43. Titration curves of mono- and polyprotic acids Flashcards
To record the titration curve, the basic rule of acid-base titrations is applied.
What is that?
the titrant solution is a monovalent strong base or acid, depending on the nature of the titrated material.
-> the pH of the reaction mixture is measured and plotted against the volume of titrant solution added.
What is the pH of neutralization point
pH = 7 in the reaction mixture,
What are inflection points?
where the function changes curvature.
2 common changes of curvature at the inflection
(A) increasingly steep → increasigly flat
(B) inreasingly flat → increasingly steep
The curvature is increasingly steep → increasingly flat inflection point(s)
-> What does it reflect?
there is only one type of salt in the reaction mixture.
Around the increasingly flat → increasingly steep inflection point(s) on the curvature
-> what do they reflect?
the pH changes the least due to the presence of a buffer in the reaction mixture
-> we are deal- ing with a weak acid.
A given buffer has the ___ capacity at such inflection point, since here the pH hardly changes upon the addition of the the strong base.
highest
If the titration curve has more than one increasingly flat → increasingly steep inflection point, what does it mean?
it can be explained with that various buffers can be prepared starting from the given weak acid, which is a property of polyprotic weak acids.
Equipment required for Recording of titration curves of acids
- buret,
- 100 ml beaker,
- 50 ml graduated cylinder,
- pH meter,
- magnetic stirrer unit + magnetic stir bar,
- graph paper,
- writing instruments.
Titration of strong acid
𝐻𝐶𝑙 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂
HCl content at the equivalent point?
- HCl is consumed up to the equivalence point
Titration of strong acid
𝐻𝐶𝑙 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂
What is present at this equivalent point
Only NaCl is present at the equivalence point , pH= 7
Titration of strong acid
𝐻𝐶𝑙 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂
What happen if we add more NaOH?
Adding more NaOH produces a rapid increase in pH
Titration of strong base
𝑁𝑎𝑂𝐻 + 𝐻𝐶𝑙 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂
What is the equivalent point?
At pH=7
Titration of weak acid
C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂
Where is the equivalent point?
At pH. = 9 -> At basic pH
Titration of weak acid
C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂
𝐂𝐇𝟑𝐂𝐎𝐎𝐇 𝐰𝐢𝐭𝐡 𝐍𝐚𝐂𝐇𝟑𝐂𝐎𝐎 forming ___
buffer
Titration of weak acid
C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂
What is present at the equivalence point?
Only 𝐍𝐚𝐂𝐇𝟑𝐂𝐎𝐎, but
CH3COO- + H2O OH- + CH3COOH
Titration of weak acid
C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂
After the equivalence point, what happen to NaOH concentration?
the NaOH concentration increases.
-> The pH is determined by the addition of NaOH, the dissociation of NaCH3COO− is suppressed.
Titration of weak acid
C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂
What does the 1st point indicate?
1st point corresponds to the midpoint of the titration, when approximately half the weak acid has been neutralized
Titration of weak acid
C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂
(1) What is the pH of Middle of the buffer range?
- > The buffer works most efficiently around (2) __
1) pH = pKa
2) pKa
3 Differences between titration curves of strong and weak acids
- The initial pH of strong acid is lower than that in a weak acids
- A buffer range is obtained only when titrating a weak acid
(There is an inflection point in the middle of the buffer range 𝑝𝐻 = 𝑝𝐾𝑎 .) - The pH at the equivalence point of the titration of a weak acid with strong base is greater than 7.00.
- > in strong acids -> equivalence point = neutralization point (pH = 7)
Dissociation of a polyprotic weak acid.
How many pKa values does it have?
What is the largest dissociation constant based on titration curve of phosphoric acid?
Phosphoric acid is triprotic
- > 3 𝒑𝑲𝒂 values
- > 𝐾𝑎1 is the largest of all the successive acid dissociation constants.
Titration curve of phosphoric acid
𝐻3𝑃𝑂4 + 3𝑁𝑎𝑂𝐻 → 𝑁𝑎3𝑃𝑂4 + 3𝐻2𝑂
Titration curve of phosphoric acid
𝐻3𝑃𝑂4 + 3𝑁𝑎𝑂𝐻 → 𝑁𝑎3𝑃𝑂4 + 3𝐻2𝑂
What do the 3 equivalence points of titration curve of phosphoric acids reflect?
Titration curve of phosphoric acid
𝐻3𝑃𝑂4 + 3𝑁𝑎𝑂𝐻 → 𝑁𝑎3𝑃𝑂4 + 3𝐻2𝑂
Describe the buffer ranges at these 3 points
