Volumetric analysis

Question 1

What is a standard solution?

Answer 1

A solution of accurately known concentration.

Question 2

A standard solution can be prepared by?

Answer 2

• 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.

Question 3

Accurate dilution?

Answer 3

Standard solutions can also be prepared by accurate dilution by pipetting an appropriate volume of a standard solution into a volumetric flask, making up to the graduation mark with solvent, stoppering and inverting.

Question 4

A primary standard must?

Answer 4

• be available in a high state of purity
• be stable when solid and in solution
• be soluble
• have a reasonably high GFM

Question 5

Examples of primary standards include?

Answer 5

• sodium carbonate, Na2CO3
• hydrated oxalic acid, H2C204H20
• potassium hydrogen phthalate, KH(CgH404)
• Silver nitrate, AgNO
• potassium iodate, KIOz
• potassium dichromate, K,Cr20,

Question 6

Acid base titrations?

Answer 6

Neutralisation reactions
Indicators are required and the choice of indicators depends upon the equivalence point.
The experiment looks for a permanent colour change (end point).

Question 7

What is the equivalence point?

Answer 7

Point at which the reaction is just complete

Question 8

Redox titrations?

Answer 8

Based on redox reactions.

Redox reactions involve the transfer of electrons from the reducing agent to the oxidising agent.

Question 9

Complexometric titrations?

Answer 9

Formation of coloured complexes is used to indicate the end point of a titration.

Complexometric titrations are useful for the determination of metal ions in solution. EDTA is generally used as it forms complexes with metal ions in a 1:1 ratio.

EDTAs use can be limited, though, as appropriate indicators are not always available. The indicator has to form a coloured complex with the metal ion. However, the indicator must bind less well with the transition metal than the EDTA does. As EDTA is added to the solution, the indicator needs to be displaced. When all the indicator attached to the metal ions has been displaced, a colour change is observed, indicating the end point of the reaction.

Most common indicator is murexide.

Question 10

Back titrations?

Answer 10

Back titrations used to find the number of moles of a substance by reacting it 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.
From the initial number of moles of that reactant, the number of moles used in the reaction can be determined. The initial number of moles of the substance being analysed can then be calculated.
A back titration is useful when trying to work out the quantity of substance in a solid with a low solubility.