Can You Do Quantitative Staircase Polarography?


While textbooks may assign staircase Polarography at a dropping mercury electrode to the history pile, it still remains one tool used in practical lab situations for the analysis of toxic heavy metals, such as Cd+2, when present at the sub-millimolar concentration range. It is as useful as it is because it has, once properly calibrated against matrix matched standards, singular precision that can be translated to equivalent accuracy. In this example you will be able to see for yourself how simple it is to determine the concentration of Cd+2 in a matrix matched unknown using only two standard solutions. In the first illustration below we see how the residual or background current from the sample matrix is identified and subtracted from the diffusion current for the reduction of Cd+2 to Cd, leaving the desired limiting current (dotted line). It is this current that is directly proportional to the concentration of Cd+2.
Cd quantitation

Following this, all that we have to do is to determine the proportionality constant relating the limiting current to the concentration. While only one standard solution is needed for this if the background current is actually zero when the Cd+2 concentration is zero, it is best to be on the cautious side and run two standard solutions. These are shown in the next two figures. From these, by measuring the limiting current on the graphs (do this on your screen), and knowing the concentrations (calculate these from the ml. of standard taken) the calibration constant can be determined. Do this now.
Cd quantitation2

Cd quantitation3


The last step is to duplicate the experimental and solution parameters of the standard solutions when doing a scan of the unknown. Since so much of the solution is matrix, and since the mercury electrode is so reproducible in its dropping character, this is not hard to do. The unknown solution scan is shown below. Determine the Cd+2 concentration in it. Note that we do not know the solution volume, so the exact mass of Cd+2 corresponding to this concentration cannot be determined. After you are done, mail your answer to me for evaluation at walters@stolaf.edu
Cd unknown

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