Gravimetric Analysis l A Gravimetric analysis is based upon the measurement of the weight of a substance that has a KNOWN composition AND IS chemically

Gravimetric AnalysisGravimetric Analysis

A Gravimetric analysis is based upon A Gravimetric analysis is based upon the measurement of the weight of a the measurement of the weight of a substance that has a substance that has a KNOWNKNOWN composition composition AND ISAND IS chemically related chemically related to the analyte.to the analyte.

Accurate and precise.Accurate and precise.

Possible sources of errors can be Possible sources of errors can be checked.checked.

It is an ABSOLUTE method.It is an ABSOLUTE method.

Relatively inexpensiveRelatively inexpensive


Precipitation methods.Precipitation methods.

Volatilisation methods.Volatilisation methods.

Electrogravimetry.Electrogravimetry.

Thermogravimetry.Thermogravimetry.



In precipitation methods, the species to In precipitation methods, the species to be determined is precipitated by a be determined is precipitated by a reagent that yields a sparingly soluble reagent that yields a sparingly soluble product that has a known composition or product that has a known composition or can be converted to such a substance.can be converted to such a substance.

Precipitation methods.Precipitation methods.

Analyte (or chemically related species) isolated Analyte (or chemically related species) isolated as a sparingly soluble precipitate of known as a sparingly soluble precipitate of known composition.composition.

Analyte (or chemically related species) isolated Analyte (or chemically related species) isolated as a sparingly soluble precipitate that can be as a sparingly soluble precipitate that can be converted by heat to species of known converted by heat to species of known composition.composition.



Determination of silver.Determination of silver. A solution of AgA solution of Ag++ is treated with an excess is treated with an excess

of NaCl or KCl solution, the precipitate is of NaCl or KCl solution, the precipitate is filtered off, washed well with water to filtered off, washed well with water to remove soluble salts, dried at 130 - 150°C remove soluble salts, dried at 130 - 150°C and weighed as AgCl.and weighed as AgCl.


Frequently the constituent being estimated is Frequently the constituent being estimated is weighed in a form other than that it was weighed in a form other than that it was precipitated in.precipitated in.

MgMg2+2+: precipitated as Mg(NH: precipitated as Mg(NH44)PO)PO44.6H.6H220 but 0 but

is weighed as magnesium pyrophosphate is weighed as magnesium pyrophosphate MgMg22PP22OO77 after ignition. after ignition.


Conditions:Conditions:1. Must be a stoichiometric reaction.1. Must be a stoichiometric reaction.

2. A stable product; no oxidation, dehydration 2. A stable product; no oxidation, dehydration or gelatinous precipitates.or gelatinous precipitates.

3. Must avoid side reactions which result in 3. Must avoid side reactions which result in coprecipitates.coprecipitates.

Accuracy Accuracy Solubility Products.Solubility Products. Solubility.Solubility. Particle size.Particle size. Coprecipitates.Coprecipitates. Drying and ignition.Drying and ignition.


Solubility ProductsSolubility Products Even the most insoluble products have at Even the most insoluble products have at

least a certain solubility. It is therefore least a certain solubility. It is therefore more correct to call these compounds more correct to call these compounds sparingly soluble substances, eg: AgClsparingly soluble substances, eg: AgCl


Equilibrium between AgCl precipitate Equilibrium between AgCl precipitate and the saturated solution.and the saturated solution.


AgCl(s) Ag+(aq) + Cl-(aq)

The corresponding thermodynamic The corresponding thermodynamic equilibrium constant Kequilibrium constant KTT is given by: is given by:


[Ag+][Cl-][AgCl] KT =

AgCl is in a solid phase thereforeAgCl is in a solid phase therefore

[AgCl] = 1[AgCl] = 1


KTSP = [Ag+][Cl-]

Solubility.Solubility. Common ion effectCommon ion effect Ionic strengthIonic strength Fractional precipitationFractional precipitation Complex ionsComplex ions TemperatureTemperature SolventSolvent


Particle size.Particle size.

Colloidal suspension (10Colloidal suspension (10-6 -6 - 10- 10-4 -4 mm diameter) mm diameter) to crystalline precipitate.to crystalline precipitate.

Depends on nucleation and particle growth.Depends on nucleation and particle growth.


Coprecipitates.Coprecipitates. Removal during precipitation of compounds Removal during precipitation of compounds

which are otherwise soluble.which are otherwise soluble. Sources:Sources:

Surface adsorptionSurface adsorption Mixed crystal formationMixed crystal formation OcclusionOcclusion Mechanical entrapmentMechanical entrapment


Drying and ignition.Drying and ignition. Removes solvents and volatilesRemoves solvents and volatiles Decomposition to known formDecomposition to known form


Inorganic:Inorganic: HH22S, AgNOS, AgNO33, HCl, BaCl, HCl, BaCl22

OrganicOrganic 2,4-DNP2,4-DNP


8-Hydroxyquinoline8-Hydroxyquinoline


Mg2+ + 2

N

OH

+ 2H+N

O

Mg

O

N

Selectivity through pH control

8-Hydroxyquinoline Examples8-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

DimethylglyoxineDimethylglyoxine


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+

SummarySummary PrinciplesPrinciples

Solution reaction between analytes and reagents to Solution reaction between analytes and reagents to give sparingly soluble products.give sparingly soluble products.

Drying or ignition of precipitates.Drying or ignition of precipitates. WeighingWeighing

ApparatusApparatus Flasks, beakers, pipettes, crucibles and filter papers.Flasks, beakers, pipettes, crucibles and filter papers. Oven or furnace and a dessicator.Oven or furnace and a dessicator. Analytical quality balance.Analytical quality balance.

SummarySummary

ApplicationsApplications Extensive numbers of inorganic ions are Extensive numbers of inorganic ions are

determined with excellent precision and determined with excellent precision and accuracy.accuracy.

Routine assays of metallurgical samples.Routine assays of metallurgical samples. Relative precision 0.1 to 1%.Relative precision 0.1 to 1%. Good accuracyGood accuracy

SummarySummary

DisadvantagesDisadvantages Careful and time consuming.Careful and time consuming. Scrupulously clean glassware.Scrupulously clean glassware. Very accurate weighing.Very accurate weighing. Coprecipitation.Coprecipitation.

Documents

Gravimetric Analysis l A Gravimetric analysis is based upon the measurement of the weight of a substance that has a KNOWN composition AND IS chemically