Researching

What does "heating to constant mass" involve?

heating a substance — allowing to cool in a desiccator to prevent absorption of water — weighing accurately— repeating the steps of heating until constant mass

What is gravimetric analysis used for?

To determine the mass of an element or compound in a substance

What happens in gravimetric analysis?

The substance is converted into another substance of known chemical composition which can be readily isolated and purified

What are the two conversion methods in gravimetric analysis?

precipitation or volatilisation

Describe precipitation conversion in gravimetric analysis.

Substance undergoes a precipitation reaction.

Precipitate is separated from the filtrate

Filtrate tested to ensure the reaction has gone to completion.

Precipitate is washed and dried before being weighed to determine the mass of the desired component.

Describe volatilisation conversion in gravimetric analysis.

Substance is heated and any volatile products (often water) are evaporated.

Substance is heated to constant mass and the final mass recorded.

What is a standard solution?

A solution of accurately known concentration

How is a standard solution prepared?

weighing a primary standard accurately

dissolving in a small volume of solvent (usually deionised or distilled water) in a beaker

transferring the solution and rinsings into a volumetric flask

making up to the graduation mark with solvent

stoppering and inverting

How else can a standard solution be prepared?

By accurate dilution by pipetting an appropriate volume of a standard solution into a volumetric flask

What are the requirements for a primary standard?

Be available in a high state of purity

Be stable when solid and in solution

Be soluble

Have a reasonably high GFM

Examples of primary standards include:

sodium carbonate

hydrated oxalic acid

potassium hydrogen phthalate

silver nitrate

potassium iodate

potassium dichromate

Why is sodium hydroxide not a primary standard?

Sodium hydroxide is not a primary standard as it has a relatively low GFM, unstable as a solid and in solution

Complexometric titrations

Based on reactions in which complexes are formed

EDTA is an important complexometric reagent and can be used to determine the concentration of metal ions in solution

Back titration use

when trying to work out the quantity of substance in a solid with a low solubility

back titration method

React with an excess volume of a reactant of known concentration.

The resulting mixture is then titrated to work out the number of moles of the reactant in excess.

The initial number of moles of that reactant, the number of moles used in the reaction can be determined.

Initial number of moles of the substance being analysed can then be calculated

What is the principle of colorimetry?

Colorimetry uses the relationship between colour intensity of a solution and the concentration of the coloured species present

How is a calibration graph created?

A colorimeter or a spectrophotometer is used to measure the absorbance of light of a series of standard solutions

How is the concentration of a test solution determined?

From its absorbance and by referring to the calibration curve.

What is required for accurate colorimetry results?

The concentration of coloured species in the solution being tested must lie in the straight line section of the calibration graph.

What is distillation used for?

The identification and purification of organic compounds.

How is boiling point used in distillation?

The boiling point of a compound is used to confirm its identity

How does distillation purify compounds?

By separating it from less volatile substances in the mixture

What is heating under reflux?

Allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping

Procedure for 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 vacuum filtration?

Carrying out filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate.

A Büchner, Hirsch or sintered glass funnel can be used during vacuum filtration.

Steps for recrystallisation.

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

How is the solvent chosen for recrystallisation?

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.

What is solvent extraction?

Isolates a solute from a liquid mixture or solution by extraction using an immiscible solvent in which the solute is soluble

Procedure for 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.

Optimizing solvent extraction.

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.

Requirements for the solvent in solvent extraction.

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

What is melting point determination?

The melting point of a substance is the temperature range over which the solid first starts to melt, to when all of the solid has melted

How is melting point used to confirm identity of a PURE compound?

Can be confirmed by melting point analysis and a comparison of the experimentally determined melting point with a literature or known melting point value

How does purity affect melting point and why?

Lowers the melting point and broadens its melting temperature range due to the disruption in intermolecular bonding in the crystal lattice.

What is a mixed melting point determination?

Mixing a small quantity of the product with some of the pure compound and determining the melting point. The melting point value and the range of the melting temperature can be used to determine if the product and the pure compound are the same substance.

What is thin-layer chromatography (TLC)?

A technique used to separate the components present within a mixture.

Substances are separated by making use of differences in their polarity or molecular size.

Describe TLC procedure.

Thin-layer chromatography (TLC) uses a fine film of silica or aluminium oxide spread over glass, aluminium foil or plastic.

A small sample of the mixture being tested is spotted onto the base (pencil) line of the chromatogram.

A solvent dissolves the compounds in the spot and carries the compounds up the chromatogram.

How far the compounds are carried depends on how soluble the compounds are in the chosen solvent and how well they adhere to the plate.

A developing agent or ultraviolet light is normally required to visualise the spots on the chromatogram.

Consistency of Rf values.

Under the same conditions (temperature, solvent, and saturation levels) a compound always has the same Rf value (within experimental error).

Confirming compound identity with TLC.

comparing the experimentally determined f R values with a literature or known value determined under the same conditions

making a direct comparison on a TLC plate between the compound being tested and the pure substance — a co-spot could be used

Assessing a pure substance with TLC.

A pure substance when spotted and developed on a TLC plate, should appear as a single spot (some impurities may not be visible by TLC analysis).

What happens if the product is impure in TLC

The presence of more than one spot