Chemical Analysis (Unit 1) Flashcards
Chromatography Technique
Chromatography is a technique for seperating and/or identifying the components in a mixture.
The basic principle is that components in a mixture have different tendencies to adsorb onto a surfuce or dissolive in a solvent.
Types of Chromatography
There are several different types of chromatography e.g paper chromatography, thin layer chromatography (TLC), gas chromatography (GC) snd high performance liquid chromatography (HPLC).
Stationary and Mobile Phases
All types of chromatography have a stationary phase (e.g paper) and a mobile phase (e.g alcohol). The stationary phase is usually a solid. The mobile pphase is usually a liquid or a gas. Mixtures seperate as the mobile phase moves through the stationary phase dragging the components of the mixture with it.
Chromatography and Intermolecular Forces
In chromatography, different molecules experience different types and strengths of intermolecular forces as the mobile phase carries them through the stationary phase. The strengths and types of intermolecular forces depend on the differences in polarity and size of the molecules being seperated.
Molecules that form stronger intermolecular forces with the mobile phase will move more quickly than other molecules that form stronger intermolecular forces with the stationary phases. For example, if the mobile phase is polar and the stationary phase is non-polar, then any polar molecules would be expected to move further and faster than non-polar molecules.
Paper Chromatography
In the seperation of dyes in ink by paper chromatography, some dyes are more attracted to the ppaer than the alcohol and tend to lag behind. Others are more attracted to the alcohol than the paper so move faster.
Gas Chromatography
Gas chromatography uses a carrier gas as the mobile phase. The stationary phase is coiled inside an oven to vaporise the sample. After seperation, a detector produces a signal whenever a componenet exits the appartus.
In gas chromatography, the identity of a component is indicated by the time it has taken to travel through the apparatus (retention time).
Larger molecules have larger retention times.
Volumetric Analysis
Titration
Volumetric analysis involves using a solution of accurately known concentration in a quantitive reaction to determine the concentration of another substance.
Standard Solution
A solution of accurately known concentration is known as a standard solution.
Preparing A Standard Solution
- Weight the sample
- Dissolve in a small volume of deionised water in a beaker
- Transfer the sample to a standard (volumetric) flask
- The beaker is rinsed and the risnings are also poured into the standrard flask.
- The flask is made up the mark adding the last few drops of water using a dropping pipette.
- The flask is stoppered and inverted several times to ensure thorough mixing of the solution.
Titration
Titration is used to accuaretly determine the volumes of solution required to reach the end-point of a reaction. The end-point is the point of at which the reaction is just complete.
An indicator is normally used to show when the end-point is reached. An indicator is a substance which changes colour at the end-point.
Titre volumes within 0.2cm3 are considered concordant.
Placing a white tile underneath the conical flask imporves the precision of the end-point.
Volumetric Analysis
In volumetric analysis:
* the volume markings on beakers provide only a rough indication of volume.
* measuring cylinders generally provide sufficient accuracy for prepartive work, but for analytical work, burettes, pipettes and volumetric flasks are more appropriate.
Titration Equations
A balanced equation for a titration reaction can be used to calculate:
* The concentration of one reactant given the concentration of the other reactant and the volumes of both solutions.
* The volume of one reactabt given the volume of the other reacatnt and the concentrations of both solutions.
Redox Titrations
Redox titration are based on redox reactions.
