Another example of acid base titration is titration between weak acid and strong base. The titration curve of 0.1 M acetic acid 100 mL that titrated with 0.1 M sodium hydroxide is shown below:
At the beginning of the titration ( before the titration is started) we only have CH3COOH 0.1 M 100 mL. Acetic acid is a weak acid so we can calculate the hydrogen ion concentration with the formula
When the titration is started some of acetic acid reacts with sodium hydroxide to yield sodium acetate and water, so in the Erlenmeyer flask contains sodium acetate, acetic acid, and water this means we have buffer system. As the titration proceeds the pH slowly increase and the ratio of [CH3COO-]/[CH3COOH] changes, and in the midpoint of the titration [CH3COO-] = [CH3COOH], and the pH is equal to pKa. We calculate the pH by pH = pKa + log ( CA- / CHA).
At the equivalence point , in the solution contains CH3COONa, because it is a salt from strong base with weak acid it will hydrolyzes partially and the pH at the equivalence will be alkaline, and the pH will be depends on the concentration of CH3COONa. The formula for calculate the pH is
When sodium hydroxide is added the concentration of CH3COO- is suppressed, and the pH of the solution is determined only with concentration excess of OH-. We calculate the pH by
[OH-] = [ excess OH-].
The indicator for this titration must have a transition range within about pH 7 to 10, so the phenolphthalein is meet s this requirement. If other indicator such as methyl red had been use as the indicator it would have changing color shortly after the titration begin and would change to the alkaline color up to pH 6, before the equivalence was eve reached.
note :
CHA = concentraion in Molar of weak acid (CH3COOH)
CA- = concentration in Molar of conjugated weak acid ( CH3COO-)
At the beginning of the titration ( before the titration is started) we only have CH3COOH 0.1 M 100 mL. Acetic acid is a weak acid so we can calculate the hydrogen ion concentration with the formula
When the titration is started some of acetic acid reacts with sodium hydroxide to yield sodium acetate and water, so in the Erlenmeyer flask contains sodium acetate, acetic acid, and water this means we have buffer system. As the titration proceeds the pH slowly increase and the ratio of [CH3COO-]/[CH3COOH] changes, and in the midpoint of the titration [CH3COO-] = [CH3COOH], and the pH is equal to pKa. We calculate the pH by pH = pKa + log ( CA- / CHA).
At the equivalence point , in the solution contains CH3COONa, because it is a salt from strong base with weak acid it will hydrolyzes partially and the pH at the equivalence will be alkaline, and the pH will be depends on the concentration of CH3COONa. The formula for calculate the pH is
[OH-] = [ excess OH-].
The indicator for this titration must have a transition range within about pH 7 to 10, so the phenolphthalein is meet s this requirement. If other indicator such as methyl red had been use as the indicator it would have changing color shortly after the titration begin and would change to the alkaline color up to pH 6, before the equivalence was eve reached.
note :
CHA = concentraion in Molar of weak acid (CH3COOH)
CA- = concentration in Molar of conjugated weak acid ( CH3COO-)
No comments:
Post a Comment