Which pH indicator acts as a Bronsted-Lowry base, accepting protons in acidic solutions and changing colour f… - Bihar NHM CHO-2025

Bihar NHM CHO-2025

Biochemistry & Nutrition

Medium

Appeared in: Bihar NHM CHO-2025

Explanation

Methyl orange functions as a Bronsted-Lowry base, which is defined as a substance that can accept a proton (H+).
In an acidic environment (specifically, at a pH below 3.1), the methyl orange molecule accepts a proton.
This protonation causes a change in the molecule's structure, leading to a visible color shift from yellow (the basic, unprotonated form) to red (the acidic, protonated form).
The transition pH range for this change is between 3.1 and 4.4, which is within the acidic range.

Option B: Thymolphthalein changes from colorless to blue, not yellow to red. Furthermore, its color change occurs in a strongly basic pH range (9.3-10.5), not an acidic one.
Option C: Although Alizarin yellow does change from yellow to red, this transition happens in a highly basic pH range (10.1-12.0). The question specifies the change must occur in acidic solutions.
Option D: Bromocresol Green changes from yellow in acidic solutions to blue in basic solutions. The final color (blue) does not match the color specified in the question (red).

Visual 1: Diagram: Illustrating the chemical structures of the yellow (basic) and red (acidic) forms of Methyl orange, showing the site of protonation.
Visual 2: Chart: A comparative chart of common pH indicators, showing their pH ranges and color changes in acidic vs. basic conditions.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Bronsted-Lowry acid-base theory and the function of pH indicators as background academic context rather than a clinical decision trigger.
Understanding pH is fundamental in nursing for interpreting arterial blood gas (ABG) results to identify acidosis or alkalosis, assessing urine pH, and understanding the absorption and action of pH-dependent medications.
The principle of titration, for which indicators are essential, is a core concept in the quality control and preparation of many pharmaceutical compounds and laboratory reagents.
What if? If a nurse needs to confirm the placement of a nasogastric feeding tube, they test the pH of the gastric aspirate. They would use a wide-range pH paper, expecting a result of less than 5.5. Using a single indicator like methyl orange would be insufficient as it only shows a change within a narrow acidic range and doesn't provide a specific pH value.

How to Approach the Question

First, break down the question into its key requirements: 1) Must be a pH indicator. 2) Must act as a Bronsted-Lowry base (accepts a proton). 3) The change must happen in an acidic solution. 4) The color change must be from yellow to red.
Review the options one by one, checking each against all four requirements.
Recall or deduce the properties of Methyl orange. It is known to be red in strong acid and yellow in weak acid/base, with a transition pH of 3.1-4.4. This fits all criteria.
Evaluate the other options to confirm they are incorrect. Thymolphthalein has the wrong colors and pH range. Alizarin yellow has the right colors but the wrong pH range (basic). Bromocresol Green has the wrong final color (blue).
This process of elimination confirms that Methyl orange is the only option that satisfies all the conditions stated in the question.

Concept Tested & Keywords

Concept Tested: Bronsted-Lowry acid-base theory and the function of pH indicators.
Stem keywords: pH indicator, Bronsted-Lowry base, protons, acidic solutions, yellow to red
Lead-in keywords: Which

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