Gravimetric Analysis and Precipitation Equilibria

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Lecture 6

• Gravimetric Analysis and Precipitation Equilibria

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Section 10.1

• Gravimetric analysis is potentially more accurate
  and more precise than volumetric analysis.
• You should get better results from part 1 of
  Experiment 2 than from part 2.
• Gravimetric analysis avoids problems with
  temperature fluctuations, calibration errors, and
  other problems associated with volumetric
  analysis.
• But there are potential problems with gravimetric
  analysis that must be avoided to get good
  results.
• Proper lab technique is critical

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Steps in a Gravimetric Analysis

• 1. Preparation of analyte solution
• Gravimetric analysis usually involves
  precipitation of analyte from solution.
• May need to separate potential interferences
  before precipitating analyte
• May require pH and/or other adjustments to
  maximize precipitate formation
• When possible, select precipitating agents that
  are selective (or specific, if possible).

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2. Precipitation

• The precipitate should
• Be insoluble, but not too insoluble
• Have large crystals
• Easier to filter large crystals
• Be free of contaminants
• The smaller the surface area the better
• The smaller the solubility, S, the greater the
  tendency to form lots of small crystals that trap
  contaminants and are difficult to filter
• The greater the solubility, the more analyte is
  left unprecipitated
• We need to strike a balance
• (but not the analytical balance, please)

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The von Weimarn Ratio

• von Weimarn ratio (Q S)/S
• A measure of relative supersaturation
• The lower the better
• If high, get excessive nucleation, lots of small
  crystals, large surface area
• If low, get larger, fewer crystals, small surface
  area
• Q concentration of mixed reactants before
  precipitation
• Keep it low by using dilute solutions, stir
  mixture well, add reactants slowly
• S solubility of precipitate at equilibrium
• Keep it high with high temperatures, adjusting pH
• Can lower S later by cooling mixture after
  crystals have formed

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Impurities in Precipitates

• Coprecipitation
• Precipitation of normally soluble compound along
  with the insoluble compound
• Precipitation of a second insoluble compound does
  not constitute coprecipitation
• But this would, of course, contaminate the
  precipitate
• The authors 4, postprecipitation (p. 319) is
  not really coprecipitation, its just the
  kinetically slow formation of a second insoluble
  compound

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Types of Coprecipitation

• Occlusion, inclusion, isomorphous replacement
  (mixed crystal formation)
• Impurities can be trapped in pockets in a rapidly
  growing crystal, or in pockets between two
  crystals that grow together, or a normally
  soluble ion can take the place of an ion in the
  precipitate (K taking the place of NH4 in
  insoluble MgNH4PO4, for example)
• Can sometimes be corrected by digestion (step 3)
  or dissolving and reprecipitating the precipitate
• Surface adsorption
• Lets look more closely at this problem

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Surface Adsorption

• Lets use AgCl as an example (Exp. 2)
• Primary adsorption layer
• Ion in excess adsorbs on the surface of the
  precipitate
• Crystal becomes positive if Ag is in excess
• Crystal becomes negative if Cl- is in excess
• Counter-ion layer (counterlayer)
• Charged crystal attracts ions of opposite charge
• For example, NO3- attracted to positive crystal,
  results in a layer of silver nitrate around the
  AgCl
• For example, Na attracted to negative crystal,
  results in a layer of NaCl around the AgCl
• The more dilute the solution, the thicker is the
  counter-ion layer

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Colloidal Precipitates (Such as AgCl

• When S is very small (very insoluble
  precipitates), cannot avoid high degree of
  nucleation and very small crystals
• Colloidal particles 1 to 100 µm in diameter
• Surface area as high as 3,000 ft2/g
• High potential for surface adsorption
• Other problems besides surface adsorption
• Colloidal precipitates cause difficulties in
  filtering and washing as well

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3. Digestion (Ostwald Ripening)

• Let precipitate stand in contact with solution,
  usually at high temperature
• Large crystals (small surface area) have lower
  free energy than small crystals (large surface
  area)
• Digestion makes larger crystals, reduces surface
  contamination, reduces crystal imperfections
• Digestion coagulates a colloid (causes the
  particles to agglomerate, or stick together), but
  surface area does not decrease as much as if
  larger crystals actually grew

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Hydrophilic vs. Hydrophobic Colloids

