Gravimetry Principle

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


Precipitation methods.

Volatilisation methods.

Electrogravimetry.

Thermogravimetry.

Precipitation:

precipitating agent

sample

dissolvedcomponentsDissolve sample

Add ppt’ing reagentFilterDryWeigh


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.


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.


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.


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.


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


Equilibrium between AgCl precipitate and the saturated solution.

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


The corresponding thermodynamic equilibrium constant KT is given by:

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


AgCl is in a solid phase therefore

[AgCl] = 1

KTSP = [Ag+][Cl-]

Gravimetric Analysis Solubility.

Common ion effectIonic strengthFractional precipitationComplex ionsTemperatureSolvent


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


Drying and ignition.Removes solvents and volatilesDecomposition to known form


Inorganic:H2S, AgNO3, HCl, BaCl2

Organic2,4-DNP


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


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.

Gravimetry Principle

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