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.

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

AgCl(s) Ag+(aq) + Cl-(aq) Gravimetric Analysis • Equilibrium between AgCl precipitate and the saturated solution.

[Ag+][Cl-] [AgCl] KT = Gravimetric Analysis • The corresponding thermodynamic equilibrium constant KT is given by:

Gravimetric Analysis • AgCl is in a solid phase therefore [AgCl] = 1 KTSP = [Ag+][Cl-]

Gravimetric Analysis • Solubility. • Common ion effect • Ionic strength • Fractional precipitation • Complex ions • Temperature • Solvent

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 adsorption • Mixed crystal formation • Occlusion • Mechanical entrapment

Gravimetric Analysis • Drying and ignition. • Removes solvents and volatiles • Decomposition to known form

Gravimetric Analysis • Inorganic: • H2S, AgNO3, HCl, BaCl2 • Organic • 2,4-DNP

O Mg + 2 N M g N + 2H 2+ + N O O H Gravimetric Analysis • 8-Hydroxyquinoline • Selectivity through pH control

Gravimetric Analysis • 8-Hydroxyquinoline Examples

C H C C C H 3 3 O N N O Ni + 2 2H+ C H C C C H 2+ + H H N i 3 3 O N N O N N H O O H C H C C C H 3 3 Gravimetric Analysis • Dimethylglyoxine • Weakly alkaline conditions • Nickel salt bright red

Summary • Principles • Solution reaction between analytes and reagents to give sparingly soluble products. • Drying or ignition of precipitates. • Weighing • Apparatus • Flasks, beakers, pipettes, crucibles and filter papers. • Oven or furnace and a dessicator. • Analytical quality balance.

Summary • Applications • Extensive 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 • Disadvantages • Careful and time consuming. • Scrupulously clean glassware. • Very accurate weighing. • Coprecipitation.

Gravimetric Analysis