Unit 3 - Industrial Chemistry (Chemical Analysis) Flashcards
(17 cards)
What is chromatography?
An important analytical technique because it allows chemists to separate substances in complex mixtures. Substances are separated as they travel in a mobile phase which passes through a stationary phase.
Different substances travel at different speeds, so some move further than others in a given time.
What is paper chromatography?
When the stationary phase is a sheet of chromatography paper. The mobile phase may either be an aqueous (water-based) liquid or a non-aqueous (carbon-based) organic solvent. For each chemical in the sample, there is a dynamic equilibrium between the stationary phase and the mobile phase. The overall separation depends upon how strongly attracted the chemicals are to the mobile and the stationary phases. This produces a chromatogram where different samples can be compared to a reference material.
What is thin layer chromatography?
When the stationary phase is a thin layer of an inert substance (e.g. silica) supported on a flat, unreactive surface (e.g. a glass plate). The distance that spots move can be compared to the overall distance the solvent has moved and comparisons and measurements made.
What are the benefits of thin layer chromatography over paper chromatography?
- The mobile phase moves more quickly through the stationary phase.
- The mobile phase moves more evenly through the stationary phase.
- There is a range of absorbencies for the stationary phase
- Produces more useful chromatograms than paper chromatography, which show greater separation of the components in the mixture - and are therefore easier to analyse.
How does size affect the distance travelled by a spot?
Larger molecules take longer to move up the chromatography paper or TLC plate, whereas smaller molecules are more mobile.
How does polarity affect the distance travelled by a spot?
Polar molecules will be more strongly attracted to polar solvents, and so would move further if a polar solvent was used as opposed to a non-polar solvent and vice versa.
What is gas chromatography?
The mobile phase is an inert gas (e.g. helium). The stationary phase is a very thin layer of an inert liquid on an inert solid support - such as beads of silica packed into a long thin tube (this flexible tube is coiled many times inside a thermostatically-controlled oven to keep it at a constant temperature). The mixture to be analysed is injected into the stream of carrier gas. As it passes along the column (long thin tube) it separates into the different substances. The amount of time that a substance takes to pass through the column is called its retention time. The retention time of an unknown substance can be compared with standard reference data to help to identify it.
What are the benefits of gas chromatography over paper or thin layer chromatography?
It’s used to separate complex mixtures. It is much better at this than thin-layer or paper chromatography. This is because it is more sensitive - allowing the determination not only of what chemicals are in the mixture, but also how much of each chemical there is.
How does affinity affect the speed of the substances?
Substances with a greater affinity (attraction) for the mobile phase reach the detector at the end of the column more quickly. Substances with a greater affinity for the stationary phase move more slowly through the column.
What is gas chromatography used for?
Used to detect banned substances in urine samples from athletes, or by forensic investigators to detect the presence of fuels that may have been used to deliberately start fires.
What are the main pieces of information that can be gathered from a gas chromatogram?
- The number of compounds in the mixture - represented by the number of peaks.
- How much of each compound is present - represented by the height of the peak (higher = more).
- The retention time - indicated by the position of the peak.
What is step one of a volumetric titration?
Prepare the standard solution, like so:
1. Measure the required mass of the substance required (weigh by difference {tare a balance with the weighing boat/crucible then transfer the required mass into the weighing boat/crucible}).
2. Dissolve in a small volume of deionised water to make up a known volume of solution (usually 100cm^3).
3. Transfer with rinsings to a volumetric flask.
4. Make up to the graduation mark with ionised water.
What is step two of a volumetric titration?
Rinse a burette with the standard solution and then fill the burette with the standard solution and record the initial volume.
What is step three of a volumetric titration?
Use a pipette to dispense a known volume of the solution with an unknown concentration into a conical flask. A safety pipette filler is used to draw solution into the pipette.
What is step four of a volumetric titration?
A few drops of an indicator may be added to the conical flask. This will show a change of colour when the titration is complete. This step is not necessary if the substance is self-indicating, such as the oxidising agent potassium permanganate, which will turn from purple to colourless.
What is step five of a volumetric titration?
The solution from the burette is run into the conical flask. The solution is added one drop at a time, with swirling to mix the solutions as the end-point is approached. Eventually, a colour change shows that the correct amount has been added to react completely with the synthesised chemical in the sample.
What is step six of a volumetric titration?
The volume of solution added from the burette is noted. The titration is repeated until concordant results (within 0·2 cm^3 of each other) are obtained. The titration results can then be used to calculate the amount of the synthesised chemical in the sample, and therefore find its purity, using the relationship n=cv and the mole ratio from the balanced equation.
