Gravimetry Flashcards
Describe precipitation gravimetry
- Analyte is converted to sparingly soluble (low solubility) precipitate
- The precipitate will be filtered and washed free of impurities
- Converted to a product of known composition by suitable heat treatment
- Weighed
precipitate > digestion > filter > washed > heat > weighed
Good precipitating reagent should be ____ and would react with the analyte to give a product that is ____ (4)
Specific or selective
- easily filtered and washed free of contaminants
- low solubility
- unreactive with atmosphere (inert)
- known chemical composition after drying or ignition
Why large precipitates are generally desirable in gravimetric analysis
- easy to filter and wash free of impurities
- usually purer
How to detect colloidal suspensions?
By Tyndall effect - scattering of light
How to filter colloidal suspensions?
Particles need to coagulate to produce larger particles
Temporary dispersion that is easier to filter
Crystalline suspension
Particle size of a precipitate is influenced by (4)
- Precipitate solubility
- Temperature
- Reactant concentrations
- Rate of mixing
The net effect of the variables that affect particle size can be accounted for, at least qualitatively, by assuming
that the particle size is related to a single property of the system called ______
relative supersaturation
Formula for relative supersaturation
RS = (Q-S)/S
Q = conc. of solute
S = equilibrium solubility
High relative supersaturation = ____ suspensions
Low relative supersaturation = ____ suspensions
high RS = colloidal
low RS = crystalline
As excess solute precipitates with time, supersaturation decreases to ____
zero
To increase the particle size of a precipitate, minimize the ___ during precipitate formation.
relative supersaturation
Precipitates can be formed in two ways: ____. Explain each
- Nucleation
- few ions, atoms, or molecule come together to form a stable solid
- form on the surface of suspended solid contaminants, such as dust particles - Particle Growth
- growth of existing nuclei
If nucleation predominates, a ____ particles is produced.
If particle growth predominates, a ____ is obtained.
nucleation = large number of small precipitates
particle growth = smaller number of larger particles
The rate of nucleation is believed to ____ enormously with increasing relative supersaturation.
Explain
increase
at high RS, nucleation happens, large number of small particles formed
How to minimize supersaturation to produce crystalline precipitates? (4)
- increase temp, increase solubility
- dilute solution
- slow addition of precipitating agent with good stirring
*4. controlling pH if the solubility of precipitate depends on pH
Precipitates that have ___, such as many sulfides and hydrous oxides,
generally form as colloids.
very low solubilities
Coagulation of colloidal precipitates can be hastened by ___
- heating
- stirring
- adding an electrolyte
Colloidal suspensions are stable because all of the particles of the colloid are _____ and thus _____
either positively or negatively charged
repel one another
Differentiate adsorption and absorption.
Adsorption is a process in which a
substance (gas, liquid, or solid) is held on the surface of a solid.
Absorption is retention of a substance within the pores of a solid.
The charge on a colloidal particle formed in a gravimetric analysis is determined by the ____ that is in
excess when the precipitation is complete.
charge of the lattice ion
Differentiate primary adsorption layer, counter-ion layer, and electric double layer
Primary adsorption layer - attached directly to the solid surface
Counter-ion layer - surrounding the charged particle that balances the charge on the surface of particle
electric double layer - primary adsorption layer + counter-ion layer
The effective charge of colloidal particles ___ and reaches ___ as the distance from surface increases.
Explain
decreases and zero
It is caused by the counter-ions in the double layer, neutralizing the effective charge
How increasing electrolyte concentration helps coagulation?
It decreases the distance between two particles by increasing the concentration of counter-ions which decreases the volume of counter-ion layer.
How heating and occasional stirring helps coagulation?
- decreases the number of adsorbed ions and thus the thickness of the double layer
- gain enough kinetic energy to overcome the barrier to close approach imposed by the double layer
A process by which
a coagulated colloid return to its dispersed state
peptization
How and when peptization usually happens?
When a coagulated colloid is washed, some electrolyte will also be washed which increases the volume of the counter-ion layer and repulsive forces will be reestablished.
How peptization is prevented since washing is crucial in ensuring the purity of the precipitate?
The precipitate will be washed with a solution with electrolyte that can be volatilized (removed) during drying or igniting step.
Describe the digestion process and the result.
Digestion 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
The result is a denser mass that is easier to filter since weakly bound water is lost from the precipitate.
Usually used for colloidal precipitate (to eliminate surface adsorption) and crystalline precipitate (dissolution and recrystallization)
After precipitate formation, how to yield a purer, more filterable product?
digestion of crystalline precipitate.
The dissolution and recrystallization that occurs continuously improves the filterability
A process in which
normally soluble compounds are carried out of solution by a precipitate
coprecipitation
Four types of coprecipitaion
- Surface adsorption
- mixed-crystal formation
- occlusion
- mechanical entrapment
Why adsorption is not a major source of contamination in crystalline precipitates?
Crystalline precipitates have low surface area in contrast with colloidal precipitates
Net effect of surface adsorption is ____
the carrying down of an otherwise soluble compound as a surface contaminant.
Normally soluble impurity is adsorbed in the surface
For a given mass of solid, the specific surface area increases dramatically as particle size ____
decreases
A drastic but effective way to minimize the effects of adsorption is ___.
Explain
reprecipitation
the filtered solid is redissolved and reprecipitated
A type of coprecipitation in which a contaminant ion replaces an ion in the lattice of a crystal
Mixed-crystal formation (inclusion)
Mixed-crystal formation happens when ___ (3)
- two-ions have the same charge
- sizes differ by no more than about 5%
- two salts must belong to the same crystal class
The extent of mixed-crystal contamination is governed by the _____ and increases as the ____ increases.
law of mass action
ratio of contaminant to analyte concentration
How to prevent mixed-crystal formation?
- different precipitating reagent must be used
- interfering ion may have to be separated before final precipitation step
type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth
occlusion (mechanical entrapment)
How din occlusion happens? Where is the occluded material greatest?
Happens when a crystal is growing rapidly. (only occurs in crystalline precipitates)
In the part of crystal that forms first
How to prevent occlusion and mechanical entrapment? (2)
- Low rate of precipitate formation (low supersaturation)
- Digestion - recrystallization open up the pockets and allow impurities to escape
Explain precipitation from homogeneous solution
the precipitating agent will be generated/added to solution gradually (homogeneously) keeping a low relative supersaturation
Differentiate
1. Particulate Gravimetry
2. Volatilization Gravimetry
3. Precipitation Gravimetry
4. Electro-gravimetry
- Particulate Gravimetry
- filtration or extraction - Volatilization Gravimetry
- conversion to a gas - Precipitation Gravimetry
- converted to a precipitate whose weight is determined - Electro-gravimetry
- deposition on an electrode by an electrical current
