Polarography (dropping mercury electrode) – application domain Polarograph-based analysis is primarily intended for which kind of samples or systems?

Introduction / Context:
Polarography is an electroanalytical technique that records current–potential curves using a dropping mercury electrode (DME) or related electrodes. It quantifies electroactive species based on diffusion-controlled waves or peaks. Understanding the sample state best suited to polarography prevents misuse and clarifies how solutions must be prepared.

Given Data / Assumptions:

• Technique operates with an electrode immersed in a conductive liquid.
• Analytes must be dissolved and electroactive within the potential window.
• Supporting electrolyte ensures controlled mass transport and ohmic behaviour.

Concept / Approach:
Polarography is fundamentally a solution-phase method. Samples are liquids (often miscible with aqueous or non-aqueous electrolytes) containing analytes that undergo reduction or oxidation at the electrode. Gases and solids can be analysed indirectly only after dissolution in a solvent/electrolyte; isotopic discrimination is not the native purpose of polarography (mass spectrometry excels there). Thus, among the options provided, “miscible liquids” most closely captures the appropriate medium for direct polarographic analysis.

Step-by-Step Solution:

Verification / Alternative check:
Analytical chemistry references place polarography under voltammetric methods for solutions; standard protocols describe preparation of liquid samples and calibration with standard additions.

Why Other Options Are Wrong:

• Gaseous mixture: Requires conversion to solution; not the direct analytical format.
• Solids: Must be dissolved or otherwise transformed for electroanalysis.
• Isotopes: Not specifically a polarographic target; mass-based techniques are preferred.

Common Pitfalls:
Assuming the “mercury” in the electrode implies gas-phase mercury or solid-sample compatibility; it simply provides a renewable liquid-metal electrode surface in solution.

Final Answer:
Miscible liquids