Question

The pH range at which the desirable colour change occurs for a particular indicator is _________.

• A. pH
• B. pF
• C. Rh
• D. pT

Hint:

When selecting an indicator for an acid-base titration, choose one whose transition index (pT) lies within the rapid pH change region of the titration curve's equivalence point.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
In acid-base titrations, indicators are weak organic acids or bases that change color over a specific pH range as they transition between their protonated and deprotonated forms.

Step 3: Detailed Explanation:
The pH range over which an indicator changes color is called its transition interval.
The specific pH value at which the indicator shows its most distinct intermediate color change is called the titration exponent or transition index, represented by the symbol pT.
Mathematically, the deprotonation of a weak acid indicator ($\text{HIn}$) can be expressed as:
\[ \text{HIn} \rightleftharpoons \text{H}^+ + \text{In}^- \]
The color change is perceived by the human eye when the concentration of the acid form ($\text{HIn}$) and base form ($\text{In}^-$) are approximately equal.
This transition midpoint occurs when:
\[ \text{pH} = \text{pK}_{\text{In}} = \text{pT} \]
At this specific pH, the indicator undergoes its most rapid and distinct color change, which helps the analyst identify the titration endpoint.
Let us review the other terms:
- pH: The negative logarithm of hydrogen ion concentration.
- pF: The logarithm of the soil moisture tension (expressed as head of water in cm), used in soil physics.
- Rh: A measure of soil redox potential or relative humidity.
Therefore, the desirable color change range is defined by pT.

Step 4: Final Answer:
The correct term is pT, corresponding to option (D).