lecturenotespolarographyandcyclic

lecturenotespolarographyandcyclic - POLAROGRAPHY: THREE...

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Unformatted text preview: POLAROGRAPHY: THREE FIGURES USED [N CLASS 4—conlacl for dropping Hg electrode — H q reservoir sample solution 09‘" Plug Dropping mercury electrode assembly and H—cell for polarographic measurements. (Redrawn, with permission, from a paper by J. J. Lingane and H. A. Laitinen, ind. Eng. Chem, Anal. Ed. 1939,11, 504.} The H—cell is comprised of two compartments, separated by a sintered-glass disk and an agar plug saturated with potassium chloride. A dropping mercury electrode lDME) consists of a length of thin—bore capillary tubing attached to the bottom of a stand tube and a mercury reservoir. Under the influence of gravity, mercury issues from the orifice of the capillary in a series of identical drops, approximately 0.5 mm in diameter and a few mm2 in area. By adjusting the height of the mercury column, one can vary the drop time [the time required for a fresh drop of mercury to emerge. grow, and fall from the tip of the capillary}; drop times range from 3 to 6 s with memory heights of 30 cm or more. Since oxygen undergoes reduction at a mercury cathode and masks the reduction of other species, nitrogen (or helium or argon) is bubbled through the sample solution to remove the oxygen and is passed over the surface of the solution during the measurements. ‘15 — 12 — 0’] 9 CI) :1 E e a — .§ '5 ’2' Lu 6 — II II D I 0 J 1 3 l i | I 0 l l I : EH2 : / .l I if L l I | __l | | U —0.2 —0.4 —-U.6 —0.8 --1.0 —1.2 - 1.4 —1.6 —1.8 E vs. SCE, Volts Representative polarograms. Curve A: Polarogram for reduction of 2.00 x 10—3 M thalliumll] in a 0.1 M potassium nitrate solution. Curve B: Residual-current curve for a 0.1 M potassium nitrate solution by itself. Note the half- wave potential, E132, which can be used to identifyr the species being reduced and the average diffusi0n current, lat-women}, which is related to the concentration of the electroactive species. CURRENT. microamperes e | t.._ l I | I l —0.3 —0.5 —0.? —0.9 —1.1 —1.3 --1.5 —l.? —1.9 E vs. SCE. volts Representative polarograms. Curve A: Polarogram for a mixture of thalliumll}, cadmiumlli}, and nickellll}, each at a concentration of 1.00 x 10"1 M, in a buffer consisting of 1 M ammonia and 1 M ammonium chloride. Curve B: Residualncurrent curve for the supporting electrolyte solution by itself. Note that a difference in half—wave potentials of 0.2 V or larger is needed to ensure good resolution of the individual waves. 4L COMMON ELECTRODES FOR CYCLIC VOLTAMMETRY Micrometer will: scale reading E HMDE ZIFOJ mm |\«'lercur\.r drop Mercu riv con tact —' y {C} Glassy carbon Il (lisk [0.3 cm diam] 5 cm lbl la] Typical electrodes for cyclic voltammetry. (a) Micrometer-type hanging mercury drop electrode {HMDE}; commercial versions of this electrode are available. {b} Glassvmcarbon disk electrode; this electrode can also be coated with a thin film of merCUry to give a mercury film electrode (MFE). {c} Platinum wire-type HMDE (DA-mm diameter Pt wire sealed into glass; a mercury drop is caught on a small glass spoon and attached via capillary,r action to the exposed cross section of the platinum wire. ...
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lecturenotespolarographyandcyclic - POLAROGRAPHY: THREE...

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