Question

How do these common analytical techniques work, and what do they tell you?

A) mp determination

B) bp determination

C) Polarimetry

D) thin layer chromatography

E) gas chromatography

Answer 1

A)

A melting point can be used to identify a substance and to get an indication of its purity. The melting point (or freezing point) of a solid is the temperature at which the solid exists in equilibrium with its liquid state under an external pressure of one atmosphere. Both the melting point range (the interval between the beginning of liquefaction and complete liquefaction) and the temperature of complete liquefaction are valuable indicators of the purity of the solid compound

B) Boiling point helps identify and characterise a compound. A liquid boils when its vapour pressure is equal to the atmospheric pressure. Vapour pressure is determined by the kinetic energy of a molecule. Kinetic energy depends on the temperature, mass and velocity of a molecule.

bp of organic ompound depends

1. The relative strength of the four intermolecular forces is: Ionic > Hydrogen bonding > dipole dipole > Van der Waals dispersion forces. ...
2. Boiling points increase as the number of carbons is increased.

C)

Whereas regular (unpolarised) light includes waves that are inclined in any direction of space, polarised light is inclined at one defined angle.

Polarised light is 'created' by eliminating all light other than that which is inclined at one angle. To do this, a very fine meshed grating – a polarisation filter – is used to filter out all waves from the regular light that do not have the same inclination as the filter's grating.

If the polarised light beam were then directed through a second grating, set at exactly 90 degrees to the first grating, no light would pass through it.

D)

Thin layer chromagraphy is a chromatographic technique used to seperate and identify the compounds in a mixture. This used the different binding affinities of the compounds in the mixture to the stationary phase and the mobile phase.

In TLC the stationary phase is coated on glass plates, metal( aluminium) plates or plastic films as a thin layer. The most commonly used substances for stationary phase are silica gel, alumina and cellulose powder.The adherence to the plate is ensured by adding a binding agent like calcium sulphate into the stationary phase. If the visualization is done under uv a fluorescent compound is also incorparate into the stationary phase. The prepared TLC plates are also known as chromaplates.when handling these chromaplates care should be taken not to touch the face of the plate to prevent damaging the stationary phase and also to prevent contaminations. They should always be handled using the edges.

The sample is spotted about 1cm above the edge of the plate. This 1cm margin should always be drawn with a pencil. Sample is spotted using a small capillary. After the spot is properly dried the plate is placed in a closed jar containing the solvent or the mobile phase. Before placing the TLC plate the jar should be allowed to saturate with the mobile phase used. The stationary phase and the mobile phase are chosen based on the compounds in the sample. When the solvent reached about 1cm below the upper edge the plate should be taken out from the jar and the solvent front should be marked with a pencil.

The mobile phase rise along the TLC plate due to the capillary action. The compunds that are soluble in the mobile phase will rise along with the mobile phase and the compounds that are bound strongly to the stationary phase will not rise with the mobile phase. For example if the compound is polar and stationary phase is also polar, and a non polar mobile phase is used, that compound will not move with the mobile phase as it is strongly bound to the stationary phase. Because of this differential distribution, the compounds in the mixture are seperated. If the compounds in the mixture are known sample can be run along with standard samples of the known compunds and this can be used in identification.

The identification and the analysis of the seperated compounds is done by using the retention factor(R_f).

R_f= Distance traveled by the compund/ Distance traveled by the solvent front

If the compound in the sample is same as the standard both will have the same Rf

E)

Gas chromatographyChromatography is a technique for separating chemical substances that relies on differences in partitioning behaviour between a flowing mobile phase and a stationary phase to separate the components in a mixture

The sample is carried by a moving gas stream through a tube packed with a finely divided solid or may be coated with a film of a liquid. Because of its simplicity, sensitivity, and effectiveness in separating components of mixtures, gas chromatography is one of the most important tools in chemistry. It is widely used for quantitative and qualitative analysis of mixtures, for the purification of compounds, and for the determination of such thermochemical constants as heats of solution and vaporization, vapour pressure and activity coefficients.

Gas chromatography is also used to monitor industrial processes automatically: gas streams are analyzed periodically and manual or automatic responses are made to counteract undesirable variations.

Many routine analyses are performed rapidly in environmental and other fields. For example, many countries have fixed moniotor points to continuously measure the emission levels of for instance nitrogen dioxides, carbon dioxide and carbon monoxide. Gas chromatography is also useful in the analysis of pharmaceutical products, alcohol in blood, essential oils and food product