• Hydrophilic (water-loving) colloids form gels and
  do not coagulate well
• Hydroxides (Al(OH)3, Fe(OH)3, for example)
  often form gels and are very difficult to filter
• Hydrophobic (water-fearing) colloids coagulate
  readily
• Luckily, AgCl is hydrophobic
• Gravimetric analysis of silver or chloride by
  AgCl precipitation is effective in spite of
  colloidal nature of precipitate

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4. Filtration

• See pp. 48-50 for filtering techniques
• Study carefully, since your results in Exp. 2
  depend on your ability to decant the solution,
  transfer the precipitate to the filter correctly
  and completely
• We will use sintered-glass filtering crucibles
  (20 each!) in Exp. 2
• Colloidal precipitates present filtering problems
  if particles are too small
• Can plug filter paper or glass or pass right
  through the filter if not coagulated well
• Hydrophobic colloids generally filter better than
  hydrophilic colloids

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5. Washing

• Removes the mother liquor
• May also remove some of the coprecipitated
  compounds
• Can cause peptization of colloids
• Diluting the counter-ion layer causes it to get
  larger, forcing coagulated colloidal particles
  apart
• And THERE THEY GO right through the filter
• Wash with a solution of a volatile electrolyte
  that will be removed in drying step

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6. Drying or Igniting

• Dry the precipitate to remove water and volatile
  electrolytes from wash solution
• Ignition (very high-temperature drying) converts
  precipitates to compounds more suitable for
  weighing
• Removes water of hydration
• Converts hygroscopic compound to non-hygroscopic
  compound
• Sometimes it even converts the filter paper to
  ash
• See page 51 for discussion

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7. Weighing

• Properly calibrated analytical balance
• Good weighing technique
• Avoid static electricity
• Review important points on p. 51 periodically
• And now, step 8, the calculations
• Chem 105 stoichiometry, anyone?

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Section 10.2

• This is just Chem 105 stoichiometry made obscure.
  If you could do gram-to-gram calculations in
  Chem 105, you can do this
• Get mole ratio from balanced equation
• Can also get mole ratio from formulas of analyte
  and precipitate
• If analyte is Bi2S3 and precipitate is BaSO4,
  mole ratio is 1 mol Bi2S3/3 mol BaSO4

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Gravimetric Factor (GF)

• This is simply the number of grams of analyte
  that correspond to 1 gram of precipitate
• How many grams Bi2S3 per gram BaSO4?
• Start with 1 g BaSO4, convert to moles, multiply
  by mole ratio (1/3), convert to grams Bi2S3
• Example 10.1 does this, but he sets it up a bit
  differently
• He uses f wt and at wt to symbolize formula
  weight and atomic weight, sometimes called molar
  mass
• Once you know gravimetric factor, can convert
  mass of precipitate directly to mass of analyte
  in one step

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Example 10.2

• This is absolutely riddled with errors in my
  printing
• Second line, put subscript 3 on (NH4)
• Molar mass of precipitate is 1876.3
• In dimensional analysis calculation at bottom of
  page
• Mass is wrong, formula is wrong, molar mass is
  wrong, mass of sample is wrong
• Other than that, it works great!
• Just do this example by your favorite Chem 105
  stoichiometry procedure

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Example 10.5

• Simultaneous equations used to solve
  stoichiometry problems
• Let x g Fe and y g Al
• Find mass of FeCl3 and Fe2O3 in terms of x
• There are 162.21 g FeCl3 for every 55.85 g Fe, so
  there are (162.21/55.85)(x) g FeCl3, or 2.904x g
  FeCl3
• Similarly, there are 1.430x g Fe2O3
• Do the same for aluminum chloride and oxide
• 4.942y g AlCl3 and 1.890y g Al2O3
• Solve 2.904x 4.942y 5.95 and 1.430x 1.890y
  2.62
• Hint I like this kind of problem!

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Sections 10.3 and 10.4

• Know where to access this information if you
  should need it in the future
• It is useful reference information for future
  analytical work you may find yourself doing
  sometime
• Do not attempt to memorize Tables 10.1 and 10.2
• Unless, of course, you really want to

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Section 10.5

• This is nothing more than a review of solubility
  and precipitation equilibria from Chem 106
• See the Whitten book, Chapter 20
• We will review Example 10.6 through 10.12 if time
  permits, but you must understand that we dont
  have time to teach Chem 106 all over again!

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Lecture 7 Assignment

• Read Chapter 5
• Its a long chapter, but mostly Chem 105 review
  with a few new things here and there
• We will cover as much as we can in one period
• Refer to the lecture notes and focus on the
  material I will emphasize