Choice of solvents and solvent effects in UV MCQs With Answer

Choice of solvents and solvent effects in UV MCQs With Answer

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Introduction

Understanding how solvents influence ultraviolet (UV) spectra is essential for reliable qualitative and quantitative analysis in Modern Pharmaceutical Analytical Techniques. In this quiz, we focus on the choice of solvents and solvent-induced spectral effects—topics that directly impact λmax selection, molar absorptivity, band shape, and baseline stability in M. Pharm laboratories. You will tackle practical and mechanistic questions: UV cut-off criteria, polarity and hydrogen-bonding effects on n→π* and π→π* transitions, solvent blank preparation, pH/ionization influences, co-solvent strategies, and common pitfalls (e.g., DMSO near 260–280 nm). Each MCQ includes the correct answer to help you self-evaluate and strengthen your command of solvent selection for robust UV-Vis spectroscopy.

Q1. What is the most important criterion when selecting a solvent for UV spectrophotometric measurement of an analyte at a specific wavelength?

  • High viscosity to minimize convection in the cuvette
  • Low UV cut-off below the target measurement wavelength and chemical inertness toward the analyte
  • High boiling point to reduce evaporation during measurement
  • High refractive index to increase signal intensity

Correct Answer: Low UV cut-off below the target measurement wavelength and chemical inertness toward the analyte

Q2. Which solvent among the following has the lowest typical UV cut-off and is therefore most suitable for far-UV measurements?

  • Ethanol (≈210 nm)
  • Toluene (≈285 nm)
  • Acetonitrile (≈190 nm)
  • Chloroform (≈245 nm)

Correct Answer: Acetonitrile (≈190 nm)

Q3. Increasing solvent polarity and hydrogen bonding typically causes what change in n→π* transitions of carbonyl compounds?

  • Bathochromic shift with hyperchromic effect
  • Hypsochromic shift with hypochromic effect
  • No change in λmax but increased band broadening
  • Bathochromic shift with hypochromic effect

Correct Answer: Hypsochromic shift with hypochromic effect

Q4. For π→π* transitions in conjugated systems, what is the general effect of increasing solvent polarity?

  • Bathochromic shift due to greater stabilization of the excited state
  • Hypsochromic shift due to ground-state stabilization
  • No shift but significant decrease in molar absorptivity
  • Peak splitting due to vibronic coupling only

Correct Answer: Bathochromic shift due to greater stabilization of the excited state

Q5. You need to measure absorbance at 230 nm. Which solvent is the safest choice to avoid solvent absorption interference?

  • Dichloromethane (≈230 nm cut-off)
  • Methanol (≈205 nm cut-off)
  • DMSO (≈268 nm cut-off)
  • Toluene (≈285 nm cut-off)

Correct Answer: Methanol (≈205 nm cut-off)

Q6. What is the primary reason protic solvents can cause a hypsochromic shift in n→π* bands?

  • They increase the oscillator strength of π→π* transitions
  • They hydrogen-bond to the nonbonding electrons, stabilizing the ground state more than the excited state
  • They decrease the dielectric constant of the medium
  • They form charge-transfer complexes with all analytes

Correct Answer: They hydrogen-bond to the nonbonding electrons, stabilizing the ground state more than the excited state

Q7. Which practice best corrects for solvent absorbance and instrument drift during UV measurements?

  • Using water as the blank for all organic solvents
  • Filling the reference cell with the same solvent (or solvent mixture) used for the sample
  • Leaving the reference cell empty to maximize throughput
  • Using a saturated solution of the analyte as blank

Correct Answer: Filling the reference cell with the same solvent (or solvent mixture) used for the sample

Q8. Below 320 nm, which cuvette material is required to avoid cuvette-induced absorption?

  • Plastic (polystyrene)
  • Ordinary glass
  • Quartz (fused silica)
  • Acrylic

Correct Answer: Quartz (fused silica)

Q9. Which empirical solvent polarity scale is commonly used to interpret solvatochromic shifts in UV-Vis spectra?

  • Refractive index (n)
  • ET(30) scale
  • Viscosity (η)
  • Surface tension (γ)

Correct Answer: ET(30) scale

Q10. The presence of an isosbestic point in a UV spectrum during a pH titration indicates which of the following?

