Polarography

Purpose: Polarography is the measurement of the current that flows in solution as a function of an applied voltage. The actual form of the observed "polarographic wave" depends upon the manner in which the voltage is applied and on the characteristics of the working electrode. The working electrode is often a Dropping Mercury Electrode (DME), and the polarographic wave thus has oscillations imposed on it from the variations in mercury drop size. This experiment employs two methods of applying the voltage, a linear sweep (DC) and a differential pulse. The working electrode employed is called a Static Mercury Drop Electrode (SMDE), and provides a more sensitive measurement of the faradaic current than the more traditional DME. The contrast between it and a standard DME is illustrated in the figure below. The SMDE is used in this experiment to obtain DC and differential pulse polarograms of various metal ions in solution, illustrating the abilities of polarography for qualitative (metal species identification) and quantitative analysis.

Consult your textbook and the Student Instructions for further information on the principles of polarography and for representative plots of polarographic (voltammetric) waves obtained through the various voltage application methods.

Equipment: PARC model 174A polarographic analyzer and 303A SMDE.

Reagents:

Samples: The instructor will supply the following:


Procedure

Consult the Student Instructions for instrument operation. Carefully turn on the Nitrogen tank. Do not exceed 4 PSI. Open the N2 tank needle valve slowly with the SMDE purge "ON", and adjust to a slow bubbling rate. Leave the N2 on for the remainder of the experiment. Start with a 5 second mercury drop time (SMDE setting, small drop size) and adjust scan rate and/or drop time as needed to obtain a good polarogram (see Figures V.3 and V.4 in the Student Instructions). If the SMDE refuses to respond to its control buttons, turn the 174A power off then on again. Start with a current sensitivity of 10FA full scale, adjusting as needed to keep the polarogram on scale but large enough for easy data analysis.


Comments

  1. Be sure that you have completely labeled each of the graphs obtained from the instrument.

  2. The instrument is quite susceptible to external shorts and open circuits. Be sure that the "zero" switch is depressed and the selector switch is "OFF" before lowering the cell and preparing for the next scan.

  3. The used solutions are best disposed of by aspirating the solutions using the water aspirator in the wet room. The trap is provided to collect Hg if it is accidentally aspirated from the pool in the bottom of the cell.

  4. Use the same precautions with mercury that you would with any extremely hazardous substance. Keep in mind that its effects are both short and long term. Immediately clean up any spills with the mercury "VacuPick", and report them to your instructor.

Report

Ede = E1/2 + 0.0591/n log10[(Id-I)/I]

where Ede is the potential of the dropping-mercury electrode, E1/2 the half wave potential, I the current at Ede (corrected for residual current), Id the diffusion current, n the number of electrons involved in reduction.