pH curves, titrations and indicators (3.1.12.5)

What are acid-base titrations used for?

Acid-base titrations are used to find the exact concentration of an acid or base.

What is a pH curve and what can you discern from them?

pH curve - plot of how pH changes as an alkali is added to an acid (or vice versa) - x-axis is the volume of alkali/acid added and y-axis is pH

You can discern:

• The pH of the acid and base added/starting (from where the curve starts and finishes)

• The volume at the equivalence point

• The volume of alkali/base at the equivalence point

• The range of pH at the vertical section of the curve

What is the equivalence point?

Equivalence point - the midpoint of the vertical section of a pH curve - when the moles of alkali added equals the moles of acid present - pH is not always 7 at equivalence point

What are the different types of pH curve?

There are 4 types of pH curve:

• Strong acid - strong base

• Strong acid - weak base

• Weak acid - strong base

• Weak acid - weak base

What are indicators?

Indicators are weak acids where HA and A^- are different colours

HA ←→ H⁺ + A^-

At low pH, HA is the main species present (since the equilibrium would shift to the left since there is a higher concentration of H⁺)

At high pH, A^- is the main species present (since the equilibrium would shift to the right since there is a lower concentration of H⁺)

How should a suitable indicator be picked?

In a titration the pH changes rapidly at the end point (when the indicator changes colour) as the last drop of acid/alkali is added. Therefore for the indicator to change colour at the equivalence point, the indicator must change colour within the vertical section.

Essentially the end point of the titration should coincide with the equivalence point.

What indicator should be used for a weak acid - weak alkali titration?

In a weak acid - weak alkali titration, there is no suitable indicator (as the pH change is too gradual close to the equivalence meter) so a pH meter should be used