Acid-base titration

Titration setup. The burette would normally be held by a clamp, not shown here. The pink is most likely caused by use of the phenolphthalein indicator.

An acid-base titration is a method in chemistry that allows quantitative analysis of the concentration of an unknown acid or base solution. It makes use of the neutralization reaction that occurs between acids and bases, and that we know how acids and bases will react if we know their formula.

Equipment

The key equipment used in a titration are:

• Burette
• Pipette
• Acid/Base Indicator (the one used varies depending on the reactants)
• Conical Flask
• Standard Solution (a solution of known concentration, a common one is aqueous Na₂CO₃)
• Solution of unknown concentration

Method

The method is as follows.

Before starting the titration we must choose a suitable indicator. The end point of the reaction, when all the products have reacted, will have a pH dependant on the relative strengths of the acids and bases. We can determine roughly the pH of the end point using the following rules:

• A strong acid reacts with a strong base to form a neutral (pH=7) solution.
• A strong acid reacts with a weak base to form an acidic (pH<7) solution.
• A weak acid reacts with a strong base to form a basic (pH>7) solution.

A suitable indicator should be chosen, that will experience a change in colour close to the end point of the reaction.

Firstly, the burette should be rinsed with the standard solution, the pipette with the unknown solution, and the conical flask with distilled water.

Secondly, a known volume of the unknown concentration solution should be taken with the pipette and placed into the conical flask, along with a small amount of the indicator chosen. The burette should be filled to the top of its scale with the known solution.

The known solution should then be allowed out of the burette, into the conical flask. At this stage we want a rough estimate of the amount of this solution it took to neutralise the unknown solution. Let the solution out of the burette until the indicator changes colour and then record the value on the burette. This is the first titre and should be discluded from any calculations.

Perform three more titrations, this time more accurately, taking into account we know roughly where the end point will occur. Take note of each of the readings on the burette at the end point, and average these at the end.

Calculations

Knowing the average of the three results recorded we can work out the number of moles, and hence the concentration of the unknown solution.

Firstly, work out and balance the equation of the reactants. It can now be determined how many moles of reactants will neutralize a mole of the solution.

We know the number of moles of the standard solution used (concentration multiplied by volume) and can then use this to determine the moles in the unknown solution. By then dividing this by the volume of the unknown solution, we can work out the concentration.

See also

• Titration