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
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.
- 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
- qualitative mixture
- quantiative unknown (single element)
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, iR, 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 (E1/2 values) for each one.
- Unknown Mixture (Qualitative/Quantitative
- 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.
- 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 E1/2 values.
- 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 E1/2 values.
The instructor will indicate which of the metals
is to be quantitated.
- 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.
- Turn off the N2 tank
and the instruments.
- Be sure that you have completely labeled
each of the graphs obtained from the instrument.
- 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.
- 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.
- 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.
- 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 Id 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 E1/2 from
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.
- The equation is the equation for a
straight line (y = mx + b), and thus a plot of Ede
against log10(Id-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: Ede = E1/2 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
- Plot log (Id-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.