27 Questions
What is the main principle behind gravimetric methods of analysis?
Determining the mass of a pure compound related to the analyte
How is lead (Pb+2) determined in water using gravimetric analysis?
By converting Pb+2 to PbCl2 and measuring its mass
Which classification of gravimetric analysis involves separating the analyte as a gas?
Volatilization method
What type of method involves separating the analyte by deposition on an electrode?
Electro-analytical method
Which property is NOT associated with gravimetric analysis?
Quick results
In precipitation gravimetry, how is the analyte typically converted for measurement?
To a sparingly soluble precipitate
What type of analysis involves changing the phase of the analyte to separate it in its pure form?
Gravimetric analysis
In gravimetric analysis, what is the purpose of digestion?
To convert the precipitate to a known composition
What is the purpose of adding oxalic acid and then ammonia in the calcium precipitation method?
To neutralize the sample
Why are precipitates consisting of large particles preferred in gravimetric work?
Because they are easier to wash
What is the purpose of igniting the dried precipitate in gravimetric analysis?
To convert the precipitate to a pure compound
What distinguishes colloidal suspensions in terms of particle size?
They have tiny particles invisible to the naked eye
Why is it important for a gravimetric precipitating agent to be selective or at least specific?
To ensure accurate quantification of the analyte
What is the role of ammonia in the calcium precipitation method?
To neutralize excess oxalic acid
Why are particles formed by nucleation often smaller than those formed by particle growth?
Because nucleation is faster than particle growth
What happens if particle growth is faster than nucleation during precipitation?
Only a few large particles form
What should a good gravimetric precipitate ideally have in terms of solubility?
Sufficiently low solubility to prevent analyte loss during filtration
What experimental variable influences the particle size of a precipitate?
Reactant concentrations
How does the relative supersaturation of the solution relate to the particle size of the precipitate?
Inversely proportional
What is the role of effective stirring in controlling particle size during precipitation?
Preventing local excesses of reagent
Which factor helps in keeping the relative supersaturation ratio low to promote particle growth?
Decreasing solute concentration
What is recommended in order to experimentally control particle size when precipitating from a solution?
Precipitate from dilute solution
How does the size of particles in a crystalline suspension compare to the dimensions mentioned in the text?
Particles are larger
What effect does filtering have on a crystalline suspension?
Promotes settling
What is the relationship between particle growth and the relative supersaturation ratio?
Inversely related
What does adding dilute precipitating reagents slowly help prevent during precipitation?
Local excesses of reagent
What characteristic of the solution plays a role in controlling the particle size?
Relative supersaturation
Study Notes
Gravimetric Methods of Analysis
- Gravimetric methods are quantitative methods that determine the mass of a pure compound related to the analyte.
- Example: Determining lead (Pb+2) in water by adding excess Cl- to precipitate PbCl2, then determining the mass of PbCl2 to calculate the amount of Pb+2 in the original solution.
Classifications of Gravimetric Methods
- Precipitation method: separating the analyte from a solution as a precipitate, converting it to a compound of known composition, and weighing it.
- Volatilization method: separating the analyte from other constituents by converting it to a gas of known chemical composition.
- Electro-analytical method: separating the analyte by deposition on an electrode using an electrical current.
Properties of Gravimetric Analysis
- Traditional method
- Accurate
- Cheap and easily available apparatus
- Simple to carry out
- Wide range of sample concentrations (ng-kg)
- No calibration required (except for the balance)
- Slow, especially when accurate results are required
Precipitation Gravimetry
- Precipitation gravimetry involves converting the analyte to a sparingly soluble precipitate, filtering, washing, and weighing the product.
- Example: determining calcium in water by adding oxalic acid, precipitating calcium oxalate, and converting it to calcium oxide by heat treatment.
Steps in Precipitation Gravimetric Analysis
- Preparation of the solution
- Precipitation
- Digestion
- Filtration
- Washing
- Drying or igniting
- Weighing
- Calculation
Mechanism of Precipitate Formation
- Nucleation: forming a stable solid from a few ions, atoms, or molecules
- Particle growth: adding molecules to a nucleus to form a crystal
- Factors affecting particle size: nucleation rate, particle growth rate, and supersaturation
Properties of Precipitating Reagents
- Ideally, reagents should react specifically or selectively with the analyte
- Example: AgNO3 is a selective reagent for 𝐀𝐠+𝟏; DMG is specific to Ni+2
Properties of Good Precipitates
- Easy to filter and wash, free of contaminants
- Sufficiently low solubility to prevent significant loss during filtration and washing
- Unreactive with atmospheric constituents
- Known chemical composition after drying or igniting
Particle Size and Filterability of Precipitates
- Large particles are desirable for gravimetric work because they are easy to filter, wash, and are usually purer
- Factors affecting particle size: nucleation rate, particle growth rate, temperature, reactant concentrations, and mixing rate
Factors Affecting Particle Size
- Precipitate solubility
- Temperature
- Reactant concentrations
- Mixing rate
- Supersaturation ratio
Learn about quantitative methods based on determining the mass of a pure compound related to the analyte, with an example being the determination of lead (Pb+2) in water by precipitating PbCl2. Explore the classifications of gravimetric methods that involve changing the phase of the analyte for separation.
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