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Tuesday, August 13, 2013



GRAVIMETRIC ANALYSIS Presentation Transcript:

2.Preparation of the solution
 Solid sample must be dissolved in a suitable solvent.
 Some form of preliminary separation may be necessary to eliminate interfering materials.
 The Purposes of Solution Preparation

i) To maintain low solubility of the precipitate.
ii) To obtain the precipitate in a form suitable for filtration.
iii) Proper adjustment of the solution condition may also mask potential interferences.

3.Factors that Must be Considered when Preparing the Solution

i) Volume of the solution during precipitation.

ii) Concentration range of the test substance.

iii) The presence and concentrations of other constituents.

iv) Temperature

v) pH

4.Formation and Properties of Precipitates
  Analyte  +  Precipitating Agent  ?  Supersaturation
   Two steps are involved in precipitation:
1. Nucleation        2.Particle Growth
The particle size of a precipitate is determined by which one is faster between these two steps.
In nucleation a few atoms, ions or molecules join together to give a stable solid called nuclei (nucleus). 
 Further precipitation then involves a competition between additional nucleation and particle growth (the process where more ions are added to the nucleus to form colloids with sizes in the range of 1-100nm in diameter).

5.If: In high supersaturated solution….
Rate of Nucleation >  Rate of Particle Growth
Precipitate containing a large number of small particles. Colloidal is formed in the solution.
In less supersaturated solution……
Rate of Nucleation < Rate of Particle Growth
Precipitate containing a smaller number of larger particles is produced.
 Precipitates with large particle are more easily handled during filtration and washing.
** In general, a dilute solution with low supersaturation enhances particles growth that results in large particle size.

6.Factors That Determine the Particle Size of precipitates
 Von Weimarn introduced the concept of relative supersaturation,
 The particle size of precipitates is inversely proportional to the relative supersaturation of the solution during precipitation.

Relative Supersaturation  =    Q  -  S
Q  =  concentration of the mixed reagent before
S  =  solubility of the precipitate at equilibrium
 (Q?S) is a measure of the degree of supersaturation.
 The rate of nucleation and the rate of particle growth depend on the supersaturation (Q?S).

 (Q-S) Becomes too Great
High relative supersaturation, nucleation is favored, many small particles (Colloidal precipitates form).
 (Q-S) is Small
The smaller will be the relative supersaturation, particle growth will predominate, few but larger particles size (Crystalline precipitates form).
 To minimize supersaturation and obtain large crystals, conditions should be adjusted so that Q will be as low as possible (Q ?) and S will be relatively large (S ?) during precipitation.

8.Several steps are commonly taken
 Precipitation from dilute solution, (Q ?).
Adding precipitating reagents slowly with effective stirring.
Stirring prevents local excesses of the reagent, (Q ? )
Precipitation from hot solution. The solubility of precipitates increases with temperature, (S?).
 After the precipitate was formed, the particle size can be improved by digestion process or by precipitation from homogeneous solution.

9.Precipitation from Homogeneous Solution
 A technique in which a precipitating agent is generated in a solution of the analyte by slow chemical reaction.
 The precipitating ion is not added to the solution but is slowly generated throughout the solution by a homogeneous chemical reaction.
 The advantages
 ?    No locally excesses of precipitating agent.
 ?    The supersaturation (Q-S) is kept low at all the time, so that the precipitate obtained is much more dense and free from impurities than precipitates formed by the conventional method.
 ?    Substances that ordinarily precipitate only as amorphous solid frequently precipitate from homogenous solution as well-formed crystals.

 Urea (NH2CONH2)
 Used for generation of hydroxide ion.
Hydrolysis of urea will increase the pH (more alkaline). The ammonia slowly liberated raises the pH of the solution, and react with metal ions to form metal hydroxides (or hydrous oxides) precipitate.
 (NH2)2 CO  +  3H2O  ?  CO2  +  2NH4+  +  2OH?
 Process is controlled by heating the solution just below 100oC and a 1-2 hr heating period is needed to complete the typical precipitation.
 Used for the precipitation of Al, Ga, Th, Bi, Fe and Sn as hydroxides.

Known as absorbed and adsorbed impurities.
Co-precipitation is a process where the impurity is precipitated along with the desired precipitate, even though the solubility of the impurity has not been exceeded.
 ?    Post Precipitation
Foreign compound precipitates on top of the desired precipitate. For example, post precipitation of magnesium oxalate occurs if a precipitate of calcium oxalate is allowed to stand too long before being filtered.   

 3 types of coprecipitation:
Surface adsorption
Mixed–crystal formation
Occlusion and mechanical entrapment
Surface Adsorption
 A process in which a normally soluble compound is carried out of solution on the surface of a precipitate.
 Adsorption is a reversible process because it is accompanied by the opposite process of desorption.
The two opposing processes lead to a state of dynamic equilibrium known as adsorption equilibrium.

13.The position of the adsorption equilibrium depends on numerous factors;
 Effects of Surface Area
The amount of a substance adsorbed is directly proportional to the total surface area of the adsorbent.
 Effect of Concentration
Adsorption of ions increases with an increase of their concentration (not proportional).
 Effect of Temperature
Adsorption is an exothermic process. Rise of temperature means decreased adsorption.
 Effects of the Nature of the Adsorbed Ions
Adsorbents with ionic crystal lattices prefer to adsorb ions, which form sparingly soluble or common ions with the precipitate. For example, BaSO4 precipitate prefers to adsorb its own common ions, Ba2+ or SO42- dependent on which is present in the solution in excess.

14.    Occurs with all precipitates especially which has a very large surface area like colloidal particle.
 The surfaces of the precipitate contain some primary adsorbed ion, either the lattice cation or the lattice anion (the excess lattice ion).
 If AgCl is precipitated by the addition AgNO3 to excess NaCl, Cl? will be adsorbed on the precipitate surface.
 The lattice ion adsorbed is called the primary layer or primary adsorbed ion. The surface of the precipitate has a minus charge (because of the Cl?).
 The particles carry the same charge on the surface and because of that they repel each other and will not easily coagulate to form larger particles.
 To balance this charge, the adsorbed ions will also attract an ion of opposite charge (called the counter ion) such as Na+, which then surround the precipitate particles.
 For a silver halide precipitate, AgX, two cases are possible.

15.If nitrate is co-precipitated, the results will  be too high  because nitrate weights more than chloride.
 A lower weight counter ion will result  in the weight of the precipitate being too low.

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