Experiment 14 -part 2
Introduction to Homogeneous precipitation,
Gravimetry and their applications in Analytical
Chemistry
Name
Student ID
：A.G. Odeesha Kumari
：FGS/MSc/IEC/2021/66
[2022.06.18]
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Objectives;
1. To illustrate the principle of homogeneous precipitation
2. Benefit of using a masking agent.
3. The effect of pH changes for generating ligands slowly
Introduction
Precipitation Gravimetry
In precipitation gravimetry, the analyte is converted to a sparingly soluble precipitate. This precipitate
is then filtered, washed free of impurities, converted to a product of known composition by suitable
heat treatment, and weighed.
Properties of Precipitates and Precipitating Reagents
Ideally, a gravimetric precipitating agent should react specifically or at least selectively with the
analyte. Specific reagents, which are rare, react only with a single chemical species. Selective
reagents, which are more common, react with a limited number of species. In addition to specificity
and selectivity, the ideal precipitating reagent would react with the analyte to give a product that is
1. easily filtered and washed free of contaminants
2. sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing
3. unreactive with constituents of the atmosphere
4. of known chemical composition after it is dried or, if necessary, ignited
Particle Size and Filterability of Precipitates
Precipitates consisting of large particles are generally desirable for gravimetric work because these
particles are easy to filter and wash free of impurities. In addition, precipitates of this type are usually
purer than are precipitates made up of fine particles.
Factors That Determine the Particle Size of Precipitates
The particle size of solids formed by precipitation varies enormously. At one extreme are colloidal
suspensions, whose tiny particles are invisible to the naked eye (10-7 to 10-4 cm in 2 diameter).
Colloidal particles show no tendency to settle from solution and are difficult to filter. At the other
extreme are particles with dimensions on the order of tenths of a millimeter or greater. The temporary
dispersion of such particles in the liquid phase is called a crystalline suspension. The particles of a
crystalline suspension tend to settle spontaneously and are easily filtered.
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The particle size of a precipitate is influenced by
• Precipitate solubility
• Temperature
• Reactant concentrations
• The rate at which reactants are mixed.
Precipitation from Homogeneous Solution
Homogeneous precipitation is a process in which a precipitate is formed by slow generation of a
precipitating reagent homogeneously throughout a solution. Solids formed by homogeneous
precipitation are generally purer and more easily filtered than precipitates generated by direct addition
of a reagent to the analyte solution.
Procedure
(A) Standardization of a KMnO4 solution
50 ml of suitable primary standard was prepared (oxalic acid, sodium oxalate, potassium tetraoxalate) and
estimated the concentration of the KMnO4 solution was provided.
(B) Determination of the concentration of Ca+2 ions using complexometric titration.
About 100ml of approximately 0.05M Ca+2 ion solution was prepared using CaCl2, Use10.0ml of this
solution and determined the concentration of Ca+2 using a complexometric titration.
(C) Determination of the concentration of Ca+2 ions using homogeneous precipitation.
Pipetted 25 ml of the 0.05M Ca+2 ion solution was prepared to a 500 ml beaker and diluted up to 130ml with
water. 2M HCl, 15 ml solution was added to adjusted the pH approximately 1.0. The solution was stirred well
using a glass rod. 15 ml of Saturated Ammonium Oxalate was added with few drops of methyl red indictor.
About 7.0 g of urea was added and stirred the solution again. The solution was boiled within 20-60 min. until
the solution become yellow. The formed precipitate was filtered through a Whatmann No. 1 filter paper. The
precipitate was washed with cold 0.1% ammonium oxalate solution and finally with little cold water. The
precipitate was transferred to a titration flask using 25 ml of warm 1M sulfuric acid and diluted to 50 ml with
distilled water. The oxalated ion concentration was estimated by titrating with standardized potassium
permanganate solution.
Results and Discussion
CaCO3 (s) + 2H+(aq) Ca2+(aq) + H2O (l) + CO2 (g) (1)- Dissolution of Ca
solution in acidic medium
(H2N)2CO (aq) + H2O(l)
Heat
2NH3 (aq) + CO2 (g)
(2)
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Very slow reaction imperceptible at room temperature and neutral pH.
Therefore to increase the rate, heat and facilitate the acidic medium. This kind of reaction is carried
invivo by the facilitation of enzyme.
2NH3(aq) + H2C2O4 (aq)
2NH4+ (aq) + C2O42- (aq) (3)
Slow production of ammonia from the reaction 3 caused the slower production of precipitate. To
increase the reaction rate facilitate the Urea.
Ca2+ (aq) + C2O42- (aq) + H2O (l)
CaC2O4H2O (s) (4)
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