This post is on Chemistry Notes on Titration pH graphs and types of indicators.
Titration pH Curve:
Before the End Point:
The pH remains high.
H⁺ ions added will neutralise OH⁻ ions.
The concentration of OH⁻ ions decreases.
At the End Point:
A single drop causes a sharp change in pH, resulting in a colour change.
Just enough H⁺ ions are added to completely neutralise the OH⁻ ions.
The end point is the midpoint of the vertical part of the pH curve.
After the End Point:
The pH remains low.
The concentration of H⁺ ions increases.
Important Note:
The pH at which the indicator changes color must match the pH range where there is a sharp change during complete neutralization.
Common Indicators and pH Ranges:
- Litmus Paper:
Blue to red in acids, red to blue in bases.
- Universal Indicator:
Full pH spectrum
- Phenolphthalein:
Colorless below pH 8.3, pink above.
- Methyl Orange:
Red below pH 3.1, orange between 3.1 and 4.4, yellow above 4.4.
Note:
- Neutral Solutions: Universal indicator (green at pH 7).
- Titrations: Strong Acid/Base: Phenolphthalein or methyl orange.
Quiz time!
Question 1) What indicator can be used to test for weak acids?:
Answer: Methyl orange. Methyl orange will be red in strong acids (pH <3 ), and orange and yellow in weak acids (pH > 4) Phenolphthalein only changes to pink at pH 8 which is far above the pH of acids.
Question 2) What color will methyl orange be in pH 7 neutral solution?
Answer: Yellow. Methyl orange turns yellow in pH > 4.
In a typical titration, the endpoint with methyl orange is observed at around pH 4.4, where it turns orange as it transitions from acidic (red) to basic (yellow). So while pH 7 is yellow, the endpoint will be orange.
Question 3: What colour will Phenolphthalein be in pH 7 neutral solution?
Answer: Colorless. Phenolphthalein is colorless until pH 8. Phenolphthalein turns pink in pH>8.
That’s it!
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