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:

• 2.0 M KCl stock solution, provided.
• 0.2 M KCl prepared from stock enough to last entire experiment, approximately 2 liters.
• Triton X-100 surfactant, 0.2%, provided
• Metal standards, the following 100 mM stock solutions are provided: Cd⁺², Ni⁺², Co⁺², Pb⁺², and Zn⁺²
• Nitrogen, oxygen free

Samples: The instructor will supply the following:

• qualitative mixture
• quantiative unknown (single element)

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.

• Shake 10 mL of 0.2 M KCl in a 50 mL flask for 4 minutes. Transfer the 0.2 M KCl to the sample cell and scan from 0.0 V to -2.0 V. Observe the oxygen double wave which results. What are the half cell reactions?
  
  
• Bubble nitrogen gas through a separate 10 mL portion of 0.3 M KCl solution for 4 minutes and repeat the same scan as in #1. The observed current is designated the residual current, i_R, and is a function of the applied voltage.
  
  
• Prepare 20 mL of a 1.0 mM Co⁺² solution in 0.2 M KCl and deaerate 10 mL for 4 minutes. Scan this solution from 0.0 V to -1.5 V.
  
  
• To the remaining 10 mL of Co⁺² solution add 0.1 mL of 0.2% Triton X-100, mix, deaerate, and repeat the scan as above. Note the any differences.
  
  
• For each of the remaining metals, prepare 10 mL of a 1.0 mM solution in 0.2 M KCl, add 0.1 mL of 0.2% Triton X-100, deaerate for 4 minutes, and scan from 0.0 V to -1.5 V, noting the location of the polarographic wave (E_1/2 values) for each one.
  
  
• Unknown Mixture (Qualitative/Quantitative Identification).
  1. Obtain from your instructor a mixture of metal ions to be identified. The mixture will contain at least two different metals. Add 0.2 mL of Triton X-100 and dilute to 20 mL with 0.2 M KCl.
     
     
  2. Deaerate 10 mL of the unknown for 4 minutes. Scan the unknown in DC mode from 0.0 V to -1.5 V and identify all the metals using their E_1/2 values.
     
     
  3. Deaerate and scan the second 10 mL portion of the unknown in differential pulse mode from 0.0 V to -1.5 V. Change the current full scale setting as needed; note the change in current sensitivity. Identify all the metals using their E_1/2 values. The instructor will indicate which of the metals is to be quantitated.
     
     
  4. Prepare a series of standard solutions of that metal in the following concentrations: 0.20, 0.50, 1.0 and 2.0 mM. Add Triton X-100 surfactant to the solutions, deaerate them, and run them in the same manner in which your unknown was run.
     
     
  5. Turn off the N₂ tank and the instruments.

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

• Comment on the removal of O₂ by use of nitrogen. How might the presence of oxygen affect the subsequent experiments which you ran?
  
  
• Comment on the use of the maxima suppressor. What would be the result of addition of an excess of this substance? What effect would its absence have had on your data?
  
  
• Compare the DC and differential pulse modes in the mixture analysis. Indicate which method is superior and in what respect and explain why. Report the identity of all the metals in your unknown sample, and any problems associated with their determination.
  
  
• Report the identity and concentration of the metal selected by the TA. Include the plot of I_d vs concentration with the position of your unknown labeled on the plot.
  
  
• Polarographic Wave Equation: Choose one of your experimental polarograms and use it to determine reversibility, n calculated, n via slope, and E_1/2 from

E_de = E_1/2 + 0.0591/n log₁₀[(I_d-I)/I]

where E_de is the potential of the dropping-mercury electrode, E_1/2 the half wave potential, I the current at E_de (corrected for residual current), I_d the diffusion current, n the number of electrons involved in reduction.

• The equation is the equation for a straight line (y = mx + b), and thus a plot of E_de against log₁₀(I_d-I)/I should produce a straight line with a slope of 0.0591/n volt. Thus the value of n can be experimentally determined.
  
  
• When the value of the logarithmic term becomes zero, the above equation becomes: E_de = E_1/2 and the half-wave potential is obtained from the point of the plot where the straight line crosses the abscissa. Substances that obey these equations (i.e., yield linear plots) are said to be reversibly oxidized, or reduced.
  
  
• Plot log (I_d-I)/I against potential (E) from your polarogram data. Select about four or five readings on each side of the half-wave potential from the graph of current versus voltage in order to calculate the values of the logarithmic term in the above equation. Indicate on the graph: 1) the value of the slope, 2) the calculated value of n, and 3) the half-wave potential.