What are the major species in solution during the titration of citric acid with a strong base?
To determine the major species in solution during the titration of citric acid with a strong base, we need to consider the acid-base reactions that occur. Citric acid (C6H8O7) is a polyprotic acid, meaning it can donate multiple protons (H+ ions) in a stepwise manner.
Citric acid has three carboxylic acid groups, so it can undergo three acid-base reactions with strong base (such as sodium hydroxide, NaOH). Let's go through each step and identify the major species formed.
Step 1: The first carboxylic acid group of citric acid reacts with NaOH:
C6H8O7 (citric acid) + NaOH (sodium hydroxide) → C6H7O7- (citrate ion) + H2O
In this step, the major species formed is the citrate ion (C6H7O7-). The hydrogen ion (H+) from the carboxylic acid group reacts with the hydroxide ion (OH-) from NaOH to form water (H2O). The remaining citrate ion carries a negative charge.
Step 2: The second carboxylic acid group of citric acid reacts with NaOH:
C6H7O7- (citrate ion) + NaOH (sodium hydroxide) → C6H6O7-2 (dihydrogen citrate ion) + H2O
In the second step, the citrate ion reacts with another hydroxide ion, forming the dihydrogen citrate ion (C6H6O7-2). Again, a water molecule is formed by the combination of H+ and OH- ions.
Step 3: The third carboxylic acid group of citric acid reacts with NaOH:
C6H6O7-2 (dihydrogen citrate ion) + NaOH (sodium hydroxide) → C6H5O7-3 (hydrogen citrate ion) + H2O
In the third and final step, the dihydrogen citrate ion reacts with a hydroxide ion to form the hydrogen citrate ion (C6H5O7-3). Once again, water is produced.
Therefore, the major species present during the titration of citric acid with a strong base are the citrate ion (C6H7O7-), dihydrogen citrate ion (C6H6O7-2), and hydrogen citrate ion (C6H5O7-3).