0.7 Titration  (Page 2/4)

Ph plots

The pH values are plotted against the volume of titrant (acid or base). Except for weak acid – weak base, there is a sharp change in pH value at the equivalence point. The plot is almost parallel to pH axis. The sharp range at equivalence point is due to the fact that pH itself is a logarithm function of hydrogen ion concentration. The gradual change away from equivalence point is explained on the basis of buffering effect of acid or base as the case be.

Strong acid – strong base

The reaction considered here is :

$$NaOH\left(aq\right)+HCl\left(aq\right)\to NaCl\left(aq\right)+H_{2}O$$

In the pH plot in which acid is titrant, pH of the solution in the flask is 14 due to strong base (NaOH). As strong acid is added, pH is almost flat till the equivalence point volume, when there is sharp drop in pH value. The extent of drop or rise depends on the reaction rate, which is very fast for strong acid- strong base reaction. The pH drop at equivalence point can induce color change in either of two indicators as drop in pH covers color change range for two indicators.

In the pH plot in which base is titrant, pH of the solution in the flask is near zero due to strong acid (HCl). As strong base is added, pH is almost flat in the beginning till the equivalence point volume, when there is sharp rise in pH value. The pH increase at equivalence point can induce color change in either of two indicators as rise in pH covers color change range for two indicators.

$$\text{volume of HCl used}=\text{volume acid required for neutralization of NaOH}$$

This means that :

$$\text{geq of HCl}=\text{geq of NaOH (indicator : methyl orange or phenolphthalein)}$$

Strong acid – weak base

The reaction considered here is :

$$N{H}_{4}OH\left(aq\right)+HCl\left(aq\right)\to N{H}_{4}Cl\left(aq\right)+H_{2}O$$

In the pH plot in which acid is titrant, pH of the solution in the flask is about 11 due to weak base ( $N{H}_{4}OH$ ). As strong acid (HCl) is added, pH drops gently till the equivalence point volume. The extent of drop in pH is lesser in comparison to strong acid - strong base reaction as reaction rate is relatively slower. The pH drop at equivalence point is shorter and can induce color change in methyl orange only.

In the pH plot in which base is titrant, pH of the solution in the flask is near zero due to strong acid (HCl). As weak base is added, pH is almost flat in the beginning till the equivalence point volume, when there is sharp rise in pH value. The pH rise at equivalence point is shorter and can induce color change in methyl oranage only.

$$\text{volume of HCl used}=\text{volume acid required for neutralization of}N{H}_{4}OH$$

This means that :

$$\text{geq of HCl}=\text{geq of NH4OH (indicator : methyl orange)}$$

Weak acid – strong base

The reaction considered here is :

$$C{H}_{3}COOH\left(aq\right)+NaOH\left(aq\right)\to C{H}_{3}COONa\left(aq\right)+H_{2}O$$

In the pH plot in which acid is titrant, pH of the solution in the flask is about 14 due to strong base (NaOH). As weak acid (HCl) is added, pH drops gently till the equivalence point volume. The extent of drop in pH is lesser in comparison to strong acid - strong base reaction as reaction rate is relatively slower. The pH drop at equivalence point is shorter and can induce color change in phenolphthalein only.