so that liquid cannot flow out of them and form a drop
Why should bores of capillaries be small
A proccess powered by adhesion that causes water molecules to move upward through a narrow tube against gravity.
what is capillary action
- make sure spot is small. keep spotting until about 1mm in diameter
- too much can cause poor separation and can cause spots to overlap
How to ensure enough sample has been spotted on spotting line (vs. having too concentrated a sample - what is the problem with this?)
have low boiling points that allow them to be easily evaporated and low viscosities to allow them to migrate rapidly
What are typical characteristics for solvents used as eluents
solvents must be able top dissolve the samples placed in the line. i.e. polar solvents used with polar compounds and nonpolar solvents used with nonpolar compound
Characteristics of solvents used with respect to samples
the more nonpolar the solvent, the faster it travels up the plate
Characteristics of solvents that allow them to migrate rapidly
- alumina is more active and better for more nonpolar substrates
- silica gel is used for more polar substrates
Adsorbent materials and specific uses (what kind of adsorbents would be better for what kind of substances)
Fastest:
alkane hydrocarbons
alkyl halides
alkenes
dienes
aromatic hydrocarbons
ethers
esters
ketones
aldehydes
amines
alcohols
phenols
carboxilic acids
sulfonic acids
Slowest:
Table 8.2 (Order of solute migration)
- to determine the number of components in a mixture
- determine the identity of two substances
- to monitor the progress of a reaction
- determine the effectiveness of a purification
- determine the appropriate conditions for a column chromatographic sep
Uses of TLC and how it is used- all 6
too polar solvent will not rise and too nonpolar will rise too quickly
Effects of using too polar or too nonpolar a solvent
certain compounds absorb UV so iodine vapor must be used which forms brown spots
Visualization techniques - why certain kind of visualization technique was used for certain types of compounds
cannot be dissolved by the eluent/solvent
Why is a pencil always used to mark a plate?
ink can be dissolved by the eluent/solvent and rise up the plate and skew results
Why is ink never used to mark the plate?
- touching the plate
- using too polar/non polar eluent
- not developing the chamber
- allowing plate to touch filter paper
- spots are below the eluent level
Sources of error for TLC
- used for separation and purification of solids and liquids.
- make sure dry method is packed well enough to prevent poor separation
-- begins with a non polar solvent, allowing the compounds to absorb to the stationary phase then polarity is increased t
Column chromatography - why used, precautions, use of TLC in column chromatography
absorption - the strength of attraction between the compounds and the stationary phase
separation - a measure of the elution or migration rate of the compounds
Define adsorption and separation in the context of chromatography
volatile solvent so it is easy to dry
Other than solubility of sample in the solvent, what is the criteria for the solvent (other than cheap and nontoxic) used for preparing the sample that impacts the experiment and why would this criteria be important in the spotting phase if you have to ma
mobile flow past stationary and must be miscible.
What must occur between a liquid phase and stationary phase in order for a mixture to be separated into its pure components?
a) the new solvent must also be soluble to the samples
b) acetone because it is more polar than di-ethyl ehter. The more nonpolar the more strongly it will absorb to the polar stationary phase
You wish to carry out a TLC experiment using ethyl acetate as your eluent. You find that all your spots remain at the spotting line even though the samples are soluble in ethyl acetate. You decide to add another solvent to ethyl acetate to help obtain mor
should stop the level at the top of the packing. Allowing the sample to run dry creates air bubbles which leads to poor separation
In column chromatography (microscale), after loading it with solvent and adsorbent and prior to loading the sample, what level should you allow the solvent to drop to? Why should you not allow the sample to run dry and what is the consequence if you had a
should fill the column with non-polar solvent then slowly add silica gel. then tap the side to allow solid to settle on the bottom. avoid air bubbles or cracks which lead to poor separation
During the packing stage of column chromatography, if you are using the dry packing method, what should you do and what should you avoid in order to ensure eventual good separation of components?
water shouldnt be used because it can dissolve some of the silica gel
Why would water not be used to elute a column (besides it polarity) in column chromatography?