ANCH WEEK 10: Gravimetric Method of Analysis

• 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
• Analytical Balance


  an instrument that yields highly accurate and precise data
• Precipitation Gravimetry


  the analyte is separated from a solution of the sample as 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. The mass of the gas then serves as a measure of the analyte concentration
• Electrogravimetry 


  the analyte is separated by deposition on an electrolyte by an electrical current. 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
• Precipitation Gravimetry


  the analyte is converted to a sparingly soluble precipitate. This precipitates is then filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment, and weighed
• 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
• Atomic Mass Spectrometry


  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
• Properties of precipitates and precipitating reagents

  1. easily filtered and washed free of constraints
  2. of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing
  3. unreactive with constituents of the atmosphere
  4. of known chemical composition after it is dried or, if necessary, ignited
• Precipitates


  consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities
• Precipitates


  are usually purer than are precipitates made up of fine particles
• Colloidal suspensions


  whose tiny particles are invisible to the naked eye
• Colloidal particles


  shows no tendency to settle from solution and are difficult to filter
• Particle size of solids


  formed by precipitation varies enormously
• Factors that determine the particles size of precipitates


  at the other extreme are particles with dimensions on the order of tenths of a millimeter or greater
• Crystalline Suspension


  temporary dispersion of such particles in the liquid phase
• Crystalline Suspension


  their particles tend to settle spontaneously and are easily filtered
• Precipitate formation


  has been studied for many years, but the mechanism of the process is still not fully understood. What is certain, however, is that the particle size of a precipitate is influenced by precipitate solubility, temperature, reactant concentrations, and the rate at which reactants are mixed.
• relative supersaturation


  The net effect of these variables can be accounted for, at least qualitatively, by assuming that the particle size is related to a single property of the system
• relative supersaturation = Q-S/S


  Q = concentration of the solute at any instant

  S = equilibrium solubility

  Q-S/S is LARGE = precipitate tends to be colloidal

  Q-S/S is SMALL = a crystalline solid is more likely
• supersaturated solution


  is an unstable solution that contains a higher solute concentration than a saturated solution. As excess solute precipitates with time, supersaturation decreases to zero
• by nucleation and by particle growth 


  effect of relative supersaturation on particle size can be explained if we assume that precipitates form in two ways. The particle size of a freshly formed precipitate is determined by the mechanism that predominates
• Experimental control of particle size


  Experimental variables that minimize supersaturation and thus produce crystalline precipitates include elevated temperatures to increase the solubility of the precipitate, dilute solutions (to minimize Q), and slow addition of the precipitating agent with good stirring. The last two measures also minimize the concentration of the solute (Q) at any given instant
• Colloidal precipitates


  Individual colloid 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
• Colloidal precipitates


  we can coagulate or agglomerate, the individual particles of most colloids to give a filterable, amorphous mass that will settle out of solution
• Coagulations of Colloids


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


  are stable because all of the particles of the colloid are either positively or negatively charged and thus repel one another and do not coagulate spontaneously. The charge results from cations or anions that are bound to the surface of the particles. We can show that 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.
• adsorption


  The process by which ions are retained on the surface of a solid
• PEPTIZATION OF COLLOIDS 


  Peptization is the process by which a coagulated colloid reverts to its original coagulated 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.
• Crystalline precipitates


  are generally more easily filtered and purified than are coagulated colloids. In addition, the size of individual crystalline particles, and thus their filterability, can be controlled to some extent.
• The particle size of crystalline solids can often be improved significantly by minimizing Q, maximizing S, or both in Equation.

  Minimization of Q is generally accomplished by using dilute solution and adding the precipitating from hot solution or by adjusting the pH of the precipitation medium.
• Digestion of crystalline precipitates 


  (without stirring) for some time after formation frequently yields a purer, more filterable product.
• The improvement in filterability results from the:

  dissolution and recrystallization.
• Coprecipitation 


  When otherwise soluble compounds are removed from solution during precipitate formation
• not coprecipitation


  Contamination of a precipitate by a second substance whose solubility product has been exceeded
• There are four types of coprecipitation:

  1. Surface adsorption
  2. Mixed-crystal formation
  3. Occlusion
  4. Mechanical entrapment
• SURFACE ADSORPTION 


  Adsorption is often the major source of contamination in coagulated colloids but of no significance in crystalline precipitates.