Topic 18.3: pH curves Flashcards
Titration definition
Technique used in analytical chemistry to determine the concentration of an unknown acid or base
Titration curve definition
Graph of the pH (vertical axis) versus the amount of reagent progressively added to the original sample
Strong Acid + Strong Base Titration Curve
a) Initial pH = pH of strong acid
b) pH changes only gradually until equivalence
c) Very sharp jump in pH at equivalence (point of inflexion)
d) After equivalence the curve flattens out at the pH of strong base
e) pH at equivalence = 7
Weak Acid + Stronger Base Titration Curve
a) Initial pH = pH of weak acid
b) pH stays relatively constant until equivalence – labelled as buffer region
c) Jump in pH at equivalence, which is not as much of a jump as for a SA – SB titration
d) After equivalence, the curve flattens out at the pH of strong base
e) pH at equivalence > 7
Half-equivalence point
It represents where half of an acid has been neutralized by base and converted into salt, while the other half of the acid in the flask remains unreacted.
What mixture is found in the half-equivalent point?
A buffer, explaining why the pH in this region is relatively resistant to change in pH on the addition of small amounts of base
(pH = pKa)
Strong Acid and Weak Base Titration Curves
a) Initial pH = pH of strong acid
b) pH stays relatively constant through the buffer region to equivalence
c) Jump in pH at equivalence from about pH 3.0 – 7.0
d) After equivalence the curve flattens out at a pH of weak base
e) pH at equivalence is <7
Weak Acid and Weak Base Titration Curves
a) Initial pH = pH of weak acid
b) Addition of base causes the pH to rise steadily
c) Change in pH at the equivalence point is much less sharp than in the other titrations
d) After equivalence the curve flattens at the pH of weak base
Definition of indicator
Weak acid / base where the components of the conjugate acid-base pair have different colors
Chemical formula of indicator
a) Weak acid
b) Weak base
a) HIn ⇌ H+ + In-
b) In- + H2O ⇌ HIn + OH-
What factor influences the change of color of indicators?
pH of the solution
Explanation of the effect of pH on the change of color of indicator
a) HIn ⇌ H+ + In-
a) Increasing [H+]: The equilibrium will shift to the left in favor of HIn
b) Decreasing [H+]: The equilibrium will shift to the right in favor of In-
When do indicators change color?
a) When the pH is equal to their pKa
b) The addition of a very small volume of acid/base will shift the equilibrium and cause the indicator to change color
Indicators used with:
a) Strong acids
b) Strong bases
a) Methyl orange
b) Phenolphthalein
End-point of an indicator
pH at which it changes color (pKa = pH)
