Titration Curves -1

Neutralisation of Acids

Acid + base --> salt + water

Amount (n) of base needed to neutralise an acid depends only on the concentration and volume and is independent of acid strength (n=c x V)
Concentrationof an acid solution is found by carrying out a titration with a base of accurately known concentration (called a standard solution). Such a titration is also called a volumetric analysis. Given the titration data, it is possible to calculate the concentration of the acid.
To determine the amount of base needed to neutalise an acid we normally use an appropriate acid-base indicator and stop the titration at the point when the indicator changes colour. This is called the end-point of the titration.
If an appropriate indicator has been used, then the end-point will also be the equivalence point of the titration. This is when the amount (n) added base is exactly equal to the initial amount (n) of acid in the flask (if strong acid, strong base). However at the equivalence point, the pH of the solution is not neccesarily neutral. This is because the salt solution formed may itself contain an ion which is acidic or basic.

Acid-base Titration Curves

A plot of the pH of acid solution against the volume of added base (or vice versa) produces what we call a pH titration curve. There are charactersitic points on the curve that can be identified. One of these is the equivalence point, the midpoint of the section where the pH of the solution rises (or falls) sharply.

Important pH values to note on an acid-base titration curve are:

The initial pH: this is the pH of the solution in the flash before any solutions formt he burette is added.
The pH at the equivalence point: this is the point halfway up the steepet part of the curve.
The pH after the equivalence point: this is pH after all the soltuion in the flask has reacted and excess solution from the burette has been added.

For titration involving a weak acid or weak base, two other places on the curve should be noted:

The buffer zone
The mid-point of the buffer zone: the point on th ecurve where half the volume needed to reach the equivalence point has been added.

Titration of a strong acid with a strong base

The example on the worksheet shows how the pH changes when a stong acid is added to a strong base or vice versa. The equi

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