AH Chemistry - Researching Chemistry - (e) - Volumetric Analysis

How do you prepare a standard solution?

To prepare a standard solution from a solid, you:

• Weigh a primary standard accurately.

• Dissolve 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.

Other bullet points

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What is a standard solution?

A standard solution is a solution of accurately known concentration.

How do you prepare a standard solution from a liquid?

To prepare a standard solution from a liquid, you:

• Pipette an appropriate volume of a standard solution into a volumetric flask.

• Make up to the graduation mark with solvent.

• Stopper and invert.

What characteristics must a primary standard have?

Primary standards must:

• Be available in a high state of purity.

• Be stable when solid and in solution.

• Be soluble.

• Have a reasonably high GFM.

What are examples of primary standards?

Examples of primary standards are:

• Sodium carbonate, Na₂CO₃

• Hydrated oxalic acid, H₂C₂O₂.H₂O

• Potassium hydrogen phthalate KH(C₈H₄O₄)

• Silver nitrate, AgNO₃

• Potassium iodate, KIO₃

• Potassium dichromate, K₂Cr₂O₇

Why is sodium hydroxide, NaOH, not a primary standard?

Sodium hydroxide, NaOH is not a primary standard because:

• It has a relatively low GFM.

• It is unstable as a solid, because it absorbs moisture, and as a solution.

Due to its relatively low GFM and instability as a solid and as a solution, what must be done to sodium hydroxide before being used in volumetric analysis?

Due to its relatively low GFM and instability as a solid and as a solution, sodium hydroxide must be standardised before being used in volumetric analysis.

What is the process of a back titration?

The process of a back titration is:

• The substance you want to find the concentration of is reacted 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.

What are back titrations useful for?

Back titrations are useful when trying to work out the quantity of substance in a solid with a low solubility.

What can complexometric titrations used to do?

Complexometric titrations can be used to determine the concentration of metal ions in a solution.