Acid-Base Titrations

Introduction

• Titration curve: a graph of volume of base vs. pH, when an acid is titrated with the base.

• We can also obtain titration curves for volume of ACID added to base, but we will omit those this term.

• Three types of titrations

  1. Strong acid-strong base

  2. Weak acid-strong base

  3. Strong acid-weak base  -  not covered this term.

• Equivalence Point: the point at which the moles of  acid = the moles of base.

  • Since  M x L = moles…

  • At the equivalence point    M x V acid  =   M x V base.

  • Polyprotic acids will have more than one equivalence point,  and the equation above will work only for the FIRST equivalence point.

Strong Acid-Strong Base Titration

• The pH is initially very low (only strong acid present).

• Any titration curve can be considered to have four parts.

  1. Initial pH: before base is added.

     1. For a strong acid, the pH is just – log[HA]

  2. Acid range: the moles of acid > moles of base.  The pH depends on the molarity of the EXCESS moles of acid.

  3. Equivalence point: for strong acid- strong base, the pH=7

  4. Base range: beyond equivalence point.

     1. Moles base>moles acid.

     2. The pH depends on the molarity of excess base.

• Indicators are weak acids.

  • The weak acid is a different color than its conjugate base.