  • Formation of multiple absorbing intermediates
  • Thermal decomposition of the analyte
  • Clean interconversion between exactly two species with constant total absorptivity at that wavelength
  • Strong scattering from suspended particles

Correct Answer: Clean interconversion between exactly two species with constant total absorptivity at that wavelength

Q11. To dissolve a poorly soluble analyte while minimizing background absorbance in the far-UV, which strategy is most appropriate?

  • Use DMSO as the primary solvent for all measurements
  • Use the minimum amount of a low cut-off co-solvent (e.g., acetonitrile) and dilute with water
  • Use toluene to improve solubility regardless of its cut-off
  • Increase temperature and ignore the solvent baseline

Correct Answer: Use the minimum amount of a low cut-off co-solvent (e.g., acetonitrile) and dilute with water

Q12. When switching the solvent for an aromatic compound from hexane to ethanol, which effect on the principal π→π* band is most likely?

  • Bathochromic shift with possible hyperchromic effect due to increased solvent polarity
  • Hypsochromic shift with hyperchromic effect
  • No change in λmax but complete loss of fine structure
  • Emergence of an n→σ* band in the visible region

Correct Answer: Bathochromic shift with possible hyperchromic effect due to increased solvent polarity

Q13. Which practice most effectively minimizes baseline noise from solvent impurities in UV measurements?

  • Using technical-grade solvents to reduce cost
  • Using UV-spectroscopy grade solvents and verifying a flat solvent blank before analysis
  • Adding salt to the solvent to increase ionic strength
  • Using plastic cuvettes to absorb impurities

Correct Answer: Using UV-spectroscopy grade solvents and verifying a flat solvent blank before analysis

Q14. You must record spectra at 210 nm in methanol (cut-off ≈205 nm). Which adjustment best reduces solvent absorbance saturation?

  • Using a 0.5 cm or 0.2 cm pathlength quartz cell
  • Switching to chloroform
  • Using glass cuvettes
  • Raising the sample concentration

Correct Answer: Using a 0.5 cm or 0.2 cm pathlength quartz cell

Q15. Which type of electronic transition is generally more sensitive to specific hydrogen-bonding effects of protic solvents?

  • σ→σ*
  • π→π*
  • n→π*
  • Charge-transfer (donor–acceptor) bands only

Correct Answer: n→π*

Q16. Relative to hexane, which solvent would be expected to induce the largest bathochromic shift for a π→π* transition?

  • Toluene
  • Ethanol
  • Water
  • Cyclohexane

Correct Answer: Water

Q17. To obtain the spectra of purely protonated and deprotonated forms of a weak acid for method development, which approach is best?

  • Record spectra at pH near the pKa only
  • Record spectra in unbuffered water
  • Record spectra in buffers at pH ≪ pKa (fully protonated) and pH ≫ pKa (fully deprotonated)
  • Use an organic solvent with no water present

Correct Answer: Record spectra in buffers at pH ≪ pKa (fully protonated) and pH ≫ pKa (fully deprotonated)

Q18. In LC–UV applications, why is acetonitrile often preferred over methanol when detection is required below ~210 nm?

  • Higher UV cut-off and higher viscosity
  • Lower UV cut-off and lower viscosity, giving a flatter baseline
  • Strong hydrogen-bond donor character enhances signals
  • It is nonflammable and safer to use

Correct Answer: Lower UV cut-off and lower viscosity, giving a flatter baseline

Q19. What is the main drawback of using DMSO as a solvent for UV measurements around 260–280 nm?

  • DMSO induces strong fluorescence that obscures UV absorbance
  • DMSO has a UV cut-off near ~268 nm and can contribute significant background absorbance
  • DMSO reacts with all carbonyl compounds
  • DMSO causes cuvette etching at room temperature

Correct Answer: DMSO has a UV cut-off near ~268 nm and can contribute significant background absorbance

Q20. How can temperature changes influence solvent effects on UV spectra of hydrogen-bonding systems?

  • Temperature has no measurable effect on UV spectra
  • Increasing temperature can weaken hydrogen bonding, causing small shifts (often reversing hypsochromic trends) and slight intensity changes
  • Increasing temperature always causes hypsochromic shifts regardless of the system
  • Only the baseline noise changes; λmax is strictly constant

Correct Answer: Increasing temperature can weaken hydrogen bonding, causing small shifts (often reversing hypsochromic trends) and slight intensity changes

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