Unit 1 - Chemical Analysis

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.

There are several different types of chromatography e.g. paper chromatography, thin layer chromatography (TLC), gas chromatography (GC) and high performance liquid chromatography (HPLC).

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 phase is usually a liquid or a gas. Mixtures separate as the mobile phase moves through the stationary phase dragging the components of the mixture with it.

In the separation of dyes in ink by paper chromatography, some dyes are more attracted to the paper (stationary phase) than the alcohol (mobile phase) and tend to lag behind. Others are more attracted to the alcohol than the paper so move faster.

Gas chromatography uses a carrier gas as the mobile phase. The stationary phase is coiled inside an oven to vaporise the sample. After separation, a detector produces a signal whenever a component exits the apparatus. In gas chromatography, the identity of a component is indicated by the time it has taken to travel through the apparatus (retention time).

• polarity of the substances - attraction for mobile/stationary phase • size of the molecules - larger molecules have longer retention times

Peaks show the concentration/quantity present. If the top of a peak can’t be seen then it can be diluted.

Volumetric analysis involves using a solution of accurately known concentration in a quantitive reaction to determine the concentration of another substance.

A solution of accurately known concentration is known as a standard solution.

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. Titre volumes within 0.2cm3 are considered concordant. Placing a white tile underneath the conical flask imporves the precision of the end-point.

• Rinse burette and pipette with solutionsto be used • Fill burette, remove funnel and any air bubbles • Read scale (bottom of meniscus)

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 OR the volume of one reactant given the volume of the other reactant and the concentrations of both solutions.

Redox titration are based on redox reactions and used to determine the unknown concentration of a solution.