Wize High School Grade 12 Chemistry Textbook > Acids and Bases
Titration Curves
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Introduction to Titrations
Acid-base titrations are neutralization reactions between a titrant (solution in the buret) and analyte (solution in the flask).
Wize Concept
Titrations always involve an acid reacting with a base!
Titrant
- Solution in the buret
- We control the amount we add of the titrant to the flask
- We know its volume, concentration, and identity Example: 1M 100mL NaOH
- It is usually a strong acid or base
Analyte
- Solution in the flask
- It is the unknown (we don't know its concentration, identity, or if it is weak or strong)
- We do know it's volume (can easily measure its volume in the flask) and its pH (we can simply use a pH meter to measure this in the flask)
- The other way to know if there's a pH change in the flask is if we see a visual change in color
- Indicators help to indicate this pH change by changing the color inside the flask
- When the color changes, we call this an end-point.
Wize Concept
We do titrations (where we add small amounts of titrant to the analyte) to determine the unknown concentration of an acid or a base!
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Labeling a Titration Curve
A plot showing the pH of the solution as a function of the quantity of acid or base added is known as a titration curve.
The titrant (substance in the buret that is being added to the analyte in the flask slowly overtime) is:
NaOH
What is the volume for the equivalence point?
25mL
What is the volume for the half equivalence point?
12.5mL
Wize Concept
@ Half-Equivalence Point: [HA] = [A-] and moles of acid=moles of conjugate base
- Exactly half of the initial amount of weak species has been reacted by the addition of a strong base or acid
- Remember according to the Henderson Hassalbalch equation:
- When the [HA]=[A-] how are pH and pKa related?
- log1=0 so pH=pKa of analyte
@ Equivalence Point: [HA] = [OH-] added and moles of acid=moles of base
- Enough titrant has been added to completely neutralize ALL of the unknown
- In other words, the acid and base have completely reacted together so there is no more acid or base left.
Finally, the pH @ Equivalence Point (pH=7, pH <7, or pH>7) is determined by the acidity/basicity of the salt produced
- Strong acid and strong base → pH @ equivalence point will be neutral (=7)
- Strong acid and weak base → pH @ equivalence point will be acidic (<7)
- Weak acid and strong base → pH @ equivalence point will be basic (>7)
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Consider the titration of 25.00 mL of 0.100 M CH3CO2H with 0.100 M NaOH. At equivalence point, what is the volume of NaOH that was added? The reaction can be represented as:
CH3CO2H + OH− ⟶ CH3CO2− + H2O
What was the volume of NaOH that was needed for this titration to go to completion?
Practice: Equivalence Point
For which type of titration will the pH be basic at the equivalence point?