Theories of acid–base indicators MCQs With Answer

Theories of acid–base indicators MCQs With Answer

Understanding acid–base indicators is essential for B. Pharm students studying analytical chemistry and pharmaceutical analysis. This introduction explores indicator behavior, including indicator dissociation, pKa, Henderson–Hasselbalch explanation of color change, transition range (pKa ±1), structural basis of color (conjugation, protonation), and practical selection for titrations (strong/weak acid–base systems). You will also encounter factors affecting indicator performance: solvent, ionic strength, temperature, and mixed indicators. Mastery of these concepts helps in accurate end-point detection during assay and quality control. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which expression best represents the equilibrium for a typical acid–base indicator HIn in water?

• HIn ⇌ H+ + In−
• HIn + H2O ⇌ InH + OH−
• HIn + H+ ⇌ InH2+
• HIn ⇌ H− + In+

Correct Answer: HIn ⇌ H+ + In−

Q2. The color change of an indicator occurs because:

• the indicator evaporates at equivalence point
• the relative concentrations of protonated and deprotonated forms change
• the indicator fluoresces under acidic conditions only
• the indicator undergoes irreversible decomposition

Correct Answer: the relative concentrations of protonated and deprotonated forms change

Q3. According to Henderson–Hasselbalch, when pH = pKa of an indicator:

• only the acidic form is present
• only the basic form is present
• concentrations of HIn and In− are equal
• the indicator is neutral and colorless

Correct Answer: concentrations of HIn and In− are equal

Q4. For reliable visual detection, the practical transition range of an indicator is approximately:

• pKa ± 0.1
• pKa ± 0.5
• pKa ± 1.0
• pKa ± 2.5

Correct Answer: pKa ± 1.0

Q5. Which indicator is appropriate for a strong acid vs. strong base titration?

• Methyl orange (range ~3.1–4.4)
• Phenolphthalein (range ~8.2–10.0)
• Thymolphthalein (range ~9.3–10.5)
• Bromothymol blue (range ~6.0–7.6)

Correct Answer: Phenolphthalein (range ~8.2–10.0)

Q6. The pH at the half-equivalence point of a weak acid titrated with a strong base equals:

• 0.5 × equivalence pH
• pKa of the weak acid
• pKb of the conjugate base
• neutral pH 7.0 always

Correct Answer: pKa of the weak acid

Q7. Which structural change typically causes a dye indicator to change color on protonation?

• Loss of a methyl group
• Change in conjugation and electronic distribution
• Formation of insoluble precipitate
• Cleavage of peptide bonds

Correct Answer: Change in conjugation and electronic distribution

Q8. An indicator with pKa 4.8 will show its mid-color at approximately:

• pH 2.8
• pH 4.8
• pH 6.8
• pH 8.8

Correct Answer: pH 4.8

Q9. Which factor does NOT significantly affect the observed transition range of an indicator?

• Solvent polarity
• Ionic strength
• Temperature
• Atmospheric pressure (within normal lab range)

Correct Answer: Atmospheric pressure (within normal lab range)

Q10. Methyl orange changes color from red to yellow as pH increases because:

• its acidic form is yellow and basic is red
• its basic form is yellow and acidic is red
• it precipitates in alkaline media
• it oxidizes irreversibly at high pH

Correct Answer: its basic form is yellow and acidic is red

Q11. Selection of an indicator for a weak acid–weak base titration is difficult because:

• the equivalence point pH may be close to the transition range of many indicators
• there is no equivalence point
• indicators are inert in such media
• the titration is always exothermic

Correct Answer: the equivalence point pH may be close to the transition range of many indicators

Q12. An indicator acts as a weak acid with pKa = 7.0. At pH 9.0, which form predominates?

• Protonated acidic form HIn
• Deprotonated basic form In−
• Both forms equally
• Neutral undissociated form only

Correct Answer: Deprotonated basic form In−

Q13. Which equation quantitatively links pH, pKa, and the ratio [In−]/[HIn]?

• pH = pKa + log([HIn]/[In−])
• pH = pKa − log([In−]/[HIn])
• pH = pKa + log([In−]/[HIn])
• pH = pKa × log([In−]/[HIn])

Correct Answer: pH = pKa + log([In−]/[HIn])

Q14. Indicator error refers to:

• the systematic difference between end point and true equivalence point
• random fluctuation in indicator color
• loss of indicator by evaporation
• color blindness of the operator

Correct Answer: the systematic difference between end point and true equivalence point

Q15. Which indicator is most suitable for titration of a weak base with a strong acid?

