Gravimetric Analysis 1-2 Flashcards
Are quantitative methods based on determining the mass of a pure compound to which the analyte is chemically related.
GRAVIMETRIC METHODS
What are the four types of gravimetric methods?
- Precipitation gravimetry
- Volatilization gravimetry
- Electrogravimetry
- Gravimetric titrimetry
It is the most common type of gravimetry. 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 analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition. The mass of gas serves as a measure of the analyte concentration.
VOLATILIZATION GRAVIMETRY
The analyte is separated by deposition on an electrode by an electric current and the mass of this product provides a measure of the analyte concentration.
ELECTROGRAVIMETRY
The mass of the reagent unknown concentration required to react completely with the analyte provides the information needed to determine the analyte concentration.
GRAVIMETRIC TRITRIMETRY
The analyt is converted to a sparingly soluble precipitate
PRECIPITATION GRAVIMETRY
The precipitate is filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment and weighed.
PRECIPITATION GRAVIMETRY
This agent of precipitation gravimetry should ideally react specifically or selectively with the analyte.
GRAVIMETRIC PRECIPITATING AGENT
In gravimetric precipitation agent of precipitation gravimetry, what kind of reagents are rare and react only with a single chemical species?
SPECIFIC REAGENTS
In gravimetric precipitation agent of precipitation gravimetry, what kind of reagents are more common and react with a limited number of species?
SELECTIVE REAGENT
Precipitating agent that would react with analyte to give a product that is **easily filtered and washed free of contaminants, low solubility, unreactive with constituents of the atmosphere, and of known chemical composition after dried or ignited.
IDEAL PRECIPITATING AGENT
What is the preferable size of particles, large or small. IT MUST BE EASY TO FILTER AND MUST BE PURER THAN FILTRATES.
LARGE PARTICLES
What are the two types of precipitates?
Colloidal and crystalline
Solid particles with less than 10^-14cm in diameters
Colloid
Tiny particles invisible to naked eye (10^-7 to 110^-4cm)
Colloidal suspension
Show no tendency to settle and difficult to filter
Colloidal particles
Colloidal suspensions may be perfectly clear in diffuse light but the presence of the second phase can be detected by shining a flashlight into the solution.
Tyndall effect
It is larger than a colloid. It describes a solution in which particles with dimensions on the order of a tenth of a millimeter or greater are temporarily dispersed in the liquid phase. The particles *tend to settle+ and easily filtered
Crystalline suspension
It is an unstable solution that contains a higher solute concentration than saturated solution. Solutions are likely because precipitation reactions are so low.
SUPERSATURATED SOLUTION
Calculates relative supersaturation. Lower value means crystalline precipitate. Higher value means colloidal precipitate.
Von Weimarn Ratio Q-S/S
The process by which a minimum number of atoms, ions, or molecules join together to give a stable solid.
Nucleation
Particle grows both by
Additional nucleation and growth of existing particles.
If _______ predominates, the resulting precipitate contains many small particles. If ______ predominates, the resulting precipitate contains large particles.
Nucleation, growth
Factors that minimize supersaturation and thus produce crystalline growth include:
Elevated temperatures, dilute solutions, slow addition of precipitating agent
TRUE OR FALSE: If solubility of the precipitate depends on pH, larger particles can be produced by controlling pH.
TRUE
Many precipitates cannot be formed as crystals under ____________.
Laboratory conditons
Can be hastened by heating, stirring, and by adding in electrolyte to the medium
Coagulation
A process by which a substance is held on the surface of a solid.
ADSORPTION
Is the retention of a substance within the pores of a solid.
Absorption
Is the attached directly to the solid surface.
Primary adsorption layer
Surrounds the charged particle
Counter-ion layer
The primarily adsorbed ions and the negative counter-ion layer constitute an ____________ that imparts stability to the colloidal suspension.
ELECTRIC DOUBLE LAYER
The higher the ______________, the thinner the distance.
electrolyte concentration
Thick electrical double layer keep them too far apart for the coagulation to occur
Concentrated solution
They can approach more closely, allowing a coagulated precipitation to appear.
Dilute solution
They can approach more closely, allowing a coagulated precipitation to appear.
Dilute solution
Coagulated colloid reverts to its original disperse state.
Peptization
Removes some electrolyte
washing
Is the process in which a precipitate is heated in the solution from which it was formed(the mother liquor) and allow to stand in contact with the solution
Digestion
How to improve the filter ability of the coagulated colloid.
Stand for an hour
Generally more easily filtered and purified than coagulated colloid.
Crystalline precipitates
Generally more easily filtered and purified than coagulated colloid.
Crystalline precipitates
To improve the particle size of crystalline solids, you must
minimize Q, maximize S, or both in the von Weimarn ratio
Is the process in which normally soluble compounds are carried out of the solution *by a precipitate.
Coprecipitation
- Surface adsorption
- Mixed crystal formation
- Occlusion
- Mechanical entrapment
Four types of Precipitation
Common and can cause significant contamination of precipitates with large specific surface areas
Surface Adsorption
A contaminant replaces an ion in the lattice of a crystal
Mixed crystal formation
A compound is trapped within a pocket formed during rapid crystal growth.
occlusion
Crystals grow together and trap a portion of the solution in a tiny pocket.
Mechanical entrapment
Can be used to improve the purity of many coagulated colloids
Digestion
The filtered solid is redissolved and reprecipitation to reduce the contaminant concentration.
reprecipitation
Coprecipitated impurities lead to a _______ if the contaminant is not a compound of the ion being determined.
Positive error
A compound contaminant may produce ______ error
Positive or negative
Is a process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution.
Homogenous precipitation
Homogenous precipitation of crystalline precipitates results in
Increase in crystal size and improvements in purity
A gravimetric precipitate is heated until its mass becomes ______.
constant
Removes the solvent and any volatile species.
Heating
Some precipitates are ignited to decompose the solid and produce a compound of known composition called the ________.
Weighing form
Is the process of recording thermal decomposition curves.
Thermogravimetric analysis
Are the mass versus temperature curves
thermograms
The combined constant factors in a gravimetric calculation are called the
Gravimetric factor
