KK2. laboratory analysis techniques

MELTING POINTS

• The temperature which the two states liquid and solid coexist in equilibrium

  • When the forces of attraction between molecules or ions overcome and a change in physical state occurs from solid to liquid

• Used to determine purity by comparing experimental melting points with known value

Principles

• Involves the measurement of the temperature at which a solid compound changes from a solid to a liquid state

• Pure substances  has a sharply defined melting point (very small temperature range)

• Impure substances have both a lower and broader melting temperature

• Impurities disrupting and weakening the overall molecular or lattice structure, meaning less energy is required to overcome the forces of attraction in the solid, thereby causing it to melt

• Stronger the forces between the atoms or ions in a compound, the higher the melting point will be

Applications

• Used in various industries to evaluate the purity of solid consumer products

• Can be used to determine the purity of pharmaceutical drugs, industrial chemicals, food additives and cosmetic ingredients

Techniques

• A solid sample under investigation is first dried and then forced into a capillary tube

• The capillary tube can then be places either in a Thiele tube, metal block or specialise cells electronic  melting point device

• Melting point is determined by recoding the temperature at the moment just before the solid 'collapses', which is equal to the meting point of the solid

• Repeated to obtain an average, and the experimental melting point is compared against the known melting point of a pure sample

  • If the range of melting point is lower and broader then that of the pure sample then the compound is not pure or is a mixture of impure compounds

  • If the melting point is lower yet well-defined, then the known substance is likely a mixture of pure compounds

SIMPLE AND FRACTIONAL DISTILATION

Distillation - used to separate and verify components of a liquid mixture based on the different boiling points

Boiling point - the temperature at which the vapour pressure of a liquid = the atmosphere pressure

• The temperature at which the forces of attraction between atoms or ions overcome and a change in physical state from liquid to gas

The stronger the forces between the atoms or ions in a compound, the higher the boiling point

Principles

• Distillation relies on boiling, evaporation and condensation to separate volatile component from non-volatile or less volatile ones

• Simple distillation - used to separate liquids with significantly different boiling points

• Fractional distillation - uses fractioning column, which allows separation of liquids with similar boiling points

If impurities are present, then the recorded boiling point will be higher (boiling point elevation) compared to its known boiling temperature

impurities

• can disrupt the regular and symmetry of the compound's crystal lattice or packaging arrangement

• Can create defects or irregularities in the lattice structure

  • Increases boiling point

• Creates disruptions in the crystal lattice and increases the energy required to break the intermolecular forces and transitions from a liquid to a vapour phase

• Can decrease the vapour pressure of the compound

  • Reduce the number of available pure compound molecules at the surface and therefore decreases the vapour pressure

• Higher temperature is required to reach the necessary vapour pressure for boiling

Applications

• Main difference is the fractionating column, which allows continuous changes from liquid to gas and back to liquid again

• Much purer fraction than when using simple distillation

  • mixtures need to be simple distilled multiple times in order to obtain a pure distillate

• Boiling points of separated liquid fractions can verify components of the mixture

• Compares them against known temperature of the pure sample

• If the temperature of the fraction is higher, then the compound is not pure