gravimetry principle
TRANSCRIPT
Gravimetric Analysis
Gravimetric Analysis A Gravimetric analysis is based upon the measurement of the weight of a substance that has a KNOWN composition AND IS chemically related to the analyte.
Gravimetric Analysis Accurate and precise.
Possible sources of errors can be checked.
It is an ABSOLUTE method.
Relatively inexpensive
Gravimetric Analysis
Precipitation methods.
Volatilisation methods.
Electrogravimetry.
Thermogravimetry.
Precipitation:
precipitating agent
sample
dissolvedcomponentsDissolve sample
Add ppt’ing reagentFilterDryWeigh
Gravimetric Analysis
In precipitation methods, the species to be determined is precipitated by a reagent that yields a sparingly soluble product that has a known composition or can be converted to such a substance.
Gravimetric Analysis
Precipitation methods.
Analyte (or chemically related species) isolated as a sparingly soluble precipitate of known composition.
Analyte (or chemically related species) isolated as a sparingly soluble precipitate that can be converted by heat to species of known composition.
Gravimetric Analysis
Determination of silver. A solution of Ag+ is treated with an
excess of NaCl or KCl solution, the precipitate is filtered off, washed well with water to remove soluble salts, dried at 130 - 150°C and weighed as AgCl.
Gravimetric Analysis
Frequently the constituent being estimated is weighed in a form other than that it was precipitated in.
Mg2+: precipitated as Mg(NH4)PO4.6H20 but is weighed as magnesium pyrophosphate Mg2P2O7 after ignition.
Gravimetric Analysis
Conditions:1. Must be a stoichiometric reaction.
2. A stable product; no oxidation, dehydration or gelatinous precipitates.
3. Must avoid side reactions which result in coprecipitates.
Gravimetric Analysis Accuracy
Solubility Products.Solubility.Particle size.Coprecipitates.Drying and ignition.
Gravimetric Analysis Solubility Products Even the most insoluble products
have at least a certain solubility. It is therefore more correct to call these compounds sparingly soluble substances, eg: AgCl
Gravimetric Analysis
Equilibrium between AgCl precipitate and the saturated solution.
AgCl(s) Ag+(aq) + Cl-(aq)
Gravimetric Analysis
The corresponding thermodynamic equilibrium constant KT is given by:
[Ag+][Cl-][AgCl] KT =
Gravimetric Analysis
AgCl is in a solid phase therefore
[AgCl] = 1
KTSP = [Ag+][Cl-]
Gravimetric Analysis Solubility.
Common ion effectIonic strengthFractional precipitationComplex ionsTemperatureSolvent
Gravimetric Analysis
Particle size.
Colloidal suspension (10-6 - 10-4 mm diameter) to crystalline precipitate.
Depends on nucleation and particle growth.
Gravimetric Analysis Coprecipitates.
Removal during precipitation of compounds which are otherwise soluble.
Sources:Surface adsorptionMixed crystal formationOcclusionMechanical entrapment
Gravimetric Analysis
Drying and ignition.Removes solvents and volatilesDecomposition to known form
Gravimetric Analysis
Inorganic:H2S, AgNO3, HCl, BaCl2
Organic2,4-DNP
Gravimetric Analysis
8-Hydroxyquinoline
Mg2+ + 2
N
OH
+ 2H+N
O
Mg
O
N
Selectivity through pH control
Gravimetric Analysis 8-Hydroxyquinoline Examples
Metal pH
Initial Ppt.
pH
Complete
Ppt
Metal pH
Initial Ppt.
pH
Complete
Ppt
Aluminium 2.9 4.7 – 9.8 Manganese 4.3 5.9 – 9.5
Bismuth 3.7 5.2 – 9.4 Molybdenum 2.0 3.6 – 7.3
Cadmium 4.5 5.5 – 13.2 Nickel 3.5 4.6 – 10.0
Calcium 6.8 9.2 – 12.7 Thorium 3.9 4.4 – 8.8
Cobalt 3.6 4.9 – 11.6 Titanium 3.6 4.8 – 8.6
Copper 3.0 >3.3 Tungsten 3.5 5.0 – 5.7
Iron(III) 2.5 4.1 – 11.2 Uranium 3.7 4.9 – 9.3
Lead 4.8 8.4 – 12.3 Vanadium 1.4 2.7 – 6.1
Magnesium 7.0 >8.7 Zinc 3.3 >4.4
Gravimetric Analysis
Dimethylglyoxine
Weakly alkaline conditions
Nickel salt bright red
Ni2+ + 2 CH3 C C CH3
N NHO OH
+
CH3 C C CH3
N N
Ni
OO
O ONN
CH3CCCH3
HH 2H+
Summary Principles
Solution reaction between analytes and reagents to give sparingly soluble products.
Drying or ignition of precipitates.Weighing
ApparatusFlasks, beakers, pipettes, crucibles and
filter papers.Oven or furnace and a dessicator.Analytical quality balance.
Summary
ApplicationsExtensive numbers of inorganic ions are determined with excellent precision and accuracy.
Routine assays of metallurgical samples.
Relative precision 0.1 to 1%.Good accuracy
Summary
DisadvantagesCareful and time consuming.
Scrupulously clean glassware.
Very accurate weighing.
Coprecipitation.