Gravimetric Analysis  A Gravimetric analysis is based upon the measurement of the weight of a substance that has a KNOWN composition to the analyte.

AND IS chemically related

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.

 Mg 2+ : precipitated as Mg(NH 4 )PO 4 .6H

2 0 but is weighed as magnesium pyrophosphate Mg 2 P 2 O 7 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 K T is given by: K T = [Ag + ][Cl ] [AgCl]

Gravimetric Analysis  AgCl is in a solid phase therefore [AgCl] = 1 K T SP = [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:  H 2 S, AgNO 3 , HCl, BaCl 2  Organic  2,4-DNP

Gravimetric Analysis  8-Hydroxyquinoline Mg 2+ + 2 OH N O N Mg N O  Selectivity through pH control + 2H +

Gravimetric Analysis  8-Hydroxyquinoline Examples

Metal pH Initial Ppt.

pH Complete Ppt Metal pH Initial Ppt.

pH Complete Ppt

Aluminium Bismuth Cadmium Calcium Cobalt Copper Iron(III) Lead Magnesium 2.9

3.7

4.5

6.8

3.6

3.0

2.5

4.8

7.0

4.7 – 9.8

5.2 – 9.4

5.5 – 13.2

9.2 – 12.7

4.9 – 11.6

>3.3

4.1 – 11.2

8.4 – 12.3

>8.7

Manganese Molybdenum Nickel Thorium Titanium Tungsten Uranium Vanadium Zinc 4.3

2.0

3.5

3.9

3.6

3.5

3.7

1.4

3.3

5.9 – 9.5

3.6 – 7.3

4.6 – 10.0

4.4 – 8.8

4.8 – 8.6

5.0 – 5.7

4.9 – 9.3

2.7 – 6.1

>4.4

Gravimetric Analysis  Dimethylglyoxine Ni 2+ + 2 CH 3 C HO N C CH 3 N OH CH 3 O H O CH 3 C C N N N Ni N C C CH 3 O H O CH 3 + 2H +  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.