Lesson 7: Precipitation and Volatilizatio Gravimetry Flashcards
A substance that is dissolved in a solution
Solute
the liquid in which asoluteisdissolvedto form a solution.
Solvent
a solution that contains less than the maximum amount of solute that is capable of being dissolved
Unsaturated Solution
a solution that has dissolved as much solute as it is capable of dissolving
Saturated Solution
interaction between solute and solution that forms ionic bond or precipitate
Precipitate Forms
a solution that contains more than the maximum amount of solute that is capable of being dissolved at a given temperature.
Supersaturated Solution
an Unstable solution that contains a higher solute concentration than a saturated solution
supersaturated solution
a quantitative method that is based on determining the mass of a pure compound to which the analyte is chemically related.
It is based on mass measurements with an analytical balance
Gravimetric Methods
the analyte is separated from a solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed.
Precipitation Gravimetry
the method in which the analyte is converted to a soluble precipitate
Precipitation Gravimetry
can be obtained by filtering, washing free of impurities, converting to a product of known composition by suitable heat treatment, and weighing.
precipitate
What are the 4 properties of precipitating agents
(1) easily filtered and can be washed free of contaminants
(2) of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing;
(3) Chemical inert or is unreactive with the constituents of the open atmosphere;
(4) Composition can be determined with high confidence after it was dried or ignited.
a few ions, atoms, or molecules (around four or five) come together to form a stable solid. Often, these nuclei form on the surface of suspended solid contaminants, such as dust particles.
Nucleation
two ways at which precipitates are formed
Nucleation and particle growth
If predominates, a precipitate containing a large number of small particles results
nucleation
if predominates, a smaller number of larger particles is produced.
particle growth
competition between additional nucleation and growth of existing nuclei
particle growth
minimum number of atoms, ions, or molecules join together to give a stable solid
Nucleation
As excess solute precipitates with time, supersaturation decreases to zero
supersaturated solution
coagulation of colloids can be hastened by what methods?
- Heating
- continuously stirring
- by adding an electrolyte into the mixture that will act as a medium
What is the reason for the stability of colloidal particles?
its particles are either positively or negatively charged and thus repel one another.
a process in which a substance (gas, liquid, or solid) is held on the surface of a solid.
adsorption
retention of a substance within the pores of a solid.
absorption
random movement of solute in solution
Brownian Motion
Positive charge
Cation
negative charge
Anion
happens when colloids are washed, because during washing, some of the electrolytes responsible for keeping the coagulated form are leached from the internal liquid in contact with the solid particles.
Peptization
process by which a coagulated colloid reverts to its original dispersed state.
Peptization
How to prevent the the effect of increasing the volume of the counter-ion layer in peptization?
solvent that can easily volatilize is used for washing, so that during the drying step, this solvent is removed from the precipitates.
What happens when electrolyte is removed in Peptization?
has the effect of increasing the volume of the counter-ion layer.
are particles with diameters ranging from order of tenths of a millimeter or greater.
crystalline suspension
of which the particles are too small (10-7 to 10-4( in diameter) that they are invisible to the naked eye.
colloidal suspensions
suspended within the mixture, they show no tendency to settle thus making them difficult to filter
colloidal suspensions
The particles of a crystalline suspension tend to settle spontaneously and are easily filtered.
crystalline suspension
the phenomenon in which the particles in a colloid scatter the beams of light that are directed at them
Tyndall effect
Factors That Determine the Particle Size of Precipitates
Precipitate Solubility
Temperature
Reactant Concentrations
Rate At Which Reactants Are Mixed
equation for relative supersaturation
Q-S / S
What is meant by Q in the formula of relative supersaturation?
the concentration of the solute
What is meant by S in the formula of relative supersaturation?
the equilibrium solubility
if (q-s/s) is large, the precipitates will tend to be _____?
colloidal
when (q-s/s) is small, then we can expect that the precipitates will be____?
crystalline in nature
How to improved the particle size of a crystalline solid?
by minimizing Q or maximizing S, or both in equation.
How to minimize Q the particle size of a crystalline solid ?
diluting the solution and adding the suitable precipitating agent slowly accompanied by good mixing should be done.
a process in which a precipitate is heated in the solution from which it was formed (the mother liquor) and allowed to stand in contact with the solution.
Digestion
How to maximize S the particle size of a crystalline solid?
S is increased by precipitating from hot solution or by adjusting the pH of the precipitation medium.
the solution from which a precipitate was formed
Mother liquor
a normally soluble compound is carried out of solution on the surface of a coagulated colloid.
Surface Adsorption
often the major source of contaminants in coagulated colloids
but this problem doesn’t arise on crystalline precipitates.
Adsorption
Explain how digestion improved the purity of many coagulated colloids.
water is expelled from the solid to give a denser mass that has a smaller specific surface area for adsorption
How to improved the purity of many coagulated colloids?
Digestion
This compound consists of the primarily adsorbed ion and an ion of opposite charge from the counter-ion layer.
Surface Adsorption
the filtered solid is re-dissolved and reprecipitated
Reprecipitation
water is expelled from the solid to give a denser mass that has a smaller specific surface area for adsorption.
Reprecipitation
contaminant ions replace an ion in the lattice of a crystal
mixed crystal formation
adds substantially to the time required for an analysis.
Reprecipitation
Requirement for the exchange in Mixed-Crystal Formation
ions should have the same charge and their ionic sizes should differ in not more than 5%.
When a crystal is growing rapidly during precipitate formation, foreign ions in the counter-ion layer may become trapped, or occluded, within the growing crystal.
Occlusion
occurs when crystals lie close together during growth. Several crystals grow together and in so doing trap a portion of the solution in a tiny pocket.
Mechanical entrapment
are at a minimum when the rate of precipitate formation is low, that is, under conditions of low supersaturation
occlusion and mechanical entrapment
There are four types of coprecipitation:
- surface adsorption,
- mechanical entrapment,
- occlusion
- mixed-crystal formation
a reaction that involves the simultaneous occurrence of nucleation, growth, coarsening, and/or agglomeration processes.
Coprecipitation
a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth.
Occlusion
a process in which normally soluble compounds are carried out of solution by a precipitate
Coprecipitation
a process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution.
Homogeneous precipitation
Two types of Gravimetric Analysis
Precipitation Method and Volatilisation method
may occur in both colloidal and crystalline precipitates, but occlusion and mechanical entrapment are confined to crystalline precipitates.
Mixed-crystal formation
nvolve the generation of a precipitating agent from a chemical reaction carried out slowly.
Precipitation in homogeneous solutions
are better suited for analysis than a solid formed by direct addition of a precipitating reagent.
both colloidal and crystalline
analysis of an ion by measuring mass of a precipitate formed from the ion
Precipitation Method
measuring mass before and after heating and removal of a volatile substance
Volatilisation method
