Gravimetric and Volumetric Analysis, Practical Techniques Flashcards
What are the steps of heating to constant mass ?
heating to constant mass:
— heating a substance
— allowing to cool in a desiccator to prevent absorption of water
— weighing
— repeating the steps of heating, cooling and weighing until no further changes in mass are observed
What is precipitation ?
In precipitation conversion the substance undergoes a precipitation reaction. The precipitate is separated from the filtrate and the filtrate tested to ensure the reaction has gone to completion. The precipitate is washed, dried to constant mass and then weighed.
What is volatilisation ?
In volatilisation conversion the substance is heated and any volatile products (often water) are evaporated. The substance is heated to constant mass and the final mass recorded
What is gravimetric analysis ?
Gravimetric analysis is used to determine the mass of an element or compound in a substance.
The substance is converted into another substance of known chemical composition, which can be readily isolated and purified.
The conversion can occur either through precipitation or volatilisation.
What makes a primary standard ?
A primary standard must:
♦ be available in a high state of purity
♦ be stable when solid and in solution
♦ be soluble
♦ have a reasonably high GFM
What equipment is used in heating under reflux ?
When carrying out heating under reflux, the reaction mixture is placed
in a round-bottomed flask with anti-bumping granules and the flask is
fitted with a condenser. The flask is then heated using an appropriate
source of heat.
What is recrystallisation ?
A technique to purify an impure solid
The solvent for recrystallisation is chosen so that the compound
being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures.
♦ dissolving an impure solid gently in a minimum volume of a hot
solvent
♦ hot filtration of the resulting mixture to remove any insoluble
impurities
♦ cooling the filtrate slowly to allow crystals of the pure compound
to form, leaving soluble impurities dissolved in the solvent
♦ filtering, washing and drying the pure crystals
What is solvent extraction ?
Solvent extraction involves isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which the solute is soluble.
When carrying out a solvent extraction, the two immiscible solvents
form two layers in the separating funnel. The solute dissolves in both
solvents and an equilibrium establishes between the two layers. The
ratio of solute dissolved in each layer is determined by the equilibrium
constant, K . The lower layer is run off into a container and the upper
layer is poured into a second container. This process is repeated to
maximise the quantity of solute extracted.
The quantity of solute extracted is greater if a number of extractions
using smaller volumes of solvent are carried out rather than a single
extraction using a large volume of solvent
What should the solvent chosen in solvent extraction be ?
♦ immiscible with the liquid mixture or solution (usually water)
♦ one in which the solute is more soluble in than the liquid mixture
or solution (usually water)
♦ volatile to allow the solute to be obtained by evaporation of the
solvent
♦ unreactive with the solute
