0.7 Titration  (Page 3/4)

In the pH plot in which base is titrant, pH of the solution in the flask is near 2-3 due to weak acid ( $C{H}_{3}COOH$ ). As strong base is added, pH is rises 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 phenolphthalein only.

$$\text{volume of}C{H}_{3}COOH\text{used}=\text{volume acid required for neutralization of NaOH}$$

This means that :

$$\text{geq of}C{H}_{3}COOH=\text{geq of NaOH (indicator : phenolphthalein)}$$

Weak acid – weak base

The reaction considered here is :

$$C{H}_{3}COOH\left(aq\right)+N{H}_{4}OH\left(aq\right)\to C{H}_{3}COON{H}_4\left(aq\right)+H_{2}O$$

There is no sharp change in pH value at equivalence point. The pH value is approximately 7 at equivalence. In this case, there is only a point of inflexion (against extended vertical shift in pH value as observed in earlier cases), which can be detected with confidence.

Two stage titration

Few reactions are completed in two stages. Corresponding to each stage, there is an equivalence point. We need to employ suitable indicator to identify completion of individual reaction. We decide selection of the indicator based on the range of pH change at equivalence volume of the titrant.

Sodium carbonate and hydrochloric acid

The reaction is completed in two stages. The first stage reaction is :

$$N{a}_{2}C{O}_3\left(aq\right)+HCl\left(aq\right)\to NaCl\left(aq\right)+NaHC{O}_3\left(aq\right)$$

Considering that HCl is the titrant, we see that sodium bicarbonate is intermediate product, which is itself a basic salt. It means that pH value remains in basic range above 7. The equivalence volume of acid corresponds to formation of sodium bicarbonate. Clearly, phenolphthalein can detect the drop of pH value at equivalence. However, this detection will not correspond to complete titration of sodium carbonate as sodium bicarbonate is further acted on by the acid according to the second stage reaction as given here :

$$NaHC{O}_3\left(aq\right)+HCl\left(aq\right)\to NaCl\left(aq\right)+C{O}_2\left(gas\right)+H_{2}O\left(liq\right)$$

The important aspect of this reaction is that carbon dioxide is acidic and as such pH of the solution further goes down. On the completion of reaction, there is sharp drop in pH value which falls within the range of color change of methyl orange. Thus, methyl orange detects the completion of reaction of sodium carbonate with hydrochloric acid.

$$\text{geq of HCl}=\text{geq of}N{a}_{2}C{O}_3\text{(indicator : methyl orange)}$$

It is clear from the description that phenolphthalein can not detect completion of reaction of acid with sodium carbonate. There is, however, an interesting aspect about the equivalence volumes involved in two stages of reaction. The first equivalence volume (25 ml) is exactly half of the second equivalence volume (50 ml). We can conclude that first equivalence volume with phenolphthalein gives exactly half of the reaction with sodium carbonate.

$$\text{geq of HCl}=\frac{1}{2}\text{geq of}N{a}_{2}C{O}_3\text{(indicator : methyl orange)}$$

This is how phenolphthalein can also be used to estimate sodium carbonate concentration – even though it does not detect completion of the reaction. It should also be emphasized that neutralization of sodium bicarbonate is detected by methyl orange - not by phenolphthalein. We may conclude that phenolphthalein can not detect neutralization of sodium bicarbonate.