lecture 6- gravimetric analysis (1h)
TRANSCRIPT
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The Process Separate - by phases
volatilization methods
precipitation methods
selective ppt'n
account for interferencesassure completeness (solubility
equilibrium)
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Precipitation:
prec ipitating agent
sample
dissolvedcomponents
Dissolve sample
Add ppt’ing
reagent Filter
Dry
Weigh
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Calculating Amount
of Precipitation Reagent
How many mL of 1% potassium chloride would be needed to precipitate all of the
silver in a 0.5 g ore sample that contains 1.5parts per thousand silver? Allow for a 15%excess of the chloride solution.
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Gravimetric Overview Measure
tools for gravimetric analysisfilter crucibles, an oven, balance
Calculate stoichiometry
Report values
% composition, ppt, ppm, ppbuncertainty
averagestandard deviation, confidence interval
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The Precipitation Process
(from solution)
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
nucleation:particles join to produce aggregates
crystal growthaggregate grows and 'fall out' of solution
We want a few big chunks of precipitate!
supersaturation: more solute than should be present in solutionrelative supersaturation:
a measure of supersaturation, (Q-S)/SQ = 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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pH control of precipitationCa2+ + C2O4
2- CaC2O4 (s)
H2C2O4 2 H+ + C2O42-
Feeder Reaction:
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Homogeneous Precipitation Fe3+ + 3 HCO2
- Fe(HCO2 )3nH2O
(NH2 )CO + 3 H2O + heat
HCOOH + OH- + CO2
+ 2 NH4
+
Feeder Reaction:
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High Electrolyte Concentration
to Aid Precipitation
Excess charge on colloid creates ionicatmosphere around particle
D.C. Harris, Quantitative Chemical Analysis, 6 th Ed., p686
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Composition by
Gravimetric AnalysisNi2+ (aq) + H2DMG Ni(DMG)2 + 2 H+
A 0.8234 g ore sample produced 0.1397 g ofbis(dimethylglyoximate) nickel (II) (FW =288.91 g/mol). Find the nickel content.
Explain how to create a large, filterableprecipitate.
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Combustion Analysis
Absorbs
water
Absorbs
CO2
D.C. Harris, Quantitative Chemical Analysis, 6 th Ed., p691
Find the empirical formula for a 13.72 mg organicsample that produced 6.97 mg of water and 28.44mg of carbon dioxide
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Gravimetric Interferences
Coprecipitated impurities
Surface adsorption
• Contaminants bound to the surface• Dry the sample
Mixed crystals
• Replacements in the crystal lattice
• Reprecipitation
Occlusions/mechanical entrapment
• Im urit ockets
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Gravimetric Overview
Simple
Cheap Glassware
Reagents
ovens, etc.
Balances
Specific Timely
Accurate
Precise
Sensitive