Chapter 13.1 Flashcards
What is Chromatography?
Chromatography is a Lab technique used to separate and analyse the substances present in a mixture.
The mixture is dissolved in a fluid called the mobile phase, which carries it through a structure holding another material called the stationary phase.
The various constituents of the mixture travel at different speeds, causing them to separate.
What can Chromatography be used to analyse?
Numerous inorganic and organic substances such as contaminants in water, toxic gases in air, additives and impurities in food, drugs in blood etc.
List the different types of Chromatography.
- Paper Chromatography
- Thin-layer Chromatography
- Column Chromatography
- Gas chromatography
- High Performance Liquid Chromatography (HPLC)
What is a Chromatograph?
The pattern of bands or spots is called a chromatograph.
What are Components?
An element of a larger whole compostion
What is Adsorption and Desorption?
Adsorption - molecules of substances adhering to the surface of solid or liquid.
Desorption- the release of an adsorbed substance from a surface.
How does chromatograph work?
All methods of Chromatography have a
- Mobile Phase (moving) = can be liquid or a gas
- Stationary Phase = Can be solid or liquid; system on which the materials to be separated are selectively adsorbed.
The mobile phase carries a substance up the stationary phase, and the substance separates into bands (bands of colours for ink)
- each band contains one component
- the pattern of spots/bands = chromatograph
As the complete are swept upwards over the stationary phase by the mobile phase, they undergo a continual process of Adsorption on the stationary phase, followed by Desorption and dissolving into mobile phase.
(For the ink/chalk chromatography) - The ability of the components to stick to the stationary phase will depend upon the polarity of the stationary phase and the component molecules.
- similarly the attraction of the components to the solvent molecules is determined by their polarity.
What does the rate of movement of each component depend on mainly?
- How strongly the component aDsorbs onto the stationary phase
- How readily the component dissolves in the mobile phase
The components separate because the undergo theses 2 processes to different degrees.
How is Paper Chromatography performed? Qualitative analysis
Paper chromatography is used to separate liquids or gases into different components.
STATIONARY PHASE; Paper.- use high quality absorbent paper, similar to filter paper.
Mobile Phase - is a solvent such as water
*qualitative analysis- identifies chemicals that are present in a mixture
Explain Thin layer chromatography - Qualitative Analysis
Similar to Paper Chromatography However;
Stationary phase; Fine powder such as Alumina (aluminium oxide) spread on a glass or plastic plate
Paper and thin layer chromatography in practice.
In both - a small spot of the solution of a sample is to be analysed is placed on one end of the chromatography paper or plate.
The position of this spot is called the ORIGIN.
The paper or plate is then placed in a container with solvent.
- origin must be a little above the level of the solvent so that the components can be transported up the paper or plate and nit dissolve into the liquid in the container.
As the solvent rises up the paper or plate, the components of each sample separate depending on their attraction to the stationary phase and their solubility in the solvent
How to identify the components of a mixture?
The components can be identified in two ways;
- Include standards known chemicals on the same chromatograph as the unknown sample and comparing the resulting positions of the unknown components with those of the known samples.
- Calculate the Retardation factor (Rf) of the sample and comparing these with the Rf values of known samples
How to identify components with the Method 1; Using Standards
- you need to know what chemicals Might be present in the sample
- if the spots from the unknown experimental sample move the same distance from the origin as the spots from the pure known sample, then that sample component is same and then identified.
- The sample and standards are ‘run’ on the same chromatograph because the distances moved from the origin will depend on the distance moved by the solvent front.
The further the solvent front is allowed to travel, the further the spot travels.
How to identify components with the Method 2; calculating Rf values.
- by comparing the distance they travel up the stationary phase to the distance travelled up the stationary phase to the distance travelled by the solvent front.
Distance the component travelled from origin Rf = ————————————————————— Distance the solvent front travelled from origin - Rf values will always be less than one
- the component most strongly adsorbed onto stationary phase moves the shortest distance and the lowest Rf value
What is Rf value and how is it compared and limitations of it.
Each component has a characteristic Rf value for the conditions under which the chromatograph was obtained.
- By comparing the Rf values of the components of a particular mixture with the Rf values of the known substances determined under IDENTICAL CONDITIONS, you can identity the components present in a mixture.
In this method the actual distance moved by the solvent font is no longer critical as the proportion of the distance moved from the origin (The Rf value ) stays the same Provided the conditions under which the chromatograph is obtained are the same.
- This means the Rf values of the unknown spots can be compared against a table of Rf values of common materials,
LIMITATIONS; However changes in the temperature, the type of stationary phase, and the amount of solvent vapour around the Paper of plate and the type of solvent will all change the Rf value for a particular chemical.
