The assay of streptomycin by polarography is a technique used to quantify the concentration of streptomycin, an antibiotic, by analyzing its electrochemical behavior in solution. Polarography is based on the reduction or oxidation of analytes at a dropping mercury electrode (DME) or other types of working electrodes under controlled conditions of potential.


Here’s an outline of the steps and principle involved in the polarographic assay of streptomycin:

1. Principle:

  • Streptomycin contains functional groups that can undergo redox reactions (such as amino groups) at the electrode surface when subjected to a specific potential.
  • In polarography, an increasing potential is applied to the working electrode, and the current generated due to the reduction (or oxidation) of streptomycin is measured.
  • The resulting current is proportional to the concentration of streptomycin in the solution.

2. Preparation of the Sample:

  • Streptomycin is dissolved in a suitable electrolyte solution that allows for good conductivity and facilitates the reduction or oxidation process. Typically, an aqueous solution with an appropriate buffer is used.
  • Sometimes, complexing agents like cobalt or copper ions may be added to enhance the electrochemical signal from streptomycin.

3. Instrumentation:

  • Working Electrode: The dropping mercury electrode (DME) is most commonly used in classical polarography because of its high sensitivity.
  • Reference Electrode: A reference electrode such as the saturated calomel electrode (SCE) or silver/silver chloride electrode is used to maintain a stable potential.
  • Auxiliary Electrode: A platinum wire or similar is used as the auxiliary electrode.

4. Procedure:

  1. The solution containing streptomycin is introduced into the polarographic cell.
  2. The electrolyte provides the ionic medium necessary for the redox reaction.
  3. A slowly increasing potential is applied to the working electrode.
  4. At a specific potential, streptomycin undergoes a reduction (or oxidation), causing a current to flow.
  5. The current is recorded as a function of the applied potential, resulting in a polarogram (current vs. potential graph).

5. Analysis:

  • The polarogram will show a current peak corresponding to the reduction or oxidation of streptomycin at a specific potential.
  • The height of the wave or peak current is directly proportional to the concentration of streptomycin.
  • A standard curve is constructed by running samples with known streptomycin concentrations. The unknown sample concentration is then determined by comparing its peak current to the standard curve.

6. Advantages:

  • Polarography is a sensitive method that can detect even small concentrations of streptomycin.
  • It is relatively simple and can be done without requiring sophisticated instrumentation.

7. Disadvantages:

  • The method may require a highly controlled environment (such as pH, temperature, and ionic strength) to avoid interference.
  • Care must be taken to avoid contamination or interaction with other electroactive species in the solution.

This assay is useful for determining the purity and concentration of streptomycin in pharmaceutical formulations.