By: Dr. O. Rajabi (Pharm.D.- Ph.D.) Associate Professor of Chemistry Department of Medicinal Chemistry Mashad University of Medical Sciences

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By Dr. O. Rajabi (Pharm.D.- Ph.D.)Associate
Professor of ChemistryDepartment of Medicinal
ChemistryMashad University of Medical Sciences
Gravimetric Analysis
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gravi metric (weighing - measure)

• Definitiona precipitation or volatilization
  method based on the determination of weight of a
  substance of known composition that is chemically
  related to the analyte
• analyte - chemical element or compound of interest

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• ReactionaA rR -----gt AaRr pptwhere
• a is of moles of analyte A
• r is of moles of reagent R
• AaRr is a pure, insoluble precipitatewhich we
  can dry and weigh or ignite to convert to
  something we can weigh
• pptprecipitate

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• 1914 Nobel Prize to T.W.Richards (Harvard
  University) for the atomic weights of Ag, Cl, and
  N
• Richards and his group determined atomic weights
  of 55 of the 92 known elements using gravimetry

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T.W.Richards

• Every substance must be assumed to be impure,
  every reaction must be assumed to be incomplete,
  every method of measurement must be assumed to
  contain some constant error, until proof to the
  contrary can be obtained.

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7 Steps in Gravimetric Analysis

• Dry and weigh sample
• Dissolve sample
• Add precipitating reagent in excess
• Coagulate precipitate usually by heating
• Filtration-separate ppt from mother liquor
• Wash precipitate (peptization)
• Dry and weigh to constant weight

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Precipitation

• Dissolve sample
• Add ppting reagent
• Filter
• Dry
• Weigh

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Suction Filtration

• Filter flask
• Buchner funnel
• Filter paper
• Glass frit
• Filter adapter
• Heavy-walled rubber tubing
• Water aspirator

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• Mother liquor

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• Identify insoluble form
• Two considerations
• Minimize errors due to limited precipitate
  solubility
• Minimize errors due to precipitation process
• Finite solubility of precipitate
• ideally, Ksp 0 (i.e., completely insoluble)
• Some come close 10-38 for Fe(OH)3 10-50
  for Ag2S
• For AgCl, Ksp 1.78 x 10-10

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• For example
• what would be the error introduced in
  gravimetric analysis by the solubility of AgCl?
• For a 0.1000 g AgCl precipitate in 200 ml H2O
• Note Error is independent of mass of
  precipitate, ? relative error will decrease as
  precipitate mass increases (i.e., 0.038 error
  for 1.000 g AgCl)

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• Precipitation process
• ideally, wed like a precipitate that forms
  quickly. This implies
• Large, pure crystals
• Low solubility
• Easily filtered
• Easily washed
• How does precipitation occur?
• As Ksp is exceeded, solution becomes
  supersaturated
• At some point nucleation begins
• At the same time, crystal growth begins

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• Two points to remember
• Crystal growth is independent of degree of
  supersaturation
• Nucleation increases with degree of
  supersaturation
• Minimization of supersaturation will produce
  the largest particles
• Two particle size classes
• Colloids
• very small
• difficult to handle experimentally
• Crystals
• large ( 10-1 mm)
• easily and rapidly filtered
• high purity

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• What affects degree of supersaturation?
• Ksp
• Temperature
• solubility ? as T ?
• Reagent addition speed
• slower addition givesprecipitation a chanceto
  begin at lowersupersaturation levels
• Solution concentration
• low reagent concentrationequals low
  supersaturation
• But, even with the above precautions, we will
  often obtain colloid instead of a crystal!

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• Keys to successful colloid precipitation
• Add precipitant slowly and in slight excess
• Digest precipitate (Heat, stir, sit)
• What about crystalline precipitate?
• Similar to colloids
• Dilute solution
• Slow precipitant addition
• Elevated temperature
• Heat unstirred
• Contaminants can escape from crystal lattice
• Increase crystal bridges

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• Particle Size / Filterability
• produce particles large enough to be 'caught
• ideally, produce crystals
• avoid colloidal suspension particle size 1 -
  100 nm

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Precipitate Formation

• crystallization
• nucleationparticles join to produce aggregates
• crystal growthaggregate grows and 'fall out' of
  solution 
• We want a few big chunks of precipitate!supersatu
  ration more solute than should be present in
  solutionrelative supersaturation a measure of
  supersaturation, (Q-S)/S Q actual solute
  concentration S equilibrium solute
  concentration

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Controlling Precipitation

• Increase S
• Increase temperature
• Decrease Q
• Dilute solution
• Well mixed (stirring)

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What Do We Get Out of Gravimetry?

• of analyte, A
• A weight of analyte x 100
  weight of sample

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How Do We Get A?

• A weight of ppt x gravimetric factor (G.F.) x
  100 weight of sample
• G.F. a FWanalyte b
  FWprecipitate
• G.F. gms of analyte per 1 gm ppt

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Gravimetric Factor

• X apples Y sugar Z apple pies
• What is this relationship in chemistry?

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The Gravimetric Factor

• G.F. a FWanalyte b
  FWprecipitate
• Analyte ppt G.F.CaO CaCO3FeS BaSO4UO2(NO
  3)2.6H2O U3O8Cr2O3 Ag2CrO4

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• Analyte ppt G.F.CaO CaCO3 CaO/CaCO3FeS BaSO4
  FeS/BaSO4UO2(NO3)2 U3O8 3UO2(NO3)2/U3O8 Cr2O3
  Ag2CrO4 Cr2O3/2Ag2CrO4

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Problem

• Consider a 1.0000 g sample containing 75
  potassium sulfate (FW 174.25) and 25 MSO4. The
  sample is dissolved and the sulfate is precipated
  as BaSO4 (FW 233.39). If the BaSO4 ppt weighs
  1.4900, what is the atomic weight of M2 in MSO4?
  
• ANS Mg2

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Answer

• The hard part is setting up the correct equation
  (good stoichiometry skills are essential here!)
• Rearranging and solving

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Problem

• A mixture of mercurous chloride (FW 472.09) and
  mercurous bromide (FW 560.99) weighs 2.00 g. The
  mixture is quantitatively reduced to mercury
  metal (At wt 200.59) which weighs 1.50 g.
  Calculate the mercurous chloride and mercurous
  bromide in the original mixture.
• ANS 0.5182 g

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Answer

• Again, important to set up correct equation
• Rearranging and solving

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Homogeneous Precipitation

• (NH2)CO 3 H2O heat
• ?
• HCOOH OH- CO2 2 NH4

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High Electrolyte Concentration to Aid
Precipitation
Excess charge on colloid creates ionic atmosphere
around particle
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Composition by Gravimetric Analysis

• Ni2 (aq) H2DMG ? Ni(DMG)2 2 H
• A 0.8234 g org sample produced 0.1397 g of
  bis(dimethylglyoximate) nickel (II) (FW 288.91
  g/mol). Find the nickel content.
• Explain how to create a large, filterable
  precipitate.

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Combustion Analysis
Find the empirical formula for a 13.72 mg organic
sample that produced 6.97 mg of water and 28.44
mg of carbon dioxide
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Gravimetric Overview

• Simple
• Cheap
• Glassware
• Reagents
• ovens, etc.
• Balances
• Specific
• Timely (1/2 day)
• Accurate
• Precise (0.1-0.3 )
• Sensitive

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