Lab D - Identification of an Unknown Using Think layer Chromatography Flashcards
Chromatography, or “Color Writing”, is a simple, economical method for ____________(1). Identification of unknowns is done through ___________(2). Chromatography helps in determining ________________(3), ________________(4), _______________(5). There is _____________(6) on sample type- it can be organic, inorganic, biological, or medical. It is a highly ____________(7), and can detect __________(8).
(1) separating components of a mixture
(2) comparisons with known reference samples
(3) the effectiveness of a purification
(4) the conditions for macroscopic separations(column chromatography)
(5) following the progress of a chemical reaction
(6) no restriction
(7) sensitive
(8) microgram amounts or less
Make a chart for chromatography, comparing the stationary and mobile phase. Include the following rows: what does it do, what is it(physical form), how does it effect sample movement, analogue to, specifically for.
Look at notes
What are the 4 types of chromatographic methods?
TLC: Think layer chromatography
LC: Liquid chromatography(SP - Silica gel or Alumina; MP-a liquid)
HPLC: high performance liquid chromatography(utilizes high pressure exerted by mechanical pumps to force ht Emobile phase through a very small diameter column packing that contains the stationary phase)
GCL Gas chromatography(MP- gas)
Silica gel is an example of an absorbent the is most commonly used as an inexpensive stationary phase. Draw its structure and explain why it is polar.
Diagram in notes
Extended covalent network of tetrahedral SI atoms bridged by O atoms - terminating in very polar Silanol(Si-OH) group, thus creating a very polar surface. The presence of these OH groups renders the surface of silica gel highly polar
Other than silica gel, what is considered another example of a good absorbent?
Aluminum oxide, alumina(Al2O3)
What is the composition of a TLC plate?
250 um silica gel layer impregnated with a fluorescent indicator on a plastic backing
What is added to help the silica gel bind to a plate? What is added to help compounds fluoresce?
CaSO4 used to help silica gel bind to a plate
ZnS used for fluorescence(absorbs at 254 nm). It helps the compound glow under UV light
What are the results of interaction with only stationary phase?
Highly polar molecules will interact strongly with the polar Si-OH bonds in the silica gel absorbent, resulting in slow movement up the TLC plate
Weakly polar molecules are help less tightly than the polar species on the silica gel absorbent - resulting in quicker movement up the TLC plate.
Draw and name the three compounds used in this lab. Rank from most to least polar.
Drawn in ntoes
Most polar is 9-hydroxyfluorene. For 9HF, it is the most polar because the hydroxyl group. Phenyl Benzoate is next most polar because ester group induced polarity. Least polar is fluorene because does not contain polar bonds.
What does Rf stand for? How do you calculate an Rf value?
Retention factor = (distance traveled by compound, or SOLUTE) / (distance traveled by solvent, or ELUENT)
How do Rf values correlate with the polarity of a compound?
More polar compounds = small Rf values = shorter the distance traveled on the TLC plate
Less polar compounds = large Rf values = longer the distance traveled on the TLC plate
What are the four steps of thin layer chromatography?
- Application of sample: use a different TLC capillary tube(open on both ends) to spot the standard solution of each solute
- Development of sample: This is when the separation actually occurs inside the TLC development chamber
- Visualization of sample: the TLC plate will be viewed under UV light
- Interpretation of results: Comparison of RF values
You should always spot on the ________ side of the TLC plate.
ROUGH
Draw the schematic for the developing chamber to run a TLC plate
Drawn in notes
Describe the process for sample application(spotting)
A. Draw the baseline lightly with pencil
1.5 cm from the bottom of the plate.
“baseline”
B. Gently and lightly draw four “x” marks
on the baseline
C. Use capillary tubes that are labelled as
TLC capillary tube (and not MP capillary tubes)
- to draw up the solution from the vial
onto the TLC plate.
(Be sure to spot lightly – to avoid making
a break in the silica gel coating
on the plate- that will hamper the separation.)
What fluent, or eluting solvent was used in this experiment?
9mL hexanes and 1mL ethyl acetate
How are spots visualized on the TLC plate?
Ultraviolet viewing cabinet,
equipped with a short wavelength
(254 nm) UV lamp
Make sure to
A. Mark solvent front. Allow solvent to evaporate from surface of TLC plate.
B. View results under UV light. Use a pencil to
mark the observed bright pink spots
Iodine Visualization
Colorless Organic Compounds (alkanes, alcohols, ethers) can also be visualized by absorption of Iodine
Vapor – Yellowish-brown colored spots form from the reaction of substances with Iodine vapor
Identical Rf values _________(1)
that compounds are the same
(1) suggest, but do not prove
how exactly does chromatography separate organic compounds?
Using a solvent(mobile phase) to move the compounds over a solid(stationary phase). The compounds are separated based on how strongly the compounds bind to the stationary phase(i.e. the silica gel) via non-covalent interactions vs how well the compounds dissolve in the solvent/eluent
Less polar compounds will move _________(1) than more polar compounds
farther up the plate
What time of interactions with SiO2 have with organic molecules
hydrogen bond
How is distance measured for the solvent front vs the spots
Spots: measured from the baseline to the centre of the spot
Solvent front: measure from the baseline to the solvent front
Less polar compounds will have higher or lower Rf values?
Higher
