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What is Titration?

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

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.

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*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 PointHalf-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.

Notes:

At this point all acetic acid-->acetation ion.

Near this point, pH chages little with +base.

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Acids, Ka, and pKa

Monoprotic AcidsDiprotic AcidsPolyprotic 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).

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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 acidpH of solution>pKa of acid

[H+] On/Off?

On

Off

Acid Protonated/Deprotonated?

Protonated

Deprotonated

Predominant form:

Protonated Acid

Conjugate Base(Deprotonated Acid)

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Source Information

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.

Wikipedia: https://en.wikipedia.org/wiki/Acetic_acid

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