• Phenolphthalein (pH ~8.2–10.0)
• Methyl orange (pH ~3.1–4.4)
• Bromothymol blue (pH ~6.0–7.6)
• Alizarin yellow (pH ~10.1–12.0)

Correct Answer: Methyl orange (pH ~3.1–4.4)

Q16. Which phenomenon explains bathochromic shifts in indicator absorption upon deprotonation?

• Decrease in conjugation length
• Increase in conjugation or resonance stabilization
• Loss of chromophore groups
• Formation of free radicals

Correct Answer: Increase in conjugation or resonance stabilization

Q17. Mixed indicators are used to:

• change color irreversibly
• broaden or sharpen the observed end-point range
• remove CO2 from solution
• stabilize pH against strong acids

Correct Answer: broaden or sharpen the observed end-point range

Q18. The vertical region (steep portion) of a titration curve is important because:

• it indicates the buffering capacity
• it defines where a small volume change gives large pH change—ideal for indicators
• it shows where the solution is neutralized to pH 7
• it is unaffected by analyte concentration

Correct Answer: it defines where a small volume change gives large pH change—ideal for indicators

Q19. Temperature affects indicator behavior mainly by altering:

• molecular weight of the indicator
• pKa value and hence transition range
• color perceived by human eye only
• indicator’s magnetic properties

Correct Answer: pKa value and hence transition range

Q20. An indicator with very small HIn concentration relative to analyte is desirable because:

• it perturbs the titration equilibrium minimally
• it accelerates the reaction kinetics
• it increases ionic strength dramatically
• it produces brighter colors

Correct Answer: it perturbs the titration equilibrium minimally

Q21. Which indicator is commonly used for titration of strong acid with strong base when equivalence pH ≈ 7?

• Phenolphthalein
• Bromothymol blue
• Methyl red
• Alizarin

Correct Answer: Bromothymol blue

Q22. The observable color difference between HIn and In− depends primarily on:

• their molar masses
• their electronic absorption spectra (different chromophores)
• solubility in organic solvents
• ability to form crystals

Correct Answer: their electronic absorption spectra (different chromophores)

Q23. Which is a limitation when using indicators in non-aqueous titrations?

• Indicators always work better in non-aqueous media
• Indicator pKa and color may shift drastically in non-aqueous solvents
• Non-aqueous solvents stabilize indicator color permanently
• There are no suitable indicators for non-aqueous media

Correct Answer: Indicator pKa and color may shift drastically in non-aqueous solvents

Q24. A pharmaceutical assay requires choosing an indicator with transition range near equivalence pH. Which principle guides this choice?

• Select indicator with pKa two units away from equivalence pH
• Select indicator whose transition range overlaps the steep region of titration curve
• Select indicator that is cheapest
• Select indicator that is insoluble in water

Correct Answer: Select indicator whose transition range overlaps the steep region of titration curve

Q25. In the presence of strong oxidizing agents, some indicators may fail due to:

• protonation only
• oxidative decomposition changing chromophore
• reversible color switching
• increased pKa without color change

Correct Answer: oxidative decomposition changing chromophore

Q26. Which descriptor best defines the endpoint in a titration using an indicator?

• the point where added titrant equals sample mass
• the observed color change point corresponding approximately to the equivalence point
• the pH where indicator decomposes
• the end of burette graduation

Correct Answer: the observed color change point corresponding approximately to the equivalence point

Q27. Why is phenolphthalein colorless in acidic solution and pink in basic solution?

• acid form lacks conjugation; base form restores conjugation producing visible absorption
• acid form precipitates; base form dissolves
• acid form fluoresces only under UV
• base form decomposes into colored product

Correct Answer: acid form lacks conjugation; base form restores conjugation producing visible absorption

Q28. When a titration curve shows a very shallow slope at equivalence, the best approach is:

• use a traditional single indicator anyway
• use instrument-based detection (pH meter) or potentiometric titration
• increase indicator concentration to amplify color change
• perform titration at elevated temperature only

Correct Answer: use instrument-based detection (pH meter) or potentiometric titration

Q29. The theoretical basis for the statement “an indicator changes color around pH = pKa ±1” assumes:

• human eye detects a tenfold ratio difference in species concentrations
• equal molar absorptivity of both forms
• only one proton is involved in titration
• indicator concentration is extremely high

Correct Answer: human eye detects a tenfold ratio difference in species concentrations

Q30. Which calculation would you perform to predict the appropriate indicator for a given titration?

• Calculate ionic strength only
• Estimate equivalence point pH from acid/base strengths and choose indicator with matching transition range
• Measure indicator solubility in organic solvents
• Always choose indicator with pKa = 7

Correct Answer: Estimate equivalence point pH from acid/base strengths and choose indicator with matching transition range

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com