Gravimetric Analysis Lecture Notes

Flashcards covering key concepts, terms, and definitions related to gravimetric analysis as discussed in the lecture notes.

Gravimetric Analysis

Quantitative methods based on determining the mass of a pure compound related to the analyte.

Precipitation Gravimetry

An analyte is separated as a precipitate and converted to a compound of known composition that can be weighed.

Volatilization Gravimetry

An analyte is converted to a gas of known chemical composition; the mass of gas is used to measure analyte concentration.

Electrogravimetry

The analyte is separated by deposition on an electrode using an electrical current.

Gravimetric Titrimetry

Uses mass of a reagent with known concentration to completely react with the analyte.

Mass Spectrometry

Technique that uses a mass spectrometer for analysis of compounds.

Precipitating Reagents

Substances that react with the analyte to form a solid precipitate.

Coagulation

Process by which colloidal particles are agglomerated, often aided by heating, stirring, or adding an electrolyte.

Colloidal Precipitates

Precipitates that are too small to be retained by ordinary filters and are stabilized by charged particles.

Crystalline Precipitates

Easily filtered and purified solids with controlled particle size.

Coprecipitation

Removal of soluble compounds during the formation of a precipitate.

Nucleation

The initial process where a minimum number of atoms, ions, or molecules join to form a stable solid.

Particle Growth

Process where dissolved solute deposits on existing nuclei, increasing their size.

Relative Supersaturation (Q - S)/S

• Measure of how much solute is available in solution compared to its equilibrium solubility.

• the way net effect is being accounted by

Peptization

The process wherein a coagulated colloid reverts to its original dispersed state.

Ignition of Precipitates

Heating that decomposes the precipitate to form a compound of known composition.

Specifically - Selectively

Ideally a precipitating agent should react ________ or at least _______ with the analyte.

Specific Reagents

• rare

Selective Reagents

• react with a limited number of species

Large Particles

• are easy to filter

• easy to wash free of impurities, usually purer than fine particles

Colloidal Suspension

• 10^-7 to 10^-4 cm in diameter, do not settle from solution

• difficult to filter

Crystalline Suspension

• size is less than or equal to 0.1 mm, tend to settle spontaneously

• easily filtered

Tyndall Effect

• when light is scattered due to colloidal particles

Colloidal

When (Q-S)/S is large, precipitate tends to be _________

Crystalline

When (Q-S)/S is small, precipitate tends to be _________

Q

Used in the formula for Relative Supersaturation:

• is the concentration of solute at any instant

S

Used in the formula for Relative Supersaturation:

• equilibrium stability

Nucleation and Particle Growth

What are the two mechanisms of precipitate formation?

Nucleation

If _________ predominates, a precipitate containing a large number of small particles is produced

Particle Growth

If ___________ predominates, a smaller number of large particles is produced

Particle Growth

Molecular Visualization:

Nucleation

Molecular Visualization:

Elevated Temperature

• increase solubility of the precipitate

Brownian Motion

• prevents gravity settling

Washing

• minimizes contamination;

• it also removes some of the electrolyte responsible for coagulation, thus increasing the volume of the counterion later, reestablishing the repulsive forces

Filterability

• this is characterized by a liquid or substance's ability to be filtered

  • is improved if allowed to stand for an hour with the hot solution

Mother Liquor

• is the hot solution which will be used to improve the filterability of a solution

  • is the original solvent from where the precipitates where formed

Digestion

• is the process which improves the purity and concentration of the precipitate by increasing the particle size of the precipitate

Minimizing Q

Improving the sizes of the particles:

• use dilute solutions, add precipitating reagent slowly, good mixing

Increasing S

Improving the sizes of the particles:

• precipitating from hot solution, adjusting pH of precipitation medium

Surface Adsorption, Mixed-Crystal Formation, Occlusion, Mechanical Entrapment

Four Types of Coprecipitation

Contamination

• is not the same as coprecipitation (is broader)

• a second substance precipitates because its Ksp has been exceeded

Surface Adsorption

• major source of contamination in coagulated colloids

• not significant in crystalline precipitates due to small surface area

Surface Adsorption

• is a normally soluble compound; i.e. an adsorbed ion with the opposite charge of the counterion layer, is carried out from solution on the surface of a coagulated colloid, which has a large surface area

Mixed-Crystal Formation

• one of the ions in the crystal lattice of a solid is replaced by an ion of a different element

Occlusion

• compounds are trapped within a pocket formed during rapid crystal growth;

• amount of this material is greatest in the part of a crystal that forms first

Mechanical Entrapment

• happens when crystals lie close together during growth, trapping a portion of the solution in a pocket

Occlusion and Mechanical Entrapment

Both _________ and ___________ happen with crystalline precipitates.

Positive error

Coprecipitated impurities:

• if contaminant is not a compound of the ion determined _________ will result

Either positive or negative errors

Coprecipitated impurities:

• when the contaminant contains the ion being determined

Heating

• removes solvent and any volatile species carried down with the precipitate

Ignition

• decomposes the precipitate to form a compound of known composition, called the weighing form