Titrations

Aliquot

an accurately measured volume of one of the solutions which is in the conical flask

Titre

A volume of the other solution from a burette which has a known concentration

Equivalence Point

The point at which the neutralisation reaction is stoichiometrically complete, when neither acid nor base remains

Indicator

A weak acid or base that changes colour when it detects a change in pH

End Point

The point at which the indicator changes colour

What is the burette rinsed with?

distilled water, followed by the solution it is filled with.

What is the pipette rinsed with?

distilled water, followed by the solution it is filled with.

What is the conical flask rinsed with?

distilled water

What is the volumetric flask rinsed with?

distilled water

Standard Solution

A solution which has an accurately known concentration

Primary Standard Solution Requirements

• It has be available with a high degree of purity and known formula.

• It must be stable: this means that it must not react with water or gases (such as CO₂) in the air.

• It must have a high molar mass (this reduces weighing error)

• It undergoes reactions according to known chemical equations.

Secondary Standard Solution

• Once the primary standard is made, it can be used to determine the concentration of an unknown solution.

• This solution in turn, can then be used to determine the concentration of a further unknown solution.

• Hence, it is called a secondary standard solution

Sources of Systematic Error in Titrations

• Choice of wrong indicator

• Not drying a primary standard sufficiently

• Perception of colour change: a titrator might consistently overtitrate

• Rinsing a burette or pipette with water only; therefore, diluting the solutions in either

Sources of Random Error in Titrations

• Reading off the burette inconsistently

• Small bubbles forming in the burette tap

• Drops of solution remaining inside the burette / side of conical flask

• Careless attention to colour change

Why are dilutions required?

• Solutions to be analysed are sometimes too concentrated and would require a volume of the known solution that would be too large

• Hence, the solution of known concentration must be diluted

Marking Key for Indicator Questions

•State the name of the indicator you would choose

• Identify the salt and name it

•Write a hydrolysis for the salt showing the cation or anion reacting with water (acting as a Bronsted/Lowry acid or base)

•Refer to the pH at equivalence point being below pH 7, at pH 7 or above

•State that the endpoint (colour change point) of the indicator has to be close to the equivalence point of the reaction

Back Titrations

• A back titration, or indirect titration, is generally a two-stage analytical technique:

• Reactant A of unknown concentration is reacted with excess reactant B of known concentration.

• A titration is then performed to determine the amount of reactant B in excess using a third reactant C.

When are back titrations used?

• one of the reactants is volatile, for example ammonia.

• an acid or a base is an insoluble salt, for example calcium carbonate

• a particular reaction is too slow

• direct titration would involve a weak acid - weak base titration (the endpoint of this type of direct titration is very difficult to observe)