaps as few as four or five) come together to form a stable solid
  • Often, these nuclei form on the surface of suspended solid contaminants, such as dust particles
  • Further precipitation then is governed by the competition between additional nucleation and growth of existing nuclei (particle growth)

  
• Particle growth


  • Moderately enhanced by high relative supersaturations
  • At low relative supersaturations, the rate of particle growth tends to predominate, and deposition of solid on existing particles occurs rather than further nucleation
• Low relative supersaturation produces crystalline suspensions
• Colloidal precipitates


  • Individual colloidal particles are so small that they are not retained by ordinary filters
  • Brownian motion prevents their settling out of solution under the influence of gravity
• Coagulation of colloids


  1. Coagulation can be hastened by heating, by stirring, and by adding an electrolyte to the medium

  2. Colloidal suspensions are stable because all of the particles of the colloid are either positively or negatively charged and thus repel one another

  3. This charge results from cations or anions that are bound to the surface of the particles

  4. Colloidal particles are charged by placing them between charged plates where some of the particles migrate toward one electrode while others move toward the electrode of the opposite charge

  5. The process by which ions are retained on the surface of a solid is known as adsorption

  
• Electric double layer


  • The primarily adsorbed ions and the negative counter-ion layer constitute an electric double layer that imparts stability to the colloidal suspension
  • As colloidal particles approach one another, this double layer exerts an electrostatic repulsive force that prevents particles from colliding and adhering

  
• Peptization


  • The process by which a coagulated colloid reverts to its original dispersed state
  • When a coagulated colloid is washed, some of the electrolyte responsible for its coagulation is leached from the internal liquid in contact with the solid particles
  • Removal of this electrolyte has the effect of increasing the volume of the counter-ion layer
  • The repulsive forces responsible for the original colloidal state are then reestablished, and particles detach themselves from the coagulated mass
• Practical Treatment of Colloidal Precipitates

  1. Colloids are best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation

  2. The filterability of a coagulated colloid often improves if it is allowed to stand for an hour or more in contact with the hot solution from which it was formed

  3. During this process, which is known as digestion, weakly bound water appears to be lost from the precipitate
• Crystalline precipitates


  • Generally more easily filtered and purified than are coagulated colloids
  • The size of individual crystalline particles, and thus their filterability, can be controlled to some extent
• Methods of Improving Particle Size and Filterability

  1. Minimize Q (the concentration of the solute at any instant) or maximize S (the equilibrium solubility), or both

  2. Q can often be minimized by using dilute solutions and adding the precipitating reagent slowly, with good mixing

  3. S is often increased by precipitating from hot solution or by adjusting the pH of the precipitation medium
• Coprecipitation


  • A process in which normally soluble compounds are carried out of solution by a precipitate
  • There are four types: surface adsorption, mixed-crystal formation, occlusion, and mechanical entrapment

  
• Surface adsorption


  Adsorption is a process in which ions or molecules in a solution accumulate on the surface of a solid, forming a thin layer