Explain why sodium carbonate against hydrochloric acid titration using phenophthalein titration do not go to completion but stop at the sodium hydrogen carbonate stage.
Carbonate becomes a diprotic acid
In order to understand why the titration between sodium carbonate and hydrochloric acid stops at the sodium hydrogen carbonate stage, we need to consider the chemical reactions involved.
Sodium carbonate (Na2CO3) is a basic compound, while hydrochloric acid (HCl) is an acidic compound. When the two are mixed together during titration, a neutralization reaction occurs.
The initial reaction is as follows:
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
In this reaction, sodium carbonate reacts with hydrochloric acid to form sodium chloride, water, and carbon dioxide. This reaction indicates that complete neutralization of sodium carbonate by hydrochloric acid should result in the formation of sodium chloride and carbon dioxide.
However, during the titration, we use phenolphthalein as an indicator. Phenolphthalein is a pH indicator that changes color in a specific pH range. It is colorless in acidic solutions and turns pink in basic solutions.
As we add hydrochloric acid to the sodium carbonate solution, the pH decreases (increasing acidity) and the phenolphthalein remains colorless. This is because sodium carbonate is highly alkaline, and it takes a certain amount of acid (HCl) to neutralize it and reach a pH level where phenolphthalein can start changing color.
Initially, the reaction proceeds as expected, and sodium carbonate is neutralized:
Na2CO3 + HCl → NaHCO3 + NaCl
Sodium carbonate (Na2CO3) is converted to sodium hydrogen carbonate (NaHCO3), while sodium chloride (NaCl) is also formed. At this stage, the phenolphthalein indicator may just start to change color, indicating the presence of a mild basic solution due to the formation of sodium hydrogen carbonate.
The reaction can be continued until all the sodium carbonate is fully converted to sodium hydrogen carbonate by the addition of more hydrochloric acid:
NaHCO3 + HCl → NaCl + H2O + CO2
However, because phenolphthalein is not sensitive to the pH change caused by the formation of sodium hydrogen carbonate, it does not provide a clear end point for the titration. Instead, the reaction often stops at the formation of sodium hydrogen carbonate (NaHCO3) without reaching full neutralization.
To obtain a more accurate end point in this titration, it is common to use a different indicator called methyl orange, which changes color in a lower pH range. Using methyl orange would allow us to detect the complete neutralization of sodium carbonate and HCl, as it would change its color when the pH reaches a lower level indicating full neutralization.
In summary, the titration between sodium carbonate and hydrochloric acid using phenolphthalein as an indicator does not go to completion because phenolphthalein is not sensitive enough to detect the formation of sodium hydrogen carbonate (NaHCO3) during neutralization.