Gravimetry Principle

Gravimetric Analysis

Gravimetric Analysis y

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 y

y

 Accurate and precise.

Possible sources of errors can be checked. y

It is an ABSOLUTE method. y

Relatively inexpensive

Gravimetric Analysis y

Precipitation methods.

y

 Volatilisation methods.

y

Electrogravimetry.

y

Thermogravimetry.

Precipitation:

Dissolve sample  Add ppt·ing reagent Filter Dry   Weigh

dissolved components sample

 precipitating agent

Gravimetric Analysis y

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 y

Precipitation methods. y

y

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

y

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 A Cl. y

Gravimetric Analysis

y

y

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

y

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 y

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

y

y

y

y

Gravimetric Analysis y

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 y

Gravimetric Analysis

y

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: y

+][Cl-] [ Ag K T = [AgCl]

Gravimetric Analysis

y

 AgCl is in a solid phase therefore [AgCl] = 1 K TSP = [Ag+][Cl-]

Gravimetric Analysis y

Solubility. Common ion effect Ionic strength Fractional precipitation Complex ions Temperature Solvent y

y

y

y

y

y

Gravimetric Analysis y

Particle size. y

y

Colloidal suspension (10 -6 - 10-4 mm diameter) to crystalline precipitate. Depends on nucleation and particle growth.

Gravimetric Analysis y

Coprecipitates. Removal during precipitation of  compounds which are otherwise soluble. Sources: Surface adsorption Mixed crystal formation Occlusion Mechanical entrapment y

y

y

y

y

y

Gravimetric Analysis 

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

y

y

Gravimetric Analysis y

Inorganic: H2S, AgNO3, HCl, BaCl2

y

y

Organic 2,4-DNP

y

Gravimetric Analysis y

8-

y r xy

i

li e

O Mg2+ + 2

 N

Mg  N

 N OH

O

Selectivity through pH control



+ 2H+

Gravimetric Analysis y

8-Hydroxyquinoline Metal

Examples

pH

pH

Initial Ppt.

Complete

Metal

pH

pH

Initial Ppt.

Complete

Ppt

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

admium

4.5

5.5 ± 13.2

Nickel

3.5

4.6 ± 10.0

alcium

6.8

9.2 ± 12.7

Thorium

3.9

4.4 ± 8.8

obalt

3.6

4.9 ± 11.6

Titanium

3.6

4.8 ± 8.6

opper

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 y

Dimethylglyoxine CH3

C

C

O  N  N  Ni

CH3

C

C

 N HO

N



CH3

H

H

CH3

C

N

O

C

CH3

Weakly alkaline conditions

 Nickel salt bright red



O

 Ni O  N

OH

CH3 H

Summary Principles Solution reaction between analytes and reagents to give sparingly soluble products. Drying or ignition of precipitates.  Weighing

y

y

y

y

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

y

y

y

y

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 

y

y

y

y

y

Summary

y

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

y

y

y