Gravimetric Analysis Very Good

8/2/2019 Gravimetric Analysis Very Good

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Gravimetric Analysis

A Gravimetric analysis is based upon

the measurement of the weight of asubstance that has a KNOWN

composition AND IS chemically related

to the analyte.


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Accurate and precise.

Possible sources of errors can bechecked.

It is an ABSOLUTE method.

Relatively inexpensive



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Precipitation methods.

Volatilisation methods.

Electrogravimetry.

Thermogravimetry.



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In precipitation methods, the species to

be determined is precipitated by areagent that yields a sparingly soluble

product that has a known composition or

can be converted to such a substance.


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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.



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Determination of silver.

A solution of Ag+ is treated with an excessof NaCl or KCl solution, the precipitate is

filtered off, washed well with water to

remove soluble salts, dried at 130 - 150C

and weighed as AgCl.


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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 pyrophosphateMg2P2O7 after ignition.


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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.


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Accuracy

Solubility Products.

Solubility.

Particle size.

Coprecipitates.

Drying and ignition.



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Solubility Products

Even the most insoluble products have atleast a certain solubility. It is therefore

more correct to call these compounds

sparingly soluble substances, eg: AgCl



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Equilibrium between AgCl precipitate

and the saturated solution.


AgCl(s) Ag+

(aq) + Cl-

(aq)


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The corresponding thermodynamic

equilibrium constant KT

is given by:


[Ag+][Cl-][AgCl]

KT =


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AgCl is in a solid phase therefore

[AgCl] = 1


KTSP = [Ag+][Cl-]


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Solubility.

Common ion effect

Ionic strength

Fractional precipitation

Complex ions

Temperature

Solvent



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Particle size.

Colloidal suspension (10-6 - 10-4 mm diameter)

to crystalline precipitate.

Depends on nucleation and particle growth.



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Coprecipitates.

Removal during precipitation of compounds

which are otherwise soluble.Sources:

Surface adsorption

Mixed crystal formation

Occlusion

Mechanical entrapment



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Drying and ignition.Removes solvents and volatiles

Decomposition to known form



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Inorganic:

H2S, AgNO3, HCl, BaCl2

Organic

2,4-DNP



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8-Hydroxyquinoline


Mg2+ + 2

N

O H

+ 2H+N

O

Mg

O

N

Selectivity through pH control


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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


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Dimethylglyoxine


Weakly alkaline conditions

Nickel salt bright red

Ni2+ + 2 C H3 C C C H3

N N

HO O H

+

C H3 C C C H3

N N

N i

OO

O ONN

C H3CCC H3

HH 2H+


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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.


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Summary

Applications

Extensive numbers of inorganic ions are

determined with excellent precision and

accuracy.

Routine assays of metallurgical samples.

Good accuracy


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Summary

Disadvantages

Careful and time consuming.

Scrupulously clean glassware.Very accurate weighing.

Coprecipitation.

Gravimetric Analysis Very Good

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