Unit 2 Flashcards
what are the most common separation techniques in the lab?
extractions
describe extractions
- allows for the transfer of the analyte(s) of interest from the sample (liquid or solid) to another phase not miscible with the first one
what is the principle behind extractions
based on the partition/distribution of the analyte between the two phases
depends on affinity with both phases
what are 2 major types of extractions?
what techniques are an example of these?
solvent extraction:
- liquid-liquid
- solid-liquid
- SFE
- LPME
solid-phase
- SPE
- SPME
brewed coffee is an example of what type of extraction?
solid-liquid extraction
describe liquid-liquid extraction (LLE)
occurs when a solute is partitioned between 2 immiscible liquids and the system is allowed to attain a thermodynamic equilibrium
in LLE, the chemical of interest is ____
solute
in LLE, what is left after the extraction?
raffinate
what are the two phases in LLE?
phase 1: raffinate
phase 2: extractant
what relationships describe the distribution of the solute between the 2 phases in LLE?
what type of constants are these?
- partition constant
- partition ratio/distribution constant
they are thermodynamic constants
define solvent
substances (usually liquids) that acts as a medium for the dissolution of solvents
define solvation
when a solute is dissolved in a solvent and the solvent molecules arrange themselves around molecules of the sulte
what does solvation involve?
- bond formation
- H bonding
- vander Waals forces
how does solvation affect entropy and thermodynamic stability of the solution?
increases entropy and thermodynamic stability
describe non-polar solvents
solvents that consist of molecules with low or no net dipole moments
describe polar solvents
solvents with large dipole moments
how are dipoles derived
from chemical bonds between atoms with large differences in electronegativity
results in a molecule with uneven distribution of the bonding electron cloud about nuclei in the molecule
in LLE, the ____ (larger/smaller) the interface area is, the faster the extraction will take place
larger interface area = faster extraction
how can you increase the interface for the mass transfer (ie. create a faster extraction)
what should you be careful of
vigorous shaking of the spearatory funnel
be careful of pressure build up
k’ is what?
capacity factor
beta is what?
phase ratio
what is the fundamental equation for separation methods driven by equilibrium?
Kd = k’ x beta
what is distribution ratio (D)?
the ratio of total analytical concentration of a solute in the extract (regardless of its chemical form) to its total analytical concentration in other phases
D = C(A phase 2) / C (A phase 1)
describe dominant chemical species
when pH < pKA
efficiently extracted in organic solvent
what is the non-dominant species
when pH > pKa
less efficiently extracted in organic solvent
what is LPME?
liquid phase microextraction
what are 3 types of LPME?
- single drop microextraction
- hollow fiber LPME
- dispersive liquid-liquid mic
what was the soxhlet extraction first used for? by who?
determination of milk fat by Franz von Soxlet
describe soxhlet extraction
- method which minimizes the use of extraction solvent by circulation
- thermally labile volatile analytes may be lost
- potential interference from contaminants from extraction thimbles; may necessitate a blank extraction prior to actual sample extraction
describe pressurized liquid extraction (PLE)
- involves use of pressurized liquids/fluids for extraction
- uses elevated temp and pressures
- allows extraction solvent to penetrate sample matrix, thus enhancing solvation of solute to be extracted
in PLE, what are 2 types of extraction?
- solubility controlled
2. diffusion controlled
what are 3 factors affecting extraction in PLE?
- solvent type
- temp
- time
in PLE, what are 3 extraction enhancers?
- drying agents
- dispersing agents
- other additives
how is microwave energy used in microwave assisted solvent extraction
to heat molecules in the sample (notably water) by ionic conduction and dipole rotation
how does microwave assisted solvent extraction (MASE) work?
within in closed vessel systems:
- heated water expands and generates pressure within the sample
- increased pressure disrupts sample matrix from inside
- analytes escape the matrix and be dissolved in the extraction solvent
how does the use of solvent in MASE compare with conventional extraction methods?
- MASE uses less solvent
- solvent used maybe be microwave absorbing or non-microwave absorbing
what is SFE?
supercritical fluid extraction
how is a supercritical fluid formed?
- when a substance is heated above its critical temp/point
- fluid would have new properties different from the gas and liquid state
how can the “solvent power” of a supercritical fluid be tuned?
by varying T and P
what is the most commonly used supercritical fluid?
what are some properties?
CO2
properties
- low Tc (critical temp)
- non-polar
- non-toxic, non-flammable, non-explosive
- relatively cheap
- easily accessible in high purity
