Below are some notes on titration that can be used for an understanding or refresher that is helpful and useful for the sciences; specifically biology, chemistry, and biochemistry.
What is Titration?
Titration is where measured amounts of base are added to measured amounts of acid in an experiment. Adding base to acid changes the pH of the solution.
In free amino acids at neutral pH:
- carboxylate group = "-" charged (acidic)
- amino group ="+" charged (basic)
Amino acids without charged groups on their side chains exist as zwitterions (no net charge).
The charge (if any) on the side chain affects the net charge on the amino acid.
What is a Titration Curve?
Titration curves of amino acids indicate the pH ranges in which titratable groups gain/lose a proton.
Curve is affected by:
- Titratable groups that can gain/lose [H+]
- Side chains of amino acids that can contribute titratable groups
The value of the pKa for each acid determines the pH values at the equivalence point and at the half-equivalence point (inflection point).
What is the Equivalence Point in a Titration Curve?
Equivalence Point= the point in the titration where the acid is neutralized.
*This point occurs when 1mol of base has been added for each mole of acid, and when all of the acetic acid (acid) has been converted to acetate ion (its conjugate base).
Acetic acid is one of the simplest carboxylic acids. It can be written as:
CH3CO2H or CH3COOH
What is the Half-Equivalence Point in a Titration Curve?
Half-Equivalence Point= the inflection point on the titration curve where the pH equals the pKa of acetic acid.
*This point occurs when 0.5mol of base has been added for each mole of acid that is present.
*Near the inflection point, pH changes minimally as more base is added.
Titration Curve: Points
|Equivalence Point||Half-Equivalence Point|
What it is:
Titration point where acid = neutralized.
Titration inflection point where pH = pKa.
How to reach it:
1 mole of base added per mole of acid.
0.5 mole of base added per mole of acid.
At this point all acetic acid-->acetation ion.
Near this point, pH chages little with +base.
Acids, Ka, and pKa
|Monoprotic Acids||Diprotic Acids||Polyprotic Acids|
Releases one [H+] ion
Releases two [H+] ions
Releases more than two [H+] ions
Has one Ka value
Have two Ka values
Have more than two Ka values
Has one pKa value
Have two pKa values
Have more than two pKa values
Amino acids and peptides act as diprotic and polyprotic acids.
When the pH of a solution is less than the pKa of an acid, the protonated form is the one that dominates.
When the pH of a solution is greater than the pKa of an acid, the deprotonated form is the one that dominates.
(The deprotonated form is the conjugate base).
Protonated and Deprotonated Forms of Acids and Conjugate Bases
A way to keep track of protonated and deprotonated forms of acids and their conjugate bases:
Protonated/Deprotonated Acid and Conjugate Bases Forms
|pH solution<pKA of acid||pH of solution>pKa of acid|
Conjugate Base(Deprotonated Acid)
Other Hubs in My "Notes on..." Series
- Notes on Water and Polarity
- Notes on Acids, Bases, and pH
- Notes on Buffers
- Notes on Amino Acids
- Notes on Peptides
The information used for this hub was taken from the following sources:
"Biochemistry" by Mary K. Campbell and Shawn O. Farrel; 7th edition.
My biochemistry lectures at school.
Knowledge and notes taken from previous courses in chemistry and biology.