Acid-Base Titration Notes

Acid-Base Titration

Define terms relevant to Acid-Base titration.
Define the term "Acid-Base titration curve" and illustrate various Acid-Base titration curves.
Define Indicators and explain how they work.

Neutralization Reaction: The reaction between an acid and a base.
Indicator: A substance that indicates when the acid and base are mixed in exactly the right proportions to "neutralize" each other.
End Point: The point in a titration when the indicator changes color, signaling the completion of the reaction. It occurs slightly after the equivalence point.
Equivalence Point: The point at which chemically equal amounts of acid and base have reacted.

Strong Acid - Strong Base (e.g., NaOH and HCl):
- Adding Acid to Alkali: pH starts high, gradually decreases, and then rapidly drops at the equivalence point (pH ~7).
- Adding Alkali to Acid: pH starts low, gradually increases, and then rapidly rises at the equivalence point (pH ~7).
Strong Acid - Weak Base (e.g., HCl and NH3):
- Adding Acid to Alkali: Rapid initial pH drop, followed by a slower decrease, with the equivalence point below pH 7.
- Adding Alkali to Acid: Gradual pH increase, followed by a sharp rise at the equivalence point (pH < 7).
Weak Acid - Strong Base (e.g., CH3COOH and NaOH):
- Adding Acid to Alkali: Gradual pH decrease, with the equivalence point above pH 7.
- Adding Alkali to Acid: Gradual pH increase, followed by a sharper rise at the equivalence point (pH > 7).
Weak Acid - Weak Base (e.g., CH3COOH and NH3):
- Adding Acid to Alkali: Gradual pH change, making it difficult to identify a clear equivalence point.

Equilibrium Reaction: HIn \leftrightarrow H^+ + In^-
- HIn represents the indicator in its acidic form (Color 1).
- In- represents the indicator in its basic form (Color 2).
In acidic media, Colour 1 is observed.
In basic media, Colour 2 is observed.
A weak organic acid is used to indicate the equivalence point of a reaction.

The indicator's color changes over a range of 1.5 to 2 pH units. A smaller range ensures greater accuracy.
Choose an indicator whose pH range of color change includes the equivalence point of the solution being titrated.

Strong Acid vs. Strong Base:
- Either indicator can detect the end-point because the curve is very steep at the equivalence point.
Strong Acid vs. Weak Base:
- Phenolphthalein is unsuitable.
- Methyl orange changes from yellow to orange near the equivalence point.
- Select an indicator that changes color on the steepest part of the curve.
Weak Acid vs. Strong Base:
- Methyl orange is unsuitable (stays yellow throughout).
- Phenolphthalein changes from pink to colorless at the equivalence point.
Weak Acid vs. Weak Base:
- Indicators are generally not used because the color change is not sharp enough.